excitation loss detection of synchronous generator …
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Power Systems Engineering Thesis
2020-03-16
EXCITATION LOSS DETECTION OF
SYNCHRONOUS GENERATOR ON
POWER SYSTEM PERFORMANCES
Ygzaw, Alganesh
http://hdl.handle.net/123456789/10370
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EXCITATION LOSS DETECTION OF SYNCHRONOUS GENERATOR ON
POWER SYSTEM PERFORMANCES
Alganesh Ygzaw
A thesis submitted to school of Research and Graduate Studies of Bahir Dar Institute
of Technology, BDU in partial fulfilment of the requirements for the degree of Masters
of Science in Electrical Engineering with specialization in Power System Engineering
in Electrical and Computer Engineering Faculty.
Advisor Name: Dr.-Ing. Belachew Banteyirga (PhD)
Bahir Dar, Ethiopia
March, 2019
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DECLARATION
I, the undersigned, declare that the thesis comprises my own work. In compliance
with internationally accepted practices, I have acknowledged and refereed all
materials used in this work. I understand that non-adherence to the principles of
academic honesty and integrity, misrepresentation/ fabrication of any
idea/data/fact/source will constitute sufficient ground for disciplinary action by
the University and can also evoke penal action from the sources which have not
been properly cited or acknowledged.
Name of the student_______________________________ Signature _____________
Date of submission: ________________
Place: Bahir Dar
This thesis has been submitted for examination with my approval as a university
advisor.
Advisor Name: __________________________________
Advisor’s Signature: ______________________________
Date: __________________________
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© 2019
ALGANESH YGZAW TEFERI
ALL RIGHTS RESERVED
iv
v
To My Brother
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ACKNOWLEDGEMENT
First of all, glory to God for all blessings I have earned in my whole life. I am very
humbled and field with a great gratitude to acknowledge the people that had a great
help in this work. Without his kindness, patience and guidance, the complete of this
work will be impossible; I would like to thank my advisor Dr.-Ing. Belachew B. I need
to give a great appreciation to my family also for their endless support and love
throughout my life. Last but not least I want to sincerely thank and acknowledge all my
friends and individuals for sharing their idea and support in every way.
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ABSTRACT
Generating steadiness of synchronous generators is highly dependent on their exciter,
as the direct current from excitation system sustains generator stator and rotor windings
magnetically coupled. However, any excitation system failure grades generator loss of
excitation and suspends power transmission from generating unit to customers. It is
typically characterized by high active power flow out of the generator with large
reactive power flow into the generator. And this power imbalance increases rotor speed
of generator beyond synchronous speed which result in voltage and current instability
in the grid connected with the generator. At this state, excitation loss protection must
isolate the faulted generator from the remaining system to avoid any damage that can
possibly happened due to excitation loss. This thesis work studies generator excitation
loss relay detection ability on IEEE 9-bus test system and Tana Beles-I power plant on
various excitation loss events. The proposed schemes have simulated and evaluated
using MATLAB/SIMULINK software.
The simulation results show that the relay tripping duration is highly dependent on
initial loading condition of the generator, type of excitation loss and reactive power
support from interconnected systems. Comparatively excitation loss relay shows a good
performance in full loss of excitation than in partial excitation loss. The relay is able to
detect any full excitation loss in less than 6.4second after failure initiated. But it detect
partial excitation loss long after the failure for heavily loaded generators and not detect
at all for lightly loaded generators. On the other hand, the relay has mal-operated for
stable and unstable power swings which are failures outside the exciter.
To overcome mal-operation of excitation loss relay, a back-up protection scheme has
proposed based on study of field voltage, quadrature-axis voltage and generator reactive
power variation in excitation loss event. The proposed algorithm limits the reactive
power consumption of excitation loss generator considering system stability. The back-
up protection has improved the excitation loss detection length to twice less for heavily
loaded generators and 16% less for medium and lightly loaded generators. It has also
differentiate system failures and excitation loss events successfully.
Key words: Excitation Loss, Excitation Loss Protection, Synchronous Generator
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Table of Contents
DECLARATION .................................................................................................... ii
ACKNOWLEDGEMENT ..................................................................................... vi
ABSTRACT ......................................................................................................... vii
LIST OF ABBREVATIONS ................................................................................. xi
LIST OF SYMBOLS ............................................................................................ xii
LIST OF FIGURES ............................................................................................. xiv
LIST OF TABLES .............................................................................................. xvi
CHAPTER-1
INTRODUCTION .................................................................................................. 1
1.1 Background ................................................................................................... 1
1.2 Excitation Systems ........................................................................................ 2
1.2.1 Types of Excitation Systems .................................................................... 3
1.2.2 Control and Protective Function of Excitation System ............................. 5
1.2.2.1 AC and DC Regulators ....................................................................... 6
1.2.2.2 Excitation System Stabilising Circuits ................................................ 6
1.2.2.3 Power System Stabiliser (PSS) ........................................................... 7
1.2.2.3 Volts-per-Hertz Limiter and Protection (V/Hz) ................................... 8
1.3 Overview of Synchronous Generator Protection............................................. 8
1.4 Excitation Loss ............................................................................................ 11
1.5 Problem Statement ....................................................................................... 13
1.6 Objective of the Study ................................................................................. 14
General Objective .......................................................................................... 14
Specific Objectives......................................................................................... 14
1.7 Scope of the Study ....................................................................................... 14
1.8 Significance of the Study ............................................................................. 14
1.9 Document Organization ............................................................................... 15
ix
CHAPTER-2
LITERATURE REVIEW ..................................................................................... 16
CHAPTER 3
SYSTEM MODELLING AND MATHEMATICAL OVERVIEW OF EXCITATION
LOSS....................................................................................................................... 20
3.1 System under Study ..................................................................................... 20
3.2 Synchronous Generator Modelling ......................................................... 21
3.3 Characteristics of Synchronous Generator in Excitation Loss Event ............. 25
3.3.1 Initial Loading Effect............................................................................. 30
3.4 Excitation Loss Protection Relay ................................................................. 32
CHAPTER 4
SIMULATION RESULTS AND DISCUSSIONS ................................................ 35
4.1 Full Loss of Excitation ................................................................................. 36
4.1.1 Field Winding Short Circuit ................................................................... 36
4.1.2 Sudden Main Circuit Breaker Failure ..................................................... 39
4.1.3 Sudden Loss of AC Voltage to Excitation System.................................. 40
4.1.4 Field Winding Open Circuit ................................................................... 42
4.2 Partial Loss of Excitation ............................................................................. 44
4.2.1 30% Field Voltage Loss ......................................................................... 46
4.2.2 50% Field Voltage Loss ......................................................................... 47
4.2.3 70% Field Voltage Loss ......................................................................... 48
4.3 Effect of Excitation Loss on Parallel Connected Generators ......................... 50
4.4 Power Swings .............................................................................................. 52
4.3.1 Short Circuit Faults................................................................................ 53
4.3.2 Outages ................................................................................................. 55
4.5 Backup Protection for Excitation Loss Detection ......................................... 57
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CHAPTER 5
CONCLUSIONS AND RECOMMENDATION ................................................... 67
5.1 Conclusion .................................................................................................. 67
5.2 Recommendations for Future Work ............................................................. 69
REFERENCES ..................................................................................................... 70
APPENDIX A ...................................................................................................... 74
A.1 Full Loss of Excitation ................................................................................ 74
A.2 Partial Loss of Excitation ............................................................................ 74
A.3 IEEE ST1A Excitation System .................................................................... 75
A.4 Two-axis Model Initial Values .................................................................... 75
APPENDIX B ...................................................................................................... 76
B.6 Excitation Loss Relay Protection Zones ...................................................... 78
APPENDIX C ...................................................................................................... 81
Physical Representation of Excitation Loss Event .............................................. 81
xi
LIST OF ABBREVATIONS
AC Alternating current
ANSI American National Standards Institute
AVR Automatic Voltage Regulator
BiT Bahir Dar Institute of Technology
CB Circuit Breaker
CLOE Complete Loss of Excitation
DC Direct current
EPS Ethiopian Power Systems
FFL Field flux linkage
FFT Fast Fourier Transform
FW Field Winding
G-1 Generator-1
G-2 Generator-2
IEEE International Electrical Engineering and Electronics
LOE Loss of Excitation
MATLAB Matrix Laboratory
MVA Mega Volt Ampere
MVAR Mega Volt Ampere Reactive
MW Mega Watt
OEL Over Excitation Relay
OOS Out of step
PID Proportional integrator differentiator
PLOE Partial Loss of Excitation
rms Root Mean Square
R-X Resistance- Reactance
SPS Stable Power Swing
SVM Space Vector Machine
UEL Under Excitation Relay
xii
LIST OF SYMBOLS
Ed′ Direct axis voltage behind transient reactance
Eq′ Quadrature axis voltage behind transient reactance
Te Electrical torque output
Tm Mechanical torque
Xd D-axis steady state reactance
Xd′ D-axis transient reactance
Xq Q-axis steady state reactance
Xq′ Q-axis transient reactance
φd D-axis stator flux linkages
φq Q-axis stator flux linkages
ωs Synchronous speed (rad/s)
∆ω Speed deviation
D Damping constant
Efd Field voltage
H Inertia constant (MWs/MVA)
Id D-axis current
Iq Q-axis current
It Generator terminal current
Pm Mechanical power input
Pt Active power
Qt Reactive power
Ra Armature resistance
s Slip
T’’d0 D-axis sub transient open loop time constant(s)
T’’q0 Q-axis sub transient open loop time constant(s)
T’d0 D-axis transient open loop time constant(s)
T’q0 Q-axis transient open loop time constant(s)
Vd D-axis terminal voltage
Vq Q-axis terminal voltage
Vref Reference voltage
Vt Generator terminal voltage
X’d D-axis transient reactance (pu)
xiii
X’q Q-axis transient reactance (pu)
Xd D-axis steady state reactance (pu)
Xq Q-axis steady state reactance (pu)
Z Terminal impedance
δ Generator’s rotor angle
xiv
LIST OF FIGURES
Figure 1: Excitation system of synchronous generator .............................................. 2
Figure 2: Excitation system control and protective circuits ....................................... 6
Figure 4: Coordination of UEL, LOE relay and stability limit ................................... 8
Figure 5: Synchronous generator protective relays .................................................. 10
Figure 6: Single line diagram of (a) IEEE-9 bus test system (b) Tana Beles-I power
plant ........................................................................................................................ 20
Figure 9: Mechanical block diagram of synchronous generator ............................... 23
Figure 10: Simplified model of ST1A excitation system ......................................... 23
Figure 11: Generator(a) Terminal and internal voltage (b) q-axis and d-axis voltage in
LOE event created at 1second .................................................................................. 26
Figure 12: Generator (a) Active and reactive power (b) d-axis and q-axis currents in
LOE event ............................................................................................................... 27
Figure 13: Generator (a) Rotor speed (b) load angle in LOE event at 1second ......... 28
Figure 14: Generator terminal (a) resistance and reactance (b) impedance in excitation
loss event ................................................................................................................. 29
Figure 15: Generator parameter variation in LOE event at various generator loading
conditions ................................................................................................................ 31
Figure 16: IEEE 9 bus system G-1parameters in G-2 LOE event ............................ 31
Figure 17: Impedance trajectory of LOE relay in different system conditions ......... 33
Figure 18: Flow chart of LOE relay ........................................................................ 34
Figure 19: Simulink model of IEEE 9-bus system with LOE event and LOE relay .. 35
Figure 20: (a) G-2 excitation current and voltage (b) rotor speed and reactive power in
field winding short circuit ........................................................................................ 36
Figure 21: Power flow direction of synchronous machines ..................................... 37
Figure 22: G-2 impedance trajectory in field winding short circuit .......................... 38
Figure 23: G-2 field voltage and current in main CB failure .................................... 39
Figure 24: G-2 impedance trajectory in main CB failure ......................................... 40
Figure 25: G-2 field voltage and terminal impedance in sudden loss of AC voltage to
exciter terminal ........................................................................................................ 41
Figure 26: G-2 impedance trajectory in sudden AC voltage loss to exciter .............. 41
Figure 27 : G-2 Field voltage and field current in field winding open circuit ........... 43
Figure 28: G-2 impedance trajectory in field winding open circuit .......................... 43
xv
Figure 29: Reactive power of G-2 in 70%Efd loss ................................................... 45
Figure 30: G-2 parameter variation in partial loss excitation ................................... 46
Figure 31: G-2 impedance trajectory in 30% field voltage loss................................ 47
Figure 32: G-2 impedance trajectory in 50% loss of excitation................................ 47
Figure 33: G-2 impedance trajectory in 70% loss of excitation................................ 48
Figure 34: Simulink model of Tana Beles-1 power plant ......................................... 51
Figure 35: Tana Beles-I (a) G-1 and (b) G-2 parameters in G-1 LOE event ............. 51
Figure 36: Tana Beles-I G-1 and G-2 impedance trajectories .................................. 52
Figure 37: Tana Beles-I G-1 and G-2 impedance trajectories in field winding (a) short
circuit with SPS (b) open with SPS .......................................................................... 52
Figure 38:G-2 impedance trajectory (a) three phase (b) two phase to ground (c) phase
to phase (d) three phase cleared after 250ms at G-2 terminal .................................... 54
Figure 39: G-2 impedance trajectory (a) three phase fault at G-2 terminal (b) L7-8
outage (c) load rejection (d) G-outage ...................................................................... 55
Figure 40: Flow chart of proposed back up protection ............................................. 57
Figure 41: Q-V curve of LOE relay and proposed back up protection ..................... 60
Figure 42: Terminal voltage reduction in LOE relay and back-up protection .......... 61
Figure 43: G-2 terminal voltage in Partial loss of excitation (a) medium load 20%Efd
loss (b) light load 30%Efd loss .................................................................................. 64
Figure 44A-1: 9-bus G-2 impedance trajectory in 90% and 70% loading ................ 74
Figure 45A-2: 9-bus G-2 impedance trajectory in (a) 90% and (b) 60% field voltage
loss .......................................................................................................................... 74
Figure 46A.3: Block diagram of IEEE ST1A excitation system .............................. 75
xvi
LIST OF TABLES
Table 1: Parameters of generators under study ........................................................ 21
Table 2: LOE relay detection ability in main causes of full excitation loss .............. 44
Table 3: LOE relay detection ability in partial loss of excitation in different loading
conditions ................................................................................................................ 49
Table 4: Performance of LOE relay in power swings .............................................. 56
Table 5: Comparison of actual and proposed excitation loss detection in field winding
short circuit ............................................................................................................. 62
Table 6: Comparison of actual and proposed excitation loss detection in field winding
open circuit .............................................................................................................. 62
Table 7: Comparison of actual and proposed excitation loss detection in partial field
voltage loss .............................................................................................................. 63
Table 8: Comparison of actual and proposed excitation loss detection in system
disturbances ............................................................................................................. 65
Table 9 B-1: IEEE 9-bus system required Machine Data ......................................... 76
Table 10 B-2: IEEE 9-bus system load data ............................................................ 76
Table 11 B-3: IEEE 9-bus Transmission line Data .................................................. 77
Table 12 B-4: IEEE 9-bus system excitation system data ........................................ 77
Table 13 B-5: Tana Beles-1 System data ................................................................. 78
1
CHAPTER-1
INTRODUCTION
1.1 Background
The versatility behaviour of electrical energy has grasp researchers attention in the
recent centuries to develop the efficiency of electric power delivery and sustain the
activities of life as easily as possible. Ever since discovered, electrical energy has
gradually improve human life to better than it was before in manufacturing, health and
generally transformation of life activities to easy and labour intensive system. In
consequence, the dependence of human life on electricity has increase gradually from
time to time and a lot of researchers have focused on improvement of reliability and
security of energy transmission from the source to customer. On the other hand
unprotected and unsecured electrical energy deliver can cause a serious damage to
properties even to life of living things. So, a modern power system concern must be
consistency and security on all part of power system (generation unit, transmission unit
and distribution unit) to make the system secure and economical since survival comes
before any gain.
The efficacy of electrical energy transmission in all part of power system is highly
dependent on the reliability of synchronous generating machines at any situation so
that a truthfulness of power transfer from the generating unit to customer is maintained
continually. Generally synchronous generators have two inputs; mechanical input from
turbine and field voltage from excitation system. And at normal condition they are able
to produce and deliver active power due to the mechanical input and reactive power
due to the field voltage. Indeed a secure and well protected excitation system should
be one of the important concerns in modern power plants.
However, any failure in excitation system grades excitation loss in the generator and
the generator will suddenly start to consume reactive power from the grid connected
with it. In this condition the generator must be isolated from the remaining system. In
actual power system industries generator terminal impedance variation is used to detect
excitation loss event. However, generator terminal impedance variation with system
disturbances threatens the accuracy of excitation loss relay to mal-operate for un-
necessary power swings that created due to failures outside the generator. In addition
this excitation loss detection method has an apprehended detection ability in partial
excitation loss and the mal-operation of the relay for system disturbances results in
2
unwanted generator tripping. And this further jeopardize stability of the gird due to
unnecessary generator outage.
1.2 Excitation Systems
One of the most significant elements of electric power system is synchronous generator
that changes the mechanical energy from turbine into electrical energy [1]. Energy
transformation is possible only if generator have excitation system which defines the
generator reactive power output values. This means that generator excitation regulation
is actually regulation of generator output energy and also impacts the stability of entire
electric power system. Excitation system is part of generating units in which it produces
flux by passing current in the field winding to supply its output to synchronous
generating machines through either brushes or slip rings to run or excite synchronous
machines [1]. Its power makes up generally 0.2-0.8% of the generator power to
maintain the terminal voltage of the generator within the accepted voltage range by
responding quickly to system component variation [2].
Limitation and
Protection unit
Measuring
elements
Controller Exciter Generator
Power system
stabilizer
Reference value
System
Figure 1: Excitation system of synchronous generator
Generally excitation system consists of two relatively independent components,
excitation regulator (AVR) and the exciter itself with requirements to keep the
generator in a condition when it is possible to transmit the power close to line power
limit, ensure sufficient dynamic stability reserve, damping power swings of generator
after any failure, maintain stability during change of properties of system and to ensure
high operating reliability of the system [2] [3]. Thus, it is able to control voltage and
3
reactive power flow by ensuring if the machine does not exceed the capability limits.
Generally, an excitation system includes the following elements.
The components of excitation system works inter-correlate to provide Direct Current
(DC) to the generator field winding [1].
Controller- It processes and amplifies input control signals to a level and form that is
appropriate to control the exciter.
Measuring elements- this includes the terminal voltage transducer which sense, rectify
and filtered the generator terminal voltage to a DC quantity and a load compensator to
compare the terminal voltage with a reference voltage.
Power system stabilizer- provides additional input signal to the regulator to damp
power system oscillations.
Limiters and protective circuits- limit the capability limit of exciters and generators.
Co-operating the above excitation system components, the main properties of excitation
system regulation should include these three characteristics: speed of system operation,
autonomy of excitation system and maximal drive security. Speed of operation is
important to maintain stability of electric power system in the meaning of reactive
power transmitting and receiving, fast de-excitation in case of internal failure and
overvoltage limitation in case of sudden unloading.
Autonomy of excitation systems means that excitation system supply must be ensured
in every condition of a drive. And drive security is function of reliabilities of all
incorporated components [4] [5].
1.2.1 Types of Excitation Systems
Excitation systems of synchronous generators can be classified in the meaning of
construction as static or rotating and according to excitation energy source as separate
excitation systems and self-excited systems [5]. In static excitation systems energy
needed for excitation is brought to generator field winding via slip-rings with carbon
brushes. To perceived use of brushes when supplying high field current to large
synchronous machines, use of brushes in static excitation systems have been eliminated
in rotating excitation systems [1]. They are brushless excitation systems but direct
measurement of generator field current and voltage is impossible in this type of exciters.
Brushless systems are used for excitation of larger generators (power over 600MVA)
and in flammable and explosive environments. Brushless system consists of Alternating
4
Current (AC) exciter, rotating Diode Bridge and auxiliary AC generator realized with
permanent magnet excitation.
Separate excitation systems are independent of disruptions and faults that occur in
electric power system, and have possibility to force excitation [4] [5]. On the other
hand, self-excited excitation systems are connected to the grid and utilizes part of
generator power. Generally there are three major groups of generator excitation
systems, with nineteen different excitation system models altogether: Direct Current
Commutator Exciters (DC), Alternator Supplied Rectifier Excitation Systems (AC) and
Static Excitation Systems (ST) [6].
DC excitation systems
These type of exciters uses direct current generators as sources of excitation power and
provided current to the rotor of the synchronous generator through slip rings. The
exciter may be placed on the same shaft with power generator or separately driven by
a motor. Nowadays, DC type exciters are mainly suppressed by the other two types and
a few new synchronous machines are being equipped with these. This group consists
of four models as described in [6].
DC1A model is used for self-excited shunt fields with voltage regulator operating in a
buck-boost mode. It represents field-controlled DC commutator exciters with
continuously acting voltage regulators that have generator output voltage as main input.
And it have improved to DC2A by adding voltage regulator output limits. On the other
case DC3A model is used in DC commutators with non-continuously acting regulators.
DC4B is newly added model that differs from DC1A in implemented controls and
contains PID controller.
AC excitation systems
In this types of excitation system AC machines are used as sources of the main
generator excitation power and rectification of AC voltage is carried out through
controlled or non-controlled rectifiers to provide DC to the generator field winding. But
these systems do not allow negative field current except AC4A model. This is the main
disadvantage of this type of systems because it does not allow de-excitation of
generator. AC1A model is used for field-controlled alternator-rectifier excitation
systems, with non-controlled rectifier in case of separate excitation. AC2A differs from
AC1A in additional compensation of exciter time and exciter field current limiting
elements. AC3A and AC4A models are used for self-excitation systems and for systems
with full thyristor bridge in the exciter output circuit respectively [7]. AC5A is
5
simplified model for brushless excitation systems with separate excitation whereas
AC6A represents field-controlled alternator-rectifier excitation systems with system
supplied electronic voltage regulators. AC7B and AC8B model incorporates newer
controls and PID controller. Here, proportional, integral and differential gains are
defined with separate constants [5] [6].
Static excitation systems
In static excitation systems all the elements are stationary. Such systems directly
provide synchronous generator field winding with excitation current by means of slip
rings and the rectifiers gain power from generator through auxiliary windings or a step-
down transformer. In such systems generator itself is power source or the generator is
self-excited. This type of excitation system consist of seven models and the possibility
to produce negative excitation current is their significant advantage. Thus, it provides
quick de-excitation which may be needed in case of generator internal fault. ST1A
model represents systems in which excitation power is supplied from generator
terminals or separate bus. Having this advantage, in this thesis work ST1A exciter
model will be used in all scenarios of the study. ST2A is model for systems that utilize
both current and voltage generator terminal quantities to comprise power source [6].
Model ST3A uses a field voltage control loop to linearize control characteristic of the
exciter and ST4B only varies from ST3A model due to usage of PI instead of lag-lead
controller. ST5B is variation of ST1A with alternative over excitation and under
excitation inputs and additional limits. Voltage regulator of ST6B model consists of a
Proportional Integrator (PI) voltage regulator with an inner loop of field voltage
regulation and pre-control. ST7B model represents static potential-source excitation
systems, with PI controller which may be turned into PID controller if phase lead-leg
filter used in series, which is typical case for brushless excitation systems.
Today, most excitation systems are AC or static types because of the fast response
ability [8].
1.2.2 Control and Protective Function of Excitation System
Capability limit of exciters and generators are limited through the limiters and
protective circuits of the exciter. This function includes set limits of field current,
terminal voltage limit, volts-per-Hertz limit, maximum and under-excitation limits.
The type of limiters and their output signals location have given in Fig. 2. For secure
6
and reliable generator operation, most of these limiting circuits are included as part of
excitation system [5].
1.2.2.1 AC and DC Regulators
The main function of AC and DC regulators is to maintain stator voltage and to hold
the field voltage at constant respectively. DC regulator is also used as back-up of AC
regulator to test and start-up and to outfit to situations when AC regulator is at fault. In
this condition the field voltage is regulated and a manual adjusting of set point is
required thus DC regulator is called as Manual control.
Voltage sensing and load
compensation
PSS
Voltage sensing
DC regulator
AC regulator
Exciter Field
shortingGenerator
Exc. Sys.
stabilizing
circuits
Overexc. limiter
Under exc.
limiter
V/Hz limiter and
protection
AC voltage adjust
DC voltage adjust
Figure 2: Excitation system control and protective circuits
1.2.2.2 Excitation System Stabilising Circuits
Stabilising circuits are used to improve the dynamic performance of the excitation
system for both AC and DC exciters by minimization of the phase shift caused by
element time constants before synchronization or after load rejection. Depend on type
excitation system, the level of stabilizing system may differ according to time constant
7
effects. The negligible inherent time delays of time constants for static excitation
systems have avoid the requirement of excitation control-system stabilization. The
general derivative feedback of excitation control can be summarized in Fig.3.
1.2.2.3 Power System Stabiliser (PSS)
Power system stabiliser is used for further improvement of power system dynamic
performance of synchronous machines. It stabilize either shaft speed, terminal
frequency or terminal power to output stabilized voltage by damping system
oscillations.
Exciter and AVR
sKF/1+sTF
fdE To generator field
Compensation
Ve
Figure 3: Excitation control system stabilization
1.2.2.4 Under Excitation Limiter (UEL)
UEL prevents generator excitation from reduction of stability limit or stator core end
region heating limit. The main control signal of UEL can be derived either from the
combination of voltage and current or active and reactive power of the generator. When
the UEL set limit is achieved, the limiter controls the excitation system until the signal
reaches below set limit. Since the limit point setting is based on the instability or stator
core heating, the limiter should coordinated with excitation loss relay and small signal
stability limit as shown in Fig.4 [5].
1.2.2.5 Over Excitation Limiter (OEL)
OEL protects generator from overheating from prolonged field overcurrent. It detects
the high field current condition and then after a time delay act through the AC regulator
to ramp down the excitation to about 110% of rated field current; if unsuccessful, trips
the AC regulator and transfers to DC regulator to reposition the set point corresponding
to rated value [5]. If this also does not reduce the excitation to a safe margin, OEL will
initiate an exciter field breaker trip and so a unit trip will be created.
8
1.2.2.3 Volts-per-Hertz Limiter and Protection (V/Hz)
These protection schemes are used to protect generator core and step-up transformer
from damage due excess overheating resulted from extreme magnetic flux of low
frequency and over voltage system condition. V/Hz limiter controls field voltage so as
to limit the generator voltage when the ratio exceed the pre-set limit. Thus the limiter
can also be used as over voltage relay for frequency greater than 60/50Hertiz.
Figure 4: Coordination of UEL, LOE relay and stability limit
1.3 Overview of Synchronous Generator Protection
[Synchronous generators supply almost all the electric power we consume today and
always there is a constant need for reduction of operational and maintenance costs of
large sized synchronous generators. The most efficient way of reducing these costs
would be continuous monitoring of the condition of these generators. This allows for
early detection of the degeneration of the generator’s health, facilitating a proactive
response, minimizing downtime and maximizing productivity [5].
Despite the efficient design and protection of synchronous generators, faults occurring
within the machine cannot be avoided completely. However, the generator protection
relays make sure the faults are eliminated within a short period of time. Since
eliminating a generator from a system may be costly and create instability in the whole
system, faults from outside should be cleared as efficiently as possible before creating
permanent damage in the generator [9]. To achieve the selectivity and sensitivity of the
9
protection relays, a proper coordination should be taken by considering abnormal
operating conditions and type of faults occurring within the machine. Abnormal
conditions of a generator can arise due to stator or rotor field failure or system
disturbances [3] [10] and protective relays for each failures using variety signals are
meant to monitor and provide proper signals to alarm or remove the generator from the
system under faulty conditions [9].
The general protective devices of synchronous generators are given in Fig.5 which
protect the generator uniquely with their own element characteristics and should work
coordinated but without overlapping. The numbering are given according to American
National Standards Institute (ANSI) standard.
Distance relay (Device 21) - it is an impedance relay which uses voltage and current
phases to measure the impedance in front of the generator. Basically this device protects
the generator from an external fault. If impedance falls into the relay characteristic,
relay will trip the generator [11].
Over excitation relay (Device 24)-when the ratio of the voltage to frequency
(volts/Hz) exceeds 1.05 pu for a generator, severe overheating can occur due to
saturation of the magnetic core of the generator and the subsequent inducement of stray
flux in components not designed to carry flux. Such over excitation most often occurs
during start-up or shutdown while the unit is operating at reduced frequencies, or during
a complete load rejection which leaves transmission lines connected to the generating
station or during excitation system failures. Over excitation relay detects this
phenomenon [12] [13].
Power direction relay (Device 32) – it is a reverse power relay which monitors the
direction of generator power to prevent any reverse flow of active power (motoring
mode of operation). Motoring is an abnormal condition that can cause serious
mechanical damage to prime mover. In some applications this relay could be used for
load shedding [14].
Excitation loss relay (Device 40) – it uses the impedance variation of generator
terminal to protect excitation loss event. This relay will be discussed in detail in this
thesis work.
Current unbalance relay (Device 46) - current unbalance relay monitors the negative
sequence component of the current and if this current exceeds from the relay setting,
relay will operate. The most common causes of unbalance current are system
asymmetries, unbalance loads, unbalance fault and open phase [12].
10
These system conditions produce negative-phase-sequence components of current
which induce a double-frequency current in the surface of the rotor. These rotor currents
may cause high and possibly dangerous temperatures in a very short time [13].
CB
64F
61
49
Field Ground
Generator Inte-
rturn
Stator Temp.
87G
Gen.Diff
60 Voltage Balance
78
40
32
21
51V
5981O/
U
24
46
Aux VTs
Sys. BackupNeg. Seq. Current
Over/Under Freq.
V/Hz
59N
51N
Gen. Neutral
Overvoltage
Loss of Feild
Loss of Sync
Over-Voltage
Reverse power
MV Line
Figure 5: Synchronous generator protective relays
Over temperature (Device 49) - this relay senses the temperature at different spots of
the generator and provides a thermal protection. Usually this relay is not used for
primary protection [6].
Time delay over current (Device 51) - monitors currents flowing through generator
windings and provide a time delay over load protection for the particular part. The relay
has an inverse time characteristic and provides a time delay which is inversely
proportional to the over load current magnitude [15]. Device 51V is the voltage
restrained time delay over current relay which provides better protection when under
voltage condition exists [16].
11
Over voltage relay (Device 59) -generator overvoltage may occur during a load
rejection or excitation control failure. In case of hydro generators, upon load rejection
the generator may speed up and the voltage can reach high levels without necessarily
exceeding the generator’s V/Hz limit. Over voltage relay is for monitoring the voltage
and if the voltage exceeds from the relay pre-set level, it will trip.
Voltage Balance Relay (Device 60) - it compares two voltages from two different set
of Voltage Transformers (VTs) and trip if these two voltages are not balanced. Most
common use of this relay is to detect VT fuse failure.
Ground fault (Device 64) -the function of this device is to detect ground fault in the
stator or rotor field winding. It is a common practice to ground all types of generators
through some form of external impedance. The purpose of this grounding is to limit the
mechanical stresses and fault damage in the generator, to limit transient voltages during
faults and to provide a means for detecting ground faults within the generator. The
magnitude of stator ground-fault current decreases almost linearly as the fault location
moves from the stator terminals ground fault near the neutral of a wye-ground fault
current becomes small toward the neutral of the generator [17].
Out-of-step relay (Device 78) - out of step relay detects generator loss-of-synchronism
condition. It contains two blinder elements supervised by a mho relay to prevent
nuisance tripping for stable swings. It detects unstable condition of generator through
prolonged system disturbances.
Over/Under frequency (Device 81) - the operation of generators at abnormal
frequencies (either over-frequency or under-frequency) generally results from full or
partial load rejection or from overloading of the generator. Full or partial load rejection
may be caused by clearing of system major system disturbance. Load rejection will
cause the generator to over-speed and operate at some frequency above normal value
[12]. Over/under frequency relay detect this conditions.
Differential Relay (Device 87) - this relay looks into a zone defined by location of
current transformers and if the input current does not match with output current in that
zone, it rapidly trips the generator [9].
1.4 Excitation Loss
Any failure in excitation system directly interrupt the generating capability of the
synchronous machine and transmission of power to the system. The phenomenon where
the generators lose its excitation is called excitation loss. In excitation loss event, the
12
excitation system fails to deliver DC current and the generator seek a way to stay
excited which causes the faulty generator to absorb a large amount of reactive power
from the system connected with it. And consequently reduces reactive power delivery
from generator to system and lead to power system voltage and current instability and
if it continues to blackout of the whole system [18] [19]. If the reduction in reactive
power continues until a pre-determined under excitation limit, the generator will lose
synchronism and result in generator rotor speed up. This causes stator overloading as a
result of reactive power decrease on the grid, heating up of rotor winding due to induced
currents, asynchronous operation and active power swings which may decrease
generator’s life time [19].
Generator instability after excitation loss may lead to complete or partial excitation loss
of the synchronous generator. Complete loss of excitation (CLOE) can occur when field
winding open or short circuit or sudden opening of the field supply breaker happen
whereas partial loss of excitation (PLOE) can occur when suddenly field voltage drop
or short-circuiting in a section of the field winding is happen.
No matter how it caused, loss of excitation (LOE) represents huge damage on the
generator and on the whole system if an early protection is apprehended.
Damage to the generator:
When a synchronous generator lost excitation; its excitation current gradually decreases
which result in reduction of internal electromotive force and the electromagnetic
relation of stator winding and rotor windings. As the interaction of stator and rotor part
of the generator decreases, the reactive power of the generator terminal starts to reduce
in value as it is dependent on electromotive force. The less interaction of rotor and
stator windings creates slip which causes rotor overheating. As the machine operates
as an induction machine after loss of excitation, large amount of reactive power
supplied by stator current is required and the stator may suffer over heating because of
this large current. Under heavy load condition, the generator may suffer from severe
mechanical stress because of the power reduction which may damage both generator
and system [2] [20].
Damage to the system:
The asynchronous behaviour of synchronous machines after excitation loss results
decline of system voltage. For some weak system, the system voltage may collapse due
to the loss of excitation of an important generator and increases the reactive power
output of other generators in the system. This may cause the overloading in some
13
transmission lines or transformers. Thus the power swing and voltage drop caused by
loss of excitation may affect the normal operating generators and lead to loss of
synchronism of some normal operating generators in the system.
As a result a well-designed and an accurate protection against LOE is needed to detect
any failure in excitation system of synchronous generators to maintain the system stable
and safe at normal and abnormal (system swings) condition of the system [19].
As studies show a mal operation of excitation loss protection devices have been the
main causes of black out in many power grids throughout the world [18] [21]. This
arises due to weak setting of LOE relays and variable behaviour of the system depend
on load condition [18] [9]. An imperfection of the protection devices also lead to sense
some external system failures which is totally different from loss of excitation but that
have similar effect on the impedance variation of the generating unit. So, it is desirable
to install an excitation system with a highest possible operating reliability since outages
or failures in excitation systems can have very unfavourable operating consequences in
the whole system including the adjacent generators.
1.5 Problem Statement
Excitation system by its behaviour not only run the generator but also receive the
generator terminal voltage back through the transducer and compares it with a stability
margin reference value to keep the system in stable condition. And the terminal voltage
of the generator mostly affected by the grid working performances, which have the
probability to affect the exciter in addition with different failures of excitation system.
The actual excitation loss relay in power system industries mal-operates in prolonged
faults. Furthermore, excitation loss detection period of the relay varies on type of
excitation loss and severe of excitation loss (complete or partial excitation loss). An
apprehended detection of partial excitation loss leads to system instabilities and
blackouts in many systems. This is due to the dependence of relay characteristic design
on generator terminal parameters. Considering the above reasons, in this work
detection ability of excitation loss relay will be studied on various types of excitation
loss and system conditions.
14
1.6 Objective of the Study
General Objective
The main objective of this thesis is to study the accuracy and performances of excitation
loss detection method of synchronous generating machines on different excitation loss
causes and power grid performances.
Specific Objectives
The main specific objectives of the work includes:
Study synchronous generator and its excitation system characteristics
Investigate loss of excitation and its causes
Examine LOE detection methods on different power system structures and
performances
Improve the conventional methods of LOE detectors to overcome mal-operation
of the relay
Modelling IEEE 9-bus system and Tana Beles-1 power plant to study excitation
loss phenomenon
1.7 Scope of the Study
Excitation loss detection performance of IEEE 9-bus test system will be studied and
simulated on different system condition and different causes of excitation loss using
MATLAB/SIMULINK programming language.
1.8 Significance of the Study
The main significances of this work are:
Identifying the drawback of actual excitation loss relay in power system
industries
Give an appropriate setting for various causes of excitation loss to
decrease the mal operation of excitation loss relay
Avoiding the threshold robustness of the LOE detecting methods which
need a tedious simulation settings of threshold set,
Differentiate system failure and loss of excitation easily without the
dependence of the grid parameters.
Improve excitation loss relay detecting duration in all excitation loss
events
15
1.9 Document Organization
This report contains five chapters and their general organization can be summarized as
following:
Chapter 1: Is the general overview part which gives a detail background to the work
including the general highlight, problem statement, objective, significance, scope and
outline of the study.
Chapter 2: Present the previous works regarding with the excitation loss detection
method improvement through different parameters of synchronous generators.
Chapter 3: Deals with the modelling and control of synchronous generator components
with and without excitation loss event.
Chapter 4: The proposed algorithms simulation results will be discussed on different
excitation loss causes and system conditions.
Chapter 5: Presents the conclusion and possible recommendations for future work.
16
CHAPTER-2
LITERATURE REVIEW
For about six centuries, various LOE detecting methods has been suggested depend on
different parameters of the generator that are assumed really sensitive in case of
excitation system failures. But still the actually excitation loss protection in power
system industries is so called impedance type protection proposed almost four decades
ago. Since the protective relays should be designed with requirements of sensitivity to
sense all possible failures of excitation systems for all types of generators and accuracy
and reliability as should be easy and less complex to set the threshold of the protective
devices after a possible failure. Even if there are generator operations where this type
of protection mal-operate, the methods that have been suggested since then no matter
how sensitive in advance they are, the vast simulation process requirement of their
algorithms makes them unpractical. Thus the protection relays on the generating unit
are expected not only to ensure the reliability of the system but also to accurately
operate in face of faulty conditions where a precise setting and a practical coordination
of protective relays is important to minimize unwanted disconnection of components
and inactivating trip when it is important to protect the system which is the power
system reliability concern above the economic issue [2] [3].
So, it is desirable to install an excitation system with a highest possible operating
reliability and a well-designed and an accurate protection or detection against LOE and
is needed to detect any failure in excitation system since outages or failures in
excitation systems can have very unfavourable operating consequences in the whole
system.
The generator terminal voltage and terminal current measurement is used to protect
against excitation loss event. In 1949, Mason [22] suggests a negative off-set mho-type
distance relay to sense the impedance variation of generator terminal point due to
excitation loss through the variation of terminal voltage and terminal current of the
generator in excitation loss event. When the impedance falls under predefined
protective zone in Resistance-Reactance (R-X) plane for a pre-set time delay
determined using the longest oscillation of swing angle, the relay detects loss of
excitation and send a trip signal to the field breaker. It is the basis for most of the
methods that have been created since, but it has high relay operation time and
difficulties in differentiating system failures like stable power swing which are failures
17
outside the excitation system. Shortly afterwards, in 1975, Berdy [20] presented a
method based on the addition of another mho unit to this protection scheme proposed
in [22]. This type of protection is the most common method of LOE protection which
detects the generator terminal parameters variation at any cause of excitation failures
and it is the actual technique used in most power system industries until now.
The above methods have a mho function characteristics that uses the current and
voltage measured at the relay point of the generator terminal to determine if the
apparent impedance plots within the mho characteristic. The relay characteristic is an
offset circle which has an angle of maximum torque that falls on the (-X) ordinate. As
viewed from the machine terminals the relay will operate for any impedance phasor
that terminates inside the circular characteristic. When the relay was introduced in
1949, it was recommended the offset be set equal to one-half of the direct axis transient
reactance (X’d/2) and the diameter of the circle set equal to the direct axis synchronous
reactance (Xd). It was shown that with the machine reactance that existed at that time,
these settings would detect a loss of excitation from any machine loading and that there
would be optimum selectivity against operation during stable power swings. Machine
direct axis synchronous reactance was in the range of 1.1 to 1.2 per unit. But, with the
improvement of synchronous machines synchronous reactance the method finds
difficulties in detecting LOE event with Xd greater than 1.6pu and lightly loaded
generators. Thus, addition of a second mho-unit relieved this problem.
Some years later, the above methods have been modified using modern computational
methods, such as neural networks [23] [24], decision tree [25] and fuzzy [26]
algorithms in protection against loss of excitation. These methods may present good
results, however require a considerable amount of training and depend on the
characteristics of the system. In 2005, S. R. Tambay and Y. G. Paithankar [27]
proposed to use a relay with quadrangular characteristic and the use of rate of change
of the reactance seen in the terminals of the machine with the help of digital relays.
Again with the advent of digital relays, another method on the basis of Space Vector
Machine (SVM) technique to discriminate between LOF and stable power swing SPS
is presented in [28]. However, both of the above mentioned schemes need a significant
amount of data for training and are dependent on the system characteristics.
In 2016, Behnam M. and Jian Guo Zhu, [29] present a setting free approach that the
rate of resistance variations at the generator terminal is introduced as an excitation loss
detector, which it will become and remain negative a short period after the event
18
occurred. Since the measured resistance has an oscillatory nature due to the speed
variation associated with slip frequency, the proposed algorithm may reset for loss of
excitation events with the high slip frequency.
On the same year, M. Abedini et al. [10], proposes a method using the rate decay of the
generator internal voltage with the field flux linkage variation. An adaptive and
threshold loss of excitation index is introduced depend on terminal voltage to
discriminate system disturbance from excitation failure such if the generator achieve
greater excitation loss index for a given samples, then loss of excitation will be
detected. This method have accurate sensing results since it uses the capability curve
of the generator, however the set points identification is a difficult task and may involve
extensive simulation processes which makes it unpractical. Those authors modify the
mentioned criterion in 2017 [21], which uses Fast Fourier Transform (FFT) coefficient
of three-phase active power to prevent the mentioned algorithm from mal-operation in
the face of SPS.
A combined index based on generator terminal voltage, reactive power and power
angle variations is presented in [18], where power angle is estimated by measuring the
rotor speed. Although this technique can be implemented by considering a special case
of the network operation, regarding the network combination is inevitable.
Excitation loss detection through generator internal parameters can be evaluated also
using flux interaction of the stator and rotor windings, internal voltage or internal
current measurements. A flux based method is presented in [30], which it uses the
installed search coils in stator slots to measure the air-gap flux. This scheme however
should normally be implemented by the generator manufacturer.
Despite the fact that the methods presented by [22] and [20] have been introduced more
than 40 years ago, they are still available in the most commercial relays for generator
protection due to the fact that the most recent methods require a considerable amount
simulation in order to obtain thresholds, especially the methods present in [10], [23]
and [25]. So, the real life implementation of recent proposed methods is still a difficult
task. The second reason is because it is really challenging to identify the best methods
among several techniques tested under different systems, load conditions, frequency
sample rate, etc. In other words, there is no paper that compares the methods under the
same test conditions. So, there is no knowledge and practical understanding about this
particular condition of LOE in the recent presented methods [2] [3].
19
In [31], different structures of a simple power system network was presented to study
the behaviour of the parallel generators in loss of excitation situation; as a result the
excitation failure in one generator have an effect on the performance of nearby
generators where the close ones were highly affected than the further generators.
20
CHAPTER 3
SYSTEM MODELLING AND MATHEMATICAL OVERVIEW
OF EXCITATION LOSS
3.1 System under Study
For this thesis work, the IEEE 9-bus test system and Tana Belese-I from Ethiopia Power
Systems (EPS) have been modelled in MATLAB/SIMULINK simulation tool to study
LOE scenarios in different system conditions. IEEE 9-bus test system includes three
generators, six transmission lines and three loads connected through nine possible
buses. Also, Tana Beles-I power plant from Ethiopian Power Systems has been used in
this thesis work to study effect of excitation loss on interconnected generators. This
generation unit consists of four identical generators connected on the same bus and the
rest of the system connected within this unit will be considered as infinite bus system.
In addition the IEEE standard models IEEE ST1A type excitation system and hydro
turbine governor from the Simulink Sim-power library have been used to study
excitation loss phenomenon [8]. The single line diagrams of IEEE 9-bus system and
Tana Beles-I have given in Fig.6 and generator under study data in table1.
Figure 6: Single line diagram of (a) IEEE-9 bus test system (b) Tana Beles-I power
plant
G-1
G-2
G-3
B-1
B-2
B-3
B-4
B-5B-6B-7
B-9
B-8
T-1
T-2
T-3L5-8
L8-9
L4-6
L5-7
L4-5
L6-9
Where
B-Bus
G-Generator
T-Transformer
L-Line
(a)
G-1
G-2
G-4
Rest of EPS
Gird G-3
Common BUs
(b)
21
Table 1: Parameters of generators under study
Generator MVA kV Xd X’d T’do
9-Bus G-2 192 18 1.72 0.23 8
Beles G-1 133 15 1.03 0.25 9.2
3.2 Synchronous Generator Modelling
A synchronous generator generally has two inputs, a torque input from a turbine
coupled to its rotor and an excitation current coupled to field winding of rotor. The
mechanical torque from turbine run the rotor and generate rotating magnet field in the
air gap which cuts the stationary coils in the stator and induce a voltage whereas the
excitation current from the excitation system produces magnetic poles in rotor [32].
Thus, the mechanical torque supplied by a turbine is converted to electrical torque
through the flux linkage and transmitted to grid as voltage and current. As
electromagnetic induction principle states, “when a coil of copper wire is rotated in a
magnetic field in such a way as to cut across the lines of magnetic force, an electric
charge is created or induced in the wires”; the synchronous generator also follows this
principle as when the magnetic circuit of the generator rotor rotates, the generator field
winding excites the rotating magnetic circuits to establish field flux linkage (FFL) and
consequently an internal voltage which have a proportion amplitude to the magnitude
of FFL is generated on the rotor field winding. This generated voltage will be delivered
to the whole system connected with the generator unit [33].
There are several kinds of synchronous generator models with different complexity and
sophistications. The mathematical model of synchronous machine can be described by
a set of differential equations representing the dynamics of the machines, exciters and
other controls and algebraic equations representing the network relation. However, at
first to model a synchronous machine mathematically all the windings that should be
included have to be identified first.
In this work, the synchronous generator behaviour and the generator parameters
calculation is analysed using a fourth order (two-axis) generator model for simplicity
of calculations as presented in [32] and Fig.7. In this generator model the transient
effects are accounted for, while the sub transient effects are neglected.
22
DC
DC
)2
('''' ])([
j
qqdqd ejEIXXE
)2
(
)(
j
qd ejII
'
djXsR eR epjX
vsj
seV
vsj
qd ejVV )(
Figure 7: Dynamic model of two-axis synchronous generator model
Where X’d is generator transient reactance, Rs generator internal resistance, Re and Xeq
are equivalent resistance and reactance of system connected with the generator
respectively.
The transient effects are dominated by the rotor circuits, which are the field circuit in
d-axis and an equivalent circuit in the q-axis formed by the solid rotor. Thus the time
constantsT’’d0 and T’’q0 are equal to zero [34]. The assumption is based on the fact that
the effect of damper windings on the transient is small enough to be negligible. The
general model of synchronous generator can be simplified as the following blocks
which work simultaneously to each other [35].
Excitation
system Rotor Electrical
Block
Torque-Angle
Loop
Terminal Voltage
(vt)
Mechanical
Torque(Tm)
Output current (It)Angular position
Field voltage
(Efd)Speed
Figure 8: Synchronous machine model block diagram
The torque angle loop in figure above represents turbine and generator mechanical
system. Inputs to this block are mechanical and electrical torques, and outputs are
rotating speed and rotor position as can represented in Fig.9 and equations (2.1-2.3).
dqqdqqdde iiXXiEiET )( '''' (2.1)
s
. (2.2)
)]([2
1.
semm DPPH
(2.3)
23
Where s is the synchronous speed, speed deviation, generator speed, mP is
mechanical power and eP is electrical power. Here the real power from the internal
source is exactly equal to the electrical torque across the air gap for this model. Then if
mechanical torque and electrical torque are balanced; the rotor will rotate at a constant
speed called synchronous speed.
D
s
0
Hs2
1
eT
mT
Figure 9: Mechanical block diagram of synchronous generator
A change in either of the torques will cause the speed variation. Another output of the
block is the rotor position. Consider any fixed point in the rotor, it will circulate in a
circle and hence the rotor position will change from 0 to 360 degree [32].
The rotor electrical block represents flux dynamics in the machine windings with
generator terminal voltage and current outputs and excitation system block compares
terminal voltage magnitude with a reference voltage and outputs field voltage. The
terminal mathematical representation for these two blocks is given as the following
equations (2.4-2.10) for both the input and output variables [36].
A
A
sT
K
1RsT1
1tV 1V
refV
fdE
Figure 10: Simplified model of ST1A excitation system
Generally, in all the thesis work the excitation voltage is considered IEEE ST1A
excitation system field voltage as can be given in equation (2.6) and Fig.10. The d and
24
q-axis voltages and field voltage of synchronous generator are also given as the
following.
]))([(1 ''
'
0
'
fddddq
d
q EiXXET
Edt
d (2.4)
)])([(1 ''
'
0
'
qqqd
q
d iXXET
Edt
d (2.5)
)][(1
fdtA
A
fd EVKT
Edt
d (2.6)
The terminal active and reactive power of generator identifies the capability of the
machine to feed the gird connected with it. Since any synchronous machine have direct
and quadrature axis, the terminal parameters of generator are also given in terms both
axis components. The terminal current, voltage and power are given as equation (2.7-
2.11).
qdt jVVV (2.7)
qdt jiiI (2.8)
Where the d-axis and q-axis currents are given as:
'
'
d
dX
VEi
And
'
'
q
ddq
X
EVi
(2.9)
qqddt iViVP (2.10)
qddqt iViVQ (2.11)
The overall dynamic representation of the two-axis model showed in Fig.7 can be
summarized in the following complex circuit in terms of the internal voltage of the
generator and system voltage assuming a single machine is connected to infinite bus
[32].
vsjjjj
s
e
qdepe
e
qdds
e
qqdqd VjiijXRjiijXRjEiXXE
)
2()
2()
2(
))(())((])([ '''''
Where Vs stands for system voltage.
Practically, the science of generating power by synchronous generators and power
consumption in customer side in any power system is defined by set of inter-related
non-linear differential equations which are dependent in time. Before t=0, the system
will be assumed an equilibrium state and calculation of these states would be important
25
for power system stability analysis. So, the initial values of two-axis model have given
in Appendix A.5.
3.3 Characteristics of Synchronous Generator in Excitation Loss Event
The main electrical and mechanical quantities of the generator including voltage,
current and rotation speed will deviate from the related steady-state values during LOE
event. In loss of excitation, the synchronous apparent power of a generator falls off to
zero within a short time [5] [2] which causes a mismatch between the mechanical power
input and the electrical power output. Generator speed increases exponentially and
eventually reaches asynchronous condition. At this point, the mechanical power
produced by the turbine equates with the asynchronously developed electrical power
[25]. The mechanical power produced is given by the reference power-setting and the
static drop of the turbine-governor. The electrical power is determined by the
parameters of the equivalent circuit and the slip. In terms of operational impedance of
an alternator, the average asynchronous torque in LOE event is given as:
))(
1()
)(
1Re(
2 00
2
jsjXjjX
VT
qd
tas (2.12)
Where Vt is terminal voltage, Re indicates real part of the equation, s 0 is
synchronous speed. Thus, an LOE occurrence can end with a decrease in terminal
voltage and output reactive power, oscillate output current and active power, and
increase the generator speed and power angle (i.e. loss of synchronism) [18].
At normal condition a synchronous generator generates active power due to the shaft
or steam and reactive power due to excitation or field current as shown in Fig.8. At this
state, the synchronous machine delivers reactive power to the system connected with
it. However, if the excitation system fail by any means; the generator losses its
excitation and the Mega Volt Ampere Reactive (MVAR) delivering to the system will
suddenly stop. Since the generator continue generating Mega-Watt (MW) power due to
the shaft, its speed increases rapidly and it only remains in synchronism until the speed
of the generator exceeds the synchronous speed [2]. However, when the synchronous
speed becomes much less than the generator speed, a slip current starts to induce in the
rotor surface which establish magnetic field and the generator lose synchronism and
finally black out may attain if it is not protected early. Because of reactive power drawn
by the LOE generator, the system voltage is immediately reduced while the armature
26
current of the generator is increased [37]. Under such circumstances, disturbance in the
power system would take place.
When synchronous generators are subjected to excitation loss event, the accurate
calculation of the machine transient performance depends on the load, open-circuit
resistance and the saturation condition of their main flux paths. From equation (2.1-
2.11), we can re-write the internal voltage, terminal voltage, power and speed of the
generator in terms of excitation voltage and current as the following equations.
)11
(1
'
'
0
'
0
''
'
0
'
ddqa
d
fd
d
d
ddqddaq
q
qd
qdt iXiRsT
Ei
sT
XXjiXiRi
sT
XXjVVV
Whereqddaq
q
qd
d iXiRisT
XXV '
'
0
'
1
and
ddqa
d
fd
d
d
ddq iXiR
sT
Ei
sT
XXV '
'
0
'
0
'
11
(2.13)
And the internal voltage of the generator in terms of field voltage, d-axis and q-axis
voltages can be stated in equation (2.14).
)11
(1 '
0
'
0
'
'
0
'
''
d
fd
d
d
ddq
q
qd
qdisT
Ei
sT
XXji
sT
XXjEEe
(2.14)
(a)
(b)
Figure 11: Generator (a) Terminal and internal voltage (b) q-axis and d-axis voltage
in LOE event created at 1second
Thus, as can be seen from equation (2.13 and 2.14) and Fig.11a, the terminal and
internal voltage of the synchronous generator varies with generator field voltage
reduction. And in case of full loss of excitation since the field voltage is null which
indeed results in reducing of terminal and internal voltage of the synchronous machine.
Comparing with the terminal voltage of a generator, the internal voltage parameters of
0 1 2 3 4 5 6 70
0.5
1
1.5
Time
internal voltage(pu)
Terminal Voltage
Internal Voltage
0 1 2 3 4 5 6 7-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
Time
q-axis voltage(pu)
d-axis voltage
q-axis voltage
27
the machine responses faster especial the q-axis voltage of the generator since the q-
axis voltage is the voltage which is proportional to field flux linkage as it have been
stated in equation(2.4) and Fig.11b. This indicates that the terminal parameters of the
generator are composted by the condition of the gird connected with the synchronous
machine since terminal voltage is the voltage delivered to the whole system.
As can be observed from Fig.12, the d-axis current increase and q-axis current remains
constant till the generator instability happen in LOE event. Thus, the active power and
reactive power can be rewritten as equations (2.15 and 2.17) using the d-axis and q-axis
currents. From equation (2.15-2.17), the reactive power will indeed reduce to negative
so does the reality in LOE event generator consume reactive power from the system.
And the active power almost remains constant till loss of synchronism. A change in the
real power demand affects essentially the frequency whereas a change in the reactive
power affects mainly the voltage magnitude and so mainly excitation loss affects
voltage parameters. If the faulted generator did not isolated from system, a loss of
synchronism will happen when the system weaken to feed the generator. The
asynchronous value of active and reactive power can be given as equation (2.16) and
(2.18) in a workable expression by avoiding sub-transient time constants for two-axis
model.
(a) (b)
Figure 12: Generator (a) Active and reactive power (b) d-axis and q-axis currents in
LOE event
2
'
0
'
'
0
'
0
'
)1
()1
)1
(( q
q
qd
d
d
fd
d
d
ddt i
sT
XXi
sT
Ei
sT
XXQ
(2.15)
0 1 2 3 4 5 6 7-1
-0.5
0
0.5
1
1.5
Time
<Output active power Peo (pu)>
Reactive Power
Active Power
0 1 2 3 4 5 6 7-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
Time
<Stator current iq (pu)>
d-axis current
q-axis current
28
])(1
)()
11[(
2 2'
2'
'
2
d
d
dd
tsT
sT
XX
VQ
(2.16)
dq
q
qd
q
d
fd
d
d
dddq
q
qd
t iisT
XXi
sT
Ei
sT
XXii
sT
XXP )
1()
11(
1 '
0
'
'
0
'
0
'
'
0
'
(2.17)
])(1
)11
[(2
)1(2'
'
'
2
d
d
dd
tsT
T
XX
VssP
(2.18)
Fig.12a illustrates the active and reactive power variation in LOE event. If the system
is able to feed the excitation system a reactive power, the generator’s parameter
remains in synchronism. However, the moment the system reaches the maximum limit
of reactive power delivering, the whole system lose synchronism. In this simulation as
can be shown in Fig.12, LOE event is created in 1second and the generator loses
synchronism after 4.23second, until this limit the active power of the generator is not
almost affected since it is the power generated due to the shaft of the machine.
qqd
dda
q
dda
iXE
ViRViR''
(2.19)
Where the d-axis and q-axis induced stator flux linkages are given as:
qqdq iXE '' And ddqd iXE '' (2.20)
(a)
(b)
Figure 13: Generator (a) Rotor speed (b) load angle in LOE event at 1second
The speed of the generator rises under loss of excitation event as shown in Fig.13a.
When the speed of the generator increases above the synchronous speed, the machine
will act like an induction generator which induces rotor surface slip currents. This is
because the speed of a rotating magnetic field is proportional to the frequency of
excitation current. Since the shaft of the generator keeps generating MW power, the
load angle 𝜃 will increase as well. The maximum mechanical power can increase until
0 1 2 3 4 5 6 70.95
1
1.05
1.1
1.15
1.2
1.25
Time
<Rotor speed wm (pu)>
Rotor speed
0 1 2 3 4 5 6 7-200
-150
-100
-50
0
50
100
150
200
Time
<Load angle delta (deg)>
Load angle
29
the load angle reaches 𝜃=90◦, after that the mechanical power will be greater than the
electrical power and the generator will loss equilibrium point between mechanical
power input and electrical power output. For the same reason, when the mechanical
power input is fixed and maximum electrical power output increase, the load angle will
increase as the intersection of mechanical power and electrical power moves up to the
peak. Generally, high increase in load angle leads to loss of stability and pole slipping
and turbine would suddenly go into over speed with AC current flowing in the rotor.
Fig.13b. shows variation of load angle with excitation loss. In LOE event, load angle
increases and 4second after the failure the load angle raises further to more than 90◦and
the generator becomes unstable.
Voltage in excitation loss depresses to such extent under voltage relays may sense it.
This reduces the terminal impedance of a generator which can be expressed in terms
of terminal voltage and current as equation (2.21).
22
2
22
2
tt
t
tt
t
QP
QVj
QP
PVjXR
I
VZ
(2.21)
Where 22
2
tt
t
QP
PVR
and
22
2
tt
t
QP
QVX
Thus, when LOE occurred, resistance of the generator declines to zero gradually
proportional to terminal voltage decay since the active power is almost unaffected by
excitation loss until the generator loss synchronism. On the other hand the terminal
reactance decreases to negative value proportional as the reactive power decline as can
be shown in Fig.14. This results in terminal impedance reduction in excitation loss
event as given in Fig.14b.
(a) (b)
Figure 14: Generator terminal (a) resistance and reactance (b) impedance in LOE
event
0 1 2 3 4 5 6 7-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
Time
Terminal Resistance
Terminal Reactance
0 1 2 3 4 5 6 70
0.2
0.4
0.6
0.8
1
1.2
1.4
Time
Terminal Impedance(pu)
Terminal Impedance
30
3.3.1 Initial Loading Effect
At normal state synchronous generator is expected to run at constant value of voltage,
limited between 95% and 100% of rated voltage. However, the variation of loading
condition results generator voltage to vary. During periods of light loads, the voltage
drop is minimal through system parts such as transformers and lines. When loading
condition increases, voltage drop increases. Initial load of synchronous generator is the
ability of the generator to produce active power to the system in normal operation.
Thus, to operate in stability the loading effect of the generator is also inter-related with
the initial field voltage. The variation in active power production ability of the
generator directly affects the initial field voltage and the initial mechanical power of
the generator. The initial mechanical power declines proportionally with the initial load
reduction as the mechanical power is equal with initial electrical power as can be shown
in equation (2.22). Thus, the influence of excitation loss decreases as the initial load
decrease. When the generator operates in light load, the duration of generator instability
during loss of excitation is much longer than in full load as the amount of reactive
power needed to excite the generator is less in value compare to full load. Thus, the
reactive power consumption in light load is so slow.
0000000 )()( qqaqddadem iiRViiRVPp (2.22)
0
''
00 )( dddqfd iXXEE (2.23)
Fig.15 shows the variation of generator parameters in full, medium and light load
conditions in LOE event created at 1second. The generator parameters vary in
magnitude relative to initial load variation. However, the reactive power reduction in
excitation loss event in light load (30%Peo) is much slower than in heavy load as can
be observed in Fig.15(c).
On the same generator and at similar excitation loss event, the duration taken to reach
reactive power consumption margin from the system differs when the loading effect
changed. In full load condition, the generator was able to consume reactive power from
the system for about 4.23second but for medium and light load conditions it takes
14second and 30.5second to loss generator stability after loss of excitation.
From the above simulations, we can see that excitation loss does not only affect the
faulted generator itself, but also affect the entire system. Since the LOE generator tries
to absorb large reactive power, the rest of the system has to produce heavy MVAR to
make up for the additional reactive power demand. If the rest of the system cannot
31
provide the desired amount of reactive power, the excitation loss condition can then
degenerate into a voltage collapse.
Field voltage
Reactive power
Terminal voltage
Rotor speed
Figure 15: Generator parameter variation in LOE event at various generator loading
conditions
Figure 16: IEEE 9 bus system G-1parameters in G-2 LOE event
0 2 4 6 8 10 12 14 160
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Time sec
Fie
ld V
oltage P
u
Vf in Full load
Vf in Medium load
Vf in Light load
0 5 10 15 20 25 30 35-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
Time sec
Genera
tor
term
inal R
eactive p
ow
er
Pu
<Output reactive power Qeo (pu)>
Qt in Full load
in Medium load
in Light load
0 5 10 15 20 25 30 350.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
Time sec
Term
inal voltage P
u
vt in Full load
in Medium load
in Light load
0 5 10 15 20 25 30 35 400.95
1
1.05
1.1
1.15
1.2
1.25
Time sec
Roto
r speed P
u
<Rotor speed wm (pu)>
Speed in Full load
in Medium load
in Light load
0 1 2 3 4 5 60
0.5
1
1.5
2
2.5
3
3.5
time sec
<Output active power Peo (pu)>
G-1 Field voltage
Reactive power
Active power
32
When the reactive power of G-2 of IEEE 9-bus test system lessen because of excitation
loss, the reactive power and field voltage of other generators on the system starts to
increases in value to compensate the loss and to keep the system in stability. But the
active power reduce in value so does bus voltages. If faulted generator was not
disconnected from the grid it also causes swings on the remaining machines as can be
showed in Fig.16.
3.4 Excitation Loss Protection Relay
In this work, the actual excitation loss detection method in power system industries
which is the impedance protection relay has used to study excitation failures in different
power system performances. This method of excitation loss protection is based on the
calculation of the impedance at generator terminals. As stated above, there are different
methods that use generator terminal impedance for loss of excitation protections; a one
circle mho relay proposed by Mason, two circle off-set mho relay proposed by Berdy
and two circle off-set mho relay with directional element and others. In modern power
system commercial industries, Berdy method is the most popular and in this thesis work
this method will be investigated in different power system conditions. The main
indicator in capturing the probability of excitation failure is the significant flow of
reactive power into the generator. This should be captured by the relay. The two circle
off-set mho relays have two protection circle zones plotted on the R-X plane of the
terminal impedance as can be observed in Fig.17. The protection zones are positioned
in the negative reactance coordinate of the R-X plane with offset value X’d/2 and with
circle zones of 1pu and Xd for zone-1 and zone-2 respectively.
As can be seen from equation (2.21) and Fig.17, in normal operating condition, the
generator generate and deliver active and reactive power to the system which means
both R and X are positive and the terminal impedance is located in the first quadrant in
R-X plane. When LOE event happened, the generator starts to draw reactive power
from the system and reactance becomes negative from the relay point of view. As a
result, the terminal impedance trajectory in R-X plane moves to the forth quadrant and
the endpoint of terminal impedance ranges between the sub transient reactance and
synchronous d-axis reactance. The endpoints depend on the initial load and type of
excitation loss cause as shown in figure below. The amount of time delay used with the
large setting should be the minimum time required to ride over transient conditions. A
time delay of 0.5 or 0.6second appears to be sufficient to ride over stable transient
33
wings. While there is no data available on the transient performance of voltage
regulators, it would appear that 1second external time delay should prevent undesired
operation due to voltage regulator undershoot [20]. In either case, when selecting a time
delay the user should determine the effect of the time delay on possible generator
damage and on the overall operation of the system.
Figure 17: Impedance trajectory of LOE relay in different system conditions
It should be noted that even in the case of a lightly loaded generator, a loss of excitation
can cause a considerable VAR drain from the system. A prolonged VAR drain may
cause the tripping of transmission lines and general system instability. As a smaller
mod zone one reach, the trajectory of the impedance entry time for the altered zone one
is higher than entry time for the initial zone two reach. These lead the usage of a typical
original zone two time delay of 1second may be insufficient to trip the generator before
the synchronous loss is taking place. A power swing conditions are selected to control
and enhance the performance of the protection scheme in steady power swing [20].
Thus, the relay is set with a time delay to distinguish between a recoverable swings and
an LOE condition for zone-2 0.5 and 1second delay have been suggested to send alarm
and trip signals consequently. But for zone-1no delay for trip signal have been
suggested.
34
Measure
Generator
terminal
parameters
Fast Fourier
Transform
R-X loci r enter
zone-1
No
Calculate R
and X
R-X loci
enter zone-
2&wait
for
1sec
LOE event trip G
No
Yes
Yes
SPS or normal
state
Figure 18: Flow chart of LOE relay
35
CHAPTER 4
SIMULATION RESULTS AND DISCUSSIONS
In this chapter, the excitation loss detection relay will be scrutinised on different causes
of excitation loss and power system disturbances. Excitation loss of generator can be
initiated through field winding short or open circuit, poor brush contact, AVR control
failure, AC voltage loss to exciter, main circuit breaker failure or slip ring flash over
[29]. Despite the cause of excitation loss, either the field voltage or field current reduce
in value or to zero depend on the type of excitation loss cause or sever of the failure.
Some of the causes of LOE have similar characteristics such as AVR failure have
similar characteristics with field winding short circuit which introduce reduction of
field voltage to zero. Accuracy of excitation loss relay to detect these causes of
excitation loss will be study on different initial loading conditions of the generator.
Figure 19: Simulink model of IEEE 9-bus system with LOE event and LOE relay
In the remaining sub-topics, excitation loss events will be study on generator two (G-
2) of IEEE 9-bus test system shown in Fig.19 except for effect of excitation loss on
parallel connected generators which will study on generator one (G-1) of Tana Beles-
I. Parameters of the generators under study is given in table1. And the required machine
data, transmission line and load data are given in Appendix B. Also, in all the
simulation scenarios of the remaining sub topics, the LOE event and system
disturbances are assumed initiating at time t=1second. The general lay out of this
chapter can summarized as the following four sub-topics.
36
Study main causes of full excitation loss
Partial loss of field voltage
LOE relay reaction on power swings and
Proposed back up protection
4.1 Full Loss of Excitation
Generator loses its excitation completely when the field voltage or field current
supplied to the synchronous generator from the excitation system is totally lost and the
excitation system fails to excite the synchronous generator completely. In this
condition the synchronous generator is able to produce active power due to the
mechanical input but it completely stops producing reactive power. Full loss of
excitation is initiated either due to field winding failure, main circuit breaker between
excitation system and generator failure or sudden AC voltage loss to exciter.
4.1.1 Field Winding Short Circuit
A short circuit on the exciter field winding is the most common type of excitation
system fault. In field winding short circuit the field voltage literally decline to zero as
shown in Fig.20a. But the field current remains high and it is able to swing when the
generator lose synchronism due to self-exciting excitation system is used and field
current is dependent on parameters of generator as given in equations (2.6), (2.23) and
(2.24).
Figure 20: (a) G-2 excitation current and voltage (b) rotor speed and reactive power
in field winding short circuit
Fig.20b shows response of generator parameters when field winding short circuit
happens at 1second. Region-(a) is the first stage of excitation loss event and the
synchronous generator starts to consume reactive power from the system. In this stage,
the system is capable of feeding reactive power to the generator and the generator
(a)
(b)
0 1 2 3 4 5 6 7-5
-4
-3
-2
-1
0
1
2
3
4
5
Time sec
Fie
ld V
olt
age a
nd c
urr
ent
Pu
<Field current ifd (pu)>
Field voltag
Field current
0 1 2 3 4 5 6 7 8-1
-0.5
0
0.5
1
1.5
Time
<Output reactive power Qeo (pu)>
Rotor speed
Reactive power
LOE
a
b
37
remains in synchronism. The reactive power amplitude consumed by the generator
could be as large as 1.56 times rated power of the generator which indicates the
machine is trying to run the faulted generator and to keep the system stable. On the
second stage shown point (b), the system reaches the maximum limit of reactive power
delivering and the generator speed rises rapidly beyond synchronous speed due to
power imbalance (excessive mechanical power of turbine). In this stage, the relation
between the stator and rotor is already very week due to under-excitation limit is
achieved. And over speed of the generator consequences the generator to loss
synchronism and bring a huge damage to the system.
The power flow direction of synchronous machine can be summarized in the four
quadrature as given in Fig.21. If the synchronous machine deliver both active and
reactive power to the system, it acts as synchronous generator and the impedance
trajectory remains in quadrant-1. However, if the machine starts to receive reactive
power from the system while delivering active power, it is acting as induction generator
and the impedance trajectory R-X loci lays on the fourth quadrant. The same for
synchronous motor principles as stated in the figure. So, in normal condition of
synchronous generator the tip of the impedance trajectory looking from the generator
terminals should remain within the first quadrant and both resistance and reactance
should be positive.
+jX+MVAR
+jR
+MW
-MVAR
-MW
-jX
-R
P
Q
Machine acts as
synchronous
Generator
Q-I
Machine acts as
an Induction
Generator
PQ
Q-IVMachine acts as
an Induction
Motor
Q
Q
P
P
Machine acts as
Synchronous
Motor
Q-III
Q-II
Figure 21: Power flow direction of synchronous machines
But on occurrence of excitation loss, the reactance becomes negative and resistance
decrease in value. So, the impedance trajectory starts moving toward to the fourth
38
quadrant and is expected to enter the protection zone of excitation loss relay to protect
the whole system from a possibly damage that can outcome due to the faulted
generator. At this stage the relay will sense the fault and will provide the proper signal
to trip or alarm after a pre-set time delay.
Fig.22 shows the impedance trajectory of G-2 for field winding short circuit in different
loading effects; heavy load, medium load and light load conditions. In this thesis work
the normal rated power is considered as heavy load, 50% and 30% of rated power as
medium load and light loading conditions correspondingly. In heavy load condition the
fault detected at 4.16second through zone-1 since the first zone of impedance protection
is modelled to protect heavy loaded generators. On the other hand, when the loading
effect starts to decrease, zone-2 is capable of detecting the LOE event with a reasonable
time delay.
Heavy load
Medium load
Light load
Tripping signal
Figure 22: G-2 impedance trajectory in field winding short circuit
For 80% initial load, the impedance trajectory of G-2 enters zone-2 and zone-1 at
4.165sand 6.415second respectively. Zone-2 sends an alarm signal after 0.5 second.
Though, the tripping signal initiates through zone 2 after 1second at 5.165second.
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
Impedance Trajectory in heavy load
in 80% load
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
Impedance Trajectory in 60% load
in 50%load
-1 0 1 2 3 4
-3
-2
-1
0
1
2
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
Impedance Trajectory in 40% load
in 30%load
0 1 2 3 4 5 6 7 8 90
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Time sec
Tripin
g S
ignal
Terminal Impedance(pu)
Triping signal in Heavy load
Triping signal in medium load
Triping signal in light load
39
As the loading effect of the generator decrease, detection duration increases. In medium
load the event is detected after 5.804second through zone-2. On the same manner, in
light load condition, the generator delivers 30 % of the rating power, and field winding
short circuit is detected after 6.286second. But, the impedance trajectory enters zone-1
after 13.43second which is much slower than in heavy load case.
It can be understood that the detection duration of LOE relay in field winding short
circuit increases as the initial load of the generator decrease but still zone-2 plays a
great role in detection of light loaded generators in short period of time compare to
zone-1. And LOE relay detection ability have shown a great response in field winding
short circuit for different loading conditions. The remaining studies of field winding
short circuit have summarized in table3 and Appendix A1 with required time reaching
of both protection zones.
4.1.2 Sudden Main Circuit Breaker Failure
When the main circuit breaker between the exciter and generator opened, the exciter
and the generator totally isolated and the generator loses its field voltage totally. In this
case, the exciter is at normal state but due to the main circuit breaker between exciter
and synchronous generator fail the field voltage entering to the generator is reduced to
zero as shown in Fig. 23. And this results in complete excitation loss in generator. The
reduction of generator field voltage to null let this cause of excitation loss to have
similar characteristics to field winding short circuit as can be observed in Fig.20a and
Fig.23.Similarly to field winding short circuit, field current is also able to swing after
generator lose synchronism since it is dependent on generator parameters.
Figure 23: G-2 field voltage and current in main CB failure
Fig.24 shows the LOE relay detection ability when the main circuit breaker between
the exciter and the generator fail at 1second. Before fault happen the generator was
0 1 2 3 4 5 6 7-5
-4
-3
-2
-1
0
1
2
3
4
5
Time sec
Fie
ld V
olt
age a
nd c
urr
ent
Pu
<Field current ifd (pu)>
Field voltag
Field current
40
delivering 0.884pu active power and 0.63pu reactive power in heavy load condition.
Again the heavy load condition is detected through zone-1 after 4.16second the same
to field winding short circuit. For 60% and 40% loading effect the failure have been
detected after 5.575second and 6.286second after the event happen through zone-2
respectively. To avoid unnecessary repetitive actions, the excitation loss causes that
have similarity with field winding short circuit have been eliminated.
Figure 24: G-2 impedance trajectory in main CB failure
4.1.3 Sudden Loss of AC Voltage to Excitation System
If the AC voltage that run the exciter interrupted by any means, the excitation system
and also the generator connected with the exciter will be faulted. In this work self-
excited static exciter is used, so sudden loss of AC voltage to the exciter is studied
when the generator terminal voltage interning to ST1A excitation system and the
exciter are isolated. Thus, AC voltage loss to the exciter also results in full loss of
excitation in generator as can be shown in Fig.25 below. Complementary to other
causes of excitation loss, excitation system got support from the other components of
Heavy load
Medium load
Light load
Tripping signal
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
Impedance Trajectory in heavy load
in 80%load
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
Impedance Trajectory in 60% load
in 50%load
-1 0 1 2 3 4-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
Impedance Trajectory in 40% load
in 30%load
0 1 2 3 4 5 6 7 8 90
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Time sec
Tripin
g S
ignal
Terminal Impedance(pu)
Triping signal in Heavy load
Triping signal in medium load
Triping signal in light load
41
the exciter in sudden AC voltage loss to exciter since the field voltage is dependent on
the initial value of field voltage and the terminal voltage of the generator as have been
stated in equation (2.6).
Figure 25: G-2 field voltage and terminal impedance in sudden loss of AC voltage to
exciter terminal
Heavy load
Medium load
Light load
Tripping signal
Figure 26: G-2 impedance trajectory in sudden AC voltage loss to exciter
0 1 2 3 4 5 6 7 8-0.5
0
0.5
1
1.5
2
2.5
Time
Terminal Impedance(pu)
Field voltage
Terminal impedance
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
Impedance Trajectory in heavy load
in 80%load-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
Impedance Trajectory in 60% load
in 50%load
-1 0 1 2 3 4
-3
-2
-1
0
1
2
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
Impedance Trajectory in 40% load
in 30%load
0 1 2 3 4 5 6 7 8 90
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Time sec
Tripin
g S
ignal
Terminal Impedance(pu)
Triping signal in Heavy load
Triping signal in medium load
Triping signal in light load
42
For simulation, let the excitation system of G-2 suddenly loss AC voltage at 1second,
and the characteristic of LOE relay on G-2 have been observed in Fig.26 below. The
relay detects the event at 4.223second through zone-1 in heavy load condition which
is 0.63second longer than the above causes.
For medium load conditions, the failure is detected after 5.878second for 50% loading
and 5.664second for 60% loading effect through zone-2. Similarly to the above cases,
in light load condition, the event is detected slowly at 6.348second after fault happened.
Comparing to the above causes of excitation loss, sudden AC voltage loss to exciter is
detected slowly. The remaining simulation results have been summarized in table2 in
different initial loading conditions.
4.1.4 Field Winding Open Circuit
In field winding open circuit, field current is terminated to null. It is associated with
inserting of an infinite discharge resistance which tends to reduce the field current to
almost null from the relationship of equation (2.24) below [2] [12]. This is the worst
cause of excitation loss event that generator lose synchronism in fraction of
microseconds in any loading conditions. As can be observed from Fig.28, comparing
the courses of generator values during loss of excitation in field winding open circuit,
system reactive power feeding limit is reached at 0.06second after fault which is almost
4seconds before in other causes of excitation loss.
fd
fd
fd
d
adufd
fd
R
EI
T
XXR
'
0 (2.24)
Despite the faster response of generator parameters in field winding open circuit, by
comparing Fig.17 and 28, it can be noticed that the peak values of active and reactive
power are smaller in the open-circuit case. From Fig.28, it can be seen that after the
fault is initiated the seen impedance by the relay moves from quadrant-one toward the
fourth-quadrant of the R_X plane and then moves back to the third-quadrant. And this
movement increases when the initial loading of the generator decreases.
LOE relay detect field winding open circuit in 0.0835second for heavy load,
0.0467second for medium and 1.131second for light initial load conditions. As shown
in table2 the impedance trajectory in field winding open-circuit fault enters the
protection zones much faster than other excitation causes in any loading condition
43
which create overvoltage in the machine. A crowbar system is proposed to change
disconnection of field winding to field winding short circuit [18]. Every time field
disconnection happen the crowbar will be activated to avoid system damage due to
overvoltage. But, without crowbar, LOE relay detects the failure in less than 1.2second
in any event which is about 5second faster than other causes of LOE event.
Figure 27 : G-2 Field voltage and field current in field winding open circuit [
Figure 28: G-2 impedance trajectory in field winding open circuit
Table 2 summarizes the detection ability of LOE relay in all main causes of full
excitation loss in different initial loading conditions. From all the causes, field winding
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5-0.5
0
0.5
1
1.5
2
Time
<Field current ifd (pu)>
Field voltage
Field current
Heavy load
Medium load
Light load
MVAR and speed
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
Impedance Trajectory in heavy load
in 80%load
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
Impedance Trajectory in 60% load
in 50%load
-1 0 1 2 3 4 5-4
-3
-2
-1
0
1
2
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
Impedance Trajectory in 40% load
in 30%load
0 1 2 3 4 5 6 7-1
-0.5
0
0.5
1
1.5
Time sec
Q,w
m p
u
<Rotor speed wm (pu)>
G-2 Reactive power
Rotor Speed
44
open circuit is the worst cause of excitation loss that the relay detects the fault in every
loading effect in short period of time comparing to others. And the remaining cause of
full LOE event; field winding short circuits, sudden loss of AC voltage to the exciter
and main circuit breaker failure have the same characteristics that the field voltage is
null and LOE relay detection ability is almost similar with micro and millisecond
differences. And it can also observe that, impedance protection of excitation loss is
highly dependent on initial generation ability of the generator. But, impedance
protection of excitation loss (ANSI 40 relay) have detect full loss of excitation in less
than 6.5second in all loading conditions.
Table 2: LOE relay detection ability in main causes of full excitation loss
4.2 Partial Loss of Excitation
Partial loss of excitation is happen when field winding voltage of the generator decrease
in value by any reason. In light load case as shown in table 3, the effect of PLOE may
not be visible by the relay and in some cases the system still remain in stability
especially until reduction of 30% of the field voltage. But in heavy loaded generators
it may cause severe damages as much as full loss of excitation. In addition to the PLOE
causes introduced above, AVR action in system disturbances is also the most common
Initial Loading(in
pu)
Type of LOE and Generator tripping
time(second)
FW
short
circuit
AC
voltage
loss
Main
CB
failed
FW open
circuit
0.8485+j0.06307 4.160 4.223 4.160 0.0835
0.7604+j0.04781 4.961 5.027 4.961 0.06485
0.6792+j0.03679 5.165 5.236 5.165 0.05115
0.5943+j0.02962 5.355 5.426 5.355 0.04555
0.5094+j0.0268 5.575 5.664 5.575 0.0481
0.4245+j0.03389 5.804 5.878 5.804 0.0467
0.3396+j0.03424 5.773 5.853 5.773 0.0341
0.2547+j0.3 6.286 6.348 6.286 1.131
45
reason of partial excitation loss. In PLOE, the filed voltage does not subject to null, so
there will be some reactive power generation but not enough to feed the system so the
generator still consumes reactive power from the system even if that is slower than full
loss of excitation. From equations (2.5-2.10) the internal voltage, terminal voltage,
generator impedance and reactive power reduced proportional to the field voltage
reduction as can be observed in Fig.30. The detection ability of LOE relay will be
studied for different percentage losses of field voltage starting from the worst partial
loss which is decrease Efd by 1.6497pu till the least loss 0.1833pu Efd reduction as can
be scrutinized in table 3.
Fig.30 summarizes characteristics of synchronous generator on 30%, 50% and 70%
loss of field voltage in heavy load condition. In all cases, the partial loss of field voltage
is happened at 1second and the normal field voltage value of G-2 right before partial
loss is 1.833pu. When the field voltage reduced to 30% of its normal value, the system
is able to feed reactive power to the synchronous generator for about 15second after
fault happen. On the other hand, when the generator lost 50% and 70% of the field
voltage, the synchronous generator was able to consume reactive power from the
system for about 9second and 7second respectively without loss of synchronism.
During partial loss of excitation, the generator can operate for longer time with
synchronism compared to the complete loss of excitation. However, both full and
partial loss of excitation on the same initial condition reaches the required value
decrement as can be compared Fig.17 and Fig.30. For both cases in heavy load, the
generator reactive power diminishes until -0.413pu before loss of synchronism. Thus,
the field voltage reduction only affects the reactive power consumption duration and
variable changing rate of the machine and the initial condition determines the generator
output during loss of excitation.
Figure 29: Reactive power of G-2 in 70%Efd loss
0 5 10 15 20 25-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
time sec
MV
AR
pu
<Output reactive power Qeo (pu)>
Reactive power in medium load
in light load
46
In case of medium and light load conditions, the system will remain in stability without
losing synchronism for about 18 and 24.6second respectively as can be shown in Fig.29
for 70% of field voltage. Generally, effect of partial excitation loss in variation rate of
generator terminal parameters highly depends on initial loading of the generator. Thus,
the slow terminal impedance decrement leads slow operation of excitation loss relay in
generator partial field voltage loss.
Field voltage
Reactive power
Terminal impedance
Rotor speed
Figure 30: G-2 parameter variation in partial loss excitation
4.2.1 30% Field Voltage Loss
In this case, the field voltage is forced to reduce 30% of its initial value. This may threat
the stability of system but except for heavy loaded generators the reduction is not
visible by LOE relay. Fig.31 shows the impedance trajectory of G-2 in 30% field
voltage loss. The field voltage reduction in pu for all the three conditions have given
in table3 below. The LOE relay successfully detects the 30% diminishing of field
voltage for heavy load after 15.210second.
0 1 2 3 4 5 6 7 8 9 100.5
1
1.5
2
Time
Fie
ld v
olta
ge
30% field voltage loss
50%field voltage loss
70%field voltage loss
0 2 4 6 8 10 12 14 16 18-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
Time
Re
active
po
we
r
<Output reactive power Qeo (pu)>
30% field voltage loss
50%field voltage loss
70%field voltage loss
0 2 4 6 8 10 12 14 16 180
0.2
0.4
0.6
0.8
1
1.2
1.4
Time
Te
rmin
al im
pe
da
nce
Terminal Impedance(pu)
30% field voltage loss
50%field voltage loss
70%field voltage loss
0 2 4 6 8 10 12 14 16 180.95
1
1.05
1.1
1.15
1.2
1.25
1.3
1.35
1.4
Time
Ro
tor
sp
ee
d
<Rotor speed wm (pu)>
30% field voltage loss
50%field voltage loss
70%field voltage loss
47
Figure 31: G-2 impedance trajectory in 30% field voltage loss
4.2.2 50% Field Voltage Loss
In 50% field voltage loss, the field voltage reduces to 0.9856pu in heavy load, 0.4545pu
in medium load and 0.2232pu in light load cases.
Heavy load
Medium load
Light load
Tripping signal(Heavy and
50%loading)
Figure 32: G-2 impedance trajectory in 50% loss of excitation
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Resistance (R) Pu
Re
acta
nce
(X
) P
u
Zone-1
Zone-2
Impedance Trajectory in Full load
in Medium load
in Light load
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
Impedance Trajectory in heavy load
in 80%load
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
Impedance Trajectory in 60% load
in 50%load
-1 0 1 2 3 4-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
Impedance Trajectory in 40% load
in 30%load
0 2 4 6 8 10 12 14 16 180
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Time sec
Tripin
g S
ignal
Terminal Impedance(pu)
Triping signal in Heavy load
Triping signal in medium load
48
Again the remaining percentage reduction values are given in table3. Fig.32 shows the
impedance trajectory of G-2 in different initial percentage load for 50% field voltage
loss. If the excitation loss generator did not isolated from the system the interconnected
generator keeps generating more reactive power and a voltage instability in the system
may lead to voltage collapse and to more severe instabilities. Thus excitation loss event
should be detect at any condition in short period of time.
Reduction half of field voltage remains un-detected for 24.15second in medium load
and not detected at all in light condition. This threat system stability and may lead to
blackout of the whole system. But in heavy load the field voltage reduction is detected
after 8.563second.
4.2.3 70% Field Voltage Loss
In 70% field voltage loss, as observed in Fig.33, the field voltage reduction is detected
5.913, 9.242 and 13.41second for heavy, medium and light load conditions
respectively.
Heavy load
Medium load
Light load Tripping signal
Figure 33: G-2 impedance trajectory in 70% loss of excitation
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Resistance (R) Pu
Re
acta
nce
(X
) P
u
Zone-1
Zone-2
Impedance Trajectory in heavy load
in 80%load
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
Impedance Trajectory in 60% load
in 50%load
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
Impedance Trajectory in 40% load
in 30%load
0 5 10 15 20 250
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Time sec
Tripin
g S
ignal
Terminal Impedance(pu)
Triping signal in Heavy load
Triping signal in medium load
Triping signal in light load
49
Thus, the relay detect the case in less than 13second in all loading conditions. However,
the stability condition of the system is threaten highly in this duration. The system
voltage is reduced to about 0.58pu in 13second which may lead to system instability.
Even if the relay detect partial loss of excitation, instability of the system should be
also the main concern to avoid unwanted blackouts and component damages.
The field voltage reduction tests showed that in some cases the impedance trajectory
does not enter the protecting zones of the relay since the remaining field voltage allows
the defective machine and the system to find a new operating point and we can see that
under the same partial field voltage loss, the system response is much different in
various initial loading effects. From above simulations, we can see that decreasing the
initial load of the synchronous generator will give the LOE relay more time before the
system collapses.
Table 3: LOE relay detection ability in partial loss of excitation in different
loading conditions
It can be conclude that the relay have a good performance in detecting field voltage
loss greater than half of the rated field voltage as have been shown in table3 and
Appendix A2. On the other hand, for partial excitation loss less than half of the rated
field voltage of the synchronous generator, it fails detecting field voltage diminishing
except for heavily loaded generators.
Initial Loading(in
pu)
Partial Loss of Excitation(% Efd loss) and G-tripping duration
[Y(sec)/N]
30% 40% 50% 60% 70% 80% 90%
0.8485+j0.06307 15.210 10.940 8.563 6.99 5.913 5.153 4.592
0.7604+j0.04781 18.210 12.51 9.569 7.878 6.816 6.077 5.493
0.6792+j0.03679 22.29 14.100 10.48 8.426 7.161 6.296 5.659
0.5943+j0.02962 N 16.64 11.81 11.81 7.681 6.657 5.918
0.5094+j0.0268 N 21.33 13.74 10.31 8.341 7.094 6.223
0.4245+j0.03389 N N 17.19 11.95 9.242 7.632 6.569
0.3396+j0.03424 N N 24.15 13.88 10.04 7.925 6.644
0.2547+j0.3 N N N 23.57 13.41 9.354 7.35
50
From the above simulations, we can see that partial or full excitation loss have much
different system responses in different initial loading conditions. The detection duration
is highly dependent on the type of excitation loss and scheduled MW output of the
generator. The duration varies inversely with the output active power. The less MW
output, there is more time before system collapse and detected by LOE relay and vice
versa. And comparing full and partial excitation loss, partial field voltage loss is
detected much slower than full excitation loss. LOE relay detect all the scenarios of
FLOE but fails to detect field voltage reduction less than half of the rated field voltage
in lightly loaded synchronous generators as have summarised in table3.
4.3 Effect of Excitation Loss on Parallel Connected Generators
Generators are rarely used in isolated situations, more commonly they are used in
parallel. In parallel connection of generators, a number of identical machines share
their power to one bus. But, to connect a number of generators in parallel, rms line
voltages and phase sequence, phase angle of corresponding phases and frequency of
the generators must be the same. In this section, Tana Beles-I power plant from
Ethiopian Power System is used to study LOE event on parallel connected generators.
This power plant contains four identical generators connected at one common bus as
shown in single line diagram in Fig.3 and Simulink model in Fig.34. As was presented
above in the discussions, if generator lost excitation it changes into significant
consumer of reactive power. If this generator is not removed, the adjacent generators
start increasing the production of reactive power up to the limit when their limiters of
rotor and stator currents act [18] [28] as can be shown in Fig.35b. This results in voltage
drop in the remaining generator terminals, but only the generator which loss excitation
G-1 absorbs reactive power. In field winding open and short circuit conditions, the
impedance trajectory of G-1 and G-2 have given in Fig.36. The trajectory of G-1 for
both failures enter the protection zones of the relay at 4.659second and 0.0731second
respectively.
On the other hand, despite the voltage drop of the other three generators the impedance
trajectories of remaining three generators for both scenarios remain outside of the
operation zone of the relay. In strong system condition of parallel connected generators
excitation loss only affects the impedance variation of the faulted generator. And the
strong help of MVAR of the remaining generators help their impedance trajectories to
stay out of the protection zones.
51
Figure 34: Simulink model of Tana Beles-1 power plant
(a)
(b)
Figure 35: Tana Beles-I (a) G-1 and (b) G-2 parameters in G-1 LOE event
The worst condition on interconnected generators have been found when LOE event
and SPS event created on the same time. In this case for both field winding short and
open circuit with SPS event have simulated at 1second. SPS event is created by three-
phase short circuit fault at the common bus cleared after 150ms by opening the circuit
breakers from the receiving and sending end. From the simulation result, it have shown
that field winding short circuit and SPS event on G-1 still keeps the impedance
trajectories of the other three generators outside the protection zones of the relay. This
is due to slow generator parameter reduction in field winding short circuit than in SPS
event so the system is able re-stabilize the system through protective devices.
0 1 2 3 4 5 6 7-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
Time
<Field current ifd (pu)>
G-1 Reactive power
Active power
Terminal voltage
Field current
0 1 2 3 4 5 6 7-0.2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Time
<Field current ifd (pu)>
G-2 Reactive power
Active power
Terminal voltage
Field current
52
Figure 36: Tana Beles-I G-1 and G-2 impedance trajectories
However, in field winding open circuit and SPS event created at the same time both
failures jeopardize the stability of the system in very short period of time, the nearby
generator G-2 impedance trajectory was able to enter the protection zones of the relay
in 1.285second even if the generator excitation system remains at normal state as
shown in Fig.37b. Thus, in parallel connected generator, field winding open circuit
with SPS condition have affected the impedance trajectories of the remaining
generators.
Figure 37: Tana Beles-I G-1 and G-2 impedance trajectories in field winding (a)
short circuit with SPS (b) open with SPS
4.4 Power Swings
Power swing is the oscillation of machine rotor angle due to power system disturbances
like a fault, generator or line outages and load propagation that alters the mechanical
equilibrium of one or more machines. Power swings can be either stable or unstable
Field winding short circuit
Field winding open circuit
(a)
(b)
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
Impedance Trajectory of G-1
Impedance T. of G-2
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
Impedance Trajectory of G-1
Impedance T. of G-2
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
Impedance Trajectory of G-1
Impedance T. of G-2
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
Impedance Trajectory of G-1
Impedance T. of G-2
53
depend on severity of disturbances and the action of protective devices. It is stable
when the rotation speed of the machine returns back to synchronous speed and it is
unstable when the machine or part of the system loses synchronism after a major
disturbance. Mostly unstable swings are caused due to prolonged disturbances. System
failures directly affect the terminal voltage of the generator and as a result all the
behaviour of the system. Thus, the excitation system compromises the terminal voltage
to keep the system in stable through voltage transducer and load compensator which is
the part of excitation system that sense and compare the generator terminal voltage
with a pre-defined reference point. Large power swings can cause unwanted relay
operations that can further aggravate the power system disturbance and results outage
in the system [38]. In spite of the excellent performance of LOE relay in most system
disturbances, the concern that the relay might operate incorrectly during stable swings
has persisted. In view of this concern, an investigation was made to determine the
proximity of stable swings to the relay characteristic. During system disturbance, the
generator impedance trajectory may enter the LOE protection zones and stays for a
short time. If the maximum waiting time exceeds tripping time delay of LOE
protection, the relay will mal-operate. Faults cleared in less time than the critical
clearing time are stable and prolonged faults are unstable. In some cases of short-circuit
faults, the impedance at generator terminal does reach the protected area, but should be
studied and coordinated to the loss of excitation tripping time.
4.3.1 Short Circuit Faults
In this sub-section the reaction of LOE relay on three-phase, phase to phase and phase
to ground faults on line 5-7 will be studied on different initial loading conditions. The
results have been summarised on table4 and discussion will be given here for rated MW
output. The pre-fault active and reactive powers of G2 in all the next scenarios are
0.8485and 0.6307 respectively for heavy load condition. In Fig.35a, a three-phase to
ground short-circuit was created on L5-7 at 1second and cleared after 150ms.
Impedance trajectory of G-2 never enter the protection zones of the relay so no action
is taken. Phase to ground (AB-G) and phase-to-phase (A-B) faults are not as severe as
three phase fault but during the faults large amount of negative sequence component
occurs in phase current. In this two cases the faults are cleared after 150ms and as given
in Fig.36b and c, the impedance trajectories of G-2 remains still out of the protection
zones. For SPS event a three phase to ground short circuit fault is created in L5-7 and
54
cleared after 150ms by opening the circuit breakers from the sending and receiving end
so that the generator experience a swing. LOE relay have not detect this scenario but
when LOE event is created after the fault cleared at 1second, the impedance trajectory
enter in the relay zones at 1.15second.
(a)
(b)
(c)
(d)
Figure 38:G-2 impedance trajectory (a) three phase (b) two phase to ground (c) phase
to phase (d) three phase cleared after 250ms at G-2 terminal
Contrary, for three-phase to ground short-circuit cleared after 250ms, the system
experiences unstable swing and the generator impedance trajectory as given in Fig.38d
swings in zone-1 for about 1.66second and in zone-2 for 0.325sec after fault happen.
The machine should be isolated from the system as soon as possible but it should not
be through LOE relay since it is not excitation loss event. In this condition, only the out
of step relay should operate and open the generator circuit breaker before LOE relay
mal-operation since the tripping time of ANSI 78 is faster than LOE relay. But if this
fails the LOE relay will mal-operate for out of step situation [18] [29].
For faults near to G-2 terminal, all short circuit fault scenarios have been depicted on
bus-7 near the low voltage output of G-2 transformer. And the impedance trajectory
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
G-2 Impedance Trajectory-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
G-2 Impedance Trajectory
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
G-2 Impedance Trajectory-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
G-2 Impedance Trajectory
55
reaches the protection zones of the relay unlike the above cases. For three-phase to
ground fault at bus-5, impedance trajectory enters zone-1 and zone-2 at 0.15 second
and 0.0157second as shown in Fig.39a. Thus, the relay mal-operate at 0.15second after
fault happen. Otherwise, for the remaining scenarios LOE relay shows a good
performance on isolating short circuit faults far from the generator.
4.3.2 Outages
In this sub-section critical line, generator and load outages will be studied to observe
their effect on detection ability of excitation loss relay.
(a)
(b)
(c)
(d)
Figure 39: G-2 impedance trajectory (a) SPS (b) L7-8 outage (c) load rejection (d) G-
outage
Outages of either of these system components results in over or under loaded condition
of synchronous generator and system frequency and voltage instabilities
consequentially. This variation also affects terminal impedance of the generator. The
critical transmission line outage is studied through outage of L8-9 which can be isolated
due to faults or Circuit breaker failures practically and either of them for simulation
purpose. And G-3 outage and 125MW load rejection from bus-5 have simulated to
study the scenarios. As have given in Fig.37b-d, it can be seen that however in this
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
Impedance Trajectory in SPS
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Resistance (R) Pu
Reacta
nce (
X) P
u
Zone-1
Zone-2
Impedance Trajectory
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
Impedance Trajectory
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
G-2 Impedance Trajectory
56
cases the terminal impedance do not enter the protection zones, but the trajectory is quit
near the operation zone.
Table 4: Performance of LOE relay in power swings
In order to ensure the reliable operation of LOE relay there is a short time delay between
entering time to the operating zone and the initiation of a tripping signal. Comparing to
critical transmission line outage, main load rejection worsens the impedance trajectory
closeness to the relay characteristics as can be observed in Fig. 36c.
Table4 shows LOE relay performance on various system performances including SPS,
OOS, generating unit outages, line outage and load propagation. As can be observed
from the results, LOE relay had actually detected a loss of synchronism which was
caused by prolonged fault clearing times even in short period of time than LOE event.
Thus, the worst swings were happened when the system impedance was low and the
System Disturbances
Fault
clearing
time(ms)
Initial Loading(in pu) and
Tripping Status Y(sec)/N
Heavy
load
medium
load
Light
load
SPS
100 N N N
150 N N N
Short-circuit and LOE at1sec 150 Y(1.15s) Y(1.25) Y(1.053)
OOS
250 Y(0.856) Y(0.601) Y(0.638)
350 Y(0.420) Y(0.401) Y(0.401)
G3-outage &100MW load
addition in bus-2
- N N N
G2-outage - N N N
G2&G3 outage - N N N
L5-7 Outage - N N N
Load Rejection at bus-6 - N N N
Load Rejection at bus-5 and
short circuit fault at L8-9
150 N N N
57
fault clearing times were greater than to the critical clearing time. And short circuit
fault at the terminal of the generator is the sever fault detected in short period in
addition to out of step condition.
4.5 Backup Protection for Excitation Loss Detection
The main factors that affect the operation range of synchronous generator are armature
current, terminal voltage, limit of stability, field current, initial loading capacity and
minimum possible excitation. Thus, any variations of these parameters jeopardize
stability of the machine and the system as whole. Thanks to the study of LOE relay in
different power system performances, the relay have been found mal-operating in some
PLOE and some power system disturbances.
Efd<Efdi
& Vi=Vlimit
Efd status
Trip G
No
Yes
Efd<Efdi &
Q<=Qlimit
No
Where ith
values are
the previous
values
Yes
Calculate
E’q threshold
and Qlimit
Delay
Power
swing
Figure 40: Flow chart of proposed back up protection
In partial loss of excitation the relay detect the event long after the fault happen. In light
load case, even in full loss of excitation detection time was long after generator loses
stability. On the other hand, in power system disturbances the relay send trip signal for
58
un-wanted system disturbances as shown in table4. In this section considering the mal-
operation of LOE relay in partial loss of excitation and power swings, a backup
protection will be proposed considering the system stability in loss of excitation and
generator parameter variation in power swing conditions.
In any LOE event, the field voltage noticeably decreases from the initial value and so
the reactive power. Here, the reactive power keeps reducing to negative value till the
generator loses synchronism if any action is not taken. In some conditions of system
disturbances the reactive power also reduce to negative but the field voltage raise in
value to pay off the terminal voltage reduction. In this section, the stability of system
in loss of excitation will be studied to calculate the reactive power margin of generator
at specified field voltage using the q-axis voltage decay in excitation loss event. The
general scheme of the proposed algorithm can be sum up as Fig.40 flow chart.
From equation (2.2) and (2.13) we can see that the q-axis voltage is highly dependent
on field components and it is reasonable that it response really fast for field failures
than other parameters of the generator. The minimum e’q reduction that lead to voltage
collapse of the system will be calculated from the terminal voltage of the generator.
This will be the threshold and minimum q-axis voltage that keep the system in stability
at any moment.
)sin()1
(
1
'
0'
'
0
'
''' t
d
fd
qaq
d
d
dd
dqddq V
sT
EiRV
XsT
XX
XvXiE
(4.1)
At specified margin of terminal voltage, the q-axis voltage can be calculated from
equation (4.1) above where the q-axis voltage is given in terms of terminal parameters
of the synchronous generator and field voltage. The threshold q-axis voltage is also
used to identify the minimum possible value of reactive power the system can feed the
faulted generator without system collapse. Limiting the reactive power consumption of
the generator will be the main concern of this proposed algorithm since the actual
excitation loss detection mal-operation is due to the algorithm fails to limit how much
reactive power should the system feed the generator so as the system does not have to
collapse before the relay detect the failure. In some literatures this concept have been
used to detect excitation loss [39] but the unpredictable behaviour of power system in
light load condition, partial loss of field and system outages have threaten them. So, in
this algorithm limiting the reactive power consumption with the internal field
component will increase the sensitivity of the method for excitation loss event than
59
system disturbances. In the remaining parts of this thesis work, the generator terminal
voltage will be limited to 0.86pu so that q-axis threshold voltage will be calculated at
given field voltage. This voltage is limited by considering the transient and sub-
transient effects of the generator model through the specified time delay of the
algorithm. Thus, studying the possible swing of generator parameters in normal state
is important to compare the field voltage of the system.
After calculating eq threshold from equation (4.1), the remaining step is to identify the
amount of reactive power consumed by the generator in the threshold quadrature
voltage and at a given active power. Synchronous generator reactive power
consumption speed in a given field voltage is highly dependent on the initial MW
output of the machine. To verify the algorithm in different loading conditions active
power status is also one factor to identify the reactive power consumed by the
generator. Thus, the reactive power of synchronous generator in equation (2.11) can be
re-formulated in terms of q-axis voltage and active power. As have discussed in
previous sections, parameters of generator in excitation loss event varies till the fault
cleared. Except the limit voltage Vlimit, all the parameters required to calculate Qlimit are
used from the online status of the generator to avoid instability of the proposed
algorithm [40]. In full loss of excitation generator lose synchronism short after wards
after loss of stability that is why LOE relay response fast in FLOE. But in partial loss
of excitation, generator loses synchronism takes place long after system loses of
stability which results in system collapse and finally blackout [4].
q
d
qqt
dddqthret ii
iVPiXiEQ )(2''
(4.2)
Where e’qthre is the threshold q-axis voltage and Pt the output active power of the
synchronous generator. This reactive power identifies the ability of the system to
recover the reactive power loss due to excitation loss generator at the same time it is
the amount of reactive power consumed by the faulted generator without voltage
collapse. If the synchronous generator model consider sub-transient components, the
generator parameters swing in normal state should be counted through a reasonable
delay for tripping the generator to avoid un-necessary relay operation. But, in this thesis
work the two-axis generator model have used so the transient characteristics of the
generator have considered with time delay of 0.081second.
60
FLOE in heavy load
50%Efd loss in heavy load
FLOE in medium load
50%Efd loss in medium load
Figure 41: Q-V curve of LOE relay and proposed back up protection
To compare the detection ability of the proposed algorithm and LOE relay, the
scenarios used above in G-2 of IEEE 9-bus test system will be repeated in three
different loading conditions; heavy load, medium load and light load conditions. Fig.41
expresses the reactive power-voltage (Q-V) curve of LOE relay and the proposed
algorithm in full and partial loss of excitation in two different loading conditions. As
can be compared from the results, heavily loaded generators consumes more reactive
power in short period of time compare to other loading conditions which result also
system instability in less time. In full loss of excitation (field winding short circuit), the
generator consumes reactive power until the terminal voltage of the generator diminish
to 0.289pu before excitation loss relay detect the event. However, in the back-up
protection the generator was able to consume reactive power until the terminal voltage
reduce to 0.86pu. Comparatively, reactive consumption amount of generator before
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
Reactive Power Q Pu
Term
inal V
oltage V
t P
u
MVAR limit of LOE relay
MVAR limit of Bacu up protection
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
Reactive power pu
Voltage p
u
MVAR limit of LOE relay
MVAR limit of back-up protection
-0.35 -0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0 0.050.82
0.84
0.86
0.88
0.9
0.92
0.94
0.96
0.98
1
1.02
Reactive Power Q Pu
Term
inal V
oltage V
t P
u
MVAR limit of LOE relay
MVAR limit of Bacu up protection
-0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0 0.050.84
0.86
0.88
0.9
0.92
0.94
0.96
0.98
1
1.02
1.04
Reactive power pu
Voltage p
u
MVAR limit of LOE relay
MVAR limit of back-up protection
61
detected through LOE relay decreases with initial loading decrease but still it puts the
system condition at risk as can be shown in Fig.41b.
In partial loss of excitation, the back-up protection have response faster than LOE relay
as given in Fig.41c-d. At the same loading condition, generator reactive power
consumption is the same despite the type of excitation loss. On the other hand on the
same type of excitation loss type, power consumption differs for various initial loading
condition as shown in Fig.41a-d. Thus, for 50%field voltage loss, the generator was
able to consume reactive power until 0.289pu terminal voltage reduction before
detected through LOE relay in heavy load condition which is similar to full loss of
excitation. Also, for the remaining loading conditions the proposed algorithm have
shown a good performance in limiting reactive power consumption before system
collapse.
Full loss of excitation
50% Efd loss
Figure 42: Terminal voltage reduction in LOE relay and back-up protection
The main role of reactive power consumption limiting in excitation loss event is to
maintain the system from voltage collapse and system loss of stability to avoid un-
necessary other protective relays operation (under-excitation relay, loss of stability
relay) and blackout. From Fig.42, it can be understand that the terminal voltage of the
generator is kept to 0.86pu in the proposed back-up protection for full and partial loss
of excitation.
Table5 and 6 shows the comparison of excitation loss relay and the proposed back-up
protection in full loss of excitation. The back-up protection have improved the time
elapsed to detect excitation loss and reactive power consumption limit of the generator.
In field winding short circuit case, the proposed algorithm detect full loss of excitation
0 2 4 6 8 10 12
x 104
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
time sec
Term
inal V
oltage p
u
Vt reduction of LOE relay in heavy load
Vt reduction of LOE relay in medium load
Vt reduction of back-up protection for both cases
0 2 4 6 8 10 12 14 16 18
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
Time
Term
inal V
oltage p
u
Vt reduction of LOE relay in heavy load
Vt reduction of LOE relay in medium load
Vt reduction of back-up protection for both cases
62
1.857second after fault happen which is 2.303second before LOE relay. Similarly, for
medium and light loaded generators the detection length have improved to about 16%
of excitation loss relay.
Table 5: Comparison of actual and proposed excitation loss detection in field
winding short circuit
Initial loading (pu)
Tripping Duration (sec)
Possible MVAR consumed by G-2
before fault detected (pu)
LOE relay Proposed LOE relay Proposed
Heavy load 4.16 1.857 -0.431 -0.194
Medium load 5.804 4.537 -0.321 -0.2688
Light load 6.286 4.104 -0.231 -0.2055
On the other hand, in field winding open circuit the proposed algorithm detects the
event in long duration than LOE relay except for light load condition and vice versa
for generator possible reactive power consumption before fault detected. Thus, the
detection duration of field winding open circuit have improved in medium load
condition and a little bit in heavy loaded generators. Thus, it can conclude field winding
open circuit highly affects the terminal impedance of the generator.
Table 6: Comparison of actual and proposed excitation loss detection in field
winding open circuit
Initial loading (pu)
Tripping Duration (sec)
Possible MVAR consumed by G-2
before fault detected (pu)
LOE relay Proposed LOE relay Proposed
Heavy load 0.0835 0.0845 -0.3175 -0.3209
Medium load 0.0467 0.2851 -0.314 -0.363
Light load 1.131 0.409 -0.2964 -0.2089
So far, the back-up LOE protection seems working perfectly for FLOE in different
loading conditions. To verify the reliability of the scheme in different LOE events, the
method have been tested in all possible partial loss of field voltage in three different
loading conditions as shown in table7. Similarly to full loss of excitation, the proposed
algorithm detect partial loss of excitation in heavy loaded generators twice less time
than LOE relay in all possible field voltage reduction. LOE relay is not able to detect
field voltage reduction until half of the rated value in medium and light loaded
generators, and the proposed method have improved this.
63
Table 7: Comparison of actual and proposed excitation loss detection in
partial field voltage loss
%Efd
loss
Initial
Loading(in
pu)
Tripping Status
Y(sec)/N
Possible MVAR consumed by G-2
before fault detected (pu)
LOE
relay
Proposed LOE relay Proposed
20% H 26.46 12.86 -0.4897 -0.204
M N N -0.2720(in30sec) -0.116(in30sec)
L N N -0.1906(in30sec) -0.05(in 30sec)
30% H 15.21 7.24 -0.4882 -0.2021
M N 29.017 -0.2735(in30sec) -0.19
L N N -0.1909(in30sec) -0.101(in30sec)
40% H 10.94 5.075 -0.4883 -0.198
M N 28.72 -0.2805(in30sec) -0.264
L N 29.803 -0.1909(in30sec) -0.1802
50% H 8.563 3.935 -0.4887 -0.1957
M 17.19 14.33 -0.2837 -0.2645
L N 15.37 -0.191 -0.1907
60% H 6.99 3.22 -0.4858 -0.1945
M 11.96 9.948 -0.2885 -0.266
L 9.673 9.402 -0.198 -0.1965
70% H 5.913 2.724 -0.482 -0.1938
M 9.242 7.648 -0.2919 -0.2669
L 7.332 7.11 -0.2076 -0.2045
80% H 5.153 2.359 -0.4792 -0.1929
M 7.632 6.216 -0.296 -0.265
L 6.143 5.697 -0.216 -0.206
90% H 4.592 2.079 -0.4768 -0.1939
M 6.569 5.241 -0.3669 -0.268
L 5.383 4.773 -0.2236 -0.2056
N.B. Where H is heavy load condition (0.8485+j0.06307pu) and M is medium load
condition (0.4245+j0.03389pu) and L is light loading condition (0.2547+j0.3pu). And
64
the negative sign in MVAR indicates the consumption of reactive power by the
generator.
(a) (b)
Figure 43: G-2 terminal voltage in Partial loss of excitation (a) medium load 20%Efd
loss (b) light load 30%Efd loss
LOE relay and also the proposed back-up protection have found not detecting 20%
field voltage for medium and light load conditions and 30%Efd loss for light load
condition. However, from the parameter variation of the generator and system as shown
in Fig.43 the terminal voltage of the generator remains in range of 0.93pu which is
stable and precise voltage range. So, the generator should not be tripped for stable case
since un-necessary eliminating of synchronous generators will further jeopardize
system stability despite its economic issue.
LOE relay have mal-operate for severe power swings as have shown in table4. To
investigate the proposed back-up protection performance during such disturbances, the
cases studied in power swing conditions have repeated with the same system conditions
and the results are compared with results of LOE relay. SPS and out of step conditions
are simulated by three-phase to ground fault at G-2 terminal with pre-fault initial
condition of 0.8485+j0.06307pu. In OOS condition the generator becomes unstable
and should be isolated from the remaining system but LOE protection should not give
any response for this condition. From the simulation results summarized in table7, all
the mal-operation of LOE relay in system disturbance have overcome through the
proposed aback-up protection. In LOE and SPS event created at 1second, LOE relay
send a trip signal after 0.15second SPS happened which is before the LOE event
detected through the relay but the back-up protection sends a trip signal after
1.857second which is the duration LOE event should be detected.
0 5 10 15 20 25 300.93
0.94
0.95
0.96
0.97
0.98
0.99
1
1.01
time sec
Vt pu
G-2 terminal voltage
0 5 10 15 20 25 300.92
0.93
0.94
0.95
0.96
0.97
0.98
0.99
1
1.01
1.02
time sec
Vt pu
G-2 terminal voltage
65
Table 8: Comparison of actual and proposed excitation loss detection in
system disturbances
Improvement of LOE relay have been the main concern of researchers for almost six
decades. However, the complexity of threshold setting in every failure, several
simulation scenarios and only testing on one common type of excitation loss were the
main disadvantages to hold them from installing in actual industries of power systems
[41]. Some recent methods have proposed setting free algorithms [29] which greatly
differentiate system disturbances and LOE event in heavily loaded generators but the
longed parameter swing in PLOE and in power swing when initial loading of the
generator decrease have not considered. In this proposed back-up protection,
considering the drawbacks of the above methods excitation loss event detection
directly from measuring of generator parameter variation have tested in all possible
scenarios of LOE event and system disturbances.
Generally from the simulation results, the proposed back up protection detects all
stages of excitation loss event much faster than excitation loss relay, have differentiated
any excitation loss event from any system disturbance such no trip signal is issued
System Disturbances
Fault clearing
time(ms)
Tripping Status
Y(sec)/N
LOE relay Proposed
SPS
100 N N
150 N N
200 Y(1.15s) N
OOS
250 Y(0.856) N
350 Y(0.420) N
G3 outage and 100MW
load addition in bus-2
- N N
G2-outage - N N
G2&G3 outage - N N
L5-7 Outage - N N
Load Rejection at bus-6 - N N
Load Rejection at bus-5 and
short circuit fault at L8-9
100 N N
SPS and LOE at 1second 200 Y(1.15s) Y(1.857)
66
through the method for any power swings. And while detecting the excitation failure,
system stability is held by limiting the reactive power consumption of the generator to
ability of the system to feed the faulted machine without system collapse. This avoids
any power system component damage and unwanted gird protective devices
functioning like under voltage relay and under excitation relay due to excitation system
failure.
67
CHAPTER 5
CONCLUSIONS AND RECOMMENDATION
5.1 Conclusion
From the excitation loss event study in synchronous generators, it has shown that
excitation loss not only imperil the faulted generator but also the whole system’s
stability due to electrical and mechanical power imbalance on the generator. This thesis
investigates detection ability of excitation loss relay in various failures of excitation
system and different power system disturbances. The relay can detect any type of full
excitation loss in less than 6.4second after fault created but for partial excitation loss
the remaining field voltage allows the defective machine and the system to find a new
operating point as a result the scenario is detected long after it happen or may be not
detected at all for lightly loaded generators. In addition to the slow operation in partial
excitation loss, the relay also response for un-wanted system disturbances especially
for stable and un-stale power swings which further threaten system stability in addition
to economic issue of un-wanted generator tripping.
A back-up protection based on the q-axis voltage and reactive power flow to the
generator have proposed to overcome the mal-operation of excitation loss relay in
power swings and slow operation of the relay in partial loss of excitation. In heavily
loaded generators, the system voltage collapse happen at 2.85second after excitation
loss initiated but excitation loss relay detects the failure at 4.16second which is
1.31second later of voltage collapse. This have been the main reasons for system
blackouts due to excitation loss event. In this proposed algorithm reactive power
consumption of the generator have been limited to the ability of the interconnected
system to feed the faulted generator without system collapse. The proposed back up
protection detects field winding short and open circuit at 1.857 and 0.153second
respectively which is 44% improvement of actual excitation loss protection. The
improvements of the back-up protection can be summarized as:
Detect excitation loss before system collapse
Improve the detection duration of impedance protection relay to twice less for
heavy loaded generators and almost 16% less for lightly loaded generators
Differentiate system failure and excitation loss
Detect possible partial loss of excitation that can lead to system instability
68
Generally it can be conclude that in both excitation loss detection methods the time
available to trip an excitation loss generator depends on initial loading of the generator,
reactive power support from interconnected systems and type of excitation loss. More
MW output of generator and full excitation loss implies less time before detected
through excitation loss protection and vice versa for weak MVAR support from the
system and partial excitation loss.
69
5.2 Recommendations for Future Work
The excitation loss detection using the generator terminal impedance is the actual
method in power system industries until now. Even if a number of improvements using
other generator parameter variation in excitation loss event have been recommended,
the set point identification is a difficult task and may involve extensive simulation
processes which make them unpractical. For future work, it is recommended to modify
the actual excitation loss protection by co-ordinating reactive power consumption limit
of synchronous generator without system loss of stability. And I recommend further
development and testing of the proposed back-up protection scheme for general power
systems.
70
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74
APPENDIX A
A.1 Full Loss of Excitation
Field winding short circuit Field winding open circuit
Figure 44A-1: 9-bus G-2 impedance trajectory in 90% and 70% loading
A.2 Partial Loss of Excitation
(a)
(b)
Figure 45A-2: 9-bus G-2 impedance trajectory in (a) 90% and (b) 60% field voltage
loss
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
G-1Impedance T. in 90% loading:
70%loading
t=4.9605st=4.092s
t=4.3352s
t=6.2556s
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
G-1Impedance T. in 90% loading:
70%loading
t=0.0106s
t=0.0105s
t=0.065s
t=0.3239s
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Resistance (R) Pu
Reacta
nce (
X)
Pu
Zone-1
Zone-2
G-2ImpedanceT Heavy L.
in Medium L.:
in Light L.
t=9.672s
t=11.954st=7.988s
t=14.728s
t=18.139s
75
A.3 IEEE ST1A Excitation System
Figure 46A.3: Block diagram of IEEE ST1A excitation system
A.4 Two-axis Model Initial Values
000 )( aqatq IjXRVE
00 qE
00 aI
00 tV
)sin( 0000 ad II
)cos( 0000 aq II
)sin( 0000 td VV
)cos( 0000 tq VV
0
'
00
'
0 ddqaqq IXIRVE
0
'
00
'
0 qqdadd IXIRVE
0
''
0
'
0 )( dddqfd IXXEE
0000000 )()( qqaqddadem iiRViiRVPp
s 0
76
APPENDIX B
Table 9 B-1: IEEE 9-bus system required Machine Data
Parameters
Unit
Synchronous
Gen. 1
Synchronous
Gen. 2
Synchronous
Gen. 3
Bus-1 Bus-2 Bus-3
Rated Power MVA 247.5 192 128
Rated voltage KV 16.5 18 13.8
H S 9.55 3.33 2.35
D
2 2 2
Xd pu 0.361 1.72 1.68
Xq pu 0.2398 1.65 1.61
X’d pu 0.1504 0.23 0.232
X’q pu 0.159 0.23 0.232
X’’d pu 0.099 0.1728 0.19
X’’q pu 0.099 0.1728 0.19
T’d0 pu 8.96 4.8 8.9
T’q0 pu 0.48 0.07 0.5
T’d pu 1.01 1.302 1.302
T’’q0 pu .001 0.0007 0.070
Table 10 B-2: IEEE 9-bus system load data
Bus P[pu] Q[pu]
5 1.25 0.50
6 0.90 0.30
8 1.00 0.35
77
Table 11 B-3: IEEE 9-bus Transmission line Data
Line R[pu/m] X[pu/m] B
[pu/m]
Total
reactance
[ohm]
Estimated length of the line
based on line reactance
values [Km] From
bus
To
bus
4 5 0.01 0.068 0.176 2645 5290
4 6 0.017 0.092 0.158 3174 6348
5 7 0.032 0.161 0.3060 3703 7406
6 9 0.039 0.1738 0.358 4761 9522
7 8 0.0085 0.0576 0.1490 4232 8464
8 9 0.0119 0.1008 0.2090 4761 9522
Table 12 B-4: IEEE 9-bus system excitation system data
Parameters
Unit
Synchronous
Gen. 1
Synchronous
Gen. 2
Synchronous
Gen. 3
Tr S 0.000 0.000 0.060
Ka pu 200 30 25
Ta S 0.395 0.400 0.200
VRmax Pu 3.84 4.590 1.00
VRmin pu -3.84 -4.590 -1.000
Ke pu 1.000 -0.020 -0.0601
Te S .0000 0.560 0.6758
Kf pu 0.0635 0.050 0.108
Tf S 1.0000 1.300 0.350
E1 pu 2.880 2.5875 2.4975
SE
0.000 0.7298 0.0949
E2 pu 3.840 3.450 3.33
SE
0.000 1.3496 0.37026
78
Table 13 B-5: Tana Beles-1 System data
Parameters
Unit
Four Synchronous Generators with
identical ratings and system data
Active Power MW 46
Reactive Power VA -27.3
Rated voltage KV 15
H S 3.14
D
0
Xd pu 1.03
Xq pu 0.7
X’d pu 0.31
X’q pu 0.159
X’’d pu 0.25
X’’q pu 0.25
T’d0 pu 0.13
T’d pu 1.01
T’’q0 pu 0.1
B.6 Excitation Loss Relay Protection Zones
%%input Parameters of a generator f=60; theta =transpose( 0:.01:(2*pi)); S=192e6; V=18e3; %rated voltage H=2; %inertia constant Xd=1.72; %pu Xq=1.65; %pu Xd1=0.23; %pu Xq1=0.23; %pu Xd2=0.1728; %pu Xq2=0.1728; %pu Ra=0.0075; %pu Td01=1.302; % second Tq01=1.00; %second Td02= 0.023; %second Tq02=0.07; Rng=15; %pu
% Transmission Line Data L=0.24; %Henry, Rl=60; %
79
%Transformer data St=100e6; % VA VHv= 230e3; % V VLv=18e3; % V XTG=0.05; %pu cooperloss= 0.001; %pu
%% Mho characteristics Modelling
% Base impedance with respect to synchronous generator Zbase= (V^2)/S;
%MHo Zone-1 Settings %radius is 1 pu
radius1= Zbase/2; %Coordinate of centre % x-coordinate is set to zero % Y-coordinate of zone-1 is given by first calculating base value of
reactance with respect to synchronous generator %actual value of X'd Xd1a= Xd1*Zbase;
YC1=-((Zbase/2)+(Xd1a/2)); % MHO ZONE 2 settings % mho circle radius for ZONE 2 is xd/2 Xda= Xd*Zbase; radius2=Xda/2; %Y-coordinate of zone-2 YC2=-(Xd1a/2+Xda/2); %Relay Settings Forward and Reverse impedances ZFR=2*Xd1a; secZFR=ZFR*Ratio; ZbaseLV=VLv^2/St; XTGa=XTG*ZbaseLV; ZRV=1.5*XTGa; secZRV=ZRV*Ratio; % X and Y-axis values of the two protection zones xs=0+radius1*cos(theta); ys=YC1+radius1*sin(theta); x2=0+radius2*cos(theta); y2=YC2+radius2*sin(theta);
%Plots % zone1=[xs,ys]; % zone2=[x2,y2]; set(gcf,'visible','on')
%simulate the Simulink model
simout=sim('ninebus100');
plot(xs,ys,'r',x2,y2,'b'); axis([-1.5 4 -3.5 1]); hold on drawnow expose ;
plot(R.signals.values,X.signals.values,'r'); xlabel('Resistance (R) Pu'); ylabel('Reactance (X) Pu');
80
legend('Zone-1','Zone-2','Impedance Trajectory'); grid on set(gca,'Xlim',[0 ntimes],... 'Ylim',[2.5 4]) set(h,'XDataSource','R.signals.values') set(h,'YDataSource', 'X.signals.values')
hold off
81
APPENDIX C
Physical Representation of Excitation Loss Event
Excitation loss phenomenon can be easily understand through human body to have a
physical view of the event. So, generally human body can be represented as
synchronous generator which have an excitation system, grid connected to it and a
power (including the voltage and current) that will be delivered to the system. Thus,
the calories taken in terms of foods or drinks and stored in stomach will be an excitation
system, the stomach itself as generator, the energy that burn from the calories and
distributed to the whole muscles to keep the body functional as power of the generator
and the whole body as the gird connected to the synchronous generator.
When the stomach emptied, the human digestion system needs another way to deliver
energy to the whole muscles of the human body. This emptiness of the calories in
stomach is called excitation loss. So when our body losses excitation, the digestion
system starts to burn the accumulated energy as fat from the body and deliver it back
to the body’s muscles so as the body will remain functional until an additional calorie
added. And our body at this condition will feel hunger, thirsty or tired. Thus
synchronous generators consume the reactive power of the system when it loses its
excitation. However, if this continues for long time till the maximum limit of re-
energizing of the body through body’s energy, the delivering of energy will suddenly
stopped and the body will be so weak that may loss stability of handling each other.
May God avoid this from all human beings but if still not avoided the body will not be
able to resist the condition. When the system reach at maximum limit of reactive power
feeding to the generator, the synchronous generator loss synchronism and if it does not
isolated the whole system will be shut down.
Feeling of tired, hunger, thirst and unbalance are the response of our body to loss of
calorie in the stomach. So, if we are able to response to our feelings we can prevent it
before it loses its balance. The parameters including current, speed, terminal and
internal voltage and impedance of the generator starts to vary when a generator loses
its excitation. Thus by sensing the parameter variation we can prevent excitation loss
which is called excitation loss detection methods.