power system stability - murdoch universitypower system stability the requirement is that the power...
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SCHOOL OF ENGINEERING AND INFORMATION TECHNOLOGY
Power System
Stability
ENG470 Engineering Honours Thesis
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By:
Gurinder Pal Singh (32534143)
15/01/2019
Unit Coordinators – Dr. Gareth Lee, Prof. Parisa Arabzadeh Bahri
Supervisor – Dr. Farhad Shahnia
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Declaration
I declare that this thesis paper titled “Power System Stability” is submitted to the Discipline
of Engineering of Murdoch University in partial fulfillment of the Bachelor of Engineering
Honours (Electrical Power and Industrial Computer System Engineering).
I confirm (Gurinder Pal Singh) that the material contained in this thesis is the result of my
research work, and the research achievements of others used in this thesis are referenced
and cited in accordance with the university plagiarism rules. This thesis is the original copy of
the thesis and has not previously been submitted to any level or higher degree.
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Copyright
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The figures from 1‐6 have been reproduced with the help of Microsoft paint. The figures from 7 – 16 have been removed from the report due to failure in contacting the owner for their permission and copyright issues, however appropriate references have been given next to the figure numbers in the report.
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Abstract
Grid instability through network disturbances is a major concern for current Power systems.
Designing a power system to enhance the voltage stability while considering major faults will
lead to an improved and stable system. Major areas of the power system, such as voltage
instability, rotor angle instability and frequency instability as well as the factors responsible
for these instabilities, their analysis techniques and preventive measures will be covered in
this report. The Power Factory simulation software will be used to test a nine‐bus system
stability by introducing the faults and performing PV and QV curves analysis. After defining
different control models of voltage regulators, governor and turbine, further analysis will be
carried out in this thesis through the Power Factory library from pre‐existing parameters.
Carrying out the fault analysis prior to the system installation will make the power system
more robust and effective in operation.
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List of Abbreviations
Avr – Automatic voltage regulator
Gov – Governor and turbine
LTC Transformer – On load tap changer Transformer
RPF – Reactive participation factor
FACTS ‐ Flexible AC Transmission Systems
SVC – Static Var Compensator
STATCOM – Static Synchronous Compensator
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List of Symbols
V = Bus Voltage
X= Impedance of the line
δ = Power angle.
ξ = Right eigenvector matrix
η = Left eigenvector matrix
Vtrk – voltage state variable measurement after sensor lag block
Vtk – Measured terminal voltage
Kak – AVR gain
Trk ‐ Time constant of sensor
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Table Contents
1. Introduction ..................................................................................................................... 13
1.1. Overview ................................................................................................................... 13
1.2. Power system Stability .............................................................................................. 14
2. Classification of power system stability ........................................................................... 16
2.1. Voltage stability ......................................................................................................... 16
2.1.1. Main causes of Voltage instability ..................................................................... 18
2.1.2. Voltage instability Scenarios .............................................................................. 18
2.1.3. Voltage stability assessment/Analysis ............................................................... 20
2.1.4. Methods for improving voltage stability ........................................................... 23
2.2. Frequency Stability .................................................................................................... 32
2.2.1 Power System balance and grid frequency ............................................................ 33
2.2.2 Power Frequency control ................................................................................... 34
2.2.3 Importance of Frequency Stability .................................................................... 36
2.3. Rotor Angle Stability .................................................................................................. 37
2.3.1. Small ‐Signal Stability ......................................................................................... 38
2.3.2. Transient Stability .............................................................................................. 39
3. Power System Simulations (power factory) .................................................................... 41
3.1 Main components of power system control ............................................................. 43
3.2 Excitation Systems ..................................................................................................... 43
3.2.1 Components of excitation system ..................................................................... 45
3.2.2 Types Excitation Systems ................................................................................... 46
3.2.3 Modelling of Excitation systems ........................................................................ 47
3.2.4 Effect of Excitation on Stability .......................................................................... 47
3.3 Models for Stability Analysis ..................................................................................... 51
3.4 System Modelling Approach ..................................................................................... 51
3.5 Stability Analysis ........................................................................................................ 53
3.5.1 PV Curve ............................................................................................................. 53
3.5.2 QV Curve ............................................................................................................ 57
3.5.3 Small Signal Stability Analysis ............................................................................ 61
3.5.4 Transient stability analysis ................................................................................. 63
4. Conclusion ........................................................................................................................ 78
5. Future scope of work ....................................................................................................... 79
6. References ....................................................................................................................... 80
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7. Appendices ....................................................................................................................... 82
Appendix 1 ‐ Avr Parameters .............................................................................................. 82
1. IEEE T1 ................................................................................................................... 82
2. IEEE T3 ................................................................................................................... 83
3. IEEE T5 ................................................................................................................... 84
4. EXAC4 ..................................................................................................................... 85
Appendix 2 – Governor Parameters .................................................................................... 86
1. HYGOV ................................................................................................................... 86
2. IEEE G1 ................................................................................................................... 87
3. IEEE G3 ................................................................................................................... 88
4. TGOV5 .................................................................................................................... 89
Appendix 3 – Nine bus system data ..................................................................................... 90
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List of Figures
Figure 1. Classification of power system stability [2] .............................................................. 16 Figure 2. 2‐Bus system [5] ........................................................................................................ 17 Figure 3. Bus voltage variation with Active and Reactive loading for 2 ‐Bus system [5] pg.4 (Figure 1.3) ............................................................................................................................... 17 Figure 4. PV Curve method [12] ............................................................................................... 21 Figure 5. VQ Curve method [12] .............................................................................................. 22 Figure 6. Power system balance (Accelerated by generation (P) decelerated by Load (L) [10].................................................................................................................................................. 34 Figure 7. Frequency Control in power system [10] pg. 5691 (Figure 4) ................................. 35 Figure 8. Timing ranges of action of Primary, Secondary and Tertiary control [10] Pg. 5691 (Figure 5) .................................................................................................................................. 36 Figure 9. Nature of small disturbance response with constant field voltage [11] pg 17 (Figure 2.4) ........................................................................................................................................... 38 Figure 10. Nature of small disturbance response with excitation control [11] Pg.17 (Figure 2.5) ........................................................................................................................................... 38 Figure 11. Rotor angle response to transient disturbance [11] pg. 19 (Figure 2.6) ................ 39 Figure 12. Nine Bus System ...................................................................................................... 42 Figure 13.Principal control of generating unit [13] pg. 233 (Figure 7.1) ................................ 43 Figure 14. Excitation control system of Synchronous generator [1] pg. [317] (Figure 8.1) .... 45 Figure 15. DC excitation system with amplidyne voltage regulator pg. [319] (Figure 8.2) ..... 46 Figure 16. Field‐control alternator rectifier system [1] (Figure 8.3) ........................................ 46 Figure 17. PV Curves ............................................................................................................... 54 Figure 18. Reactive Power Compensation ............................................................................... 55 Figure 19. Critical Scalable demand (Contingencies) ............................................................... 56 Figure 20. QV Curves ................................................................................................................ 57 Figure 21 Critical bus voltage and reactive power .................................................................. 58 Figure 22. QV Curve Bus 6 at different Loads .......................................................................... 59 Figure 23 Contingencies ........................................................................................................... 60 Figure 24. Eigenvalues plot ...................................................................................................... 61 Figure 25. Bar plot of Participation factor ............................................................................... 62 Figure 26. Composite model of Generator 2 ........................................................................... 64 Figure 27. 3 Phase balanced fault at Line 5‐7 .......................................................................... 65 Figure 28. Rotor Angle response of Generator 2 with different Avr ....................................... 66 Figure 29. Speed of Generator 2 with different Avr ................................................................ 66 Figure 30 Active Power of Generator 2 with different Avr ...................................................... 67 Figure 31. Reactive Power of Generator 2 with different Avr ................................................ 67 Figure 32. Excitation voltage of Generator 2 with different Avr ............................................. 68 Figure 33. Terminal voltage of Generator 2 with different Avr ............................................... 68 Figure 34. Turbine power of Generator 2 with different Avr .................................................. 69 Figure 35. Current Magnitude of Generator 2 with different Avr ........................................... 69 Figure 36. Rotor Angle response of Generator 2 with different governors ............................ 70 Figure 37. Speed of Generator 2 with different governors ..................................................... 70 Figure 38. Active power of Generator 2 with different governors .......................................... 71 Figure 39. Reactive power of Generator 2 with different governors ...................................... 71 Figure 40. Excitation voltage of Generator 2 with different governors .................................. 72
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Figure 41. Turbine Power of Generator 2 with different governors ....................................... 72 Figure 42. Terminal voltage of Generator 2 with different governors .................................... 73 Figure 43. Current Magnitude of Generator 2 with different governors ................................ 73 Figure 44. Generator Speed G1, G2, G3 .................................................................................. 74 Figure 45. Reactive Power (G1, G2, G3) ................................................................................... 74 Figure 46. Rotor angle (G1, G2, G3) ......................................................................................... 75 Figure 47 Turbine Power (G1, G2, G3) ..................................................................................... 75 Figure 48. Excitation Voltage (G1, G2, G3) .............................................................................. 76 Figure 49. Current Magnitude (G1, G2, G3) ............................................................................. 76 Figure 50.Active Power (G1, G2, G3) ....................................................................................... 77 Figure 51. Terminal voltage (G1, G2, G3) ................................................................................ 77 Figure 52. Global type Avr ‐ IEEE T1 ......................................................................................... 82 Figure 53. Global type Avr ‐ IEEE T3 ......................................................................................... 83 Figure 54. Global type Avr ‐ IEEE T5 ......................................................................................... 84 Figure 55. Global type Avr ‐ EXAC4 .......................................................................................... 85 Figure 56. Global type Gov ‐ HYGOV ........................................................................................ 86 Figure 57. Global type Gov ‐ IEEE G1 ....................................................................................... 87 Figure 58. Global type Gov ‐ IEEE G3 ....................................................................................... 88 Figure 59. Global type Gov ‐ TGOV5 ........................................................................................ 89 Figure 60. Nine Bus system simulation data ............................................................................ 90
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1. Introduction
1.1. Overview
In order to understand the stability it is essential to become familiar with the basic operations
of a power system. There are three main components of a power system i.e. generation,
transmission and distribution. Electrical power is generated mostly from synchronous
machines. The primary sources of energy (fossil, hydraulic) are converted into mechanical
energy through prime movers. Mechanical energy is used by synchronous generators to
produce electrical power and most of the electric power systems are three phase AC systems
operating at constant voltage. Three phase equipment is also used by generation and
transmission facilities along with industrial residential and commercial loads that are equally
distributed among all the phases to form a three‐phase balanced system [1]. A power system
entirely relies upon its voltage, frequency and rotor angle stability. The main causes behind
the instabilities, analysis techniques and methods used to improve the overall system stability
will be discussed in this report.
This report will explore the main aspects of power system instabilities by
reviewing existing literature, analysing through simulation and finally correcting the grid
instability with regard to voltage, frequency, and rotor angle. A small nine bus transmission
network using Power Factory to analyse and match the theoretical factors that determine the
reliability and operation as mentioned in current literature. The Power Factory provides
numerous inbuilt global models of Automatic voltage regulator (Avr), Governor and Turbine
(Gov), and power system stabilizer (PSS) etc. to reduce the complexity and avoid building the
differential functions from scratch. The system will be analysed under four random avr and
gov models by introducing a three‐phase fault at one of the transmission lines. The response
of one of the generator will be examined on the basis of its variables like rotor angle, speed,
output powers, terminal and excitation voltages. The ideal combination of avr and gov will be
used to carry out PV and QV curve analysis to find the weakest bus in the system. Lastly, the
performance of that bus will be analysed by varying the load and reactive power
compensation. Stability of the system will be enhanced by using various Avr, Gov models
through testing pre‐existing parameters.
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1.2. Power system Stability
The requirement is that the power system must be able to operate at equilibrium state under
normal conditions and maintain the equilibrium state after any disturbance introduced in the
system. The synchronous machines plays a crucial role in maintaining the power system
stability. The dynamic of generator rotor angles and power angle relationships are main
components which influence the stability. Such disturbances has led many power system to
collapse. For power systems to operate securely, the stability is considered a prime concern
when operated under system overloads [2].
There are various reasons which has led system instability problem to become a greater
concern than the past:
Increasing number of interconnections
Introduction of new technologies
Environmental impact on transmission expansion
More power consumption in heavy loading areas
New system loading patterns due to advancement in electricity market
More utilization of induction machines
Wind generators penetration and local uncoordinated controls in the system [2].
Power systems are exposed to a wide range of disturbances like variations in loads,
transmission line short circuits, and faults in major equipment like generators. The system is
required to respond and operate satisfactorily after such disturbances. Designing a power
system which can stay stable after all types of disturbances is practically impossible and
uneconomical. On the basis of high occurrence probability, contingencies are selected while
designing the system.
The disturbance response of a system has an impact on the majority of its equipment.
Assuming an outage of a critical element after a fault due to the activation of a protective
device will vary the system power flows, bus voltages and the rotor speed of the machines.
The voltage regulators present in the generators and transmission network will get actuated
after variations in the voltage. The change in rotor speed will actuate the prime mover
governors, the fluctuations in frequency and voltage will affect the loads and lead the
protective devices to trip the individual equipment according to the impact. These events will
result in instability.
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A new equilibrium state will be achieved, if a system is stable after following a disturbance
while preserving its integrity. In order to maintain the continuity in the operation of the rest
of the system, some loads and generator may get isolated. The interconnected systems
sometime split into two or more islands to maintain the power continuity for the majority of
loads. On the contrary, an unstable system will lead to a situation, like continuous increase in
the rotor angle and downfall in the bus voltages [3]. Unstable condition may result in the
cascading outages and shut down of major portion of the power system.
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2. Classification of power system stability
Due to the number of multi‐dimensional instabilities occurring in the power system, there is
a need to classify them. The classification of power system stability simplifies the
representation, analysis and treatment of specific problems effectively. The major areas of
the power system has been classified (figure 2.1) into the following categories [3].
2.1. Voltage stability
Voltage stability is the capacity of a power system to maintain an acceptable level of voltage
at all the busbars after a disturbance happens within the system. Voltage instability leads to
a drop in bus voltage to unacceptable values which results in voltage collapse. Voltage
instability is considered a menace for system operation and is the reason behind the major
blackouts across the world [4]. The power systems that are heavily loaded, faulted or have
reactive power shortages often face this problem.
The following two bus system example (Figure 2) with formulas is used to clarify the concept
of voltage stability. The load is considered as constant power type and the equation for real
power and reactive power transfer from bus 1 to 2 is given by:
Figure 1. . Classification of power system stability [2]
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𝑃 sinδ (1)
𝑄 cos (2)
where, E = ∠δ E is the voltage at bus 1,
V = The voltage at bus 2,
X= impedance of the line (neglecting resistance),
δ = power angle.
Replacing the P and Q equation with v = V/E, p = P.X/E2 and q = Q.X/E2
The result will be
p = v sin
q = ‐ v2 + v cos
After squaring the above two equations and squaring them to calculate the positive real
solutions of ‘v’:
𝑣 𝑞 𝑝 𝑞 (5)
Figure 3. Bus voltage variation with Active and Reactive loading for 2 ‐Bus system [5] pg.4 (Figure 1.3)
Figure 2. 2‐Bus system [5]
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The above plot represents the voltage on p‐q‐v plane. There are two solutions of v
corresponding to each point (p,q), the high voltage or stable solution and the lower or
unstable solution. The point along the equator where two solutions of v are equal shows the
maximum power limits. Beginning from the upper part of the surface, the system gets closer
to the maximum point with increasing p or q or both. The voltage becomes unstable if these
values becomes more than the maximum point [5].
2.1.1. Main causes of Voltage instability
Typically, loads are the main contributing factor to voltage instability. During any disturbance,
the power consumption of a load is compensated by the action of motor slip, distribution
voltage regulators, and tap‐changing transformers. Re‐established loads raise the pressure on
the high voltage network by over utilization of reactive power and lead to further reduction
in the voltage profile of the network. The moment when load dynamics try to restore power
consumption by crossing the limits of the respective generation and transmission network, it
leads to critical situations that lead to instability [2]. In a real power system, there are multiple
factors which cause voltage instability like transmission network capacity, reactive and
voltage control limits of the generator, loads voltage sensitivity, reactive compensation
devices, and voltage control devices like transformers and under‐load tap changers [5].
2.1.2. Voltage instability Scenarios
2.1.2.1. Short Term Voltage Instability
Short term voltage instability has a time frame of about 10 seconds to correct. During a step
drop in voltage, the active power (P) of the motor instantly drops as the square of the voltage
V (constant impedance nature), then regains the value close to its pre‐disturbance value
within time frame of a second. The core variable in this course is known as rotor slip. In case
of a motor with minimal stator losses and constant mechanical torque, a constant active
power is restored [8]. Considering these losses and real life torque behaviours, P is steady
state dependant with respect to V. Reactive power Q is steady state dependant but a little
more complex. Initially, the reactive power drops quadratically with V until it reaches
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minimum and then rises up to the level where the motor stalls because of low voltage. The
stalling voltage in large three phase industrial motors can be as low as 0.7 pu, whereas in
smaller electrical appliances or high loaded motors it can be higher. The induction motors
plays a significant role in the load restoration of the systems having peak load in summer,
with air conditioning on large scale. In the case of short term dynamics, it is very hard to tell
the difference between angle and voltage instabilities as there are few examples to explain
the pure voltage instability cases.
Suppose a large induction motor as the load of the system. In a general situation voltage
collapse situation may be one or the combination of both cases.
The maximum load power which a generation system can supply decreases after a line
outage. If the maximum load power gets smaller than the power, the motor is likely
to restore.
In case of a short circuit near the motor, the motor decelerates. The motor becomes
incapable of restoring the speed if the fault is not cleared quickly which leads to the
collapse in voltage of the system [8].
2.1.2.2. Middle Term Voltage Instability
In a middle term voltage instability the time frame is generally 2‐3 minutes. This type of
instability contains high loads and high power imports from isolated generation and a large
disruption. Due to the voltage sensitivity of the loads the system is transiently stable. The
large disturbances like loss of major generator in the loading area or major transmission line
leads to loss in reactive power in large amount and cause voltage sags in the load areas. The
bulk power is delivered by the tap changes, LTC transformers and distribution voltage
regulators tends to restore distribution voltage after sensing the low voltages and hence
restore the power levels of the loads. The loads restoration further increase the sags of the
voltage in the transmission network. The local generators gets overexcited and heavily
loaded, but the field currents are returned to their rated values by the over excitation limiters
as the ability of the time –overload in the system expires. The distant generators must supply
the reactive power which is then not sufficient and ineffective. The generation and
transmission system is then incapable to upkeep the loads and reactive losses. This results in
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partial or complete voltage collapse in the system following by the voltage decay. The last
stage may include the stalling of the motor and the operation of the protective relays [8].
2.1.2.3. Long Term Voltage Instability
Long term voltage instability develops over a longer period of time which is caused by very
large load build up like in peak times of the day or a large instant increase in the power
transfer. The load build up is usually measured in megawatts per minute and sometime it may
be too fast. There are some operator actions like timely application of reactive power
equipment or load shedding which may take place to avoid the instability [8]. During the long
term voltage instability there are some factors which should be taken into consideration like
time –overload limit of the transmission line and loss of load diversity due to the low
voltage(due to constant energy and load controlling by thermostatics).
2.1.3. Voltage stability assessment/Analysis
The main motive behind voltage stability analysis is to design and develop the necessary tools
to prevent voltage collapse and improve stability. Power system analysis has become a recent
prime focus due to:
More centralised generation means limited voltage controlled buses due to huge
power plants and increased electrical distance between the generation and load side.
Excessive use of shunt capacitor compensation.
Reducing gap between the nominal and instability values of a system.
Events occurred around the world (USA, Japan, France, etc.) related to instability [2].
There are a number of conventional techniques which are used in the investigation of voltage
stability. Those analysis techniques are :
PV curve method.
QV‐Curve method and reactive power reserve.
Continuation power flow technique.
Approach based on singularity of power flow Jacobian matrix at the collapse point of
voltage.
2.1.3.1. PV Curve method
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PV curve method is most used among all steady voltage analysis methods which helps in
finding the active power margin that is left before the point of voltage instability [5]. The
voltage of the critical bus is examined after altering the real power consumption in the radial
system. The 2‐ Bus system in figure ‐2 can be used to understand this method more efficiently.
The real and reactive power equations of the system can be normalised to get a positive real
solution of v as shown in equation 1 and 2.The v‐p curve shown below is generated by using
the PV‐ curve method and it shows that for each load the component (p,q) is not dependent
on the bus voltage. There is a rapid voltage drop with increasing load at the knee point of the
curve. A sufficient margin from the instability point is necessary for a secure and healthy
operation of a system.
2.1.3.2. VQ curve method and reactive power reserve
The most accepted method to analyse the voltage instability during the post transient period
is the VQ curve method. In this method, there is no need to represent the system into a two
bus system like with the PV curve method. The plot between voltage and the reactive power
of the critical bus is generated. An experimental generator with minimal active power and no
reactive power limits is attached with the test bus. Assuming the test bus as generator bus,
power flow analysis is done for a range of voltage values. Then a plot against the reactive
power and specified voltage is generated from the power flow analysis. The point where
reactive power becomes zero the experimental generator is taken away from the test bus.
There is a direct relationship between the voltage stability and the existing reactive power
reserve, which is accessible from the V‐Q curve. The MVAR gap between the peak point of
Figure 4. PV Curve method [12]
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the V‐Q curve and the operating point or the point where the characteristics of the capacitor
are tangent to the V‐Q curve is known as the reactive power margin [5]. The slope of the right
side V‐Q curve is used to evaluate the stiffness of the bus. The bigger slope means that bus is
less stiff and it is more close to voltage collapse.
The figure below shows the voltage stability limit of the system at the bottom of the curve
where dq/dv = 0. It also represent the minimal reactive power required for the stable
operation of the system. The right side of the curve demonstrates that v is increasing with
rising q. So, this stable area is suitable for the operation of reactive power control devices.
2.1.3.3. Approach based on singularity of power flow Jacobian matrix:
2.1.3.3.1. Q‐V curve Modal Analysis
The weak buses in a system are identified by modal analysis method. In this, the critical
Eigenvalue of the reduced Jacobian matrix is calculated. If all the eigenvalues have positive
real values, the system is considered as stable from voltage perspective. A zero eigenvalue
will represent a stable system which is on the verge of instability whereas one negative value
to a unstable system [6]. The event of voltage instability is assessed by the position if
eigenvalues on the graph. The reactive participation factor is determined from the left and
right eigenvalues which is useful to predict the voltage collapse point of the system. If ‘k’ and
‘i’ are the bus number and mode of the bus respectively, the participation factor is calculated
by the following formula:
RPFi = Pki = ξki ηik (6)
Figure 5. VQ Curve method [12]
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Where,
ξ = Right eigenvector matrix
η = Left eigenvector matrix
The voltage collapse is caused by the critical modal reactive power and the RPF helps in
finding the busses affecting the voltage stability. This factor is also used to calculate the
amount of contribution made by those buses towards the voltage collapse.
2.1.3.3.2. The Continuation Power Flow Method (CPF)
The singularity of jacobian matrix and incapability of the conventional power flows to
congregate at full loading point has made CPF technique more useful than other power
analysis methods [6]. In this method predictor‐corrector steps are involved in conventional
power flow equations, where the load is incremented after each predictor step and a new
solution is predicted. Then a corrector step followed by the calculation of exact same value
by Newton Raphson power flow method. The sequence gets repeated until the maximum
loading point is achieved. This technique is very useful in calculating the loading margin of a
power system [6].
2.1.4. Methods for improving voltage stability
Voltage stability is achieved by managing the production, flow and utilization of the reactive
power within a power system [2]. To operate a system efficiently, it is important to keep the
voltage level to its nominal value in the entire power system. Conventional methods have
accomplished this for transmission and distribution networks by treating them differently.
The node voltage of transmission networks is maintained within the allowed range of
deviation with the help of centralized power plants. There is a limited use of dedicated voltage
control devices in this approach [6].
Presently, grids in the power system make use of shunt capacitors and mechanically
controlled circuit breakers (MCCBs). Within limits, the static reactive power sources like shunt
capacitors assist the system to maintain the voltage profile. During emergencies, when there
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is urgent demand of reactive power these devices fail to maintain the voltage profile unless
they are converted to pseudo‐dynamic sources by mechanically switching. Specially in the
case of a voltage depression volt‐ampere‐reactive (VAr) output is proportional to the output
voltage’s square [2]. In addition, voltage stability is also achieved by the rescheduling the
power generation, Under Voltage Load Shedding, installation of Synchronous Condensers and
blocking of Tap Changer under reverse Operation[7]
2.1.4.1. FACT Devices
Reactive power imbalance is the major cause of voltage instability in the power system
network. The only way to prevent voltage collapse is by supplying additional reactive power
or by removing reactive power load. Fixed compensators like capacitors and inductors are
being used to compensate the reactive power in the system. These reactive power
compensating devices are not fully compatible with the dynamics of modern type loads. So,
there exists a need for more responsive and variable devices to provide reactive power
compensation [6].
Flexible AC Transmission Systems (FACTS) devices are used to maintain the voltage profile
and performance of the power system. These devices also ensures the reliability and security
of the system along with the capability to manipulate the power flows [7].
FACT devices are divided into following types:
2.1.4.1.1. Shunt Fact devices.
These devices are mainly used to compensate reactive power and therefore also used for
voltage control. Due to their influence on line effective impedance, these devices can control
the stability and power flow. Mostly, reactive power compensation is counted in series by the
manufacturers which is used in fixed configuration like FACT devices. This is because of the
similar system setup and knowledge required for the other FACT devices. The shunt and series
configuration of the FACT devices is gaining more popularity than others. The energy market
activities leads to higher volatility of power flows which need better use of the transmission
capacity. The power flow control devices transfer power flows from the over loaded parts of
the systems to less the less loaded section of the power systems.
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a) Static Var Compensator (SVC)
SVC, a shunt connected device can either be used in voltage control mode or Var control
mode. The static term is used to represent that there is no rotating part present in the device
unlike the synchronous machine. In the voltage control mode, SVC control the reactive power
in the system from its location. To control the voltage fluctuation and voltage regulation, SVC
can draw leading or lagging Var. The reactive power is injected if there is a dip in the voltage
and the reactive power is absorbed if there is rise in the voltage. Therefore, according to the
requirement of the system SVC can be used as a source or sink reactive Var[7].
b) Static Synchronous Compensator (STATCOM)
STATCOM is a broadly used dynamic shunt power compensator for regulating the reactive
power in the transmission and distribution network. These devices are small in size, have
faster speed and the wide range of operation has a great benefit in the performance and are
very effective in compensating reactive power. These devices can suppress harmonic current
and provide voltage support for the transmission system[7]. The use of these devices is
gaining more popularity with time.
2.1.4.1.2. Series FACT devices
There are numerous benefits of series compensation in AC transmission system like more
power transfer ability and better transient stability. These series devices are improved from
mechanical switching compensations to Thyristor Controlled Series Compensation or Voltage
Source Converter based devices. Reactive power compensation is used in order to reduce
power transmission losses, control the transmission capability of a system and to sustain the
voltage. The series compensation devices are used to manipulate the impedance of
transmission line with the help of inductive or capacitive compensation[7].
a) Thyristor Controlled Series Capacitor
Thyristor‐controlled reactor (TCR) are used in parallel with capacitor segments of series
capacitor bank in a TCSC controller. A broad range of capacitive reactance can be smoothly
controlled with the combination of TCR and capacitor and interchanged to a state where
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bidirectional pairs of thyristor conduct continuously and inject the inductive power into the
transmission lines. The problems like transient stability, dynamic stability, steady state
stability and voltage stability can be solved with TCSC very economically and effectively in the
case of lengthy transmission lines[7].
b) Static Synchronous Series Capacitor (SSSC)
Static Synchronous Series Capacitor is a series connected FACT device. These devices are
operated on similar principle as of STATCOM, but due to the need of a proper platform,
mounting and protection make them more complex devices. A SSSC is comprised of a VSC, DC
link capacitor and a coupling transformer. A SSSC has the ability to produce a compensating
voltage which is controllable in both inductive and capacitive range. Along with this, the SSSC
is capable of interfacing with DC power supply for providing the compensation. To achieve
this compensation, a phase voltage is injected into the transmission line through a coupling
transformer, so the active power flow can be controlled directly. The voltage is injected in
series with the transmission line at 90 degree with the line current. The voltage can also be
injected in quadrature with the line current in inductive or capacitive mode according to the
requirements[7].
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2.1.4.1.3. Combined Shunt and Series Facts Devices
a) Unified Power Flow Controller (UPFC)
This controller is a novel power transmission controller. This controller provides full
flexibility of controlling the transmission parameters, voltage, line impedance and phase
angle dynamically. The series devices inside UPFC act as controllable voltage source Vc.
The shunt device acts as regulator for the dc link voltage which regulates the voltage by
controlling the active power drawn from the transmission line [7].
b) Dynamic Power Flow Controller (DPFC)
This controller is comprised of one shunt and many converters connected in series. The
shunt converter acts similar to STATCOM. The series converter uses multiple single phase
converters instead on one high rated converter. All the converters inside the DPFC act as
independent converters and own a dc capacitor to meet the dc voltage supply [7].
2.1.4.1.4. Optimization Techniques in Power System
It is necessary to choose an appropriate location for the placement of FACT devices on the
transmission line in order to maintain the power system stability. There are various
recommended and currently used optimization techniques in the existing power systems.
However, due to the advancement in the computer engineering and the complications
related to the optimization has resulted in the need of programming techniques for finding
the appropriate location to place the FACT devices. It involves dynamic programming,
Lagrange multiplier methods, Heuristic techniques and techniques like generic algorithm[7].
These optimization methods are used along with other effective systems like artificial neural
network (ANN), expert systems (ES), tabu search algorithms (TS) and Fuzzy logic (FL).
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2.1.4.1.4.1. Optimal allocation and sizing of FACTS devices
There are certain issues which should be addressed prior installing the FACT devices into the
system. These issues are mainly the type, capacity and the ideal location of the device in the
power system. Supposing that the price of any device varies accordingly to its capacity, it is
not ideal to install a device which is relatively bigger for its intended purpose. In case of larger
capacity series connected FACT device connected with less rated transmission line, the line
will prevent the device from being used at its full potential and hence will not be very
economic. On the other hand, if the device is comparatively smaller than the rated line it will
not be able to handle as much power of the line and also reduced the rating of the associated
transmission line[8].
The finest FACT device selection that will have maximum impact in the desired effect makes
big difference in the optimization. For example, reactive power control and voltage support
are achieved by the type ‘A’ FACT devices more effectively than others. In case of high reactive
power flow in the transmission lines type B devices does not perform well as type ‘A’ devices.
The selection of these devices makes a significant difference in the relative cost of the devices.
The cost of the device is inversely proportional to the level of technology used in the device.
Thus, SVC (type A device) is the cheapest whereas the UPFC, (type C device) is the more
expensive among the FACT devices. The desired effect and characteristic of the specific
system determines the location of FACT devices. The possible way to determine the optimal
location of the device through simulating the operation of the device on all possible locations
of installation. This task is very time consuming and due to this there are few guidelines to
recognize the possible selective locations in the system which are most suitable for the FACT
controllers.
The guidelines are mentioned below:
• Prevention of loop flows:
The most suitable location to place the FACT controller in the power system to avoid loop
flow depends mainly on the site of loop flows. The transmission line on which loop flows
occurs with either being enforced to zero or by sending opposite to the direction of loop flow
is one of the ideal line to place the FACT device. The effectiveness of the device for this
purpose will reduce significantly if there is an existence of a parallel path between the buses
like a transmission line. This will happen due to the little amount of power transfer that is
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being directed through the FACT device will be shunted back to the other bus with that
parallel path between them. For this purpose, type B or C devices will be an ideal choice to
make[8].
• Electronic fence
This theory of electric fence is an effort by a utility to guard its property rights by putting in
measures to passively prevent other utilities from using its transmission system.
In such a case, there would be limited access points available as tie lines which connect the
neighbouring systems to the utility and in this way makes it possible to reduce activity on the
grid and thereby control the power flowing through the utility. This would need to be
implemented on all but one of the tie lines since this would make it more affordable since the
cost is predictively high. Furthermore, costs could be lowered by strategically placing fewer
devices provided it is considered a low interconnection location. FACTS devices are placed in
key locations where those tie‐lines are of high loading while such devices in weaker tie‐lines
are less likely to be used and have very little effect. For two or more parallel paths, a single
device becomes obsolete since the flow will then occur over the uncontrolled tie‐lines[8].
• Enhanced economic operation
To increase the system’s ability in power transmission, there are two methods of placing
FACTS devices within the transmission system which would also allow for additional
generation units. The first would be to place the device in an underused line which would
then allow more power to be pushed through it. The second method is to place the device in
a high to heavily loaded line to restrict the power flow in that line. This alters the proportion
of power flow throughout the lines and allows more power to flow through the underutilised
lines while also protecting the lines from being over loaded. For this application, type B or C
devices could be used[8].
• Obtaining a specific operating condition
Desired operating conditions can also be achieved by placement of FACTS devices such as
under‐voltage correction or forcing certain amounts of current through a line. The selection
of the correct device would be determined by its location condition. For under‐voltage, type
A would be more suited for that bus since it is the first candidate site for this method. Type
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B or C can be used at other location if the utility choses to control the flow at that location.
The objective of FACTS will have the greatest impact when placed in an optimally suited
location and so one location may not be suitable for another due to different condition. Each
Utilities systems are unique having their own characteristics and so optimal locations would
vary between utilities[8].
2.1.4.2. Generator AVR’s
Generator AVRs are considered very important means for controlling the voltage in the power
system. The terminal voltages of the generators are kept constant under normal conditions.
On the occurrence of a voltage stability problem due to the change in reactive power, the
generators can supply more power to the system in the range of field limits of the current.
The AVRs works on the exciter side of the synchronous generators. Field voltage is supplied
by the exciter in the field winding. The voltage can be controlled by the exciter while being in
the capability limits of the generator[9].
2.1.4.3. Under Load Tap Changers
The utilization of different voltage levels across the system is enabled by the transformers.
Along with the voltage transformation, control of voltage and reactive power flow is achieved
with the help of transformers. From the perspective of power system, variations in the system
voltages is compensated by changing the ratio of the transformers. The situation where a
frequent changes in the ratio is required due to instantaneous changes in the loads (like daily
variations), ULTC is used. Hence, ULTC are used frequently in order to prevent voltage
instability. Usually, the ratios of the transformers are varied between the range of ±15 percent
with the help of the taps[9].
2.1.4.4. Load Shedding During Contingencies
Load shedding reduces the risk of voltage collapse due to the voltage instability. This method
of preventing the voltage instability during the contingencies is the most economical way in
the industry. The load shedding can be manual or automatic. There are several V‐Q curves
studies performed by the system planners in order to find the load amount shedding required
to retain the voltage profile during the contingencies. Heavy loading conditions are main
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cause of incident like voltage collapse and therefore system load peak and generation sources
decides the load shedding amount [9].
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2.2. Frequency Stability
It is the ability of a power system to keep its frequency stable after a disturbance or system
instability. Frequency stability of a system relies upon the capability to maintain the
equilibrium between the system generation and load demand with losing minimal amount of
loads. There are multiple reasons which can lead to loss of frequency stability of a power
system. Instability may occur due to the electrical deficiency like the loss of generation after
quick imbalance between the generation and load side. Severe system instabilities are a result
of large deviations in system frequency and voltage along with power flows and other
parameters. Several issues related to operation control coordination, protection devices,
equipment response and generation reserve are caused as a result of frequency instability in
a system.
To understand the instability problem and develop a solution when considering the physical
nature of the instability, the size of disturbance and time frame is deliberately estimated.
According to IEEE/VIGRE task Force, the frequency stability would be classified as short term
and long term phenomena where for short term, frequency instability islands are created by
inadequate power generation and by implication insufficient load shedding which leads to
frequency decay and momentary blackouts. In the long term case such as steam turbine over
speed control, and boiler‐reactor protection and control, the response time can vary between
10s to a few minutes. System frequency instability and the lack of corrective measures as a
result of the imbalance between production and consumption, potentially influence the
voltage magnitude in overloaded or islanded situations by decrease or increasing the voltage.
Another important aspect effecting dynamic system frequency behaviour is the structure of
the power system which shows the importance of the relationship between the electrical
network structures and network risks. Electrical power system structures that have direct
impact on frequency stability in generation and transmission are the transformer limit and
transformer capacity, load demand and load nature. Power system structures and grid
performance have implications in the management and mitigation of failure within the
electrical grid[10].
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2.2.1 Power System balance and grid frequency
All power systems use synchronous generators to generate electricity and feed the electrical
grids. The frequency of the system is created and sustained by the rotating machines. It is
very important to maintain an equilibrium state between the power generation and
consumption while the dynamics of the electrical system deals with small disturbances. The
frequency of the electrical grid stays stable until there is a proper balance between both sides,
however the kinetic energy stored in the rotating machines is used to compensate the impact
of the disturbance on the balance between the generation and load side. There is plenty
energy stored to withstand the grid for cycles to seconds (depends upon the imbalance
amount). Hence there is a deviation in the set point of the system or grid frequency. The
system frequency is directly proportional to the generation. When the frequency increases
generation has increased and conversely, as it decreases, generation also decreases. To avoid
blackouts and damaged equipment, the frequency must not deviate but remain within
specific limits.
To ensure this a number of control strategies can be employed into the electrical power
system. The figure below shows the balance concept in the AC systems. The power
equilibrium can be represented as a flywheel that rotates with a specific frequency (50Hz for
Australia, Europe and Malaysia, 60Hz for North America). In this model, the flywheel is
accelerated continuously by several generators connected within the same grid such that
energy is added to the flywheel while the flywheel is also decelerated at the same time by the
loads in that extract that same energy. The model shows that the flywheel will continue to
spin at a constant value if the sum of the accelerated and decelerated powers are close or
equal to zero. When a frequency change occurs between the power generated and load, it’s
because of the mismatch of accelerating and decelerating powers that do not sum to zero. In
such a case, the rotational frequency of the wheel will change. The rate of change of
frequency during a certain event is inversely proportional to the inertia of the flywheel. The
inertia in the AC power system is represented by the total inertia of all rotating mass
connected to the power system using the synchronous machines[10].
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2.2.2 Power Frequency control
To maintain electrical power system stability, the frequency and voltage level of the system
must be close to their steady state limit values. Due to the continuous change in the load
demand, the system frequency of the system is normally imbalanced. There must be an
equilibrium between the generated power and consumed power to avoid power deficiency
in the electrical power system. The system frequency increases if the generation is more than
the load demand and decreases when load demand is more than the generated power. There
is direct relation between the frequency and rotational speed of the generator.
f = (p x n)/60 (7)
Where
p = Number of poles of generator
f = System frequency
n= Rotational speed of the synchronous machines
Practically, the frequency of the power system can be controlled by changing the generator
speed. To monitor and sense the speed constantly, generators are installed with the
governors. During an increase in load of an isolated electrical system having a single generator
Figure 6. Power system balance (Accelerated by generation (P) decelerated by Load (L) [10]
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unit, the inertia of the generator initially compensates the increase in the energy demand.
This will result a decrease in generator’s speed and therefore decreased system frequency
[10].
The governor plays a crucial role in regulating the turbine speed by controlling the turbine
gate. The system frequency increases with increasing turbine speed. The system frequency
recovers within standard range. In case of interconnected systems, which are installed with
controlling mechanism to recover frequency of the system during uneven conditions. The
following control actions are taken in order to improve the system frequency and to avoid the
risk of power system blackout.
The figure above shows that the power deviation in the system is tackled by the primary
control with re‐establishing the equilibrium between the generation and load demand.
Normally, this control action has 50 Hz set point. Then an action is responded by the primary
controllers of the other generators in the system within 0‐10 seconds of time. The output
power is controlled by the controller until it gets balanced with the demand power.
The secondary controller comes into action for restoring the system frequency and the
power deviation to their normal values. The secondary control plays important role to cover
any disturbances which may cause trouble with production, transmission and consumption.
The secondary control stays in action for several minutes and timely linked with the primary
control. Lastly, the tertiary control acts after 15 minutes to re‐establish remaining power and
system deviation to supply enough secondary control reserve at right time. At certain working
points of generators units and involved loads there is need of manual or automatic power
changes. These manual or automatic power connected under tertiary control is called as
tertiary control reserve.
Deviation between the mean frequency and nominal frequency of 50 Hz can lead to
inconsistency between the universal and synchronous time. This offset act as a performance
indicator for primary, secondary and tertiary control power equilibrium and must be less than
30 seconds[10].
The picture below demonstrate the timings of different action of primary, secondary and
tertiary control.
Figure 7. Frequency Control in power system [10] pg. 5691 (Figure 4)
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Current power system networks work closer to the stability limits to cope with increasing
electricity demand in energy utilisation.
2.2.3 Importance of Frequency Stability
In order to keep the frequency of the system in a narrow band, there are various factors which
should be taken into account.
The generators in traditional power stations are installed with auxiliary electric motor
drives and these drives determines the performance of the generators. The tasks like
delivering air and fuel to the boiler, oil bearings and cooling services for whole system
are performed by these auxiliary drives. These drives will get affected significantly by
the reduced speed of the generator due to the low frequency. There will be a
reduction in the output of the power station and this event will result in cascading
shutdowns of the power stations.
The steam turbines will get damaged if the frequency goes lower than 47 Hz whereas
the hydro plants and thermal units remain unaffected due to the robustness. The
frequencies lower than 45 Hz results in tragic consequences like disconnections.
The power transformers may get overloaded during the high deviations in the
frequency due to their sensitiveness with the system frequency.
A fixed speed is required to ensure the operation of electric motors at a practically
constant speed. An AC motor is used in various consumer appliances which operates
them at almost fixed rate.
The main frequency can be used for various timing processes which occur inside
electronic appliances[10].
Figure 8. Timing ranges of action of Primary, Secondary and Tertiary control [10] Pg. 5691 (Figure 5)
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2.3. Rotor Angle Stability
This stability mainly involves the electromechanical oscillations inherent in power system. The
key factor to this problem is the relation between the rotor oscillations and the output power
of the synchronous machines. In order to understand this relation it is necessary to be familiar
with synchronous machines characteristics.
There are two main circuits of synchronous machine: the field and armature. The field is
on the rotor and armature is on the stator. The direct current power is supplied to the field
windings and the load power is consumed from the terminals of the armature. The rotor is
rotated by the prime mover (turbine) to generate a rotating magnetic field in the field
windings which induces alternating voltage. The frequency of the induced voltage relies on
the speed and poles of the machine. The mechanical speed of the rotor and electrical voltage
frequency are synchronized at 60 Hz in USA and 50 Hz in most other countries.
During the interconnection of two or more synchronous machines, it is important the stator
voltage and current have same frequency and the rotor mechanical speed of both machines
is coordinated with this frequency. The electrical torque at the output generator is changed
by adjusting the mechanical torque input. By adjusting the mechanical input torque the rotor
is shifted to a new position relative to the stator’s magnetic field [11].
The power transfer between the motor and generator depends on the angular separation
‘δ’ between the rotors of both machines. This angular separation is because of the
Generator internal angle
Angular difference between terminal voltages of the generator and motor
Internal angle of the motor
The rotor angle stability has been divided into two parts: Small Signal stability and Transient
Stability.
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2.3.1. Small ‐Signal Stability
It is the capability of the power system to maintain the synchronism after being subjected to
small disturbances. The main reason for these disturbances the minor variations in load and
generation side. There could be two possible forms which may occur in an unstable system
1. Steady increase in rotor angle due to insufficient synchronizing torque
2. Rotor oscillation of increasing amplitude due to insufficient damping torque
The response of a power system to these disturbances is determined by the transmission
system strength, initial operating conditions and the controls used for the generator
excitation.
In the absence of automatic voltage regulators, the lack of synchronizing torque is the cause
behind the instability of a generator connected radially to a large power system[11]. This
instability can be seen in the below figure.
In the presence of continuously acting avr the system oscillations are damped to ensure small
signal stability which can be seen in the below picture.
The current power systems can face small signal instabilities due to the lack of oscillation
damping in the system. The following types of oscillations are major concern in the system
Local modes or machine system modes: This type of oscillations are related to the
swinging units of the generating stations relative to the rest of power system.
Interarea modes: These oscillations are linked with internal swinging of the machines
against the machines in other parts.
Figure 9. Nature of small disturbance response with constant field voltage [11] pg 17 (Figure 2.4)
Figure 10. Nature of small disturbance response with excitation control [11] Pg.17 (Figure 2.5)
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Control mode: These oscillations includes the generating units and other controls in
the power system.
Torsional modes: These type of oscillations are related to the rotational parts of
turbine – governor shaft system.
2.3.2. Transient Stability
This is the capability of the system to remain stable under severe transient disturbances.
These disturbances leads to rotor angle excursions in the power systems and is influenced by
nonlinear power‐angle relationship. The initial operating conditions and scale of disturbance
determines the stability of the system. The disturbance may lead to different steady state
operation of the system as compared to the state before the disturbance.
The power systems are designed to maintain the stability for certain set of events. These
events includes short circuits of different types like phase to ground, phase to phase to ground
or phase to phase. These short circuits are expected to occur on transmission lines, buses or
on transformers. The figure below shows the stable and unstable behaviour of the
Synchronous machines. The case‐1 show that the rotor angle shoots up to maximum and then
oscillates until steady state with decreasing amplitude. This case is called transient stable. The
case 2 shows the steady increase of the rotor angle until synchronism is lost. This type of
instability is known as first swing instability. The case 3 represents the stable system in first
swing and becomes unstable due to increased number of oscillations while approaching to
the end state. These type of instabilities takes place when post fault steady state condition
itself a small signal unstable.
Most of the transient stability analysis are carried out for 3‐5 seconds after the
disturbance, but may extend to 10 seconds depending on the size of the system and inter‐
area modes of oscillations [11].
Figure 11. Rotor angle response to transient disturbance [11] pg. 19 (Figure 2.6)
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The transient stability of a system is analysed by using conventional and integration methods.
The conventional methods are energy based methods which shows the stability of a system
by the comparison of accelerating and decelerating areas. The system is considered as stable
if both areas are equal [12]. The conventional methods are listed below
Equal area criterion
Single machine equivalent method
Direct Lyapunov’s theory
The integration methods are based on time response waveform of rotor angle. The
integration methods are given below
Runge‐Kutta method
Implicit Trapezoidal rule
Mixed Adams‐BDF (Backward Differentiation Formulae) technique
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3. Power System Simulations (power factory)
A Nine bus system has been chosen from the power factory examples for simulation side of
the project. The system was introduced in the in the book Power System Control and Stability
by P. M. Anderson and A. A. Fouad [13]. The system contains 9 buses, 3 generators, 3 loads,
6 transmission lines and 3 transformers representing a simplified transmission network. The
data related to all the buses, lines, generators and loads has been mentioned in the manual
(9_Bus_System) of this system[14]. This example is chosen as the base for all the simulations
for power system stability analysis. Prior to the simulations it is important to gain the
knowledge of major models of the generator which need to be defined before carrying out
the transient and small signal stability analysis like Automatic voltage regulators, Turbine and
Gov models. Different models have been defined in the composite model of a generator to
observe the system behaviour after a three phase fault. These models will now be discussed,
how they are defined in Power Factory environment and their effects on the stability of power
system.
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Figure 12. Nine Bus System
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3.1 Main components of power system control
There are mainly three control systems which directly influence the performance of a
Synchronous generator i.e. the boiler control, governor and exciter. The below figure show
the basic structure and functionality of major elements of power generation. The generating
unit has been assumed lossless. This assumption seems reasonable after comparing the total
turbine and generator losses with the total output.
Therefore, the input power as steam must be converted into electrical power on the terminals
of the generator as shown in the figure 19. The governor is responsible for controlling the
steam power input to the turbine. The amount generated EMF of the generator is controlled
by the excitation system and due to this it controls the output voltage along with the power
factor and magnitude of the current. Since excitation systems have been given much
importance in the literature for their role in the stability of a power system, excitation systems
have explained further as follows.
3.2 Excitation Systems
Excitation system supplies direct current to the field winding of the synchronous machine.
The control and protection tasks are handled by the excitation system to enhance the
performance of the power system. This is achieved by manipulating the field voltage and
thereby the field current. The main control functions consists of the flow of the reactive
power, voltage control and power system enhancement. The capability limits of the
synchronous machines, excitation systems and other equipment in the power system are
maintained by the protective tasks of the excitation system. The considerations of the
synchronous generator and the power system determine the performance needs of the
excitation system[1].
The excitation system is required to supply and manipulate the field
current of the synchronous generator in accordance to the varying output while being in the
Figure 13.Principal control of generating unit [13] pg. 233 (Figure 7.1)
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continuous capability of the generator. In addition to this, the excitation system must respond
to the transient instabilities with field‐forcing according to the instantaneous and short term
capabilities of the synchronous generators. There are many factors which affects the
capabilities of the generator like rotor insulation failure from high field voltage, rotor heating
from high field current, stator heating from high armature current loading, core end heating
during period of under excitation and heating from extra flux. The thermal limits of
synchronous machines have time dependent properties and the short term overload abilities
of the generators may extend up to one minute. The most effective way to utilize the
excitation system is to meet the system demand by taking full advantage of the capability
limits of the generator [1].
The power system needs the excitation system for effective voltage
control and stability. The excitation system should be able to enhance the transient stability
and small‐signal stability. There has been a continuous growth in the use of excitation system
for the purpose of system stability. In the past, excitation system were controlled manually
for maintaining the voltage and reactive power loading across the system. The first
automated voltage control was very slow, but later continuous and fast acting regulators for
enhancing the small‐signal and transient stability were developed. There has been a
significant change in the excitation systems with time. The use of excitation system was
extended by introducing two auxiliary stabilizing signals and terminal voltage error signal for
controlling the voltage for damping the system oscillations. This section of the excitation
control is known as Power System Stabilizer (PSS). The current excitation systems are able to
response instantaneously with high ceiling voltages. The overall dynamic system performance
is substantially improved by the use of high field forcing capability and use of auxiliary
stabilizing signals[1].
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3.2.1 Components of excitation system
Exciter ‐ It supplies the dc power to the field winding of the synchronous machine,
constituting the power stage of the excitation system.
Regulator ‐ It is used for processing and amplifying the input control signals to make
them ideal for the control of the exciter. It consists of regulating and excitation system
stabilizing functions (rate feedback or lead lag compensation).
Terminal voltage transducer and load compensator ‐ It is used to sense, rectify and
filter the generator terminal voltage into dc voltage and compare it with a reference
value.
Power system stabilizer ‐ It helps in damping the power system oscillations by
providing additional input signals to the regulator. Rotor speed deviation, accelerating
power and frequency deviation are some input signals.
Limiters and protective circuits ‐ These devices include various control and protective
functions which help to ensure that the exciter and synchronous generator limits are
not exceeded from their capability limits. The frequently used functions are field –
current limiter, maximum excitation limiter, terminal voltage limiter, volts‐per‐Hertz
regulator and protection and under excitation limiter[1].
Figure 14. Excitation control system of Synchronous generator [1] pg. [317] (Figure 8.1)
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3.2.2 Types Excitation Systems
The excitation systems are broadly categorised into three systems on the basis of the power
source used for the excitation.
3.2.2.1 DC excitation systems
This type of excitation systems use dc generators as excitation power and supply the current
to the rotor of the synchronous machine through slip rings. The exciter may be operated by
motor or generator shaft which can either be self or separately excited. These types of
systems were used in past and they are now replaced by ac or static type excitation
systems[1].
3.2.2.2 AC excitation systems
These types of excitation systems use alternators as sources of main generator excitation
power. The exciter and turbine generator shares the same shaft in the operation. The
controlled or non –controlled rectifies the ac output of the exciter to supply dc current to the
generator field. In the past, a combination of magnetic and rotating amplifiers as regulators
were used in the excitation systems. Electronic amplifier regulators are now used in most new
systems.
Depending on the arrangement of rectifier, method of controlling the
exciter control and excitation source can lead to various forms of excitation systems[1]. Few
of the ac excitation systems are listed below
Stationary rectifier systems
Rotating rectifier systems
Figure 15. DC excitation system with amplidyne voltage regulator pg. [319] (Figure 8.2)
Figure 16. Field‐control alternator rectifier system [1] (Figure 8.3)
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3.2.2.3 Static excitation systems
These types of systems contains static components. The controlled or uncontrolled rectifiers
directly supply the excitation current to the main generator’s field through slip rings. The
transformers stepdown the voltage to supply power to the rectifiers which is supplied from
the main generator. There are mainly three types of static excitation systems
Potential ‐ source controlled‐rectifier systems
Compound ‐ source rectifier systems
Compound ‐ controlled rectifier excitation systems
3.2.3 Modelling of Excitation systems
Voltage stability, regulation and performance of a synchronous machine are significantly
influenced by the control of its excitation system. The behaviour of whole power system is
largely dependent on the excitation system of its generators, for example, inter‐area
oscillations and excitations of a generator are directly linked to each other. The main
components in excitation system control are PSS, an excitation system stabilizer, an AVR and
terminal voltage transducer and load compensator. The power industry uses various types of
excitation systems but IEEE type ST1A are most common type among all systems[2].
3.2.4 Effect of Excitation on Stability
The excitation system plays an important role in improving the stability of a power system. It
was found in past investigations that existing high gain continuous acting voltage regulators
help in increasing the steady state power limits of the network. It was also noticed that no
load and under load conditions required different values of voltage regulator gain for better
performance. The types of instabilities offered by old voltage regulators were uncovered by
the engineers in early 1950s which led stabilising feedback circuits to come in use. There were
disruptions caused in the parallel operation of large systems due to the growing oscillations
in large interconnected system in 1960s. The engineers found that inherently weak natural
damping of large coupled systems was causing such situations of negative damping which was
further affected by the regulator gain. The team learned that the artificial signals introduced
by the excitation system could enhance the damping of the system. This method proved very
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beneficial to deal with growing oscillation problems faced in the power system of North
America[13].
After this success of improving the dynamic stability of the system in certain aspects with
excitation system led many engineers to discover the further capabilities of this approach.
Due to the presence of small effective time constants in the control loop of excitation
systems, it was supposed that excitation control could deliver big contribution to control the
system with a small input of control energy. This control was also limited in its effectiveness.
The next hurdle was to determine this limit i.e., figuring out the design and control
parameters of the excitation system which can deliver best results in a cost effective way.
3.2.4.1 Transient stability and dynamic stability considerations
A machine subjected to a fault during transient stability which stays for a short period of time
and results in decreased terminal voltage and capability to transfer synchronizing power of
the machine. Assuming one machine –infinite bus, the power transfer approximation is given
by the equation:
P = (Vt V∞/x) sinδ
Where Vt is the terminal voltage of the machine and V∞ is infinite bus voltage. It can be seen
that the decrease in the value of Vt will decrease the P in corresponding amount. In order to
avoid this reduction in the value of P need quick actions from excitation system to uphold the
field to ceiling and hence holding the Vt at acceptable value. Actually, voltage regulators
require to be very fast for this action and have high ceiling voltage which make the holding of
Vt at the desired level more effective. In addition to this, another response from the excitation
is required when there is a jump in the reactance x after fault due to switching. The machine’s
ability to transfer the power from turbine to the terminals is significantly affected due to these
violent changes. [13].
In case of dynamic stability of the system, the problems are different from the transient
problems in many ways and it requires different excitation actions as compared to actions
required in transient stability. The dynamic stability basically means the ability to adjust of all
machines in the system after small changes in load.
Suppose a constant load fed by a multimachine feeding system. Now assuming that load
increased by a small amount at an instant at any point in the system, the largest change in
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the load will be noticed by the closest machine in the control group whereas the farthest
machine in the control group will notice minimal change. Additionally, this change will be
noticed by a set of machines in the system control group.
Due to the increase in the load, there will be a sudden increase in the power output
requirement of these machines. This power demand will be first met by the stored energy of
the control group machines, since step changes in power to turbines is not possible. So the
rotating energy will be supplied to compensate this power demand until the governors take
action to adjust the power input to various generators. If we examine the behaviour of the
machines in this period of time before the governors comes into action we will see that the
machine voltages, currents and speeds will not be the same for all machines. This is because
every machine in the control group have differences in their size, unit, design and electrical
location with respect to the load. Thus, all the machines in the control group respond
according to the amount to load increase which is calculated from the impedances they see
from the terminals and the unit size. Every unit response with its own natural frequency and
oscillates until damping forces decay these oscillations. It can be concluded that a change in
load is responsible for setting up all types of oscillatory responses and the system rings for a
time with multiple frequencies present, these induced changes give rise to their own
interaction with machines present in the surroundings[13].
The electromechanical system in the past had a substantial deadband in the voltage regulator
and the excitation of these machines remain unchanged unless generators are very close to
the load change. The machines closer to the load change are compensated by the increased
excitation but with a slow response. The excitation system in the present face a different type
of problem. These systems acknowledge the change in the load quickly either by the change
in the terminal voltage or current or both. Hence, the excitation system responds accordingly
to the oscillation of each unit, as there is a change in the speed voltage the terminal voltage
will also change. In addition to this, the oscillating control group machines react with each
other and their actions are accompanied by the excitation change[13].
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3.2.4.2 Excitation system effects on transient Stability
In transient stability, the performance of the system is analysed after it has been subjected
to severe impacts. The behaviour and ability to remain stable during and after the impacts is
the main concern related to this stability. The time period of few seconds during the first
swing is very important in this study. During this time there is a sudden change in the output
power of the generator which results in the change in the speed of the rotor and causes high
risk to the synchronism of the system. The machine behaviour and dynamic relations of the
power network are major factors which influence the outcome. The assumption has been
made that there is no change in the power supplied by the prime movers during this period
of time. So the effect of the excitation systems on such transient relies upon its capability to
assist the generator in maintaining the output power during this course[13].
The major factors which are responsible in affecting the behaviour of the system during
severe transient (first swing transients) are listed below.
The type of disturbance, its origin and the duration greatly influence the transient
behaviour of the system.
The capability of the transmission network to maintain the synchronizing forces
generated by the disturbance during the transient.
The parameters related with generator and turbine.
There are some other system parameters which are affecting these factors which have been
listed below:
The parameters of the synchronous machine which are mainly the inertia constant,
the transient reactance and open circuit time constant of direct axis, and the capability
of the excitation system to maintain the flux level of the synchronous machine and
increase the power output during the transient.
The impedances values of the transmission network during the normal, pre‐fault and
post fault situations. The flexibility in switching out the unhealthy sections in the
network is necessary so that the transfer admittances are maintained among the
synchronous machines after the isolation of the fault.
The equipment and the protective relaying scheme which can isolate the faulted
section in minimal amount of time to avoid the disruption.
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3.3 Models for Stability Analysis
There are predefined models in the power factory which are used for the stability analysis
calculations. These models include standard IEEE definitions for the controllers, prime movers
and other related devices and functions. The appendix D in the power factory manual have
all standard models that are present in the global library.
Defining the models for simulation purpose is very useful in the planning purposes. The
predefined parameters in the models provide a favourable and justifiable nature of the
analysed system. This method is also used for the operational analysis and the system
behaviour should be same as of a real system.
There are certain systems and configurations like wind generators, HVDC system which does
not have IEEE models and therefore it requires effective tools to define models by the user.
Power factory provides a platform to create exact models for this purpose.
Sometime the manufacturers offer the precise models of the controllers along with actual
parameters which can be actually defined inside a power factory by building a new block
diagram which represent the actual controller or device instead using existing IEEE models.
This feature inside power factory helps in system modelling with high precision. System
operation performance and optimization studies are carried out by the consultants to find
the need of appropriate methods and tools for building better transient models for stability
analysis. This consists analysis of complex operations and planning problems of special
devices. This need for the system led to the development of DIgSILENT Power factory time
domain modelling feature which is highly flexible and precise.
3.4 System Modelling Approach
System modelling for the analysis of system stability plays a crucial role in the power system
analysis. On the basis of the accuracy of the designed model, any results can be produced
with its justification like large signal validity, available parameters of the system and applied
faults or tests etc. This is one of the complex aspects in the transient stability study. The large
set of time domain models, which may be the combination of other models, represent the
other aspect of complexity in the transient stability study. These time domain models are
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joined together to form a single large transient model to obtain basic set of differential
equations.
Considering the complexity of this transient analysis problem, hierarchical system modelling
approach is used by the power factory modelling philosophy in which the graphical and script
based modelling methods are combined. The base of the modelling approach used in the
power factory is made up of basic hierarchical levels of time domain modelling:
DSL Block Definitions: These definitions are based on the DIgSILENT Simulation
language (DSL) which are used to form building blocks to represent the differential
equations and transfer functions of transient models.
The Built in models and common models: These elements are inbuilt transient Power
factory models used for standard power equipment like generator, motors and VAR’s.
These models utilize the DSL definitions and are very important in user defined
transient models.
The composite models: These models utilise the common frames which are used for
connecting various in built models or common models.
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3.5 Stability Analysis
3.5.1 PV Curve
The nine bus system has been simulated with the help of the PV curve tools in power factory
in order to obtain simulated PV curves and critical scalable demand of all the busbars. The
curves in the figure below shows the amount of the loading that can be increased before the
bus voltage collapse.
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The figure above shows PV curves of the critical bus (Bus8) of the system and all other the
buses (Buses 1‐14) of the network. According to the power factory calculation the critical
scalable load demand at bus 8 is 575.7 MW.
Figure 17. PV Curves
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In order to improve the voltage profile at this bus reactive power compensation has been
used. The bus voltage has been examined after adding a shunt from 1 Mvar to 3 Mvar. The
load capability of the bus has been enhanced after reactive power compensation. The results
can be seen in the below figure.
3.5.1.1 Contingencies
Contingency analysis is considered as crucial analysis in the power system. It is necessary for
the industry planners and operators to analyse the power system and long term effects
considering the new generation facilities and estimated growth in the load. The power factory
provides the flexibility of analysing the system not only in the base case topology, but also
analyse the system which could be the result of the possible contingent scenario. The n‐1 rule
is often used by the power system planners which expects a power system to operate after a
single transmission or generation loss in a stable and secure manner [15].
This system has been analysed under a number of contingencies. The system has been
observed under outage of various components like lines, generators and transformers. It can
Figure 18. Reactive Power Compensation
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be seen in the below figure, the outage component and the critical scalable demand of the
busbars linked to the outage.
Figure 19 Critical Scalable demand (Contingencies)
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3.5.2 QV Curve
The picture illustrates that the outage of various components in the system will decrease the
critical scalable demand at the busbars. The critical contingence is at bus 7 when there is an
outage of line 5‐7.
Figure 20 QV Curves
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The figure above shows the qv curves of critical busbar (Busbar 6) and all other busbars
(busbars 1‐9) respectively. The minimum point of Q‐V curve is below the horizontal axis which
represents a positive reactive power margin. The part of the curves below the x axis
represents the voltage stability limits and the maximum MVAR load the bus can hold before
collapse. According to the power factory calculations, the maximum MVAR load and the
critical bus voltage of all busbars have been shown in the above picture.
Figure 21 Critical bus voltage and reactive power
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These power margins have been analysed by changing the loading amount on the busbars
and the effect can be seen in the below figure. The load has been varied from 60% to 140 %
in steps of 20 %.
Figure 22. QV Curve Bus 6 at different Loads
‐250
‐200
‐150
‐100
‐50
0
50
100
0.6 0.7 0.8 0.9 1 1.1
Mvar
Voltage (p.u)
QV Curve Bus 6
140% Load
120% Load
100% Load
80% Load
60% Load
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3.5.2.1 Contingencies
The system has been analysed for various contingencies on the critical bus and the results are
shown below:
Figure 23 Contingencies
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3.5.3 Small Signal Stability Analysis
Small signal stability has been analysed with the help of modal analysis of the system. The
Eigen values plot and bar plot of participation factor can be seen in the below pictures. The
stable eigen values can be seen on the left side of the graph in green colour.
Figure 24. Eigenvalues plot
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Figure 25. Bar plot of Participation factor
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3.5.4 Transient stability analysis
Four random gov models have been chosen from the powerfactory library which are IEEE G1,
IEEE G3, HYGOV and EXAC4. IEEE T1 avr model have been used with all gov models for carrying
out analysis due to better terminal voltage response among other avr. Similarly, four random
avr models have been chosen from the library named as IEEE T1, IEEE T3, IEEE T5 and EXAC4.
The HYGOV Gov model is used with all four avr models for the analysis due to better rotor
angle response among others. The parameters of PSS model STABNI have been left blank to
analyse the avr and gov response of the generator 2 more clearly. Parameters of used avr and
gov models are shown in appendix 1 and 2. Generator 1 and generator 3 does not contain any
gov and avr model to keep the system simple for analysis. Three phase balanced fault has
been defined on line 5‐7 which can be seen in the figure 36. This fault is same in all the
simulations carried out in this report. The simulation time has been set for 100 seconds to see
the steady state of the system. The transient state of the system can simply be seen by
changing the scaling of the x axis of the respective graph (in word format). The figures 35 and
46 show the sample composite model of generator 2 and fault analysis of nine bus system.
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Figure 26. Composite model of Generator 2
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The effect of chosen avr and gov models on the stability curves of generator 2 has been
shown in following figures.
Figure 27. 3 Phase balanced fault at Line 5‐7
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3.5.4.1 Response of Generator 2 with different Avr models and HYGOV gov (Power factory)
Figure 28. Rotor Angle response of Generator 2 with different Avr
0
20
40
60
80
100
120
‐10 0 10 20 30 40 50
Degrees
Time(s)
Rotor Angle (G2) with HYGOV
IEEE T5 Avr
IEEE T3 Avr
IEEE T1 Avr
EXAC4 Avr
Figure 29. Speed of Generator 2 with different Avr
0.996
0.998
1
1.002
1.004
1.006
1.008
1.01
1.012
1.014
1.016
‐10 0 10 20 30 40 50 60 70 80
Spee
d (P.u)
Time(s)
Generator Speed (G2) with HYGOV
IEEE T5 Avr
IEEE T3 Avr
IEEE T1 Avr
EXAC4 Avr
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Figure 30 Active Power of Generator 2 with different Avr
0
50
100
150
200
250
‐10 0 10 20 30 40 50 60
Power (MW)
Time(s)
Active Power Positive Seq.(G2) with HYGOV
IEEE T5 Avr
IEEE T3 Avr
IEEE T1 Avr
EXAC4 Avr
‐100
‐50
0
50
100
150
200
250
300
350
‐10 0 10 20 30 40 50
Mvar
Time (s)
Reactive Power Positive Seq. (G2) with HYGOV
IEEE T5 Avr
IEEE T3 Avr
IEEE T1 Avr
EXAC4 Avr
Figure 31. Reactive Power of Generator 2 with different Avr
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Figure 32. Excitation voltage of Generator 2 with different Avr
0
1
2
3
4
5
6
‐10 0 10 20 30 40 50
Voltage (p.u)
Time (s)
Excitation Voltage (G2) with HYGOV
IEEE T5 Avr
IEEE T3 Avr
IEEE T1 Avr
EXAC4 Avr
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
‐5 0 5 10 15 20 25 30 35 40
Voltage (p.u)
Time(s)
Terminal Voltage (G2) with HYGOV
IEEE T5 Avr
IEEE T3 Avr
IEEE T1 Avr
EXAC4 Avr
Figure 33. Terminal voltage of Generator 2 with different Avr
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Figure 34. Turbine power of Generator 2 with different Avr
0
0.2
0.4
0.6
0.8
1
1.2
‐10 0 10 20 30 40 50 60
Power (P.u)
Time (s)
Turbine Power (G2) with HYGOV
IEEE T5 Avr
IEEE T3 Avr
IEEE T1 Avr
EXAC4 Avr
Figure 35. Current Magnitude of Generator 2 with different Avr
0
2
4
6
8
10
12
14
16
18
20
‐10 0 10 20 30 40 50 60
Curren
t (kA)
Time (s)
Current Magnitude (G2) with HYGOV
IEEE T5 Avr
IEEE T3 Avr
IEEE T1 Avr
EXAC4
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3.5.4.2 Response of Generator 2 with different Gov models and IEEE T1 Avr (Power factory)
Figure 36. Rotor Angle response of Generator 2 with different governors
0
10
20
30
40
50
60
70
80
90
100
‐5 0 5 10 15 20 25 30 35 40
Degrees
Time(s)
Rotor Angle (G2) with IEEE T1
IEEE G1 Gov
IEEE G3 Gov
HYGOV Gov
TGOV5 Gov
Figure 37. Speed of Generator 2 with different governors
0.995
1
1.005
1.01
1.015
1.02
1.025
1.03
‐10 0 10 20 30 40 50 60 70 80
Spee
d (P.u)
Tme(s)
Generator Speed (G2) with IEEE T1
IEEE G1 Gov
IEEE G3 Gov
HYGOV Gov
TGOV5 Gov
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Figure 38. Active power of Generator 2 with different governors
0
50
100
150
200
250
‐10 0 10 20 30 40 50
Power (MW)
Time(s)
Active Power Positive Seq.(G2) with IEEE T1
IEEE G1 Gov
IEEE G3 Gov
HYGOV Gov
TGOV5 Gov
Figure 39. Reactive power of Generator 2 with different governors
‐50
0
50
100
150
200
250
300
350
‐10 0 10 20 30 40 50
Mvar
Time (s)
Reactive Power Positive Seq. (G2) with IEEE T1
IEEE G1 Gov
IEEE G3 Gov
HYGOV Gov
TGOV5 Gov
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Figure 40. Excitation voltage of Generator 2 with different governors
0
1
2
3
4
5
6
‐10 0 10 20 30 40 50
Voltage (p.u)
Time (s)
Excitation Voltage (G2) with IEEE T1
IEEE G1 Gov
IEEE G3 Gov
HYGOV Gov
TGOV5 Gov
Figure 41. Turbine Power of Generator 2 with different governors
0
0.2
0.4
0.6
0.8
1
1.2
‐10 0 10 20 30 40 50 60
Power (P.u)
Time (s)
Turbine Power (G2) with IEEE T1
IEEE G1 Gov
IEEE G3 Gov
HYGOV Gov
TGOV5 Gov
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Figure 42. Terminal voltage of Generator 2 with different governors
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
‐5 0 5 10 15 20
Voltage (p.u)
Time(s)
Terminal Voltage (G2) with IEEE T1
IEEE G1 Gov
IEEE G3 Gov
HYGOV Gov
TGOV5 Gov
Figure 43. Current Magnitude of Generator 2 with different governors
0
2
4
6
8
10
12
14
16
18
20
‐10 0 10 20 30 40 50
Curren
t (kA)
Time (s)
Current Magnitude (G2) with IEEE T1
IEEE G1 Gov
IEEE G3 Gov
HYGOV Gov
TGOV5 Gov
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3.5.4.3 Response of Generators – G1, G2, G3
Avr slot ‐ IEEE T1 and Gov slot ‐ HYGOV
Three Phase fault – Line 5 ‐ 7
0.996
0.998
1
1.002
1.004
1.006
1.008
1.01
1.012
1.014
‐10 0 10 20 30 40 50
Spee
d (p.u)
Time(s)
Generator Speed (G1, G2, G3)
G1
G2
G3
Figure 44. Generator Speed G1, G2, G3
‐100
0
100
200
300
400
500
‐5 0 5 10 15 20
Mvar (p.u)
Time(s)
Positive Seq. Reactive Power (G1, G2, G3)
G1
G2
G3
Figure 45. Reactive Power (G1, G2, G3)
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‐20
0
20
40
60
80
100
‐10 0 10 20 30
Angle (Deg)
Time(s)
Rotor Angle (G1, G2, G3)
G1 (Ref Machine angle inRad)
G2 (With reference toreference machine angle)
G3 (With reference toreference machine angle)
Figure 46. Rotor angle (G1, G2, G3)
0
0.2
0.4
0.6
0.8
1
1.2
‐5 0 5 10 15 20
Power (p.u)
Time(s)
Turbine Power (G1, G2, G3)
G1
G2
G3
Figure 47 Turbine Power (G1, G2, G3)
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0
1
2
3
4
5
6
‐5 0 5 10 15 20
Voltage (p.u)
Time(s)
Excitation Voltage(G1, G2, G3)
G1
G2
G3
Figure 48. Excitation Voltage (G1, G2, G3)
0
2
4
6
8
10
12
14
16
18
20
‐5 0 5 10 15 20 25
Curren
t(kA
)
Time(s)
Current Magnitude (G1 G2, G3)
G1
G2
G3
Figure 49. Current Magnitude (G1, G2, G3)
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‐50
0
50
100
150
200
250
‐5 0 5 10 15 20 25 30
Power (MW)
Time(s)
Positive Seq. Active Power (G1 G2, G3)
G1
G2
G3
Figure 50.Active Power (G1, G2, G3)
0
0.2
0.4
0.6
0.8
1
1.2
‐5 0 5 10 15 20
Term
inal Voltage (p.u)
Time(s)
Terminal Voltage (G1 G2, G3)
G1
G2
G3
Figure 51. Terminal voltage (G1, G2, G3)
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4. Conclusion
The report has described the major components of the generation side like Avr, excitation
systems, gov models and their effects on the power system on the basis of their selection.
The role of gov and avr models in the power system stability has been demonstrated through
the power factory simulations.
In addition to this, major types of instabilities, analysis tools, their causes and methods to
improve them has been presented from current literature in this report. After power factory
simulations and observing the response of generator 2 it can be concluded that:
Stability analysis with random Avr
The transient states of generator 2 with IEEE T5 and IEEE T3 avr models have
responded to the line fault by showing more oscillations when compared to IEEE T1
and EXAC4. T3 and T5 avr stability curves have taken more time to reach stability than
the other two avr models (Figures 37‐44). This is due to the existence of different
excitation systems in the model most likely owing to different response models and
parameters used by each manufacturer. It can also be seen that all the avr models
have returned to their set points at stability which demonstrates the main objective
of the avr to maintain the required voltage profile of the system to upkeep the load
demand of the network.
Choosing an ideal excitation system depends upon the operating conditions, system
requirements and the economic situation. After considering the transient behaviour
of all avr models and time taken to become stable, IEEE T1 avr has been chosen for
final analysis (Figures 53‐60) which includes responses of all three generators after
introducing the same line fault.
Stability analysis with random Gov models
The gov models IEEE G1 and TGOV5 have shown similar behaviour whereas IEEE G3 and
HYGOV models have similar response in terms of rotor angle, active power, and turbine
power (Figures 45‐52).However, there is a difference in the steady state values of these gov
models which is due to variation in turbines, their capacity and fuel source used like coal,
hydro or nuclear. Considering the amount of oscillations and time to become stable HYGOV
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model has been selected for final simulation with IEEE T1 avr model which are simulated from
figures 53 to 60 with all three generators.
5. Future scope of work
In this report the behaviour of the nine bus system has been analysed by choosing different
types of inbuilt global models of avr and gov available in power factory environment.
Further research can be done on the power system by:
Modifying the parameters of the selected avr and gov model. The differential
equations which exists in background of composite model could be explored for
manipulating the responses of individual components in the models.
Outage of power source, loads and other transmission lines.
Introducing unbalanced faults for gaining more knowledge and experience.
Introduction of PSS model and parameter settings to damp electro‐mechanical
oscillations of the generator for the protection of shaft line and grid stabilisation.
Exploring the loading and reactive power compensation for testing the robustness of
the system.
The stability analysis has been done on just one generator to focus on the marginal
differences in the transient and stability states of the stability control.
Defining avr and gov models in the other two generators, analyse and observe the
difference in overall stability and compare the current network stability in this thesis.
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6. References
[1] P. Kundur, N. J. Balu, and M. G. Lauby, Power system stability and control. McGraw‐hill New York, 1994.
[2] J. Hossain and H. R. Pota, "Power system voltage stability and models of devices," in Robust control for grid voltage stability: high penetration of renewable energy: Springer, 2014, pp. 19‐59.
[3] P. Kundur et al., "Definition and classification of power system stability," IEEE transactions on Power Systems, vol. 19, no. 2, pp. 1387‐1401, 2004.
[4] A. F. M. Nor, M. Sulaiman, A. F. A. Kadir, and R. Omar, "Voltage Instability Analysis for Electrical Power System Using Voltage Stabilty Margin and Modal Analysis," Indonesian Journal of Electrical Engineering and Computer Science, vol. 3, no. 3, pp. 655‐662, 2016.
[5] S. Chakrabarti, "Notes on power system voltage stability," Dept. of EE, IIT, Kanpur http://home. iitk. ac. in/~ saikatc/EE632_files/VS_SC. pdf (assessed on 1st Nov 2011), 2010.
[6] M. Chakravorty and S. Patra, "Voltage stability analysis using conventional methods," in Signal Processing, Communication, Power and Embedded System (SCOPES), 2016 International Conference on, 2016, pp. 496‐501: IEEE.
[7] T. Madhuranthaka and T. G. Manohar, "A Review on Power System Voltage Stability and Optimization Techniques," International Journal of Engineering Research and Applications, vol. 6, no. 11, pp. 06‐14, 2016.
[8] A. M. M. Mohammed and T. Sharaf, "Improving the Voltage Stability of Electrical Power Systems Using Shunt Facts Devices," Master thesis, Cairo University November, 2009.
[9] S. B. Bhaladhare, A. Telang, and P. P. Bedekar, "PV QV Curve‐A Novel Approach for Voltage Stability Analysis," NCIPET, 2013.
[10] B. S. Abdulraheem and C. K. Gan, "Power system frequency stability and control: Survey," International Journal of Applied Engineering Research, vol. 11, no. 8, pp. 5688‐5695, 2016.
[11] H. H. Al Marhoon, "A practical method for power systems transient stability and security," University of New Orleans, 2011.
[12] Y. ALShamli, N. Hosseinzadeh, H. Yousef, and A. Al‐Hinai, "A review of concepts in power system stability," in 2015 IEEE 8th GCC Conference & Exhibition, 2015, pp. 1‐6: IEEE.
[13] P. M. Anderson and A. A. Fouad, Power system control and stability. John Wiley & Sons, 2008.
[14] D. PowerFactory.''DIgSILENT PowerFactory,User Manual,'' Nine Bus System Documentation [Online] https://www.digsilent.de/en/downloads.html.
[15] P. W. Corporation. Manual https://www.powerworld.com/WebHelp/Default.htm#MainDocumentation_HTML/QV_Curves.htm [Online].
[16] J. Machowski, J. Bialek, J. R. Bumby, and J. Bumby, Power system dynamics and stability. John Wiley & Sons, 1997.
[17] G. Rogers, Power system oscillations. Springer Science & Business Media, 2012.
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[18] P. Kundur, M. Klein, G. Rogers, and M. S. Zywno, "Application of power system stabilizers for enhancement of overall system stability," IEEE Transactions on Power Systems, vol. 4, no. 2, pp. 614‐626, 1989.
[19] X.‐Y. Gui, W. Hu, F. Liu, and S.‐W. Mei, "Governor control design based on nonlinear
hydraulic turbine model," Dianli Xitong Zidonghua(Autom. Electr. Power Syst.), vol. 29, no. 15, pp. 18‐22, 2005.
[20] J. F. Kennedy, "Electronic governor control," ed: Google Patents, 1981. [21] C. Lim, "A self‐tuning stabiliser for excitation or governor control of power systems,"
IEEE Transactions on Energy Conversion, vol. 4, no. 2, pp. 152‐159, 1989. [22] Q. Lu, C. Sun, and Y. Sun, "Nonlinear governor control for hydroturbine generator
sets," in TENCON'93. Proceedings. Computer, Communication, Control and Power Engineering. 1993 IEEE Region 10 Conference on, 1993, vol. 4, pp. 38‐42: IEEE.
[23] Y. Guo, D. J. Hill, and Y. Wang, "Global transient stability and voltage regulation for power systems," IEEE Transactions on Power Systems, vol. 16, no. 4, pp. 678‐688, 2001.
[24] E. Larsen and D. Swann, "Applying power system stabilizers Part III: Practical considerations," IEEE Transactions on Power Apparatus and systems, no. 6, pp. 3034‐3046, 1981.
[25] H. A. Moussa and Y.‐n. Yu, "Optimal power system stabilization through excitation and/or governor control," IEEE Transactions on Power Apparatus and Systems, no. 3, pp. 1166‐1174, 1972.
[26] A. R. Perrins, "Automatic voltage regulator," ed: Google Patents, 1966. [27] P. W. Sauer and M. Pai, "Power system dynamics and stability," Urbana, 1998. [28] Y. L. Tan and Y. Wang, "Transient stabilization using adaptive excitation and dynamic
brake control," Control Engineering Practice, vol. 5, no. 3, pp. 337‐346, 1997. [29] T. Van Cutsem and C. Vournas, Voltage stability of electric power systems. Springer
Science & Business Media, 2007. [30] Y. Wang, D. J. Hill, R. H. Middleton, and L. Gao, "Transient stability enhancement and
voltage regulation of power systems," IEEE Transactions on Power systems, vol. 8, no. 2, pp. 620‐627, 1993.
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7. Appendices
Appendix 1 ‐ Avr Parameters
1. IEEE T1
Figure 52. Global type Avr ‐ IEEE T1
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2. IEEE T3
Figure 53. Global type Avr ‐ IEEE T3
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3. IEEE T5
Figure 54. Global type Avr ‐ IEEE T5
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4. EXAC4
Figure 55. Global type Avr ‐ EXAC4
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Appendix 2 – Governor Parameters
1. HYGOV
Figure 56. Global type Gov ‐ HYGOV
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2. IEEE G1
Figure 57. Global type Gov ‐ IEEE G1
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3. IEEE G3
Figure 58. Global type Gov ‐ IEEE G3
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4. TGOV5
Figure 59. Global type Gov ‐ TGOV5
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Appendix 3 – Nine bus system data
Figure 60. Nine Bus system simulation data