thesis final
DESCRIPTION
Actual script of the thesis submitted for degree requirement. Chapter 1 is brief introduction to FACTS family.Chapter 2 is about STATCOM, SSSC and UPFC.Chapter 3 about basic power systems.Chapter 4 and onwards are simulation work done.contact if you want the simulation files at [email protected];)TRANSCRIPT
FACTS Devices and Effect of UPFC
on 500 KV Transmission System
Project Advisor:
Assistant Prof. Adnan Bashir
Group Members:
Laeeq Ahmad Faiz 2008-RCET-EE-11
Junaid-ur-Rehman 2008-RCET-EE-13
Hafiz Muhammad Abdullah 2008-RCET-EE-24
Muhammad Ismail Khan 2008-RCET-EE-28
Department of Electrical Engineering,
Rachna College of Engineering and Technology
Gujranwala
FACTS Devices and Effect of UPFC on
500 KV Transmission System
This work is submitted as a discourse to the Department of Electrical
Engineering, Rachna College of Engineering and Technology,
Gujranwala Pakistan, for the partial fulfillment of the requirement of the
degree of
Bachelors
in
Electrical Engineering
Approved on _______________
Internal Examiner Assistant Prof. Adnan Bashir (Project Advisor) Department of Electrical Engineering, RCET Gujranwala
Signature ______________________
External Examiner Name _________________________
Signature ______________________
Department of Electrical Engineering,
Rachna College of Engineering and Technology, Gujranwala
Declaration
We declare that the work contained in this thesis is our own, except where
explicitly stated otherwise. In addition this work has not been submitted to
obtain another degree or professional qualification.
Signed:
Laeeq Ahmad Faiz ___________
Junaid-ur-Rehman ___________
Hafiz Muhammad Abdullah ___________
Muhammad Ismail Khan ___________
Date:_______________
ACKNOWLEDGEMENT
To our beloved parents who prayed for our success day and night and
supported our every decision with affection and love and to our respected
teachers whose guidance and advice has brought us to the point where we
have deep understanding of things and can call ourselves professionals.
And to our fellow students who were there for us through every thick
and thin and who made it possible to spend those tiring curricular years in a
healthy and light mood.
To our respect project supervisor Sir Adnan Bashir who guided us
through this tough subject and made possible for this project to see its
finishing line.
And finally to our team members who worked whole heartedly with a
team spirit encouraging each other and supporting when then there were times
of no hope.
To everyone who shared a smile or a tear.
EXECUTIVE SUMMARY
Flexible AC Transmission is an emerging technology in the Power World
which uses power electronic devices for reactive compensation. FACTS devices can
be utilized to control power flow and enhance system stability. Particularly with the
deregulation of electricity market, there is an increasing interest in using FACTS
devices in the operation and control of power systems with new loading and power
flow conditions. A better utilization of the existing power systems, to increases their
capacity and controllability by installing facts devices becomes imperative. Due to
the present situation there are two main aspects which should be considered in using
FACTS devices. The first aspect is the flexible power operation according to the
power flow control capability of FACTS devices the other aspect is the improvement
in transient and steady state stability of power systems. Facts devices are the right
equipment to meet these challenges. Facts devices can be effectively utilized for the
steady state power control and dynamic control of power systems.
One of the more intriguing and potentially most versatile classes of FACTS
device is the Unified Power Flow Controller (UPFC).The UPFC can provide
simultaneous control of all basic power system parameters (transmission) voltage,
impedance and phase angle). The controller can fulfill functions of reactive shunt
compensation, series compensation and phase shifting meeting multiple control
objectives. From a functional perspective, the objectives are met by applying a
boosting transformer injected voltage and an exciting transformer reactive current.
The injected voltage is inserted by a series transformer.
In addition to allow control of the line active and reactive power, the UPFC
provides an additional degree of freedom. Its shunt converter operating as a
STATCOM controls voltage by absorbing or generating reactive power. Both the
series and shunt converters use a Voltage-Sourced Converter (VSC) connected on the
secondary side of a coupling transformer. The VSCs use forced-commutated power
electronic devices (GTOs, IGBTs or IGCTs) to synthesize a voltage from a DC
voltage source.
We have performed UPFC simulation in Simulink (Matlab) in which the
power flow control, voltage regulation and stability provided by UPFC has been
verified.
TABLE OF CONTENTS
Chapter # 1 ---------------------------------------------------------------------------------------------- 1
INTRODUCTION TO FACTS DEVICES ------------------------------------------------------------------------ 1
1.1 BACKGROUND ------------------------------------------------------------------------------------------ 1
1.2 FLEXIBLE ALTERNATING CURRENT TRANSMISSION SYSTEMS -------------------------------- 2
1.3 INHERENT LIMITATIONS OF TRANSMISSION SYSTEMS ------------------------------------------ 4
1.4 AN OVERVIEW OF FACTS CONTROLLERS ---------------------------------------------------------- 5
Chapter # 2 -------------------------------------------------------------------------------------------- 12
INTRODUCTION TO UPFC ------------------------------------------------------------------------------------ 12
2.1 STATIC SYNCHRONOUS COMPENSATOR(STATCOM) -------------------------------------------- 13
2.2 STATIC SYNCHRONOUS SERIES COMPENSATOR(SSSC) ----------------------------------------- 14
2.3 TECHNICAL ADVANTAGE OF UPFC ----------------------------------------------------------------- 16
Chapter # 3 -------------------------------------------------------------------------------------------- 21
ELEMENTARY KNOWLEDGE TO UNDERSTAND FACTS ------------------------------------------------- 21
3.1 THE SYMMETRICAL SYSTEM ------------------------------------------------------------------------ 24
3.2 LOADS AND PHASOR DIAGRAMS ------------------------------------------------------------------- 25
3.3 FERRANTI EFFECT------------------------------------------------------------------------------------- 27
3.4 SYNCHRONISM: --------------------------------------------------------------------------------------- 28
3.5 VOLTAGE PROFILE ------------------------------------------------------------------------------------ 28
Chapter # 4 -------------------------------------------------------------------------------------------- 29
POWER FLOW CONTROL OF 500/230 KV GRID WITH UPFC -------------------------------------------- 29
SIMULATION ----------------------------------------------------------------------------------------------- 30
500/230 KV GRID WITHOUT UPFC ---------------------------------------------------------------------- 31
500/230 KV GRID WITH UPFC---------------------------------------------------------------------------- 34
SIMULATION RESULTS: ----------------------------------------------------------------------------------- 41
NETWORK WITHOUT UPFC ------------------------------------------------------------------------------- 41
WITH UPFC ------------------------------------------------------------------------------------------------- 43
BOTH BYPASSED (SIMULATION DIAGRAM) ----------------------------------------------------------- 44
BOTH BYPASSED (OBSERVATIONS) -------------------------------------------------------------------- 45
FIRST CONNECTED AND SECOND BYPASSED (SIMULATION DIAGRAM) ------------------------- 47
FIRST CONNECTED AND SECOND BYPASSED (OBSERVATIONS) ----------------------------------- 48
BOTH CONNECTED (SIMULATION DIAGRAM) -------------------------------------------------------- 50
BOTH CONNECTED (OBSERVATIONS) ------------------------------------------------------------------ 51
REMARKS: -------------------------------------------------------------------------------------------------- 53
Chapter # 5 -------------------------------------------------------------------------------------------- 54
VOLTAGE REGULATION OF 500KV TRANSMISSION SYSTEM ----------------------------------------- 54
VOLTAGE REGULATION: --------------------------------------------------------------------------------- 56
REMARKS: -------------------------------------------------------------------------------------------------- 57
Chapter # 6 -------------------------------------------------------------------------------------------- 58
STABILITY OF 500KV TRANSMISSION SYSTEM ---------------------------------------------------------- 58
VOLTAGE WITHOUT UPFC (OBSERVATIONS) --------------------------------------------------------- 60
VOLTAGE WITH UPFC (OBSERVATIONS) -------------------------------------------------------------- 61
REMARKS: -------------------------------------------------------------------------------------------------- 62
Chapter # 7 -------------------------------------------------------------------------------------------- 63
CONCLUSION -------------------------------------------------------------------------------------------------- 63
LIST OF FIGURES
FIG. 1.1 OPERATIONAL LIMITS OF THE TRANSMISSION LINES .............................................. 4
FIG. 1.2 OVERVIEW OF MAJOR FACT DEVICES ........................................................................... 6
FIG. 1.3 TYPICAL SVC CONFIGURATIONS .................................................................................... 9
FIG. 2.1 THE UNIFIED POWER FLOW CONTROLLER (UPFC) ................................................... 12
FIG. 2.2 STATCOM ............................................................................................................................ 13
FIG. 2.3 STATIC SYNCHRONOUS SERIES COMPENSATOR ...................................................... 14
FIG. 2.4 EFFECT OF SSSC ON TRANSMISSION LINE VOLTAGES ........................................... 15
FIG. 2.5 UPFC INSTALLED IN A TRANSMISSION LINE ............................................................. 17
FIG. 2.6 SINGLE PHASE EQUIVALENT CIRCUIT ........................................................................ 18
FIG. 2.7 (A) ACTIVE/REACTIVE POWER CONTROL (B) VOLTAGE REGULATION .............. 18
FIG. 2.8 COMPARISON OF UPFC WITH OTHER FACTS TECHNIQUES ................................... 20
FIG. 3.1 BASICS OF POWER FLOW ................................................................................................ 24
FIG. 3.2 SIMPLE POWER SYSTEM .................................................................................................. 25
FIG. 3.3 PHASOR DIAGRAM, RESISTIVE LOAD. ......................................................................... 25
FIG. 3.4 PHASOR DIAGRAM, INDUCTIVE LOAD. ....................................................................... 26
FIG. 3.5 PHASOR DIAGRAM, CAPACITIVE LOAD. ..................................................................... 26
FIG. 3.6 EFFECT OF RESISTIVE AND INDUCTIVE LOAD ON SYSTEM VOLTAGE ............... 27
FIG. 4.1 CASE OF STUDY ................................................................................................................. 29
FIG. 4.2 SYSTEM MODELED ON SIMULINK (WITHOUT UPFC) ............................................... 31
FIG. 4.3 ACTIVE POWER METERING WITH RESPECT TO TIME (WITHOUT UPFC) ............. 32
FIG. 4.4 REACTIVE POWER METERING WITH RESPECT TO TIME (WITHOUT UPFC) ........ 33
FIG. 4.5 SYSTEM MODELED WITH UPFC ..................................................................................... 34
FIG. 4.6 SETTINGS OF THE TIMER BLOCK .................................................................................. 35
FIG. 4.7 REF ACTIVE POWER.......................................................................................................... 36
FIG. 4.8 ACTIVE POWER W.R.T TIME (WITH UPFC) .................................................................. 37
FIG. 4.9 REACTIVE POWER W.R.T TIME (WITH UPFC) ............................................................. 38
FIG. 4.10 CASE MODIFIED (NO UPFC INSTALLED) .................................................................... 40
FIG. 4.11 ACTIVE POWER READING BUS 1-5 (WITHOUT UPFC) ............................................. 41
FIG. 4.12 ACTIVE POWER READING BUS 6-9 (WITHOUT UPFC) ............................................. 42
FIG. 4.13 CASE MODIFIED (UPFC INSTALLED BOTH BYPASSED) ......................................... 44
FIG. 4.14 ACTIVE POWER READING BUS 1-5 (WITH UPFC BOTH BYPASSED) .................... 45
FIG. 4.15 ACTIVE POWER READING BUS 6-9 (WITH UPFC BOTH BYPASSED) .................... 46
FIG. 4.16 CASE MODIFIED (WITH UPFC 1ST
CONNECTED 2ND
BYPASSED) ........................... 47
FIG. 4.17 ACTIVE POWER READING BUS 1-5 (UPFC 1ST
CONNECTED 2ND
BYPASSED) ..... 48
FIG. 4.18 ACTIVE POWER READING BUS 6-9 (UPFC 1ST
CONNECTED 2ND
BYPASSED) ..... 49
FIG. 4.19 CASE MODIFIED (WITH UPFC BOTH CONNECTED) ................................................. 50
FIG. 4.20 ACTIVE POWER READING BUS 1-5 (WITH UPFC BOTH CONNECTED) ................ 51
FIG. 4.21 ACTIVE POWER READING BUS 6-9 (WITH UPFC BOTH CONNECTED) ................ 52
FIG. 5.1 CASE MODIFIED FOR ANALYSIS OF VOLTAGE REGULATION ............................... 54
FIG. 6.1 CIRCUIT BREAKER INSTALLED AT DOUBLE CIRCUIT TRANSMISSION LINE .... 59
FIG. 6.2 BLOCK PARAMETERS OF CIRCUIT BREAKER ............................................................ 59
FIG. 6.3 BUS VOLTAGES WITHOUT UPFC ................................................................................... 60
FIG. 6.4 BUS VOLTAGES WITH UPFC ........................................................................................... 61
List of Abbreviations and Acronyms
FACTS Flexible alternating current transmission systems
UPFC Unified Power Flow Controller
STATCOM Static synchronous compensator
SSSC Static synchronous series compensator
DVR Dynamic Voltage Restorer
SVR Static Voltage Restorer
SVR Static Voltage Restorer
TCSC Thyristor Controlled Switched Capacitor
PST Phase Shifting Transformers
IPFC Interline Power Flow Controller
GUPFC Generalized Unified Power Flow Controller
PS Phase shifter
LTC Load Tap changer
TCSC Thyristor-controlled series capacitor
IPC Interphase power controller
SVC Static VAR compensator
HVDC High-voltage direct-current
0
ABSTRACT
The maintenance and reliability of the power system has become a
major aspect of study. The encouragement to the construction of HV lines, the
amount of power transmission/km on HV line and the amount of power
transaction as seen from economic side is much responsible for concern
towards congestion in power system. The solution is the use of FACTS
devices especially the use of UPFC.
In this paper the performance of unified power flow controller is
investigated in controlling the flow of power over the transmission line.
Voltage sources model is utilized to study the behavior of the UPFC in
regulating the active, reactive power and voltage profile. Finally, by help of
modeling of a power system in MATLAB, and by installing UPFC in
transmission link, its use as power flow controller and voltage injection is
seen. Conclusion is made on different results to see the benefit of UPFC in
power system.
1
Chapter # 1
Introduction to FACTS devices
1.1 Background
The electricity supply industry is undergoing a profound transformation
worldwide. Market forces, scarcer natural resources, and an ever increasing demand
for electricity are some of the drivers responsible for such an unprecedented change.
Against this background of rapid evolution, the expansion programs of many utilities
are being thwarted by a variety of well-founded, environmental, land-use, and
regulatory pressures that prevent the licensing and building of new transmission lines
and electricity generating plants. An in-depth analysis of the options available for
maximizing existing transmission assets, with high levels of reliability and stability,
has pointed in the direction of power electronics. There is general agreement that
novel power electronics equipment and techniques are potential substitutes for
conventional solutions, which are normally based on electromechanical technologies
that have slow response times and high maintenance costs.
An electrical power system can be seen as the interconnection of generating
sources and customer loads through a network of transmission lines, transformers,
and ancillary equipment. Its structure has many variations that are the result of a
legacy of economic, political, engineering, and environmental decisions. Based on
their structure, power systems can be broadly classified into meshed and longitudinal
systems. Meshed systems can be found in regions with a high population density and
where it is possible to build power stations close to load demand centers.
Longitudinal systems are found in regions where large amounts of power have to be
transmitted over long distances from power stations to load demand centers.
Independent of the structure of a power system, the power flows throughout
the network are largely distributed as a function of transmission line impedance; a
2
transmission line with low impedance enables larger power flows through it than
does a transmission line with high impedance. This is not always the most desirable
outcome because quite often it gives rise to a myriad of operational problems; the job
of the system operator is to intervene to try to achieve power flow redistribution, but
with limited success. Examples of operating problems to which unregulated active
and reactive power flows may give rise are: loss of system stability, power flow
loops, high transmission losses, voltage limit violations, an inability to utilize
transmission line capability up to the thermal limit, and cascade tripping.
In the long term, such problems have traditionally been solved by building
new power plants and transmission lines, a solution that is costly to implement and
that involves long construction times and opposition from pressure groups. It is
envisaged that a new solution to such operational problems will rely on the
upgrading of existing transmission corridors by using the latest power electronic
equipment and methods, a new technological thinking that comes under the generic
title of FACTS – an acronym for flexible alternating current transmission systems.
1.2 Flexible alternating current transmission systems
In its most general expression, the FACTS concept is based on the substantial
incorporation of power electronic devices and methods into the high-voltage side of
the network, to make it electronically controllable. Flexible AC Transmission
Systems, called FACTS, got in the recent years a well known term for higher
controllability in power systems by means of power electronic devices. Several
FACTS-devices have been introduced for various applications worldwide. A number
of new types of devices are in the stage of being introduced in practice. Even more
concepts of configurations of FACTS-devices are discussed in research and
literature.
In most of the applications the controllability is used to avoid cost intensive
or landscape requiring extensions of power systems, for instance like upgrades or
additions of substations and power lines. FACTS-devices provide a better adaptation
to varying operational conditions and improve the usage of existing installations.
3
The basic applications of FACTS-devices are:
• power flow control,
• increase of transmission capability,
• voltage control,
• reactive power compensation,
• stability improvement,
• power quality improvement,
• power conditioning,
• flicker mitigation,
• interconnection of renewable and distributed generation and storages.
In all applications the practical requirements, needs and benefits have to be
considered carefully to justify the investment into a complex new device.
Figure 1.1 shows the basic idea of FACTS for transmission systems. The
usage of lines for active power transmission should be ideally up to the thermal
limits. Voltage and stability limits shall be shifted with the means of the several
different FACTS devices. It can be seen that with growing line length, the
opportunity for FACTS devices gets more and more important. The influence of
FACTS-devices is achieved through switched or controlled shunt compensation,
series compensation or phase shift control. The devices work electrically as fast
current, voltage or impedance controllers. The power electronic allows very short
reaction times down to far below one second.
In the following a structured overview on FACTS-devices is given. These
devices are mapped to their different fields of applications. Detailed introductions in
FACTS-devices can also be found in the literature [1]-[5] with the main focus on
basic technology, modeling and control.
4
Fig. 1.1 Operational limits of the transmission lines for different voltage levels
1.3 Inherent limitations of transmission systems
The characteristics of a given power system evolve with time, as load grows
and generation is added. If the transmission facilities are not upgraded sufficiently
the power system becomes vulnerable to steady-state and transient stability
problems, as stability margins become narrower.
The ability of the transmission system to transmit power becomes impaired by
one or more of the following steady-state and dynamic limitations.
angular stability
voltage magnitude
thermal limits
transient stability
dynamic stability
These limits define the maximum electrical power to be transmitted without
causing damage to transmission lines and electric equipment. In principle, limitations
5
on power transfer can always be relieved by the addition of new transmission and
generation facilities. Alternatively, FACTS controllers can enable the same
objectives to be met with no major alterations to system layout. The potential
benefits brought about by FACTS controllers include reduction of operation and
transmission investment cost, increased system security and reliability, increased
power transfer capabilities, and an overall enhancement of the quality of the electric
energy delivered to customers.
1.4 An overview of facts controllers
The development of FACTS-devices has started with the growing capabilities
of power electronic components. Devices for high power levels have been made
available in converters for high and even highest voltage levels. The overall starting
points are network elements influencing the reactive power or the impedance of a
part of the power system. Figure 1.2 shows a number of basic devices separated into
the conventional ones and the FACTS-devices.
For the FACTS side the taxonomy in terms of 'dynamic' and 'static' needs
some explanation. The term 'dynamic' is used to express the fast controllability of
FACTS-devices provided by the power electronics. This is one of the main
differentiation factors from the conventional devices. The term 'static' means that the
devices have no moving parts like mechanical switches to perform the dynamic
controllability. Therefore most of the FACTS-devices can equally be static and
dynamic.
The left column in Figure 1.2 contains the conventional devices build out of
fixed or mechanically switchable components like resistance, inductance or
capacitance together with transformers. The FACTS-devices contain these elements
as well but use additional power electronic valves or converters to switch the
elements in smaller steps or with switching patterns within a cycle of the alternating
current. The left column of FACTS-devices uses Thyristor valves or converters.
These valves or converters are well known since several years. They have
low losses because of their low switching frequency of once a cycle in the converters
or the usage of the Thyristors to simply bridge impedances in the valves.
6
Fig. 1.2 Overview of major fact devices
The right column of FACTS-devices contains more advanced technology of
voltage source converters based today mainly on Insulated Gate Bipolar Transistors
(IGBT) or Insulated Gate Commutated Thyristors (IGCT). Voltage Source
Converters provide a free controllable voltage in magnitude and phase due to a pulse
width modulation of the IGBTs or IGCTs. High modulation frequencies allow to get
low harmonics in the output signal and even to compensate disturbances coming
from the network. The disadvantage is that with an increasing switching frequency,
the losses are increasing as well. Therefore special designs of the converters are
required to compensate this.
In each column the elements can be structured according to their connection
to the power system. The shunt devices are primarily for reactive power
compensation and therefore voltage control. The SVC provides in comparison to the
mechanically switched compensation a smoother and more precise control. It
improves the stability of the network and it can be adapted instantaneously to new
situations. The STATCOM goes one step further and is capable of improving the
power quality against even dips and flickers.
The series devices are compensating reactive power. With their influence on
the effective impedance on the line they have an influence on stability and power
7
flow. These devices are installed on platforms in series to the line. Most
manufacturers count Series Compensation, which is usually used in a fixed
configuration, as a FACTS-device. The reason is, that most parts and the system
setup require the same knowledge as for the other FACTS-devices. In some cases the
Series Compensator is protected with a Thyristor-bridge. The application of the
TCSC is primarily for damping of inter-area oscillations and therefore stability
improvement, but it has as well a certain influence on the power flow.
The SSSC is a device which has so far not been build on transmission level
because Series Compensation and TCSC are fulfilling all the today's requirements
more cost efficient. But series applications of Voltage Source Converters have been
implemented for power quality applications on distribution level for instance to
secure factory in-feeds against dips and flicker. These devices are called Dynamic
Voltage Restorer (DVR) or Static Voltage Restorer (SVR).
More and more growing importance are getting the FACTS-devices in shunt
and series configuration. These devices are used for power flow controllability. The
higher volatility of power flows due to the energy market activities requires a more
flexible usage of the transmission capacity. Power flow control devices shift power
flows from overloaded parts of the power system to areas with free transmission
capability.
Phase Shifting Transformers (PST) are the most common device in this sector.
Their limitation is the low control speed together with a high wearing and
maintenance for frequent operation. As an alternative with full and fast
controllability the Unified Power Flow Controller (UPFC) is known since several
years mainly in the literature and but as well in some test installations. The UPFC
provides power flow control together with independent voltage control. The main
disadvantage of this device is the high cost level due to the complex system setup.
The relevance of this device is given especially for studies and research to figure out
the requirements and benefits for a new FACTS-installation. All simpler devices can
be derived from the UPFC if their capability is sufficient for a given situation.
Derived from the UPFC there are even more complex devices called Interline Power
Flow Controller (IPFC) and Generalized Unified Power Flow Controller (GUPFC)
8
which provide power flow controllability in more than one line starting from the
same substation.
FACTS controllers intended for steady-state operation are as follows:
Thyristor-controlled phase shifter (PS):
This controller is an electronic phase-shifting transformer adjusted by
thyristor switches to provide a rapidly varying phase angle.
Load tap changer (LTC):
This may be considered to be a FACTS controller if the tap changes are
controlled by thyristor switches.
Thyristor-controlled reactor (TCR):
A thyristor-controlled reactor (TCR) is a reactance, which is connected in
series with a bidirectional thyristor valve. The thyristor-controlled reactor is an
important component of a Static VAR Compensator.
The thyristor valve is phase-controlled. By phase-controlled switching of the
thyristor valve, the value of delivered reactive power can be set. Thyristor-controlled
reactors can also be used for limiting voltage rises when circuits are open.
Thyristor-controlled series capacitor (TCSC):
This controller consists of a series capacitor paralleled by a thyristor-
controlled reactor in order to provide smooth variable series compensation.
Interphase power controller (IPC):
This is a series-connected controller comprising two parallel branches, one
inductive and one capacitive, subjected to separate phase-shifted voltage magnitudes.
Active power control is set by independent or coordinated adjustment of the two
phase-shifting sources and the two variable reactance. Reactive power control is
independent of active power.
9
Static VAR compensator (SVC):
SVCs can be used to perform a wide range of compensation tasks in large
transmission systems. Requirements vary greatly and are sometimes contradictory.
The control system can be designed so that priorities can be flexibly assigned to one
task or another, depending on current conditions in the power system. Fig. 1.3 shows
some typical SVC configurations. The selection of the individual configuration
depends on factors like investment costs, losses and availability figures.
Fig. 1.3 Typical SVC configurations
Static compensator (STATCOM):
A static synchronous compensator (STATCOM), also known as a "static
synchronous condenser" ("STATCON"), is a regulating device used on alternating
current electricity transmission networks. It is based on a power electronics voltage-
source converter and can act as either a source or sink of reactive AC power to an
electricity network. If connected to a source of power it can also provide active AC
power. It is a member of the FACTS family of devices.
Usually a STATCOM is installed to support electricity networks that have a
poor power factor and often poor voltage regulation. There are however, other uses,
10
the most common use is for voltage stability. A STATCOM is a voltage source
converter (VSC)-based device, with the voltage source behind a reactor. The voltage
source is created from a DC capacitor and therefore a STATCOM has very little
active power capability. However, its active power capability can be increased if a
suitable energy storage device is connected across the DC capacitor. The reactive
power at the terminals of the STATCOM depends on the amplitude of the voltage
source.
The response time of a STATCOM is shorter than that of an SVC, mainly
due to the fast switching times provided by the IGBTs of the voltage source
converter. The STATCOM also provides better reactive power support at low AC
voltages than an SVC, since the reactive power from a STATCOM decreases linearly
with the AC voltage (as the current can be maintained at the rated value even down
to low AC voltage).
Solid-state series controller (SSSC):
The Static Synchronous Series Compensator (SSSC) is a device that belongs
to the Flexible AC Transmission Systems (FACTS) family using power electronics
to control power flow and improve power oscillation damping on power grids. The
SSSC injects a voltage in series with the transmission line where it is connected. The
SSSC contains a solid-state voltage source inverter connected in series with the
transmission line through an insertion transformer. This connection enables the SSSC
to control power flow in the line for a wide range of system conditions.
Unified power flow controller (UPFC):
This consists of a static synchronous series compensator (SSSC) and a
STATCOM, connected in such a way that they share a common DC capacitor. The
UPFC, by means of an angularly unconstrained, series voltage injection, is able to
control, concurrently or selectively, the transmission line impedance, the nodal
voltage magnitude, and the active and reactive power flow through it. It may also
provide independently controllable shunt reactive compensation.
11
Power electronic and control technology have been applied to electric power
systems for several decades. HVDC links and static VAR compensators are mature
pieces of technology:
High-voltage direct-current (HVDC) link:
This is a controller comprising a rectifier station and an inverter station,
joined either back-to-back or through a DC cable. The converters can use either
conventional thyristors or the new generation of semiconductor devices such as gate
turn-off thyristors (GTOs) or insulated gate bipolar transistors.
12
Chapter # 2
Introduction to UPFC
The Unified Power Flow Controller (UPFC) proposed by Gyugyi [1] is the most
versatile FACTS controller for the regulation of voltage and power flow controller
in a transmission line. It consists of two voltage source converters (VSC) one shunt
connected and the other series connected. The DC capacitors of the two converters
are connected in parallel (see Fig. 2.1).
Fig. 2.1 the Unified Power Flow Controller (UPFC)
If the switches 1 and 2 are open, the two converters work as STATCOM and SSSC
controlling the reactive current and reactive voltage injected in shunt and series
respectively in the line. The closing of the switches 1 and 2 enable the two converters
to exchange real (active) power flow between the two converters. The active power
can be either absorbed or supplied by the series connected converter. As discussed in
the previous chapter, the provision of a controllable power source on the DC side of
13
the series connected converter, results in the control of both real and reactive power
flow in the line (say, measured at the receiving end of the line). The shunt connected
converter not only provides the necessary power required, but also the reactive
current injected at the converter bus. Thus, a UPFC has 3 degrees of freedom unlike
other FACTS controllers which have only one degree of freedom (control variable).
2.1 Static Synchronous Compensator(STATCOM)
The Static Synchronous Compensator (STATCOM) is a shunt device of the
Flexible AC Transmission Systems (FACTS) family using power electronics to
control power flow and improve transient stability on power grids. The STATCOM
regulates voltage at its terminal by controlling the amount of reactive power injected
into or absorbed from the power system. When system voltage is low, the
STATCOM generates reactive power (STATCOM capacitive). When system voltage
is high, it absorbs reactive power (STATCOM inductive).
The variation of reactive power is performed by means of a Voltage-Sourced
Converter (VSC) connected on the secondary side of a coupling transformer. The
VSC uses forced-commutated power electronic devices (GTOs, IGBTs or IGCTs) to
synthesize a voltage V2 from a DC voltage source. The principle of operation of the
STATCOM is explained on the figure below showing the active and reactive power
transfer between a source V1 and a source V2. In this figure, V1 represents the
system voltage to be controlled and V2 is the voltage generated by the VSC
Fig. 2.2 STATCOM
14
Operating Principle of the STATCOM
In steady state operation, the voltage V2 generated by the VSC is in phase
with V1 (=0), so that only reactive power is flowing (P=0). If V2 is lower than V1, Q
is flowing from V1 to V2 (STATCOM is absorbing reactive power). On the reverse,
if V2 is higher than V1, Q is flowing from V2 to V1 (STATCOM is generating
reactive power).
A capacitor connected on the DC side of the VSC acts as a DC voltage
source. In steady state the voltage V2 has to be phase shifted slightly behind V1 in
order to compensate for transformer and VSC losses and to keep the capacitor
charged.
2.2 Static Synchronous Series Compensator(SSSC)
The Static Synchronous Series Compensator (SSSC) is a series connected
FACTS controller based on VSC and can be viewed as an advanced type of
controlled series compensation, just as a STATCOM is an advanced SVC.The SSSC
injects a voltage Vs in series with the transmission line where it is connected.
The schematic of a SSSC is shown in Fig.2.2 (a). The equivalent circuit of the
SSSC is shown in Fig 2.2 (b).
Fig. 2.3 Static Synchronous Series Compensator
15
The magnitude of Vq can be controlled to regulate power flow. The winding
resistance and leakage reactance of the connecting transformer appear in series with
the voltage source Vq.
If there is no energy source on the DC side, neglecting losses in the converter and
DC capacitor, the power balance in steady state leads to
Re[ VqI* ] = 0 (2.1)
The above equation shows that Vq is in quadrature with I. If Vq lags I by 90±, the
operating mode is capacitive and the current (magnitude) in the line is increased with
resultant increase in power flow. On the other hand, if Vq leads I by 90±, the
operating mode is inductive, and the line current is decreased.
As the SSSC does not use any active power source, the injected voltage must
stay in quadrature with line current. By varying the magnitude Vq of the injected
voltage in quadrature with current, the SSSC performs the function of a variable
reactance compensator, either capacitive or inductive.
The variation of injected voltage is performed by means of a Voltage-Sourced
Converter (VSC) connected on the secondary side of a coupling transformer. The
VSC uses forced-commutated power electronic devices (GTOs, IGBTs or IGCTs) to
synthesize a voltage V_conv from a DC voltage source.
Fig. 2.4 Effect of SSSC on transmission line Voltages
A capacitor connected on the DC side of the VSC acts as a DC voltage
source. A small active power is drawn from the line to keep the capacitor charged
and to provide transformer and VSC losses, so that the injected voltage Vs is
practically 90 degrees out of phase with current I. In the control system block
16
diagram Vd_conv and Vq_conv designate the components of converter voltage
V_conv which are respectively in phase and in quadrature with current.
2.3 Unified Power Flow Controller
Line outage, congestion, power system stability loss and cascading line
tripping are the major issues where capability and utilization of FACTS are noticed.
Representative of the last generation of FACTS devices is the Unified Power Flow
Controller (UPFC). The UPFC is a device which can control simultaneously all three
parameters of line power flow (line impedance, voltage and phase angle). Such
"new" FACTS device combines together the features of two "old" FACTS devices:
the Static Synchronous Compensator (STATCOM) and the Static Synchronous
Series Compensator (SSSC). In practice, these two devices are two Voltage Source
Inverters (VSI‟s) connected respectively in shunt with the transmission line through a
shunt transformer and in series with the transmission line through a series
transformer, connected to each other by a common dc link including a storage
capacitor.
2.3.1 Theme
The shunt inverter is used for voltage regulation at the point of connection
injecting an opportune reactive power flow into the line and to balance the real
power flow exchanged between the series inverter and the transmission line. The
series inverter can be used to control the real and reactive line power flow inserting
an opportune voltage with controllable magnitude and phase in series with the
transmission line. Thereby, the UPFC can fulfill functions of reactive shunt
compensation, active and reactive series compensation and phase shifting. Besides,
the UPFC allows a secondary but important function such as stability control to
suppress power system oscillations improving the transient stability of power system.
As the need for flexible and fast power flow controllers, such as the UPFC, is
expected to grow in the future due to the changes in the electricity markets, there is a
corresponding need for reliable and realistic models of these controllers to
investigate the impact of them on the performance of the power system.
17
2.3.2 Structure
The general structure of the UPFC contains two "back to back" voltage source
converters using insulated gate bipolar transistor (IGBT) or Integrated Gate
Commutated Thyrister (IGCT) with a common DC link (Fig. 2). First converter is
connected as parallel and another converter as series with transmission line. The
shunt converter is used to provide active power demanded by the series converter
through a common DC link. The series converter provides the main function of the
UPFC by injecting an AC voltage with controllable magnitude and phase angle. The
transmission line current flows through series converter and therefore, it exchanges
the active and reactive power with the AC system. Generally, this structure (Fig.2)
enables voltage control by the shunt inverter and independent active and reactive
power flow control by the series inverter.
Fig. 2.5 UPFC installed in a transmission line
In the parallel branch of UPFC the active power is controlled by the phase angle of
the converter output voltage. In the series branch of UPFC the active and reactive
power flows in the transmission line are influenced by the amplitude as well as the
phase angle of the series injected voltage. Therefore, the active power controller can
significantly affects the reactive power flow and vice versa [5].
18
2.3.3 Phasor diagram representation
Single phase circuit representation is given below with UPFC installed in the power
system (Fig. 3). The voltages at the midpoint of transmission line is marked as VM,
whereas the voltage injected by UPFC with controllable magnitude and phase is
marked as Vc .
Fig. 2.6 Single phase equivalent circuit
The shunt inverter in UPFC is operating in such a way to inject a controllable current
IC into the transmission line. This current consists of two components with respect to
the line voltage:
1) the real or direct component Id
2) reactive or quadrature component Iq
The following phasor diagram (Fig. 4) is well explaining the effect of direct and
quadrature components.
Fig. 2.7 (a) Active/Reactive Power control (b) Voltage regulation
19
2.4 Technical Advantage of UPFC
The UPFC is versatile and multifunction power flow controller with
capabilities of terminal voltage regulation, series line compensation and phase angle
regulation. Besides the above mentioned functions, UPFC has additional features
making it very popular among the available FACTS devices. The following are the
additional features UPFCs offer:
Optimal Power Flow:
A UPFC can be controlled in a power system to satisfy the following
objectives simultaneously [5] Regulating power flow through a transmission line
(over-load relief, loop-flow minimization, contractual power fulfillment etc.)
Minimization of power losses without generator rescheduling
Reliability:
The load carrying capacity of the system at a given risk level is significantly
affected by the employment of the UPFC. This is particularly true at high risk levels.
The increase in load carrying capacity due to the employment of the UPFC is
extremely dependent on the risk criterion. The impact of employing the UPFC is
greater using the LOLE criterion than for the UPM or SM. This reduces the customer
interruption cost. For a given peak load, the system risk associated with utilizing a
TCSC is higher than using a UPFC.
Dynamic Security:
For a long time, preventive control has been considered as the only strategy
to control dynamic security, since the instability occurs rapidly and no manual
intervention is possible after the onset of contingency. Preventive control obtained by
rescheduling of active power is generally characterized by a higher production cost
than the one obtained by economic dispatch. At this stage of technology, complete
automatic corrective control is feasible. In order to implement these remedial actions,
fast actuators are needed. UPFC controllers can control the security of the network
under large perturbations control actions associated to generation and load.
20
Harmonic Isolator:
The UPFC as harmonic isolator uses the series voltage source in another
mode. In this mode the voltage harmonics associated with the non-linear load are
isolated. The isolating voltage source now prevents the load harmonics from
penetrating back into the system onto the voltage receiving bus. This injected voltage
source can also be used to isolate incoming network harmonics from penetrating into
local harmonic filters and sensitive loads.
The table given below dictates the technical supremacy of UPFC over the rest of
FACTs family.
Fig. 2.8 Comparison of UPFC with other Facts techniques
21
Chapter # 3
Elementary knowledge to understand facts
In an ideal AC power system the voltage and frequency at every supply point would
be constant and free from harmonics, and the power factor would be unity. In
particular these parameters would be independent of the size and characteristics of
consumers' loads. In three-phase systems, the phase currents and voltages must also
be balanced. The stability of the system against oscillations and faults must also be
assured.
The maintenance of constant frequency requires an exact balance between the
overall power supplied by generators and the overall power absorbed by loads,
irrespective of the voltage. However, the voltage plays an important role in
maintaining the stability of power transmission, as we shall see. Voltage levels are
very sensitive to the flow of reactive power and therefore the control of reactive
power is important. This is the subject of reactive compensation. Where the focus is
on individual loads, we speak of load compensation.
Load compensation is the management of reactive power to improve the
quality of supply at a particular load or group of loads. Compensating equipment such
as power-factor correction equipment is usually installed on or near to the consumer's
premises. In load compensation there are three main objectives:
1. power-factor correction
2. Improvement of voltage regulation
3. Load balancing.
Power-factor correction and load balancing are desirable even when the
supply voltage is `stiff ': that is, even when there is no requirement to improve the
voltage regulation. Ideally the reactive power requirements of a load should be
provided locally, rather than drawing the reactive component of current from a remote
22
power station. Most industrial loads have lagging power factors; that is, they absorb
reactive power. The load current therefore tends to be larger than is required to supply
the real power alone. Only the real power is ultimately useful in energy conversion
and the excess load current represents a waste to the consumer, who has to pay not
only for the excess cable capacity to carry it, but also for the excess Joule loss in the
supply cables. When load power factors are low, generators and distribution networks
cannot be used at full efficiency or full capacity, and the control of voltage throughout
the network can become more difficult. Supply tariffs to industrial customers usually
penalize low power-factor loads, encouraging the use of power-factor correction
equipment.
The most obvious way to improve voltage regulation would be to `strengthen'
the power system by increasing the size and number of generating units and by
making the network more densely interconnected. This approach is costly and
severely constrained by environmental planning factors. It also raises the fault level
and the required switchgear ratings. It is better to size the transmission and
distribution system according to the maximum demand for real power and basic
security of supply, and to manage the reactive power by means of compensators and
other equipment which can be deployed more flexibly than generating units, without
increasing the fault level. Similar considerations apply in load balancing.
Most AC power systems are three- phase, and are designed for balanced
operation. Unbalanced operation gives rise to components of current in the wrong
phase-sequence (i.e. negative- and zero-sequence components). Such components can
have undesirable effects, including additional losses in motors and generating units,
oscillating torque in AC machines, increased ripple in rectifiers, malfunction of
several types of equipment, saturation of transformers, and excessive triplen
harmonics and neutral currents.
The harmonic content in the voltage supply waveform is another important
measure in the quality of supply. Harmonics above the fundamental power frequency
are usually eliminated by filters. Nevertheless, harmonic problems often arise together
with compensation problems and some types of compensator even generate
harmonics which must be suppressed internally or filtered.
23
The ideal compensator would
(a) supply the exact reactive power requirement of the load;
(b) present a constant-voltage characteristic at its terminals; and
(c) be capable of operating independently in the three phases.
In practice, one of the most important factors in the choice of compensating
equipment is the underlying rate of change in the load current, power factor, or
impedance. For example, with an induction motor running 24 hours/day driving a
constant mechanical load (such as a pump), it will often suffice to have a fixed power-
factor correction capacitor. On the other hand, a drive such as a mine hoist has an
intermittent load which will vary according to the burden and direction of the car, but
will remain constant for periods of one or two minutes during the travel. In such a
case, power-factor correction capacitors could be switched in and out as required.
An example of a load with extremely rapid variation is an electric arc furnace,
where the reactive power requirement varies even within one cycle and, for a short
time at the beginning of the melt, it is erratic and unbalanced. In this case a dynamic
compensator is required, such as a TCR or a saturated-reactor compensator, to provide
sufficiently rapid dynamic response. Loads that require compensation include arc
furnaces, induction furnaces, arc welders, induction welders, steel rolling mills, mine
winders, large motors (particularly those which start and stop frequently), excavators,
chip mills, and several others. Non-linear loads such a s rectifiers also generate
harmonics and may require harmonic filters, most commonly for the 5th and 7th but
sometimes for higher orders as well.
The power-factor and the voltage regulation can both be improved if some of
the drives in a plant are synchronous motors instead of induction motors, because the
synchronous motor can be controlled to supply (or absorb) an adjustable amount of
reactive power and therefore it can be used as a compensator. Voltage dips caused by
motor starts can also be avoided by using a `soft starter', that is, a phase-controlled
thyristor switch in series with the motor, which gradually ramps the motor voltage
from a reduced level instead of connecting suddenly at full voltage.
24
3.1 The symmetrical system
The symmetrical system is an important example indeed the simplest example
of an interconnected power system as shown in Figure. It comprises two synchronous
machines coupled by a transmission line. It might be used, for example, as a simple
model of a power system in which the main generating stations are at two locations,
separated by a transmission line that is modeled by a simple inductive impedance jX.
The loads (induction motors, lighting and heating systems, etc) are connected in
parallel with the generators, but in the simplest model they are not even shown,
because the power transmission system engineer is mostly concerned with the power
flow along the line, and this is controlled by the prime-movers at the generating
stations (i.e. the steam turbines, water turbines, gas turbines, wind turbines etc.).
Fig. 3.1 Basics of Power Flow
Although the circuit diagram of a symmetrical system just looks like two generators
connected by inductive impedance, power can flow in either direction. The
symmetrical system can be used to derive the power flow equation, which is one of
the most important basic equations in power system operation:
25
3.2 Loads and Phasor diagrams
A resistive load R on an AC power system draws power and produces a phase
angle shift d between the terminal voltage V and the open-circuit voltage E. d is called
the load angle (see Figure). The voltage drop across the Thevenin equivalent A purely
inductive load draws no power and produces no phase-angle shift between V and E
(see Figure). The terminal voltage V is quite sensitive to the inductive load current
because the volt-drop jX I is directly in phase with both E and s V. You might ask,
`what is the use of a load that draws no power?' One example is that shunt reactors are
often used to limit the voltage on transmission and distribution systems, especially in
locations remote from tap-changing transformers or generating stations. Because of
the shunt capacitance of the line, the voltage tends to rise when the load is light (e.g.
at night). By connecting an inductive load (shunt reactor), the voltage can be brought
down to its correct value. Since the reactor is not drawing any real power (but only
reactive power), there is no energy cost apart from a small amount due to losses in the
windings and core.
Fig. 3.2 Simple Power System
Fig. 3.3 Phasor diagram, resistive load.
26
Fig. 3.4 Phasor diagram, inductive load.
A purely capacitive load also draws no power and produces no phase-angle
shift between V and E: i.e. d 0. The system volt-drop jX I is directly in anti-phase
with Es and V, and this causes the terminal voltage V to rise above E. Again you
might ask what is the use of a load that draws no power?' An example is that shunt
capacitors are often used to raise the voltage on transmission and distribution systems,
especially in locations remote from tap-changing transformers or generating stations.
Because of the series inductance of the line, the voltage tends to fall when the load is
heavy (e.g. mid-morning), and this is when shunt capacitors would be connected.
Shunt reactors and capacitors are sometimes thyristor-controlled, to provide rapid
response. This is sometimes necessary near rapidly-changing loads such as electric arc
furnaces or mine hoists. Of course the use of thyristors causes the current to contain
harmonics, and these must usually be filtered.
Fig. 3.5 Phasor diagram, capacitive load.
We have seen that when the load power and current are kept the same, the
inductive load with its lagging power factor requires a higher source voltage E, and
the capacitive load with its leading power factor requires a lower source voltage.
Conversely, if the source voltage E were kept constant, then the inductive load would
have a lower terminal voltage V and the capacitive load would have a higher terminal
voltage.
27
3.3 Ferranti effect
The Ferranti effect is a increase in voltage occurring at the receiving end of a
long transmission line, relative to the voltage at the sending end, which occurs when
the line is energized but there is a very light load or the load is disconnected.
This effect is due to the voltage drop across the line inductance (due to
charging current) being in phase with the sending end voltages. Therefore inductance
is responsible for producing this phenomenon.
The Ferranti Effect will be more pronounced the longer the line and the higher
the voltage applied.[2] The relative voltage rise is proportional to the square of the
line length.
Fig. 3.6 Effect of Resistive and Inductive load on system Voltage
The voltage `profile' and the stability of a transmission line or cable can be
improved using `reactive compensation'. In the early days reactive compensation
took the form of fixed-value reactors and capacitors, usually controlled by
mechanical switchgear. Synchronous condensers and large generators were used in
cases where it was necessary to vary the reactive power continuously. Since the
1970s power-electronic equipment has been developed and applied to extend the
range of control, with a variety of methods and products.
28
3.4 Synchronism:
The basis of AC transmission is a network of synchronous machines
connected by transmission links. The voltage and frequency are defined by this
network, even before any loads are contemplated. All the synchronous machines must
remain constantly in synchronism: i.e. they must all rotate at exactly the same speed,
and even the phase angles between them must not vary appreciably. By definition, the
stability of the system is its tendency to recover from disturbances such as faults or
changes of load.
The power transmitted between two synchronous machines can be slowly
increased only up to a certain level called the steady-state stability limit. Beyond this
level the synchronous machines fall out of step, i.e. lose synchronism. The steady
state stability limit can be considerably modified by the excitation level of the
synchronous machines (and therefore the line voltage); by the number and
connections of transmission lines; and by the pattern of real and reactive power flows
in the system, which can be modulated by reactive compensation equipment.
A transmission system cannot be operated too close to the steady-state
stability limit, because there must be a margin to allow for disturbances. In
determining an appropriate margin, the concepts of transient and dynamic stability are
useful. Dynamic stability is concerned with the ability to recover normal operation
following a specified minor disturbance. Transient stability is concerned with the
ability to recover normal operation following a specified major disturbance.
3.5 Voltage profile
It is obvious that the correct voltage level must be maintained within narrow
limits at all levels in the network. Under voltage degrades the performance of loads
and causes over current. Overvoltage is dangerous because of the risks of flashover,
insulation breakdown, and saturation of transformers. Most voltage variations are
caused by load changes, and particularly by the reactive components of current
flowing in the reactive components of the network impedances. If generators are close
by, excitation levels can be used to keep the voltage constant; but over long links the
voltage variations are harder to control and may require reactive compensation
equipment.
29
Chapter # 4
Power Flow Control of 500/230 kV Grid with UPFC
Fig. 4.1 Case of Study
To Relieve Power Congestion on a 500/230 kV Grid
A UPFC is used to control the power flow in a 500 kV /230 kV transmission
systems. The system, connected in a loop configuration, consists essentially of five
buses (B1 to B5) interconnected through transmission lines (L1, L2, L3) and two 500
kV/230 kV transformer banks Tr1 and Tr2. Two power plants located on the 230-kV
system generate a total of 1500 MW which is transmitted to a 500-kV 15000-MVA
equivalent and to a 200-MW load connected at bus B3. The plant models include a
speed regulator, an excitation system as well as a power system stabilizer (PSS). In
normal operation, most of the 1200-MW generation capacity of power plant #2 is
30
exported to the 500-kV equivalent through three 400-MVA transformers connected
between buses B4 and B5. For this demo we are considering a contingency case
where only two transformers out of three are available (Tr2= 2*400 MVA = 800
MVA).
Using the load flow option of the powergui block, the model has been
initialized with plants #1 and #2 generating respectively 500 MW and 1000 MW and
the UPFC out of service (Bypass breaker closed or simply „1‟). The resulting power
flow obtained at buses B1 to B5 is indicated by numbers on the circuit diagram. The
load flow shows that most of the power generated by plant #2 is transmitted through
the 800-MVA transformer bank (899 MW out of 1000 MW), the rest (101 MW),
circulating in the loop. Transformer Tr2 is therefore overloaded by 99 MVA. The
demonstration illustrates how the UPFC can relief this power congestion.
The UPFC located at the right end of line L2 is used to control the active and
reactive powers at the 500-kV bus B3, as well as the voltage at bus B_UPFC. It
consists of a Phasor model of two 100-MVA, IGBT-based, converters (one
connected in shunt and one connected in series and both interconnected through a
DC bus on the DC side and to the AC power system, through coupling reactors and
transformers). Parameters of the UPFC power components are given in the dialog
box. The series converter can inject a maximum of 10% of nominal line-to-ground
voltage (28.87 kV) in series with line L2. The numbers on the diagram show the
power flow with the UPFC in service and controlling the B3 active and reactive
powers respectively at 687 MW and -27 Mvar.
SIMULATION
This is the Simulink model of above 500/230kv grid station. Effect of UPFC
is studied such that first it is simulated without UPFC and active power on all 5 buses
is noted. Then UPFC is brought into the system and active power is again noted in a
similar fashion.
31
500/230 kV Grid without UPFC
The following is the simulation of the above example without UPFC. Graphs
of active power and reactive power are shown. Moreover, active power is also
mentioned on the buses in the diagram.
Fig. 4.2 System modeled on Simulink (Without UPFC)
32
500/230 kV Grid without UPFC
Active Power (MW) along y axis on bus no 1 to bus no 5
Fig. 4.3 Active Power Metering with respect to time (Without UPFC)
33
500/230 kV Grid without UPFC
Reactive Power (MVAR) along y axis on bus no 1 to bus no 5
Fig. 4.4 Reactive Power Metering with respect to time (Without UPFC)
34
500/230 kV Grid with UPFC
Fig. 4.5 System modeled with UPFC
Note:
By-Pass Breaker:
When we select by-pass breaker „1‟, UPFC is bypassed.
When we select by-pass breaker „0‟, UPFC is connected into the system.
In this example, breaker is made 0 at 5 seconds i.e. UPFC is brought into system at 5
second.
35
Figure below shows the timer block and its setting in this simulation
Fig. 4.6 Settings of the Timer block
36
Reference Active Power
The reference active power is the power settings that UPFC block is going to
adjust over time. We have seen before that the power flowing through the bus B3 is
587 MW at steady state. Now for the elaboration purpose we adjust that to 687 to
obtain a 100 MW increase in power flowing through that bus as simulation proceeds
to 5 seconds. This is shown in the following figure.
Reference Active Power is increased from 5.87 pu to 6.87 pu i.e. 100MW
increase.
Fig. 4.7 Ref active power
37
500/230 kV Grid with UPFC
Active Power (MW) along y axis on bus no 1 to bus no 5 respectively
Fig. 4.8 Active Power w.r.t time (With UPFC)
To be observed:
The active power through bus 3 which increases from 570MW to 670 MW in
the interval of 5s to 6s
38
500/230 kV Grid with UPFC
Reactive Power (MVAR) along y axis on bus no 1 to bus no 5
Fig. 4.9 Reactive power w.r.t time (With UPFC)
To be observed:
As the reactive power was set to -27MW via Qref it is maintained at it
after the switching of UPFC into the system
39
Remarks:
We see that active power on all the buses is changed. On bus 1 it becomes
195MW and on bus 4 it is reduced from 900 MW to 800 MW, thus preventing
transformer Tr 2 from overloading.
This is a great advantage o UPFC. We can easily control the direction of
active power in a power system. This was a basic example which is easy to
understand. UPFC performs equally well in complex power network. We will show
this in the upcoming examples.
40
To elaborate the effect of UPFC more explicitly, system is made more complex as
shown below. Now there are total 9 buses.
Active power on all buses is mentioned on the buses.
Network without UPFC
Fig. 4.10 Case Modified (No UPFC installed)
41
Simulation Results:
Active Power Reading
Network without UPFC
B1 to B5:
Active Power (MW) along y axis on bus no 1 to bus no 5
Fig. 4.11 Active Power Reading bus 1-5 (without UPFC)
42
Network without UPFC
B6 to B9:
Active Power (MW) along y axis on bus no 6 to bus no 9
Fig. 4.12 Active Power Reading bus 6-9 (without UPFC)
43
With UPFC
Now two UPFC‟s are connected, one on each branch.
Three cases are considered:
Both UPFC‟s bypassed
First connected and second bypassed
Both connected
Pattern of active power flow is different each time. When UPFC is bypassed, less
power lows through its branch. By connecting UPFC , large amount of power stars
flowing through its branch.
This is shown below.
44
Both Bypassed (Simulation diagram)
Fig. 4.13 Case Modified (UPFC installed both bypassed)
45
Both Bypassed (Observations)
B1 to B5:
Active Power (MW) along y axis on bus no 1 to bus no 5
Fig. 4.14 Active Power Reading bus 1-5 (with UPFC both bypassed)
46
Both Bypassed (Observations)
B6 to B9:
Active Power (MW) along y axis on bus no 6 to bus no 9
Fig. 4.15 Active Power Reading bus 6-9 (with UPFC both bypassed)
47
First connected and second bypassed (Simulation diagram)
Fig. 4.16 Case modified (with UPFC 1st connected 2nd bypassed)
48
First connected and second bypassed (Observations)
B1 to B5:
Active Power (MW) along y axis on bus no 1 to bus no 5
Fig. 4.17 Active Power reading bus 1-5 (with UPFC 1st connected 2nd bypassed)
49
First connected and second bypassed (Observations)
B6 to B9:
Active Power (MW) along y axis on bus no 6 to bus no 9
Fig. 4.18 Active Power reading bus 6-9 (with UPFC 1st connected 2nd bypassed)
50
Both connected (Simulation diagram)
Fig. 4.19 Case modified (with UPFC both connected)
51
Both connected (Observations)
B1 to B5:
Active Power (MW) along y axis on bus no 1 to bus no 5
Fig. 4.20 Active Power reading bus 1-5 (with UPFC both connected)
52
Both connected (Observations)
B6 to B9:
Active Power (MW) along y axis on bus no 6 to bus no 9
Fig. 4.21 Active Power reading bus 6-9 (with UPFC both connected)
53
Remarks:
Pattern of active power flow is different each time. When UPFC is bypassed, less
power flows through its branch. By connecting UPFC, large amount of power stars
flowing through its branch.
Hence flow of active power is easily controlled with the help of UPFC.
54
Chapter # 5
Voltage regulation of 500kv transmission system
In this chapter we have connected a complex network of different transmission lines
and generating stations and performed the overall analysis of change in pu voltage
levels on the busses in the system.
Below is the system designed used for the analysis
Fig. 5.1 Case modified for analysis of voltage regulation
55
The above system has 4 transmission lines. All lines have 1 UPFC each.
We will see effect of UPFC on bus voltages and active and reactive power flow
among themselves.
We will specifically focus on voltage regulation. Simulation is carried out by first by
connecting all UPFC‟s and then bypassing all UPFC‟s. The readings are tabulated as
shown in the table.
56
Voltage Regulation:
With UPFC Without UPFC
BUS NO. V (p.u.) V (p.u.)
1 1.001 1.017
2 1.003 1.013
3 1.002 1.008
4 0.9894 0.9912
5 0.9985 1.001
6 0 1.013
7 1.003 1.008
8 1.002 0.9912
9 0.9894 1.001
10 0.9985 0
11 1.001 1.017
12 0.9985 0
13 1.003 1.013
14 1.002 1.008
15 0.9894 0.9912
16 0.9985 1.001
17 0 1.013
18 1.003 1.008
19 1.002 0.9912
20 0.9894 1.001
57
Remarks:
From above table we have seen that
when we are not using UPFC, voltage at different buses is not very close to 1pu.
When we are using UPFC, voltage at buses is very close to 1pu.
This shows that UPFC is very helpful for us in maintaining voltage close to unity in
spite of heavy load.
58
Chapter # 6
Stability of 500kv transmission system
Suppose in the above system, one 65km line gets out o service due to fault or
repairing purpose. Now the system will definitely become unstable or less stable and
unbalanced due to the dynamic change. As a result, voltage on all buses will be
changed drastically and may cause damage which is undesirable. This problem can
be settled with UPFC. UPFC maintains the voltage on all the buses and reduce the
oscillations produced as a result o uneven change in power system.
This is shown in simulation. The circuit breaker trips the line after 1 second.
Circuit breaker is used to get one line out of the system.
59
Fig. 6.1 circuit breaker installed at double circuit transmission line
Following block set the timing of the circuit breaker.
Fig. 6.2 Block parameters of circuit breaker
60
Voltage without UPFC (Observations)
Voltage (pu) along y axis on bus no 1 to bus no 5
Fig. 6.3 Bus voltages without UPFC
61
Voltage with UPFC (Observations)
Voltage (pu) along y axis on bus no 1 to bus no 5
Fig. 6.4 Bus Voltages with UPFC
62
Remarks:
We see that at 1 second when one line got out of the system, there was severe
oscillation in the system voltage. But with UPFC the system voltage did not change
and remained close to 1 pu.
63
Chapter # 7
CONCLUSION
Flexible AC Transmission is an emerging technology in the Power World
which uses power electronic devices for reactive compensation It is meant to
enhance controllability and increase power transfer capability of the network.The
Unified Power Flow Controller (UPFC) is the most versatile member of the FACTS
family to control power flow on power grids. A UPFC is an electrical device for
providing fast-acting reactive power compensation on high-voltage electricity
transmission networks. It is a versatile controller which can be used to control active
and reactive power flows in a transmission line. Moreover, with UPFC the voltage
regulation can be achieved and system stability can be increased.
In this project, we have observed the impact of UPFC upon the 500kv
transmission system. We have observed that UPFC can control the direction of active
power low. We have simulated two examples in chapter 4 which show that UPFC
increases the amount active power in the line to which it is connected. Thus we can
control the flow of active power and prevent our transmission system from the
congestion. In this way performance of the power system will increase with UPFC.
That was our first objective which we achieved successfully.
Then we simulated a complex 500kv system and applied UPFC on various
branches. We observed that the voltages at different buses were not very close to 1
pu in the system without UPFC. However, in the system with which UPFC is
connected, bus voltages are very close to 1 pu. This proves that UPFC is very useful
and suitable in voltage regulation. In this way performance of the power system will
increase. That was our second objective which we achieved successfully.
Then we checked the stability of the 500kv system with UPFC. We
proceeded with the example of chapter 4 in which we made one 500kv line (out of
64
two), out of service with the help of circuit breaker and observed the impact of that
drastic change on the system bus voltages. We observed that in the system without
UPFC the bus voltages collapsed at the instant the line got out of service and
oscillations were produced in the bus voltages. Certainly this phenomenon is very
undesirable because it will make the system unbalanced and unstable. The system
performance will also become poor. However, with UPFC the results obtained are
amazingly different and favorable. Bus voltages remain very close to 1 pu before and
after the removal of line and even a single severe glitch was not observed. This is a
great advantage of UPFC that it maintains the system stability in the changing
circumstances and hence improves the performance of the power system. That was
the third and final objective of our project which we achieved successfully.
Hence we conclude that with UPFC the amount and flow of active power can
be controlled, voltage regulation can be achieved and system stability can be
increased. Thus we can say that UPFC is quite useful to improve the performance of
the power system and we recommend Pakistan WAPDA authorities to implement
UPFC in their existing power system to upgrade it and avail of its benefits.
65
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