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

References

1) John J. Grainger, William D. Stevenson, ―Power System Analysis‖, Mittal Press, New Delhi

India

2) Muhammad H. Rashid, ―Power Electronics Circuits Devices & Applications, 3ed.

3) Flexible AC Transmission Systems: Modelling and Control- Xiao-Ping Zhang, Christian

Rehtanz, Bikash Pal

4) Juergen Schlabbach, Karl-Heinz Rofalski , ―Power System Engineering‖, WILEY VCH

Verlag GmbH & Co. KgaA.

5) Karris, ―Introduction to Simulink with Engineering Applications‖, Orchard Publications

2008.

6) FACTS CONTROLLERSIN POWER TRANSMISSION AND DISTRIBUTION- K. R. Padiyar

7) ― MATLAB R2007b version 7.5.0.342‖ (help)

8) http://www.energy.siemens.com/hq/en/power-transmission/facts/

9) http://www.energy.siemens.com/hq/pool/hq/power-

transmission/FACTS/FACTS_Technology_.pdf

10) S. Muthukrishnan and Dr. A. Nirmal Kumar, “Comparison of Simulation and Experimental

Results of UPFC used for Power Quality Improvement”, International Journal of Computer

and Electrical Engineering, Vol. 2, No. 3, June, 2010

11) Ali Ajami, S.H. Hosseini, and G.B. Gharehpetian, “Modelling and Controlling of UPFC for

Power System Transient Studies”, ECTI TRANSACTIONS ON ELECTRICAL ENG.,

ELECTRONICS, AND COMMUNICATIONS VOL.5, NO.2 August 2007

12) S. Tara kalyani, g. Tulasiram das, “simulation of real and reactive power flow control with

UPFC connected to a transmission line”, journal of theoretical and applied information

technology

66

13) Nashiren.F. Mailah, Senan M. Bashi, “Single Phase Unified Power Flow Controller (UPFC):

Simulation and Construction”, Euro Journals Publishing, Inc. 2009

14) Vibhor Gupta, “Study and Effects of UPFC and its Control System for Power Flow Control

and Voltage Injection in a Power System”, International Journal of Engineering Science and

Technology Vol. 2(7), 2010, 2558-2566

15) Satakshi Singh, “ The Unified Power Flow Controller Optimal Power Flow Model”,

International Journal of Scientific and Research Publications, Volume 2, Issue 8, August

2012

16) Samina Elyas Mubeen, R. K. Nema, and Gayatri Agnihotri, “Power Flow Control with UPFC

in Power Transmission System”, World Academy of Science, Engineering and Technology

47 2008

17) S. Muthukrishnan, Dr. A.Nirmalkumar, G. Murugananth, “Modeling and Simulation Five

Level Inverter based UPFC System”, International Journal of Computer Applications (0975 –

8887)Volume 12– No.11, January 2011

18) N.M. Tabatabaei , Gh. Aghajani ,J.M. Tabatabaei, “REACTIVE POWER CONTROL WITH

SHUNT FACTS DEVICES”, “Technical and Physical Problems of Power Engineering”

(TPE-2008) 4-6 September 2008

19) K. Manoz Kumar Reddy, “ Simulation of Real, Reactive Power and regulation with upfc”,

International Journal of Scientific and Research Publications, Volume 2, Issue 4, April 2012

20) R. Orizondo, and R. Alves, “UPFC Simulation and Control Using the ATP/EMTP and

MATLAB/Simulink Programs”, IEEE PES Transmission and Distribution Conference and

Exposition Latin America, Venezuela 2006

21) K. Manoz kumar reddy, “ Multiple Control Functions of UPFC”, International Journal of

Advanced Research in Computer Science and Software Engineering, Volume 2, Issue 4,

April 2012