A

SEMINAR ABSTRACT

ON

TRANSMISSION SYSTEM APPLICATION

REQUIREMENT FOR FACTS CONTROLLERS

Guided By: Submitted By:

Dr. Mool SinghProfessor

Dhanraj MeenaO50215,E2IV yr , B.Tech

DEPARTMENT OF ELECTRICAL ENGINEERING

MALAVIYA NATIONAL INSTITUTE OF TECHNOLOGY

JAIPUR (RAJASTHAN)-30201

MALAVIYA NATIONAL INSTITUTE OF TECHNOLOGY

(DEEMED UNIVERSITY)

JAIPUR (RAJASTHAN)-302017

CERTIFICATE

It is certified that Mr. DHANRAJ MEENA, Student of B.Tech.

(ELECTRICAL ENGG.), has worked for his seminar “TRANSMISSION

SYSTEM APPLICATION REQUIREMENT FOR FACTS CONTROLLERS” under my

guidance being submitted in partial fulfillment of award of degree of B.Tech.

MNIT, Jaipur during the session 2008-09.

Place: Jaipur Dr. Mool SinghDate: Professor Dept. of Electrical Engineering, MNIT, Jaipur

2

ACKNOWLEDGEMENT

I have worked for my seminar entitled “TRANSMISSION SYSTEM APPLICATION

REQUIREMENT FOR FACTS CONTROLLERS” under the guidance of Dr Mool Singh,

Professor, Department of Electrical Engineering, MNIT, Jaipur.

Firstly, I would like to express my deep gratitude to, Dr Mool Singh,Professor

Department of Electrical Engineering, for his valuable guidance given in each and every step of

my work, without which it would have been incomplete. I would like to thank to Sh. V.K. Jain,

Head of the Department of Electrical Engineering, Malaviya National Institute of Technology,

Jaipur for giving me this opportunity to do this work. I would also like to thank to U.G. Co-

ordinators, Mrs. Nikita Jhajharia, Reader and Mr. V.S. Pareek, Reader who helped me for the

completion of my report. Last but never the least, I would like to state my deep gratitude for all

the support given required from time to time, by my parents and all my friends.

Dhanraj meena

3

ABSTRACT

FACTS technology is being promoted as a means to extend the capacity existing power

transmission network to their thermal limits without the necessity of adding new transmission

lines. Another advantage of facts lies in their ability to improve damping and to controlled flow

of power through selected corridors in a network. FACTS, an acronym which stands for

flexible AC transmission system, is an evolving technology based solution envisioned to help

utility industry to deal with changes in the power delivery business. FACTS devices aim

principally to control the three main parameters directly affecting AC power transmission

namely voltage, phase angle and impedance. This work discusses system planning issues and

requirements for applications of FACTS Controllers into electric transmission networks. It lists

applications and discusses differences between traditional equipment and FACTS Controllers.

Characteristics of models for FACTS Controllers are described. This work also provides

guidance on how to incorporate FACTS Controllers into the traditional planning process. It

includes detailed discussions of the various types of FACTS Controllers, their functions and

applicability. With these tools, the transmission planner now has additional options available to

improve overall transmission system usage while maintaining system reliability.

4

CONTENTS

CHAPTER ……………………………………………………..……………...….PAGES

1. INTRODUCTION…………………………………………..………………...…….1-2

2. FLEXIBLE A.C. TRANSMISSION SYSTEM……………………..……………..3-6

2.1 Main FACTS Devices…………………………………………………………......3

2.2 Basic characteristics of FACTS…………………………………………….……..4

2.3 Opportunities for FACTS…………...……………………………………….…....4

2.4 Benefits of FACTS Technology…………………………………………….…….6

2.5 Comparative Technical Benefits of FACTS for Different Applications…………..6

3. TYPES OF FACTS CONTROLLERS…………………………………………….7-9

3.1 Series Controller………………………………………………………………...…..7

3.2 Shunt Controller………………………………………………………………….....7

3.3 Combined Series-Series Controller…………………………………………….......8

3.4 Combined Series-Shunt Controller……………………………………………...…9

4. TRANSMISSION SYSTEM APPLICATION REQUIREMENT FOR FACTS

CONTROLLERS…….………………………………………………..……………10-19

4.1 FACTS Controller Applications…………………………………………………...10

4.2 Over View of FACTS Controllers………………………………………………………13

4.3 FACTS Controller Models ………………………………………………………..18

4.4 Operational Requirements for FACTS Controller………………………………….....18

5. CONCLUSION………………………………………..………………….…………..20 REFERENCES………………………………………………………………..............21

5

CHAPTER-1

INTRODUCTION

In recent years environment, way leave and high cost problems have delayed the

construction of new transmission. This has highlighted the need to change the traditional

system concepts and achieve better utilization of existing lines.

FACTS technology is based on the use of power electronic controlled devices for

allowing transmission circuits to be used to their maximum thermal capability. FACTS

technology is being promoted as a means to extend the capacity existing power transmission

network to their thermal limits with out the necessity of adding new transmission lines. Another

advantage of facts lies in their ability to improve damping and to controlled flow of power

through selected corridors in a network. FACTS, an acronym which stands for Flexible AC

Transmission System, is an evolving technology-based solution envisioned to help the utility

industry to deal with changes in the power delivery business. The term FACTS refers to

alternating current transmission systems incorporating power electronic-based and other static

controllers to enhance controllability and increase power transfer capability. Technology

concepts were conceived in the 1980’s and projects sponsored by the Electric Power Research

Institute (EPRI) demonstrated many of these concepts with laboratory scale circuits. In the

early 1990’s development of higher power electronic switching devices had progressed to the

point that utility size installations were feasible. It is hoped that growth in demand for these

products will spur continued development in the power electronic devices allowing larger

sizes and more economical installations.

In particular the FACTS devices aim principally to control the three main parameters directly

affecting a.c. power transmission namely voltage, phase angle, and impedance. None of the

FACTS devices is new in basic concept but the use of advanced power electronics has

revolutionized the design and engineering of them. HVDC is a parallel technology using

advanced power electronics and is not normally included as a FACTS technology.

The constraints preventing use of full Thermal Capability on conventional a.c. circuits are Poor

power sharing in parallel circuits under different network operating conditions, Transient

Dynamic & voltage instability, Voltage control and associated reactive power flow problems

6

and Fault level constraints. Although various Technologies Available for Improving Circuit

Utilization like Changes to network configuration, HVDC, FACTS. But the advantages of

Facts Technology over other solutions to network reinforcement are that it Has potential to

control flows as required, Less environmental impact than most alternative techniques of

transmission reinforcement, Depending on cost-benefit analysis. Could cost less than

alternatives.

Satisfactory damping of power oscillations is an important issue addressed when dealing with

the rotor angle stability of power systems. This phenomenon is well-known and observable

especially when a fault occurs. To improve the damping of oscillations in power systems,

supplementary control laws can be applied to existing devices.

These supplementary actions are referred to as power oscillation damping (POD) control. In

this work, POD control has been applied to two FACTS devices, TCSC and UPFC. The design

method utilizes the residue approach. The presented approach solves the optimal siting of the

FACTS as well as selection of the proper feedback signals and the controller design problem.

In case of contingencies, changed operating conditions can cause poorly damped or even

unstable oscillations since the set of controller parameters yielding satisfactory damping for

one operating condition may no longer be valid for another one. In this case, an advantage can

be taken from the wide area monitoring platform, to re-tune the POD controller’s parameters. A

lately developed algorithm for on-line detection of electromechanical oscillations based on

Kalman filtering techniques has been employed. It gives the information about the actual

dominant oscillatory modes with respect to the frequency and damping as well as about the

amplitude of the oscillation obtained through on-line analysis of global signals measured at the

appropriate place in the power system. This has further been used as a basis for the fine

adaptive tuning of the POD parameters.

7

CHAPTER-2

FALEXIBLE A.C.TRANSMISSION SYSTEM

FACTS are Alternating current transmission systems incorporating power electronic-based and

other static controllers to enhance controllability and increase power transfer capability

2.1 Main FACTS devices:

Thyristor Controlled Series Compensator (TCSC):

A device for modifying the line reactance. Current technology comprises a series capacitor

for reducing line reactance with thyristor control modules for Setter power flow and

stability control.

Static VAR Compensator (SVC) & Advanced Static VAR Compensator :

A device for oroviding controllable shunt capacitive or inductive reactance usins power

electronic control.

Thyristor Controlled Power Angle Regulator (TCPAR) :

A development of existing quadrature boosting device.. for controlling power line flows

using power electronics to improve performance characteristics.

Some other devices considered under FACTS technology:

Thyristor Controlled System Oscillation Damper (NGH Damper):

This device adds circuit resistance to inhibit Slow (up to 50Hz) oscillations on a long

heavily loaded line.

Dynamic Load Brake:

A large thyristor controlled resistor used to maintain generator transient fault stability.

Thyristor Controlled Fault Current Limiter:

Negative Sequence & Harmonic Compensator:

A thyristor controlled device for balancing three phase supplies or modifying harmonics.

A High Energy Surge Arrestor:

Series and parallel stacked metal oxide high voltage surge arrestors in combination with

thyristors, for establishing the precise and reproducible surge voltage threshold for

operation

8

2.2 Basic Characteristics of FACTS Devices:

All three basic FACTS devices have the ability to control flow5 at the operators discretion

in an a.c. transmission circuit and thereby achieve better circuit utilization. The choice of

device depends on the particular application and costs.

Assuming sufficient thermal capability the theoretical maximum transmittable power in a

transmission line is V'/X. In practice lines can rarely operated close to this limit for

stability reasons. A sufficient margin is needed to recover from transient disturbances

(transient instability). Minor disturbances can lead to increasing power oscillation (dynamic

instability).

Both shunt and series line compensation can increase the maximum transmittable Dower.

Thus it is expected that, with suitable and fast control can improve both transient and

dynamic stability performance. Phase shifter controls could also be designed to aid

stability.

2.3 Opportunities for FACTS

What is most interesting for transmission planners is that FACTS technology opens up new

opportunities for controlling power and enhancing the usable capacity of present, as well as

new and upgraded, lines. The possibility that current through a line can be controlled at a

reasonable cost enables a large potential of increasing the capacity of existing lines with larger

conductors, and use of one of the FACTS Controllers to enable corresponding power to flow

through such lines under normal and contingency conditions.

These opportunities arise through the ability of FACTS Controllers to control the interrelated

parameters that govern the operation of transmission systems including series impedance, shunt

impedance, current, voltage, phase angle, and the damping of oscillations at various frequencies

below the rated frequency. These constraints cannot be overcome, while maintaining the

required system reliability, by mechanical means without lowering the useable transmission

capacity. By providing added flexibility, FACTS Controllers can enable a line to carry power

closer to its thermal rating. Mechanical switching needs to be supplemented by rapid-response

power electronics. It must be emphasized that FACTS is an enabling technology, and not a one-

on-one substitute for mechanical switches.

9

The FACTS technology is not a single high-power Controller, but rather a collection of

Controllers, which can be applied individually or in coordination with others to control one or

more of the interrelated system parameters mentioned above. A well-chosen FACTS Controller

can overcome the specific limitations of a designated transmission line or a corridor. Because

all FACTS Controllers represent applications of the same basic technology, their production

can eventually take advantage of technologies of scale. Just as the transistor is the basic

element for a whole variety of microelectronic chips and circuits, the thyristor or high-power

transistor is the basic element for a variety of high-power electronic Controllers

FACTS technology also lends itself to extending usable transmission limits in a step-by-step

manner with incremental investment as and when required. A planner could foresee a

progressive scenario of mechanical switching means and enabling FACTS Controllers such that

the transmission lines will involve a combination of mechanical and FACTS Controllers to

achieve the objective in an appropriate, staged investment scenario.

Some of the Power Electronics Controllers, now folded into the FACTS concept predate the

introduction of the FACTS concept. Notable among these is the shunt-connected Static VAR

Compensator (SVC) for voltage control. The first series-connected Controller, NGH-SSR

Damping Scheme, a low power series capacitor impedance control scheme showed that with an

active Controller there is no limit to series capacitor compensation. Even prior to SVCs, there

were two versions of static saturable reactors for limiting over voltages and also powerful

gapless metal oxide arresters for limiting dynamic over voltages. Research had also been

undertaken on solid-state tap changers and phase shifters. However, the unique aspect of

FACTS technology is that this umbrella concept revealed the large potential opportunity for

power electronics technology to greatly enhance the value of power systems, and thereby

unleashed an array of new and advanced ideas to make it a reality.

It is also worth pointing out that, in the implementation of FACTS technology, we are dealing

with a base technology, proven through HVDC and high-power industrial drives. Nevertheless,

as power semiconductor devices continue to improve, particularly the devices with turn-off

capability, and as FACTS Controller concepts advance, the cost of FACTS Controllers will

continue to decrease. Large-scale use of FACTS technology is an assured scenario.

10

2.4 Benefits of FACTS Technology

Within the basic system security guidelines the FACTS Controllers enable the transmission

owner to obtain, on case by case basis, one or more of the following benefits:

■ Control of power flow as ordered. The use of control of the power flow may be to follow a

contract, meet the utilities' own needs, ensure optimum power flow, ride through emergency

conditions, or a combination thereof.

■ Increase the loading capability of lines to their thermal capabilities, including short term and

seasonal.

■ Increase the system security through raising the transient stability limits, limiting short-circuit

currents and overloads, managing cascading blackouts and damping electromechanical

oscillations of power systems and machines.

■ Provide secure tie line connections to neighboring utilities and regions thereby decreasing

overall generation reserve requirements on both sides.

■ Provide greater flexibility in siting new generation.

■ Reduce reactive power flows, thus allowing the lines to carry more active power.

■ Reduce loop flows.

■ Increase utilization of lowest cost generation. One of the principal reasons for transmission

interconnections is to utilize lowest cost generation. When this cannot be done, it follows that

there is not enough cost-effective transmission capacity. Cost-effective enhancement of

capacity will therefore allow increased use of lowest cost generation.

■ Environment friendly, contains no hazardous material and produce no waste or pollution.

2.5 Comparative Technical Benefits of FACTS for Different Applications.

FACTS Load flow

control

Voltage

Control

Transient

Stability

Dynamic

Stability

SVC * * * * * * *

TCSC * * * * * * * *

STATCOM * * * * * * * *

UPFA * * * * * * * * * * *

11

CHAPTER-3

TYPES OF FACTS CONTROLLERS

In general, FACTS Controllers can be divided into four categories:

• Series Controllers

• Shunt Controllers

• Combined series-series Controllers

• Combined series-shunt Controllers

Fig.1: General symbol for FACTS Controller.

3.1 Series Controllers

The series Controller could be a variable impedance, such as capacitor, reactor, etc., or a

power electronics based variable source of main frequency, subsynchronous and harmonic

frequencies (or a combination) to serve the desired need. In principle, all series Controllers

inject voltage in series with the line. Even a variable impedance multiplied by the current flow

through it, represents an injected series voltage in the line. As long as the voltage is in phase

quadrature with the line current, the series Controller only supplies or consumes variable

reactive power. Any other phase relationship will involve handling of real power as well.

Fig.2: Series Controller.

12

3.2 Shunt Controllers

As in the case of series Controllers, the shunt Controllers may be variable impedance, variable

source, or a combination of these. In principle, all shunt Controllers inject current into the

system at the point of connection. Even a variable shunt impedance connected to the line

voltage causes a variable current flow and hence represents injection of current into the line. As

long as the injected current is in phase quadrature with the line voltage, the shunt Controller

only supplies or consumes variable reactive power. Any other phase relationship will involve

handling of real power as well.

Fig.3: Shunt Controller.

3.3 Combined series-series Controllers

This could be a combination of separate series controllers, which are controlled in a

coordinated manner, in a multiline transmission system. Or it could be a unified Controller, in

Which series Controllers provide independent series reactive compensation for each line but

also transfer real power among the lines via the power link. The real power transfer capability

of the unified series-series Controller, referred to as Interline Power Flow Controller, makes it

possible to balance both the real and reactive power flow in the lines and thereby maximize the

utilization of the transmission system. Note that the term "unified" here means that the dc

terminals of all Controller converters are all connected together for real power transfer.

13

Fig.3: Unified series-series Controller.

3.4 Combined series-shunt Controllers

This could be a combination of separate shunt and series Controllers, which are controlled in a

coordinated manner, or a Unified Power Flow Controller with series and shunt elements. In

principle, combined shunt and series Controllers inject current into the system with the shunt

part of the Controller and voltage in series in the line with the series part of the Controller.

However, when the shunt and series Controllers are unified, there can be a real power exchange

between the series and shunt Controllers via the power link.

Fig.4 (a): Coordinated series and shunt Controller. Fig.4(b): Unified series-shunt Controller.

14

CHAPTER-4

TRANSMISSION SYSTEM APPLICATION REQUIREMENT

FOR FACTS CONTROLLERS

Defining applications for FACTS Controllers requires an understanding of the planning

process for transmission systems. These controllers must function and be economical in a

deregulated market environment as well as meet the traditional requirements forsystem

security, reliability and sufficient capacity to meet the needs of customers. Therefore the focus

of the work became a discussion of the changing utility environment, the choices of solutions

for power transfer and voltage control issues and the system requirements that FACTS

Controllers must meet. In addition to describing the planning process, functions performed by

each of the FACTS Controllers are briefly described and their basic circuits shown. This work

focuses on how the transmission system planning process can define and justify FACTS

applications.

The main sections of the work are:

Transmission Planning Environment

Transmission Planning and System Control

Overview of FACTS Controllers

FACTS Controller Models

Operational Requirements for FACTS Controllers.

4.1 FACTS Controller Applications

The simplest way to identify the potential roles to be played by FACTS Controllers is to

examine their functions as they relate to conventional equipment. The definition of FACTS

systems incorporates both power electronic-based and other static controllers to enhance

controllability and increase power transfer capability. One of the system planners’ tasks is to

determine which combinations of Controllers provide both the capacity to supply the reactive

power, dynamic reserve and continuous regulation needed for the application. Table 1 lists the

main functions that can be performed by FACTS Controllers and show both FACTS and other

conventional equipment that performs these functions. A characteristic of FACTS controllers is

15

the ability to have control algorithms structured to achieve multiple objectives. Since FACTS

Controllers have control systems with embedded digital processors, it is possible to switch

between control algorithms and to include different types of nonlinear limiting functions. Both

shunt connected and series connected controllers can be programmed to assist in one or more of

these functions. They will have varying effectiveness depending on the power rating of the

individual controller; it’s location in the network, and the desired function. In addition to the

primary control function, these controllers can also provide damping to system oscillations.

The effectiveness of this damping control is highly dependent of the location of the controller

in the network. The value of FACTS applications lies in the ability of the transmission system

to reliably transmit more power or to transmit power under more severe contingency conditions

with the control equipment in operation. If the value of the added power transfer over time is

compared to the purchase and operational costs of the control equipment, relatively complex

and expensive applications may be justified. Other economic considerations include the market

structure, transmission tariff, and identification of winners and losers. Realization of the value

added by a proposed transmission project often requires a coordinated implementation of

conventional transmission equipment, possibly including transmission line segments, FACTS

Controllers, coordinated control algorithms and special operating procedures.

16

Function Non FACTS Control Methods FACTS Controllers

Voltage

Control

Electric generators

Synchronous Condensers

Conventional Transformer tap-

changer Conventional Shunt

Capacitor/Reactor Conventional

Series Capacitor/Reactor

Static Var Compensator (SVC)

Static Synchronous Compensator

Unified Power Flow Controller (UPFC)

Superconducting Energy Storage (SMES)

Battery Energy Storage System (BESS)

Convertible Static Compensator (CSC

Active

and

Reactive

Power

Flow

Control

Generator schedules

Transmissionline switching

Phase Angle Regulator (PAR)

Series Capacitor (switched or fixed)

High Voltage Direct Current

Transmission (HVdc)

Interphase Power Controller (IPC)

Thyristor controlled Series Capacitor

(TCSC)

Thyristor Controlled Series Reactor

(TCSR)

Thyristor Controlled Phase Shifting

Transformer (TCPST) ,UPFC

Static Synchronous Series Compensator

(SSSC)

Interline Power Flow Controller (IPFC)

Transient

Stability

Braking Resistor

Excitation Enhancement

Special Protection Systems

Independent Pole Tripping

Fast Relay Schemes

Fast Valving Line Sectioning HVdc

Thyristor Controlled Braking Resistor

(TCBR)

SVC, STATCOM, TCSC, TCPST, UPFC

BESS, SMES, SSSC, CSC, IPFC

Dynamic

Stability

Power System Stabilizer

HVdc

TCSC, SVC, STATCOM, UPFC, SSSC,

TCPST, BESS, SMES, SSSC,CSC, IPFC

Short

Circuit

Current

Limiting

Switched series reactors

Open circuit breaker arrangements

Thyristor switched series reactor, TCSC,

IPC, SSSC, UPFC; These are secondary

functions of these controllers and their

effectiveness may be limited.

Table 1: System Control Functions

17

4.2 Over View of FACTS Controllers

This section of the work shows the circuit diagram and describes the operation of the FACTS

Controllers that have been commercially applied.

The Controllers described are:

Static Var Compensator (SVC)

Static Synchronous Compensator (STATCOM)

Superconducting Magnetic Energy Storage (SMES)

Battery Energy Storage System (BESS)

Thyristor Controlled Series Capacitor (TCSC)

Static Synchronous Series Compensator (SSSC)

Unified Power Flow Controller (UPFC)

Interphase Power Controller (IPC)

Static Var Compensator (SVC)

The Static Var Compensator used for transmission system applications is a dynamic source of

leading or lagging reactive power. It is comprised of a combination of reactive branches

connected in shunt to the transmission network through a step up transformer. The SVC is

configured with the number of branches required to meet a utility specification as indicated in

Figure 5. This specification includes required inductive compensation and required capacitive

compensation. The sum of inductive and capacitive compensation is the dynamic range of the

SVC. One or more thyristor-controlled reactors may continuously vary reactive absorption to

regulate voltage at the high voltage bus. This variation is accomplished by phase control of

the thyristors, which results in the reactor current waveform containing harmonic components

that vary with control phase angle. A filter branch containing a power capacitor and one or

more tuning reactors or capacitors is included to absorb enough of the harmonic currents to

meet harmonic specifications and provide capacitive compensation. The thyristor switch

capacitor is switched on or off with precise timing to avoid transient inrush currents.

18

Figure5. : Circuit diagram of a SVC containing a thyristor controlled reactor, a thyristor

Switched capacitor and a double tuned filter

Static Synchronous Compensator (STATCOM)

The STATCOM shown in Figure 6 performs the same voltage regulation and dynamic control

functions as the SVC. However, its hardware configuration and principle ofoperation are

different. It uses voltage source converter technology that utilizes power electronic devices

(presently gate turn-off thyristors (GTO), GCTs or insulated gate bi-polar transistors (IGBT))

that have the capability to interrupt current flow in response to a gating command. Analogous

to an ideal electro magnetic generator, the STATCOM can produce a set of three alternating,

almost sinusoidal voltages at the desired fundamental frequency with controllable magnitude.

The angle of the voltage injected by the STATCOM is constrained to be very nearly in-phase

with the transmission network at the point of connection of the coupling transformer. When the

voltage is higher in magnitude than the system voltage, reactive current with a phase angle 90

degrees ahead of the voltage phase angle flows through the coupling transformer. This is

analogous to the operation of a shunt capacitor. When the generated voltage is lower than

system voltage, the current phase angle is 90 degrees behind the voltage phase angle that is

analogous to the operation of a shunt reactor. The slight deviation in voltage phase angle

absorbs power needed for the losses in the circuit. For high power applications a number of six

or twelve pulse converters are operated in parallel to meet both the current rating requirement

19

and the harmonic requirement of the network. Two different switching patterns, phase

displaced converters with electronic devices switched once per cycle and pulse width

modulation, have been used to form the sinusoidal waveform.

Figure 6: STATCOM circuit diagram

Thyristor Controlled Series Capacitor (TCSC)

The thyristor controlled series capacitor (TCSC) is placed in series with a transmission line and

is comprised of three parallel branches: a capacitor, a thyristor pair in series with a reactor

(TCR), and a metal oxide varistor (MOV) that is required to protect against overvoltage

conditions. (See Figure 7. The TCSC can function as a series capacitor if the thyristors are

blocked or as a variable impedance when the duty cycle of the thyristors is varied.

Applications of TCSCs currently in service provide impedance variations to damp inter-area

system oscillations. The most economical installations often contain one segment of thyristor-

controlled capacitors in series with one or more segments ofconventionally switched series

capacitors.

20

Figure 7: One Line Diagram of the TCSC

Static Synchronous Series Compensator (SSSC)

A static synchronous series compensator (SSSC) is connected in series with a transmission line

and is comprised of a voltage source converter operated without an external electric energy

source. (See Figure 8) This configuration serves as a series compensator whose output voltage

is in quadrature with, and controlled, independently of the transmission line current.

Figure 8: Circuit diagram for a Static Synchronous Series Compensator(SSSC)

The purpose of the SSSC is to increase or decrease the overall reactive voltage drop across the

line and thereby control the transmitted real electric power. The SSSC may include transiently

rated energy storage or energy absorbing equipment to enhance the dynamic behaviour of the

power system by additional temporary real power compensation, to increase or decrease

momentarily, the overall real (resistive) voltage drop across the line. This action controls the

reactive power flow on the line.

21

Unified Power Flow Controller (UPFC)

The Unified Power Flow Controller (UPFC) provides voltage, and power flow control by using

two high power voltage source converters (VSC) coupled via a dc capacitor link. Figure.9

shows the two interconnected converters. VSC 1 is connected like a STATCOM and VSC 2 is

connected as a SSSC in series with the line. With the dc bus link closed, the UPFC can

simultaneously control both real and reactive power flow in the transmission line by injecting

voltage in any phaseangle with respect to the bus voltage with the series converter. The shunt-

connected converter supplies real power required by the series connected converter. With its

remaining capacity the shunt converter can regulate bus voltage The UPFC circuit can be

reconfigured by use of external switches and possibly additional transformers to form

STATCOM, SSSC, or coupled SSSC circuits. Similar to other inverter based FACTS

Controllers, FACTS controllers that contain energy storage are coupled to the AC network

through an AC-DC inverter. In addition they have a DC-DC power circuit to interface the

energy storage (to date either a superconducting magnet or a battery) to the DC bus of the

inverter. This equipment has been applied at the distribution voltage level. Energy limitations

for storage systems have limited applications to short-term backup for critical loads and to

dynamic damping of system oscillations during transient conditions.

Figure 9: Circuit Diagram of a Unified Power Flow Controller

22

4.3 FACTS Controller Models

For FACTS Controllers to be included in transmission system plans, there must be appropriate

models for all the analyses that are normally performed. To date only the SVC typically has

an embedded model in the most widely used power flow (load flow) software. Some of the

other controllers are represented by user defined models and others by models for electric

machines or static inductors and capacitors. This lack of explicit models extends to the

software used in many transmission control centers and it is an impediment to defining new

applications and to operating FACTS equipment .

Models for dynamic simulation studies have, to date, been made by the equipment suppliers.

These models often represent the FACTS Controllers in extreme detail requiring simulation

time steps that are too small for the models to be easily incorporated into software that

simulates large electric networks. There is a need for more general models that are compatible

with the simulation software that is more widely used. These models would:

Represent the power circuit equations algebraically in the appropriate software routines.

Represent inverters as voltage sources.

Allow voltage changes that affect the network to occur at simulation time steps rather

than continuously.

Represent control functions using differential equation and logic algorithms

For specialized studies including harmonic analyses and analysis of switching phenomenon

there must be models that are compatible with EMTP or EMTDC analysis software. These

models must be made with the cooperation of the equipment manufacturer. They can represent

inverters either as AC voltage sources or by detailed switching circuits. If the switching circuit

is employed, the model must also represent the switching control logic including phase locked

loop synchronizing circuits. The logic that protects inverter valves from overcurrents must also

be included in these models. This level of detail is normally required only for design studies or

for detailed analysis of operating issues with the equipment.

4.4 Operational Requirements for FACTS Controller

When a utility, transmission coordinating council, or regional transmission organization (RTO)

considers the addition of FACTS Controllers, the consideration usually involves a number of

system requirements to assure the reliability and security of the installation. Since most of the

23

FACTS Controllers contain a computer based control system, they can be programmed to both

perform their primary function and also manage the operation of conventional transmission

equipment. Formalized procedures must be developed to define system conditions where

coordinated operation of the FACTS Controller and other transmission equipment is needed.

These formalized procedures are often published and increasingly are available to other system

planners as well to the general public. These controllers are expected to function during both

normal and transient conditions in the electric system. To meet this expectation requires design

and certification procedures based upon:

Directly measured system dynamics.

Assured resources for the prompt detection, analysis and correction of anomalous

controller effects.

Performance monitors that communicate information to system analysts

Tests for commissioning and periodic certification

Information exchange among grid operators.

Although these formalized guidelines are necessary to assure the security of the network, they

place significant demands for information about component reliability and system reliability for

FACTS Controllers. They also place demands for careful study of interactions in the

transmission system and definition of system contingencies that most stress the application.

Much of the required information is not well known during the development phase of a FACTS

Controller and engineering judgment and cooperation between equipment designers and system

engineers is essential in early applications. For FACTS Controllers to become widely used

they must:

Meet the availability and maintenance requirements that are expected for other power

electronic equipment used in the system.

Contain operator interface software and displays that clearly show the operating state of

the Controller and allow the operator to readily change reference settings, control

modes or limits.

Meet automatic startup, shutdown and mode change requirements.

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CHAPTER-5

CONCLUSIONS

Although in the deregulated electric system environment, transmission system planning is more

difficult, the industry has always sought the application of equipment that will maximize the

use of available transmission lines. FACTS Controllers are just additional options available to

the planning engineer. Specifically, they are a new generation of power electronic based

equipment with the same function as conventional equipment but with enhanced controllability

and speed of response. Traditional planning methods still apply. Equipment selection will

depend on function, availability, cost, applicability, reliability, and robustness in the face of

future uncertainties. Transmission networks operating at current flow levels near the thermal

limits of transmission lines require large amounts of reactive power. They also require that this

reactive power is properly distributed throughout the network and that a portion be dynamic to

prevent voltage collapse during system contingencies. The allowed transmission limits are

defined both by rules intended to meet reliability requirements and the physical limits of the

system. The value of FACTS Controllers increases as the operating limits of the system

approach the physical limits. This work provides guidance on how to incorporate FACTS

Controllers into the traditional planning process. It includes detailed discussions of the various

types of FACTS Controllers, their functions and applicability, as well as commentary on

appropriate models for the necessary planning analyses. With these tools, the transmission

planner now has additional options available to improve overall transmission system usage

while maintaining system reliability.

25

REFERENCES

1. A Special Publication for System Planners IEEE WG15.05.13, “Transmission System

Application Planning Requirements for FACTS Controllers”, 2006

2. N. G. Hingorani and L. Gyugyi, Understanding FACTS: Concepts and Technology of

Flexible AC Transmission Systems. Piscataway, NJ: IEEE Press, 1999.

3. Arabi, S., Kundur, P., and Adapa, R., "Innovative Techniques in Modeling UPFC for Power

System Analysis," Paper PE-231-PWRS-0-10-1998 IEEE PES Winter Meeting, New Yor

January-February 1999.

4. G.D. Galanos, et al, “Advanced Static Compensator For Flexible AC Transmission”. IEEE

Transactions on Power Systems, Vol8, No. 1, Feb. 1993, pp. 113-121.

5. L. Gyugyi, “Flexible AC transmission systems (FACTS),” in Inst.Elect. Eng. Power and

Energy Series 30, Y. H. Song and A. T. Johns,Eds. London, U.K., 1999, ch.1.

6. Larsen, F, et al., “Benefits of Thyristor Controlled Series Compensation”, Cigre

Conference 1992, paper 14/37/38-04.

7. Hingorani, N.G., “High Power Electronics and Flexible AC Transmission System”, IEEE

Power Eng. REV., July 1988.

8. A Special Publication for System Planners IEEE WG15.05.13, “Transmission System

Application Planning Requirements for FACTS Controllers”, 2006

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