A

SEMINAR REPORT

“FLEXIBLE AC TRANSMISSION SYSTEM”

INDEX

SR.NO. CHAPTER NAME PAGE NO.

1 1.INTRODUCTION 1

2 2.TYPES OF FACTS CONTROLLERS 32.1series Controller2.2 Shunt Controllers 2.3 Combined Series –Series Controllers 2.4 Combined Series-Shunt Controllers2.5 Benefits Of Facts Controllers

3 3. CONTROL OF POWER SYSTEM 11

4

5

6

3.1 Generation ,Transmission ,Distribution3.2 Power System Constraints 3.3 Controllability Of Power System3.4 Benefits Of Control Of Power System 3.5 Benefits Of Utilising Facts Devices3.6 Classification3.6.1 First Generation Of Facts 3.6.11 Static Var Compensator3.6.12 Thyristor Controlled Series Capacitor3.6.2 Secind Generation Facts 3.6.21static Compensation 3.6.22 Static Synchronous Series Compensator3.6.23 Unified Power Flow Controller

APPLICATIONS 4.1facts Application To Steady State Power System Problems 4.2facts Application To Optimal Power Flow

CONCLUSION

REFERENCES

19

20

21

LIST OF FIGURES

Sr.No. Fig. No. Name Of Figure Page No.

1 Fig.2.1 Single line diagram of series compensator 3

2 Fig.2.2 Diagram of parallel compensator 4

3 Fig .3.1 Block diagram of power system 11

4 Fig. 3.2 Single line diagram and phasor diagram 13

5 Fig.3.3 Unified power flow controller 17

CHAPTER 1: INTRODUCTION

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 unprecedented change. Against this background of rapid

evolution, the expansion programs of many utilities are being thwarted by a variety of well-

founded, environment, land-use, and regulatory pressures that prevent the licensing and building

of new transmission lines and electricity generating plants.

The ability of the transmission system to transmit power becomes impaired by one or

more of the following steady state and dynamic limitations: (a) angular stability, (b) voltage

magnitude, (c) thermal limits, (d) transient stability, and (e) dynamic stability. These limits

define the maximum electrical power to be transmitted without causing damage to transmission

lines and electrical equipment .In principle, limitations on power transfer can always be relieved

by the addition of new transmission lines and generation facilities. Alternatively, flexible

alternating current transmission system (FACTS) controllers can enable the same objectives to

be met with no major alterations to power system layout. FACTS are alternating current

transmission systems incorporating power electronic-based and other static controllers to

enhance controllability and increase power transfer capability

.

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. FACTS controllers aim at increasing the control of power flows in the high-

voltage side of the network during both steady state and transient conditions. The concept

of FACTS as a total network control philosophy was introduced in 1988 by Dr. N.

Hingorani .Owing to many economical and technical benefits it promised, FACTS received

the support of electrical equipment manufacturers, utilities, and research organizations

around the world. This interest has led to significant technological developments of

FACTS controllers. Several kinds of FACTS controllers have been commissioned in

various parts of the world. The most popular are: load tap changers, phase-angle regulators,

static VAR compensators, thyristor controlled series compensators, interphase power

controllers, static compensators, and unified power flow controllers.

In this paper, the state of the art in the development of FACTS controllers is

presented. The paper presents the objectives, the types, and the benefits of FACTS

controllers. Moreover, various FACTS controllers are described, their control attributes are

presented, and their role in power system operation is analyzed.

Objectives of FACTS controllers

The main objectives of FACTS controllers are the following:

1. Regulation of power flows in prescribed transmission routes.

2. Secure loading of transmission lines nearer to their thermal limits.

3. Prevention of cascading outages by contributing to emergency control.

4. Damping of oscillations that can threaten security or limit the usable line capacity.

CHAPTER 2: TYPES OF FACTS CONTROLLERS

2.1 Series controllers

The series controller could be a variable impedance, such as capacitor,

reactor, or a power electronics based variable source of main frequency, sub-synchronous

and harmonic frequencies (or a combination) to serve the desired load. In principle, all

series controllers inject voltage in series with 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.

Series controllers include SSSC, IPFC, TCSC, TSSC, TCSR, and TSSR.

Fig2.1: Single line diagram of series compensator.

2.2 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

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

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

.

Fig2.2: Diagram of parallel compensator

2.3 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 proper link.

The real power transfer capability of the unified series-series controller, referred to as

IPFC, makes it possible to balance both real and reactive power flow in the lines and

thereby maximize the utilization of the

transmission system. The term “unified” here means that the dc terminals of all controller

converters are all connected together for real power transfer.

2.4 Combined series-shunt controllers:-

This could be a combination of separate shunt and series

controllers, which are controlled in a coordinated manner, or a UPFC 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 proper link.

Combined series-shunt controllers include UPFC, TCPST, and TCPA

FACTS CONTROLLERS

STATCOM:

STATCOM is a static synchronous generator operated as a shunt-

connected static var compensator whose capacitive or inductive output current can

be controlled independent of the ac system voltage. The use of STATCOM as a

FACTS controller is proposed .

SVC:

SVC is a shunt-connected static var generator or absorber whose output is

adjusted to exchange capacitive or inductive current so as to maintain or control

specific parameters of the electrical power system (typically bus voltage). SVC is

an important FACTS controller already widely in operation. Ratings range from

60 to 600 MVAR , SVC can be considered as a “first generation” FACTS

controller and uses thyristor controllers. It is a shunt reactive compensation

controller consisting of a combination of fixed capacitor or thyristor-switched

capacitor in conjunction with thyristor-controlled reactor.

TCR:

TCR is a shunt-connected thyristor-controlled inductor whose effective

reactance is varied in a continuous manner by partial-conduction control of the

thyristor valve. TCR has been used as one of the economical alternatives of

FACTS controllers .

TSC:

TSC is a shunt-connected thyristor-switched capacitor whose effective

reactance is varied in a stepwise manner by full- or zero-conduction operation of

the thyristor valve .

TSR:

TSR is a shunt-connected thyristor-switched inductor whose effective

reactance is varied in a stepwise manner by full- or zero-conduction operation of

the thyristor valve .

TCBR:

TCBR is a shunt-connected thyristor-switched resistor, which is controlled to

aid stabilization of a power system or to minimize power acceleration of a

generating unit during a disturbance

.

SSSC:

SSSC is a static synchronous generator operated without an external electric

energy source as a series compensator whose output voltage is in quadrature with,

and controllable independently of, the line current for the purpose of increasing or

decreasing the overall reactive voltage drop across the line and thereby

controlling the transmitted electric power . The SSSC may include transiently

rated energy storage or energy absorbing devices to enhance the dynamic

behavior of the power system by additional temporary real power compensation,

to increase or decrease momentarily, the overall real (resistive) voltage drop

across the line.

TCSC:

TCSC is a capacitive reactance compensator, which consists of a series

capacitor bank shunted by a thyristor-controlled reactor in order to provide a

smoothly variable series capacitive reactance. The description of the first TCSC

installation is given .

TSSC:

TSSC is a capacitive reactance compensator, which consists of a series

capacitor bank shunted by a thyristor-switched reactor to provide a stepwise

control of series capacitive reactance.

TCSR:

TCSR is an inductive reactance compensator, which consists of a series

reactor shunted by a thyristor-controlled reactor to provide a smoothly variable

series inductive reactance .

TSSR:

TSSR is an inductive reactance compensator, which consists of a series

reactor shunted by a thyristor-controlled reactor to provide a stepwise control of

series inductive reactance .

TCPST:

TCPST is a phase-shifting transformer adjusted by thyristor switches to

provide rapidly variable phase angle . This controller is also referred to as

TCPAR.

UPFC:

UPFC is a combination of STATCOM and a SSSC which are coupled via a

common dc link to allow bidirectional flow of real power between the series

output terminals of the SSSC and the shunt output terminals of the STATCOM

and are controlled to provide concurrent real and reactive series line

compensation without an external electric energy source. The UPFC, by means of

angularly unconstrained series voltage injection, is able to control, concurrently or

selectively, the transmission line voltage, impedance, and angle or, alternatively,

the real and reactive power flow in the line. The UPFC may also provide

independently controllable shunt reactive compensation. The UPFC proposed by

Gyugyi is the most versatile FACTS controller for the regulation of voltage and

power flow in a transmission line.

GUPFC:

GUPFC can effectively control the power system parameters such as bus

voltage, and real and reactive power flows in the lines . A simple scheme of

GUPFC consists of three converters, one connected in shunt and two connected in

series with two transmission lines terminating at a common bus in a substation. It

can control five quantities, i.e., a bus voltage and independent active and reactive

power flows in the two lines. The real power is exchanged among shunt and series

converters via a common dc link.

IPC:

IPC is a series-connected controller of active and reactive power consisting, in

each phase, of inductive and capacitive branches subjected to separately phase-

shifted voltages. The active and reactive power can be set independently by

adjusting the phase shifts and/or the branch impedances, using mechanical or

electronic switches. In the particular case where the inductive and capacitive

impedance form a conjugate pair, each terminal of the IPC is a passive current

source dependent on the voltage at the other terminal and the practical design

aspects of a 200 MW prototype for the interconnection of the 120 kV networks

were described. However, the original concept proposed has undergone

modifications that are described

.

TCVL:

TCVL is a thyristor-switched metal-oxide varistor used to limit the voltage

across its terminals during transient conditions .

TCVR:

TCVR is a thyristor-controlled transformer that can provide variable in-

phase voltage with continuous control.

IPFC:

IPFC is a combination of two or more SSSCs that are coupled via a common

dc link to facilitate bi-directional flow of real power between the ac terminals of

the SSSCs and are controlled to provide independent reactive compensation for

the adjustment of real power flow in each line and maintain the desired

distribution of reactive power flow among the lines. The IPFC structure may also

include a STATCOM, coupled to the IPFC common dc link, to provide shunt

reactive compensation and supply or absorb the overall real power deficit of the

combined SSSCs.

2.5 Benefits Of Facts Controllers:

FACTS controllers enable the transmission owners to obtain, on a case-by-

case basis, one or more of the following benefits:

2.5.1Cost:

Due to high capital cost of transmission plant, cost considerations

frequently Over weigh all other considerations. Compared to alternative methods

of solving transmission loading problems, FACTS technology is often the most

economic alternative.

2.5.2 Convenience:

All FACTS controllers can be retrofitted to existing ac transmission plant with

varying degrees of ease. Compared to high voltage direct current or six-phase

transmission schemes, solutions can be provided without wide scale system

disruption and within a reasonable timescale.

2.5.3 .Environmental impact:

In order to provide new transmission routes to supply an ever increasing

worldwide demand for electrical power, it is necessary to acquire the right to

convey electrical energy over a given route. It is common for environmental

opposition to frustrate attempts to establish new transmission routes. FACTS

technology, however, allows greater throughput over existing routes, thus meeting

consumer demand without the construction of new transmission lines. However,

the environmental impact of the FACTS device itself may be considerable.

2.5.4. Control of power flow to follow a contract, meet the utilities own needs,

ensure optimum power flow, minimize the emergency conditions, or a

combination thereof.

2.5.5. Contribute to optimal system operation by reducing power losses and

improving voltage profile.

2.5.6. Increase the loading capability of the lines to their thermal capabilities,

including short term and seasonal.

2.5.7 Increase the system security by raising the transient stability limit, limiting

short-circuit currents and overloads, managing cascading blackouts and damping

electromechanical oscillations of power systems and machines.

2.5.8. Provide secure tie line connections to neighboring utilities and regions

thereby decreasing overall generation reserve requirements on both sides.

CHAPTER 3: CONTROL OF POWER SYSTEMS

3.1 Generation, Transmission, Distribution

In any power system, the creation, transmission, and utilization of

electrical power can be separated into three areas, which traditionally determined

the way in which electric utility companies had been organized. These are

illustrated in Figure 1 and are:

• Generation

• Transmission

• Distribution

Fig 3.1 : Block Diagram of power system

Although power electronic based equipment is prevalent in each of these three

areas, such as with static excitation systems for generators and Custom Power

equipment in distribution systems, the focus of this paper and accompanying

presentation is on transmission, i.e. moving the power from where it is generated

to where it is utilized.

3.2 Power System Constraints

As noted in the introduction, transmission systems are being pushed closer

to their stability and thermal limits while the focus on the quality of power

delivered is greater than ever. The limitations of the transmission system can take

many forms and may involve power transfer between areas or within a single area

or region and may include one or more of the following characteristics:

• Steady-State Power Transfer Limit

CHAPTER 4:APPLICATIONS

4.1 Facts Applications To Steady State Power System Problems

For the sake of completeness of this review, a brief overview of the FACTS

devices applications to different steady state power system problems is presented in this

section. Specifically, applications of FACTS in optimal power flow and deregulated

electricity market will be reviewed.

4.2 FACTS Applications to Optimal Power Flow

In the last two decades, researchers developed new algorithms for solving the optimal

power flow problem incorporating various FACTS devices . Generally in power flow

studies, the thyristor controlled FACTS devices, such as SVC and TCSC, are usually

modeled as controllable impedance controllable sources. The Interline Power Flow

Controller (IPFC) is one of the voltage source converter(VSC) based FACTS Controllers

which can effectively manage the power flow via multi-line Transmission System.

•Better utilization of existing transmission system assets

•Increased transmission system reliability and availability

•Increased dynamic and transient grid stability and reduction of loop flows

•Increased quality of supply for sensitive industries

• Voltage Stability Limit

• Dynamic Voltage Limit

• Transient Stability Limit

• Power System Oscillation Damping Limit

• Inadvertent Loop Flow Limit

• Thermal Limit

• Short-Circuit Current Limit

• Others

Each transmission bottleneck or regional constraint may have one or more of

these system-level problems. The key to solving these problems in the most cost-

effective and coordinated manner is by thorough systems engineering analysis.

3.3 Controllability of Power Systems

To illustrate that the power system only has certain variables that can be

impacted by control, we have considered here the power-angle curve, shown in

Figure 2. Although this is a steady-state curve and the implementation of FACTS

is primarily for dynamic issues, this illustration demonstrates the point that there

are primarily three main variables that can be directly controlled in the power

system to impact its performance. These are:

• Voltage

• Angle

• Impedance

Fig 3.2: Single line diagram with phasor diagram

3.4 Benefits of Control of Power Systems

Once power system constraints are identified and through system

studies viable solutions options are identified, the benefits of the added power

system control must be determined. The following offers a list of such benefits:

• Increased Loading and More Effective Use of Transmission Corridors

• Added Power Flow Control

• Improved Power System Stability

• Increased System Security

• Increased System Reliability

• Added Flexibility in Starting New Generation

• Elimination or Deferral of the Need for New Transmission Lines

3.5 Benefits of utilizing FACTS devices

The benefits of utilizing FACTS devices in electrical transmission systems can be

summarized as follows

•Better utilization of existing transmission system assets

•Increased transmission system reliability and availability

•Increased dynamic and transient grid stability and reduction of loop flows

•Increased quality of supply for sensitive industries

3.6 Classification

There are different classifications for the FACTS devices: Depending on the

type of connection to the network FACTS devices can differentiate four

categories

• serial controllers

• derivation controllers

• serial to serial controllers

• serial-derivation controllers

Depending on technological features, the FACTS devices can be divided into two

generations

• first generation: used thyristors with ignition controlled by gate(SCR).

. second generation: semiconductors with ignition and extinction controlled

by gate (GTO´s , MCTS , IGBTS , IGCTS , etc).

3. 6.1 First Generation Of Facts

3.6.2 Static VAR Compensator (SVC)

A static VAR compensator (or SVC) is an electrical device for providing fast-

acting reactive power on high-voltage electricity transmission networks. SVCs are

part of the Flexible AC transmission system device family, regulating voltage and

stabilizing the system. The term "static" refers to the fact that the SVC has no

moving parts (other than circuit breakers and disconnects, which do not move

under normal SVC operation). Prior to the invention of the SVC, power factor

compensation was the preserve of large rotating machines such as synchronous

condensers. The SVC is an automated impedance matching device, designed to

bring the system closer to unity power factor

3. 6.3 .Thyristor-Controlled Series Capacitor (TCSC)

TCSC controllers use thyristor-controlled reactor (TCR) in parallel with

capacitor segments of series capacitor bank. The combination of TCR and

capacitor allow the capacitive reactance to be smoothly controlled over a wide

range and switched upon command to a condition where the bi-directional

thyristor pairs conduct continuously and insert an inductive reactance into the

line. TCSC is an effective and economical means of solving problems of transient

stability, dynamic stability, steady state stability and voltage stability in long

transmission lines. TCSC, the first generation of FACTS, can control the line

impedance through the introduction of a thyristor controlled capacitor in series

with the transmission line

3.6.4 Thyristor-Controlled Phase Shifter (TCPS)

In a TCPS control technique the phase shift angle is determined as a nonlinear

function of rotor angle and speed. However, in real-life power system with a large

number of generators, the rotor angle of a single generator measured with respect

to the system reference will not be very meaningful.

3.6.5 SECOND GENERATION OF FACTS

3.6. 6 Static Compensator (STATCOM)

The emergence of FACTS devices and in particular GTO thyristor-based

STATCOM has enabled such technology to be proposed as serious competitive

alternatives to conventional SVC [21] A static synchronous compensator

(STATCOM) 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, the most common use is for voltage

stability.

3.6.7 Static Synchronous Series Compensator (SSSC)

This device work the same way as the STATCOM. It has a voltage source

converter serially connected to a transmission line through a transformer. It is

necessary an energy source to provide a continuous voltage through a condenser

and to compensate the losses of the VSC. A SSSC is able to exchange active and

reactive power with the transmission system. But if our only aim is to balance the

reactive power, the energy source could be quite small. The injected voltage can

be controlled in phase and magnitude if we have an energy source that is big

enough for the purpose. With reactive power compensation only the voltage is

controllable, because the voltage vector forms 90º degrees with the line intensity.

In this case the serial injected voltage can delay or advanced the line current. This

means that the SSSC can be uniformly controlled in any value, in the VSC

working slot.

3.6.8 Unified Power Flow Controller (UPFC)

A unified power flow controller (UPFC) is the most promising device in the

FACTS concept. It has the ability to adjust the three control parameters, i.e. the

bus voltage, transmission line reactance, and phase angle between two buses,

either simultaneously or independently. A UPFC performs this through the

control of the in-phase voltage, quadrature voltage, and shunt compensation. The

UPFC is the most versatile and complex power electronic equipment that has

emerged for the control and optimization of power flow in electrical power

transmission systems. It offers major potential advantages for the static and

dynamic operation of transmission lines. The UPFC was devised for the real-time

control and dynamic compensation of ac transmission systems, providing

multifunctional flexibility required to solve many of the problems facing the

power industry. Within the framework of traditional power transmission concepts,

the UPFC is able to control, simultaneously or selectively, all the parameters

affecting power flow in the transmission line. Alternatively, it can independently

control both the real and reactive power flow in the line unlike all other

controllers.

Fig 3.3: Unified power flow controller

CONCLUSION

This paper has presented various FACTS controllers and analyzed their

control attributes and benefits. The flexible ac transmission system

(FACTS), a new technology based on power electronics, offers an

opportunity to enhance controllability, stability, and power transfer

capability of ac transmission systems. The application of FACTS

controllers throws up new challenges for power engineers, not only in

hardware implementation, but also in design of robust control systems,

planning and analysis.

REFERENCES

1) Y. N. Yu, Electric Power System Dynamics. Academic Press, 1983.

2) P. W. Sauer and M. A. Pai, Power System Dynamics and Stability. Prentice Hall 1998.

3) J. R. Smith, G. Andersson, and C. W. Taylor, “Annotated Bibliography on Power System Stability Controls: 1986- 1994”, IEEE Trans. on PWRS, 11(2)(1996), pp. 794

4) N. G. Hingorani and L. Gyugyi, Understanding FACTS: Concepts and Technology of Flexible AC Transmission Systems. New York: IEEE Press, 2000.

5) N. G. Hingorani, “FACTS-Flexible AC Transmission System”, Proceedings of 5th International Conference on AC and DC Power Transmission-IEE Conference Publication 345, 1991, pp. 1–7.

6) N. G. Hingorani, “Flexible AC Transmission”, IEEE Spectrum, April 1993, pp. 40– 45.

7) N. G. Hingorani, “High Power Electronics and Flexible AC Transmission System”, IEEE Power Engineering Review, July 1988

8) R. M. Mathur and R. S. Basati, Thyristor-Based FACTS Controllers for Electrical Transmission Systems. IEEE Press Series in Power Engineering, 2002.

9) Yong Hua Song and Allan T. Johns, Flexible AC Transmission Systems (FACTS). London, UK: IEE Press, 1999.