44

CHAPTER 3

FLEXIBLE AC TRANSMISSION SYSTEM DEVICES

3.1 INTRODUCTION

Modern power system networks are very large with hundreds of

buses and mechanical controlling is not sufficient for them. There is a

widespread use of microelectronics, computers and high-speed

communications for control and protection of present day transmission

systems; however, when operating signals are sent to the power circuits,

where the final power control action is taken, the switching devices are

mechanical and there is little high-speed control. Another problem with

mechanical devices is that control cannot be initiated frequently, because

these mechanical devices tend to wear out very quickly compared to static

devices. In effect, from the point of view of both dynamic and steady-state

operation, the system is really uncontrolled. Power system planners,

operators, and engineers have learned to live with this limitation by using a

variety of ingenious techniques to make the system work effectively, but at a

price of providing greater operating margins and redundancies. These

represent an asset that can be effectively utilized with prudent use of FACTS

technology on a selective, as needed basis.

In recent years, greater demands have been placed on the

transmission network, and these demands will continue to increase because of

the increasing number of nonutility generators and heightened competition

among utilities themselves. Added to this is the problem that it is very

difficult to acquire new rights of way (ROW). Increased demands on

transmission system, absence of long-term planning, and the need to provide

45

open access to generating companies and customers, all together have created

tendencies toward less security and reduced quality of supply.

The FACTS technology is essential to alleviate some but not all of

these difficulties by enabling utilities to get the most service from their

transmission facilities and enhance grid reliability. It must be stressed,

however, that for many of the capacity expansion needs, building of new lines

or upgrading current and voltage capability of existing lines and corridors will

be necessary.

The FACTS is a concept based on power-electronic controllers,

which enhance the value of transmission networks by increasing the use of

their capacity. As these controllers operate very fast, they enlarge the safe

operating limits of a transmission system without risking stability. Needless to

say, the era of the FACTS was triggered by the development of new solid-

state electrical switching devices. Gradually, the use of the FACTS has given

rise to new controllable systems.

3.2 BASIC CONCEPT OF FACTS

Assuming the line to be lossless and ignoring the effect of line

charging, the real power flow (P) is given by,

P=V1V2

Xsin( 1 2) (3.1)

where, X is the series line reactance. Assuming V1 and V2 to be held constants

(through voltage regulators at the two ends), the power injected by the power

station determines the flow of power in the line. The difference in the bus

angles is automatically adjusted to enable P = PG (Note that usually there

could be more than one line transmitting power from a generating station to a

load centre).

46

We may like to control the power flow in an AC transmission line

to (a) enhance power transfer capacity and or (b) to change power flow under

dynamic conditions (subjected to disturbances such as sudden increase in

load, line trip or generator outage) to ensure system stability and security. The

stability can be affected by growing low frequency, power oscillations (due to

generator rotor swings), loss of synchronism and voltage collapse caused by

major disturbances. The maximum power (Pmax) transmitted over a line is

Pmax=V1V2

Xsin max (3.2)

where, max (30o - 40o) is selected depending on the stability margins and the

stiffness of the terminal buses to which the line is connected. For line lengths

exceeding a limit, Pmax is less than the thermal limit on the power transfer

determined by the current carrying capacity of the conductors (Note this is

also a function of the ambient temperature). As the line length increases, X

increases in a linear fashion and Pmax reduces as shown in Figure 3.1.

The series compensation using series connected capacitors

increases Pmax as the compensated value of the series reactance (XC) is given

by

Xc=X(1-kse) (3.3)

where, kse is the degree of series compensation. The maximum value of kse

that can be used depends on several factors including the resistance of the

conductors. Typically kse does not exceed 0.7.

47

Figure 3.1 Power transfer capacity as a function of line length

Fixed series capacitors have been used since a long time for

increasing power transfer in long lines. They are also most economical

solutions for this purpose. However, the control of series compensation using

thyristor switches has been introduced only 10-15 years ago for fast power

flow control. The use of Thyristor Controlled Reactors (TCR) in parallel with

fixed capacitors for the control of XC also helps in overcoming a major

problem of Sub Synchronous Resonance (SSR) that causes instability of

torsional modes when series compensated lines are used to transmit power

from turbo generators in steam power stations. In tie lines of short lengths, the

power flow can be controlled by introducing Phase Shifting Transformer

(PST) which has a complex turns ratio with magnitude of unity. The power

flow in a lossless transmission line with an ideal PST shown in Figure 3.2 is

given by

P= V1V2

Xsin( ± ) (3.4)

where, = 1 - 2.

PMAXStability Limit

Line Length

Thermal Limit

48

Figure 3.2 A lossless line with an ideal PST

Again, manually controlled PST is not fast enough under dynamic

conditions. Thyristor switches can ensure fast control over discrete (or

continuous) values of , depending on the configuration of PST used. Pmax

can also be increased by controlling (regulating) the receiving end voltage of

the AC line. When a generator supplies a unity power factor load, the

maximum power occurs when the load resistance is equal to the line

reactance. It is to be noted that V2 varies with the load and can be expressed

as

V2=V1 Cos( 1 2) (3.5)

Substituting Equation (3.5) in (3.1) gives

P=V1

2sin[2( 1 2)]2X

(3.6)

By providing dynamic reactive power support at bus (2) as shown in

Figure 3.3, it is possible to regulate the bus voltage magnitude. The reactive

power (QC) that has to be injected is given by

V1L 1 V2L 2

1 : e

jX

49

QC=V2

2-V1V2cos( 1 2)X

(3.7)

Figure 3.3 Transmission line compensated by controllable reactivepower source at receiving end

Comparing Equation (3.6) with (3.1), it can be seen that the

maximum power transfer can be doubled just by providing dynamic reactive

power support at the receiving end of the transmission line shown in

Figure 3.3. This is in addition to the voltage support at the sending end. It is to

be noted that while steady state voltage support can be provided by

mechanically switched capacitors, the dynamic voltage support requires

synchronous condenser or a power electronic controller such as SVC or Static

Synchronous Compensator (STATCOM).

3.3 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.

Load at unity

power factor

V1L 1 V2L 2

jX

QC

50

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.

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 Micro-

electronic 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.

51

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. FACTS

technology has also provided an impetus and excitement perceived by the

younger generation of engineers, who will rethink and re-engineer the future

power systems throughout the world.

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.

3.4 PARAMETERS CONTROLLED BY FACTS

CONTROLLERS

FACTS controllers are capable of controlling the following

parameters

Solve power transfer limit and stability problems

Thermal limit

Voltage limit

Stability limit

Transient stability limit

Small signal stability limit

Voltage stability limit

52

Increase power transfer capability of a line

Mitigate Sub Synchronous Resonance (SSR)

Power quality improvement

Load compensation

Limit short circuit current

Increase the loadability of the system

Rapid control of reactive power flow

3.5 BENEFITS OF FACTS CONTROLLERS

Providing greater flexibility

Control of power flow as ordered

Increase the voltage stability and enhance the static stability

Reduce real power loss and improve the voltage profile

Increase the utilization of lowest cost production

Reduce loop flows

Reduce reactive power flows

Provides secure tie line connections to neighboring utilities

Increase the loading capability of lines to their capabilities

including short term and seasonal

Improved steady state system performance

Reduced environment impacts

FACTS controller requires minimal maintenance

Reduced power system oscillation

53

Increase the system security

To provide controllable compensation to a power system in

order to increase the power transmission capability

It is possible to maintain constant power flow in a

transmission line

It is possible to vary the apparent impedance

3.6 POSSIBILITIES OF POWER FLOW CONTROL

Control of the line impedance X (e.g., with a thyristor-

controlled series capacitor) can provide a powerful means of

current control.

When the angle is not large, which is often the case, control of

X or the angle substantially provides the control of active

power.

Control of angle (with a Phase Angle Regulator, for example),

which in turn controls the driving voltage, provides a powerful

means of controlling the current flow and hence active power

flow when the angle is not large.

Injecting a voltage in series with the line, and perpendicular to

the current flow, can increase or decrease the magnitude of

current flow. Since the current flow lags the driving voltage

by 90 degrees, this means injection of reactive power in series,

(e.g., with static synchronous series compensation) can

provide a powerful means of controlling the line current, and

hence the active power when the angle is not large.

Injecting voltage in series with the line and with any phase

angle with respect to the driving voltage can control the

54

magnitude and the phase of the line current. This means that

injecting a voltage phasor with variable phase angle can

provide a powerful means of precisely controlling the active

and reactive power flow. This requires injection of both active

and reactive power in series.

Because the per unit line impedance is usually a small fraction

of the line voltage, the MVA rating of a series controller will

often be a small fraction of the throughput line MVA.

When the angle is not large, controlling the magnitude of one

or the other line voltages (e.g., with a thyristor-controlled

voltage regulator) can be a very cost-effective means for the

control of reactive power flow through the interconnection.

Combination of the line impedance control with a series

controller and voltage regulation with a shunt controller can

also provide a cost-effective means to control both the active

and reactive power flow between the two systems.

3.6.1 Objectives of Series Compensation

Series compensation is more effective in controlling the actual

transmitted power which, at a defined transmission voltage, is ultimately

determined by the series line impedance and the angle between the end

voltages of line.

It was always recognized that ac power transmission over long lines

was primarily limited by the series reactive impedance of the line. Series

capacitive compensation was introduced decades ago to cancel a portion of

the reactive line impedance and thereby increase the transmittable power.

Subsequently, within the FACTS initiative, it has been demonstrated that

55

variable series compensation is highly effective in both controlling power

flow in the line and in improving stability. Controllable series line

compensation is a cornerstone of FACTS technology. It can be applied to

achieve full utilization of transmission assets by controlling the power flow in

the lines, preventing loop flows and, with the use of fast controls, minimizing

the effect of system disturbances, thereby reducing traditional stability margin

requirements.

3.6.2 Objectives of Shunt Compensation

It has long been recognized that the steady-state transmittable

power can be increased and the voltage profile along the line be controlled by

appropriate reactive shunt compensation. The purpose of this reactive

compensation is to change the natural electrical characteristics of the

transmission line to make it more compatible with the prevailing load

demand. Thus, shunt connected, fixed or mechanically switched reactors are

applied to minimize line overvoltage under light load conditions, and shunt

connected, fixed or mechanically switched capacitors are applied to maintain

voltage levels under heavy load conditions.

The ultimate objective of applying reactive shunt compensation in a

transmission system is to increase the transmittable power. This may be

required to improve the steady-state transmission characteristics as well as the

stability of the system. Var compensation is thus used for voltage regulation

at the midpoint (or some intermediate) to segment the transmission line and at

the end of the (radial) line to prevent voltage instability, as well as for

dynamic voltage control to increase transient stability and damp power

oscillations.

56

3.7 TYPES OF FACTS CONTROLLERS

In general, FACTS controllers can be classified into four types

depending on the manner in which it is connected to the power system.

Series controllers

Shunt controllers

Combined series-series controllers

Combined series-shunt controllers

Depending on the power electronic devices used for the purpose of

controlling, the FACTS controllers can be classified as

Variable impedance type

Voltage Source Converter (VSC) based

The variable impedance type controllers include:

Static Var Compensator (SVC), (shunt connected)

Thyristor Controlled Series Capacitor or Compensator

(TCSC), (series connected)

Thyristor Controlled Phase Shifting Transformer (TCPST) of

Static PST (combined shunt and series)

The VSC based FACTS controllers are:

Static synchronous Compensator (STATCOM) (shunt connected)

57

Static Synchronous Series Compensator (SSSC) (series

connected)

Interline Power Flow Controller (IPFC) (combined series-series)

Unified Power Flow Controller (UPFC) (combined shunt-

series)

Some of the special purpose FACTS controllers are

Thyristor Controller Braking Resistor (TCBR)

Thyristor Controlled Voltage Limiter (TCVL)

Thyristor Controlled Voltage Regulator (TCVR)

Interphase Power controller (IPC)

NGH-SSR damping controller

3.7.1 Series Controllers

The series Controller shown in Figure 3.4 could be variable

impedance, such as capacitor, reactor, etc., or power electronics based variable

source of main frequency, sub synchronous and harmonic frequencies (or a

combination) to serve the desired need. In principle, all series controllers

inject voltage in series with the line. Even 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.

58

Figure 3.4 Series controller

3.7.2 Shunt Controllers

As in the case of series controllers, the shunt controller shown in

Figure 3.5 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 variable shunt impedance connected to the line

voltage causes a variable current flow and hence represents injection of

current into the line.

Figure 3.5 Shunt controller

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.

Line

i

Line

59

3.7.3 Combined Series-Series Controllers

This could be a combination of separate series controllers shown in

Figure 3.6, 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.

Figure 3.6 Combined series-series controllers

Note that the term "unified" here means that the dc terminals of all

controller converters are all connected together for real power transfer.

Line

DC power

link AC

line

s

60

3.7.4 Combined Series-Shunt Controllers

This could be a combination of separate shunt and series

controllers, which are controlled in a coordinated manner shown in

Figure 3.7, or a Unified Power Flow Controller with series and shunt

elements.

Figure 3.7 Combined series-shunt controllers

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.

3.8 APPLICATIONS OF FACTS DEVICES

The basic applications of FACTS-devices are:

Power flow control

i

Line

DC power

link

61

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

3.9 STATIC VAR COMPENSATOR

A Static Var Compensator is a shunt connected FACTS controller

for providing fast – acting reactive power on high voltage electricity

transmission networks. SVCs are part of the Flexible AC System device

family, regulating voltage and stabilizing the system.

The term ‘static’ refers to the fact that the SVC has no moving

parts, the SVC is an automated impedance matching device, designed to bring

the system closer to the unity power factor.

3.9.1 Definition

SVC means a shunt-connected static var generator or absorber

whose output is adjusted to exchange capacitive or inductive current so as to

62

maintain or control specific parameters of the electrical power system

(typically bus voltage).

3.9.2 Objectives of SVC

Increase power transfer in long lines

Improve stability with fast acting voltage regulation

Damp low frequency oscillations due to swing (rotor) modes

Damp sub-synchronous frequency oscillations due to torsional

modes

Control dynamic over voltages

3.9.3 Operating Principle

The SVC is an automated impedance matching device consists of a

group of shunt-connected capacitors and reactors banks with fast control

action by means of thyristor switching circuits designed to bring the system

closer to the unity power factor. From the operational point of view, the SVC

can be considered as a variable shunt reactance that adjusts automatically

according to the system operative conditions. Depending on the nature of the

equivalent SVC’s reactance, i.e., capacitive or inductive, the SVC draws

either capacitive or inductive current from the network. Suitable control of

this equivalent reactance allows voltage magnitude regulation at the SVC

point of connection. They also be placed near high and rapidly varying loads

where they can smooth flicker of voltage.

3.9.4 Analysis of SVC

The location of SVC is important in determining its effectiveness.

Ideally, it should be located at the electrical centre of the system or midpoint

of a transmission line. For example, consider a symmetric lossless

63

transmission line with SVC connected at the midpoint shown in Figure 3.8.

Without SVC, the voltage at the midpoint is given by,

Vmo=Vcos /2cos /2

(3.8)

Where, = l is the electrical length of the line, l is the length of the line and

is the phase constant given by

lc=2 f lc (3.9)

Where, l and c are positive sequence inductance and capacitance of the line

per unit length, f is the operating frequency.

Figure 3.8 A transmission line with SVC connected at midpoint

It can be shown that the voltage variation in the line (due to

variation in ) is maximum at the midpoint. SVC helps to limit the variation

by suitable control. The steady state control characteristics of SVC are shown

in Figure 3.9 where ADB is the control range. OA represents the

characteristic where the SVC hits the capacitor limit; BC represents the SVC

at its inductor limit. Note that SVC current is considered positive when SVC

susceptance is inductive. Thus

SVC

P P

ISVC

VL VmL /2 VL0

64

ISVC=-BSVCVSVC (3.10)

The slope of OA is BC (susceptance of the capacitor) and the slope

of OBC is BL (susceptance of the reactor). A positive slope (in the range of

1-5%) is given in the control range to

(a) enable parallel operation of more than one SVC connected at

the same or neighboring buses and

(b) prevent SVC hitting the limits frequently.

Figure 3.9 Control characteristics of SVC

The steady state value of the SVC bus voltage is determined from

the intersection of the system characteristic and the control characteristic

shown in Figure 3.10. The system characteristic is a straight line with

negative slope and is defined by

VSVC=VTH-XTHISVC (3.11)

A

C

D Vref

0

B

VSVC

ISVC

65

where, VTH and XTH are the Thevenin voltage and reactance viewed from the

SVC bus. For the system shown in Figure 3.8, we have

VTH=Vmo=Vcos( 2 )

cos( /2) (3.12)

VTH=Zn

2tan

2 (3.13)

Where, Zn is the surge impedance defined by

Zn=lc

(3.14)

3.9.5 Expression for Voltage and Power

Control Range: The SVC control range is described by

VSVC=Vref+XsISVC (3.15)

Figure 3.10 Determination of operating point of SVC

VSVCo

0

VSVC

ISVCo ISVC

System

Characteristics

66

where, Xs is the slope of the control characteristic. Vref is the SVC voltage

(corresponding to point D) when ISVC = 0.

Combining Equations (3.11) and (3.15), we get

VSVC=Vm=VTHXS

XS+XTH+

VrefXTH

XS+XTH (3.16)

The expression for power flow in the line is given by

P=VmVsin( /2)

Znsin( /2) (3.17)

With Vref = V, it can be shown that P is given by,

P=kP0+ 1-k P1 (3.18)

Here,

P0=V2sinZnsin

, P1=V2sin( /2)Znsin( /2)

(3.19)

and

k=Xs

XS+XTH (3.20)

Remarks

1. P0 is the power flow in the line without SVC and P1 is the

power flow in the line when SVC maintains a constant voltage

V at the midpoint (Xs = 0)

2. k 1 as Xs

67

3. For small values, it can be assumed that sin , sin /2 /2,

Cos /2 1.

In this case,

P0=V2

XLsin , P1=

2 V2

XLsin

2 (3.21)

Where, XL = ( l)d is the total reactance of the line (d is the length of the line).

At SVC limits: When the SVC hits the limit it can be represented as a fixed

susceptance (BSVC) where BSVC = BC at capacitive limit. At the inductive

limit, BSVC = - BL.

Substituting ISVC from Equation (3.10) in Equation (3.11), we get

VSVC=VTH

(1-XTHBSVC)=

Vcos /2(1-XTHBSVC)cos /2

(3.22)

The power flow in the line is given by

P=P0

(1-XTHBSVC)=

V2sinZn(1-XTHBSVC)sin

(3.23)

3.9.6 Configuration of SVC

The two most popular configuration of SVC are

1. Fixed Capacitor-Thyristor Controlled Reactor (FC-TCR)

2. Thyristor Switched Capacitor - Thyristor Controlled Reactor

(TSC- TCR)

68

The second type is more flexible than the first one and requires

smaller rating of the reactor and consequently generates fewer harmonic. The

schematic diagram of a TSC - TCR type SVC is shown in Figure 3.11.

Figure 3.11 A Typical SVC (TSC-TCR) configuration

This shows that the TCR and TSC are connected on the secondary

side of a step-down transformer. Tuned and high pass filters are also

connected in parallel which provide capacitive reactive power at fundamental

frequency. The voltage signal is taken from the high voltage SVC bus using a

potential transformer.

The TSC is switched in using two thyristor switches (connected

back to back) at the instant in a cycle when the voltage across valve is

minimum and positive. This results in minimum switching transients. In

steady state, TSC does not generate any harmonics. To switch off a TSC, the

gate pulses are blocked and the thyristors turns off when the current through

them fall below the holding currents. It is to be noted that several pairs of

thyristors are connected in series as the voltage rating of a thyristor is not

adequate for the voltage level required.

L LC C

Filter

SVC HV Bus

69

However, the voltage ratings of valves for a SVC are much less

than the voltage ratings of a HVDC valve as a step down transformer is used

in the case of SVC. To limit di/dt in a TSC it is necessary to provide a small

reactor in series with the capacitor.

3.9.7 Types of SVC Controllers

The following are the basic types of reactive power control

elements which make up all or part of any static Var system.

Saturated reactor (SR)

Thyristor controlled reactor (TCR)

Thyristor switched capacitor (TSC)

Thyristor controlled transformer (TCT)

Self or line commutated converter (SCC/LCC)

3.9.8 Types of SVC Models

Two models of SVC are usually implemented for load flow

analysis of a power system. They are,

3.9.8.1 Firing angle model

The equivalent susceptance, Beq which is function of a changing

firing angle, is made up of the parallel combination of Thyristor Controlled

Reactor (TCR) equivalent admittance and a fixed capacitive susceptance. This

model provides information on the SVC firing angle required to achieve a

given level of compensation.

70

3.9.8.2 Variable susceptance model

A changing susceptance BSVC represents the fundamental frequency

equivalent susceptance of all shunt modules making up the SVC. This model

is an improved version of SVC models.

3.9.9 Applications of SVC

The major application of SVC is for rapid voltage regulation

and control of dynamic (temporary) over voltages caused by

load throw off, faults or other transient disturbances. The

dynamic reactive control at the load bus increases power

transfer and can solve the problem of voltage instability

(collapse) caused by contingency conditions.

It is to be noted that steady state voltage regulation can be

achieved by mechanically switched capacitors and reactors.

However, fast voltage regulation is required to prevent

instability under transient conditions. Thus, generally, a SVC

is operated with minimum reactive power output under normal

conditions. This is achieved by the Susceptance Regulator

which ensures that full dynamic range is available for control

under contingency conditions.

The fast controllability provided by the thyristor switches can

be also utilized to improve system stability (both transient and

small signal). The use of auxiliary damping controllers can

help damp low frequency, inter area power oscillations that

can appear at stressed operating conditions (involving high

loading of tie lines).

71

The location of SVC is an important issue. If the objective is

to compensate a long transmission line, the SVC is to be

located at the midpoint of the line (if a single SVC is to be

used). For very long lines, multiple SVC at regular intervals

can be applied. For example, if two SVCs are to be used, one

is located at a distance d/3 from the sending end while the

other is located at a distance, d/3 from the receiving end (d is

the length of the line).

When SVCs are applied to improve the power transfer in a

transmission network, the location can be determined by the

sensitivity of voltage at the critical buses with respect to the

reactive power injection ( Vi/ Qj). In general, it can be stated

that a bus with low short circuit level can be a candidate bus.

Incidentally a synchronous condenser can raise the fault level

while providing controllable reactive power. It is to be noted

that a SVC does not raise the fault level which can be a

blessing as the requirement of the fault current interruption

capability of circuit breakers does not go up. On the other

hand, for reactive power control at HVDC converter stations

with low Short Circuit Ratios (SCR), the synchronous

condenser improves voltage regulation and system

performance.

However, a SVC has several advantages over synchronous

condenser namely,

(a) Faster response under transient conditions

72

(b) There are no moving parts, hence requires less maintenance

(c) There are no problems of loss of synchronism

(d) As mentioned earlier, a SVC does not contribute to short

circuit currents.

3.10 THYRISTOR CONTROLLED SERIES CAPACITOR

The basic Thyristor-Controlled Series Capacitor (TCSC) scheme

was proposed in 1986 by Vithayathil with others as a method of “rapid

adjustment of network impedance." It consists of the series compensating

capacitor shunted by a Thyristor-Controlled Reactor. In a practical TCSC

implementation, several such basic compensators may be connected in series

to obtain the desired voltage rating and operating characteristics.

3.10.1 Definition

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.

3.10.2 Operating Principle

A TCSC is a series-controlled capacitive reactance that can provide

continuous control of power on the ac line over a wide range. From the

system viewpoint, the principle of variable-series compensation is simply to

increase the fundamental-frequency voltage across a fixed capacitor (FC) in a

series compensated line through appropriate variation of the firing angle, .

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Figure 3.12 Basic thyristor-controlled series capacitor scheme

This enhanced voltage changes the effective value of the series-

capacitive reactance. A simple understanding of TCSC functioning can be

obtained by analyzing the behavior of a variable inductor connected in

parallel with an FC, as shown in Figure 3.12. The equivalent impedance, Zeq,

of this LC combination is expressed as

Zeq= j1C

||( L)=-j1

C- 1L

(3.24)

The impedance of the FC alone, however, is given by –j(1/ C). If

C – (1/ L) > 0, (1/ L) > 0 or, in other words, L > (1/ C), the reactance

of the FC is less than that of the parallel-connected variable reactor and that

this combination provides a variable-capacitive reactance. Moreover, this

inductor increases the equivalent-capacitive reactance of the LC combination

above that of the FC.

If C – (1/ L) = 0, a resonance develops that results in an infinite-

capacitive impedance - an obviously unacceptable condition. If however, C

– (1/ L) < 0, the LC combination provides inductance above the value of the

fixed inductor. This situation corresponds to the inductive-vernier mode of the

TCSC operation.

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In the variable-capacitance mode of the TCSC, as the inductive

reactance of the variable inductor is increased, the equivalent-capacitive

reactance is gradually decreased. The minimum equivalent-capacitive

reactance is obtained for extremely large inductive reactance or when the

variable inductor is open-circuited, in which the value is equal to the

reactance of the FC itself.

The behavior of the TCSC is similar to that of the parallel LC

combination. The difference is that the LC-combination analysis is based on

the presence of pure sinusoidal voltage and current in the circuit, whereas in

the TCSC, because of the voltage and current in the FC and TCR are not

sinusoidal because of thyristor switching.

3.10.3 Modes of TCSC Operation

There are essentially three modes of TCSC operation; these are

illustrated in Figure 3.13 and described as follows;

3.10.3.1 Bypassed-thyristor mode

In this bypassed mode, the thyristors are made to fully conduct with

a conduction angle of 1800. Gate pulses are applied as soon as the voltage

across the thyristors reaches zero and becomes positive, resulting in a

continuous sinusoidal of flow current through the thyristor valves. The TCSC

module behaves like a parallel capacitor–inductor combination. However, the

net current through the module is inductive, for the susceptance of the reactor

is chosen to be greater than that of the capacitor.

Also known as the TSR mode, the bypassed thyristor mode is

distinct from the bypassed-breaker mode, in which the circuit breaker provided

across the series capacitor is closed to remove the capacitor or the TCSC module

in the event of TCSC faults or transient over voltages across the TCSC.

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This mode is employed for control purposes and also for initiating

certain protective functions. Whenever a TCSC module is bypassed from the

violation of the current limit, a finite-time delay, Tdelay, must elapse before the

module can be reinserted after the line current falls below the specified limit.

(a) The bypassed-thyristor mode

(b) The blocked-thyristor mode

(c) The partially conducting thyristor (capacitive-vernier) mode

(d) The partially conducting thyristor (inductive-vernier) mode

Figure 3.13 Different operating modes of a TCSC

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3.10.3.2 Blocked-thyristor mode

In this mode, also known as the waiting mode, the firing pulses to

the thyristor valves are blocked. If the thyristors are conducting and a

blocking command is given, the thyristors turn off as soon as the current

through them reaches a zero crossing. The TCSC module is thus reduced to a

fixed-series capacitor, and the net TCSC reactance is capacitive. In this mode,

the dc-offset voltages of the capacitors are monitored and quickly discharged

using a dc-offset control without causing any harm to the transmission-system

transformers.

3.10.3.3 Partially Conducting Thyristor or Vernier Mode

This mode allows the TCSC to behave either as a continuously

controllable capacitive reactance or as a continuously controllable inductive

reactance. It is achieved by varying the thyristor-pair firing angle in an

appropriate range. However, a smooth transition from the capacitive to

inductive mode is not permitted because of the resonant region between the

two modes.

A variant of this mode is the capacitive-vernier-control mode, in

which the thyristors are fired when the capacitor voltage and capacitor current

have opposite polarity. This condition causes a TCR current that has a

direction opposite that of the capacitor current, thereby resulting in a loop-

current flow in the TCSC controller. The loop current increases the voltage

across the FC, effectively enhancing the equivalent-capacitive reactance and

the series-compensation level for the same value of line current. To preclude

resonance, the firing angle of the forward-facing thyristor, as measured

from the positive reaching a zero crossing of the capacitor voltage, is

constrained in the range min 1800. This constraint provides a continuous

vernier control of the TCSC module reactance. The loop current increases as

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is decreased from 1800 to min. The maximum TCSC reactance permissible

with = min is typically two-and-a-half to three times the capacitor reactance

at fundamental frequency.

Another variant is the inductive-vernier mode, in which the TCSC

can be operated by having a high level of thyristor conduction. In this mode,

the direction of the circulating current is reversed and the controller presents

net inductive impedance.

3.10.4 Equivalent Circuits for TCSC

(a) Basic equivalent circuit

(b) Capacitive operation

(c) Inductive operationFigure 3.14 Equivalent circuits for TCSC

IL

XTCR

XC

ITCR

IL

XTCR

XC

ITCR

IL

XTCR

XC

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Consider the equivalent circuit of the TCSC modelled as a

capacitor in parallel with a variable inductor shown in Figure 3.14(a). The

impedance of TCSC (ZTCSC) is given by

ZTCSC =-jXCjXTCR

jXTCR-XC=

-jXC

(1- XCXTCR

) (3.25)

The current through the TCR (ITCR) is given by

ITCR=-jXC

j(XTCR-XC)IL=

IL

(1- XTCRXC

) (3.26)

Since the losses are neglected, the impedance of TCSC is purely

reactive. The capacitive reactance of TCSC is obtained from 3.25 as

XTCSC=XC

(1- XCXTCR

) (3.27)

Note that XTCSC is capacitive as long as XC < XTCR. XTCR = when

the thyristors are blocked and ITCR = 0. For the condition when XC < XTCR,

ITCR is 1800 out of phase with the line current IL. In other words, IL is in phase

with – ITCR.

For the condition where XC > XTCR, the effective reactance of

TCSC (XTCSC) is negative implying that it behaves like an inductor. In this

case, IL and ITCR are in phase. The capacitive and the inductive operation of

TCSC are shown in Figure 3.14(b) and (c) respectively.

3.10.5 Control of TCSC

The control of TCSC also includes protective functions (protective

bypass). The control functions are partitioned into two levels - common

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(to all modules) and the module (level). Commands for the control flow from

the common level to the module levels while the status information is sent

back from each module level.

There are three basic functions at each module level. These are

(a) Reactance control

(b) SSR damping control (involving modulation of the reactance)

(c) Bypass (for protection)

The controller also ensures that the transients associated with mode

transitions are minimized. The module controller executes the ordered change

to reactance within one half cycles. This includes bypassing, reinsertion and

setting the vernier without overshoot. The common level receives signals of

line current and TCSC voltage to generate feedback signals for closed-loop

control functions. It also receives commands from energy management centre

for setting power order.

The major control functions in a TCSC are,

Power Scheduling Control

Power Swing Damping Control (PSDC)

Transient Stability Control (TSC)

Sub-synchronous Damping Control (SSDC)

3.10.6 Types of TCSC Models

A TCSC involves continuous-time dynamics, relating to voltages

and currents in the capacitor and reactor, and nonlinear, discrete switching

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behavior of thyristors. Deriving an appropriate model for such a controller is

an intricate task.

3.10.6.1 Variable-reactance model

A variable-reactance TCSC model for transient and oscillatory

stability studies used widely for its simplicity. In this quasi-static

approximation model, the TCSC dynamics during power-swing frequencies

are modeled by a variable reactance at fundamental frequency. It is assumed

that the transmission system operates in a sinusoidal steady state, with the

only dynamics associated with generators and PSS. This assumption is valid,

because the line dynamics are much faster than the generator dynamics in the

frequency range of 0.1–2 Hz that are associated with angular stability studies.

The variable-reactance TCSC model assumes the availability of a continuous-

reactance range and is therefore applicable for multi-module TCSC

configurations. This model is generally used for inter-area mode analysis, and

it provides high accuracy when the reactance-boost factor (= XTCSC/ XC) is

less than 1.5.

3.10.6.2 Transient-stability model

In the variable-reactance model for stability studies, a reference

value of TCSC reactance is generated from a power-scheduling controller

based on the power-flow specification in the transmission line. The reference

value may also be set directly by manual control in response to an order from

an energy-control center, and it essentially represents the initial operating

point of the TCSC; it does not include the reactance of FCs (if any). The

reference value is modified by an additional input, from a modulation

controller for such purposes as damping enhancement. Another input signal,

which is applied at the summing junction, is the open-loop auxiliary signal,

which can be obtained from an external power-flow controller.

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A desired magnitude of TCSC reactance is obtained that is

implemented after a finite delay caused by the firing controls and the natural

response of the TCSC. This delay is modeled by a lag circuit having a time

constant of typically 15–20 ms. The resulting XTCSC is added to the Xfixed,

which is the reactance of the TCSC installation’s FC component. To obtain

per-unit values, the TCSC reactance is divided by the TCSC base reactance,

Zbase, given as

Zbase=(kVTCSC)2

MVAsys (3.28)

where,

kVTCSC = the rms line–line voltage of the TCSC in kilovolts (kV)

MVAsys = the 3-phase MVA base of the power system

The TCSC model assigns a positive value to the capacitive

reactance, so Xtotal is multiplied by a negative sign to ensure consistency with

the convention used in load-flow and stability studies. The TCSC initial

operating point, Xref, for the stability studies is chosen as

Xref=Xtotal-Xfixed (3.29)

During periods of over current, only some TCSC modules move

into the bypassed mode, for the bypassing of a module causes the line current

to decrease and thus reduces the need for the remaining TCSC modules to go

into the bypass mode.

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3.10.6.3 Long-term-stability model

The capability curves of the TCSC depend on the duration for

which the voltage- and current-operating conditions persist on the TCSC. In

general, two time-limited regions of TCSC operation exist: the transient-

overload region, lasting 3–10 s, and the temporary-overload region, lasting 30

min; both are followed by the continuous region. For long-term dynamic

simulations, an overload-management function needs to be incorporated in the

control system.

This function keeps track of the TCSC variables and their duration

of application, and it also determines the appropriate TCSC overload range,

for which it modifies the Xmax limit and Xmin limit. It then applies the same

modifications to the controller. However, the model is used widely in

commercial stability programs because of its simplicity, and it is also used for

system-planning studies as well as for initial investigations of the effects of

the TCSC in damping-power oscillations.

3.10.6.4 Advanced transient-stability studies model

An alternate TCSC model for transient-stability studies has been

developed that effectively solve the differential equations pertaining to the

TCSC capacitor and the TCR. The TCSC model is invoked at every

half-cycle of the line current. A variable is used to store the instantaneous

capacitor voltage at the line zero crossing at the end of each half-cycle to be

used as the initial condition for the next sample process. The TCR is

represented by a current source updated by the fundamental component of

TCR current that the model calculates at each half-cycle. Also, the model

incorporates the effects of both thyristor firing and synchronization. The

triggering instant is a function of the signal that is used for synchronization,

such as the TCSC voltage or line current. The model is compatible with

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conventional transient-stability programs in that it updates the capacitor

voltage at every half-cycle while the stability program updates the line current

with the same frequency. It is also flexible enough to integrate not only

controls for minimizing the TCSC-response delay but higher-order controls as

well.

3.10.7 Advantages of TCSC

Use of thyristor control in series capacitors potentially offers the

following little-mentioned advantages:

Rapid, continuous control of the transmission line series-

compensation level.

Dynamic control of power flow in selected transmission lines

within the network to enable optimal power-flow conditions

and prevent the loop flow of power.

Damping of the power swings from local and inter-area

oscillations.

Suppression of sub-synchronous oscillations. At sub-

synchronous frequencies, the TCSC presents an inherently

resistive–inductive reactance. The sub-synchronous

oscillations cannot be sustained in this situation and

consequently get damped.

Decreasing dc-offset voltages. The dc-offset voltages,

invariably resulting from the insertion of series capacitors, can

be made to decay very quickly (within a few cycles) from the

firing control of the TCSC thyristors.

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Enhanced level of protection for series capacitors. A fast by-

pass of the series capacitors can be achieved through thyristor

control when large over voltages develops across capacitors

following faults. Likewise, the capacitors can be quickly

reinserted by thyristor action after fault clearing to aid in

system stabilization.

Voltage support. The TCSC, in conjunction with series

capacitors, can generate reactive power that increases with

line loading, thereby aiding the regulation of local network

voltages and, in addition, the alleviation of any voltage

instability.

Reduction of the short-circuit current. During events of high

short-circuit current, the TCSC can switch from the

controllable-capacitance to the controllable-inductance mode,

thereby restricting the short-circuit currents.

3.10.8 Applications of TCSC

The major objective in applying TCSC is to increase power

transfer capacity in critical transmission lines (typically tie

lines) under contingency conditions. Under normal steady

state conditions, series compensation using fixed capacitors

may be adequate unless SSR becomes a problem. A TCSC

may be used in such cases to damp (mitigate) SSR by

converting a part of the fixed compensation to controllable

series compensation.

Often, the contingency conditions are also accompanied by

low frequency oscillations that can threaten dynamic security.

85

Sometimes, transient stability may be affected. Thus, it

becomes necessary to provide Power Oscillation Damping

(POD) using appropriate control signals synthesized from

local measurements. Typically active power or line current has

been suggested as input to POD.

In the restructured electricity supply regime, the problem is to

in- crease the Available Transfer Capability (ATC) which is

defined as a measure of transfer capability for transfers of

power for further commercial activity, over and above already

committed uses. The optimal size of a FACTS controller such

as a TCSC is defined as that which results in the minimum

cost of enhancing ATC.

An important consideration in the application of TCSCs is

their location. Based on DC power flow analysis, a technique

is suggested that ranks the effectiveness of the location of a

TCSC. The norm of the sensitivity vector [ PL / Xj] when

the ith element of the vector is given by PLi/Xj, determines the

rank. Here, PLi is the change in the power flow in ith line

when there is a change in the reactance of line j. The sum

norm of a vector ‘v’ is defined by

Norm[v]= |v(i)|n

i=1

(3.30)

The line which results in maximum value of the norm is the

most effective location. It is to be noted that within a line, the

location of the controller is not significant. The ratings of the

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TCSC are determined from power flow, SSR and transient

stability studies.

3.11 PROPOSED APPROACH

In this work, the SVC and TCSC devices are considered

individually and in a combined manner. The combination is based on two

controllers having both series and shunt devices. The combination is the

variable susceptance model of SVC and variable reactance model of TCSC.

The improvement in voltage stability and active power loss minimization are

compared when different type of FACTS Controllers is used. The detailed

static models of SVC and TCSC are discussed in chapter 5.

3.12 CONCLUSION

This chapter gives an introduction and importance of the FACTS

devices with their classification. The power control concept, opportunities,

possibilities of power flow control and benefits of FACTS devices are also

discussed. The configuration and model of SVC and TCSC are discussed in

detail, which are considered in this research work.