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Methods of voltage control

Control of voltage levels is accomplished by controlling the production, observationand flow of reactive power at all level in the system.

Automatic voltage regulators (ACR) control field excitation to maintain a scheduledvoltage level at the terminals of generators. Additional means are usually required tocontrolled voltage throughout the system.

The devices used for this purpose may be classified as Sources or sinks of reactive power, such as shunt capacitor, shunt reactors,

synchronous condensers and static var compensator (SVC).

Line reactance compensators, such as series capacitor. Regulating transformer such as step changing transformer and boosters.

Shunt capacitor compensation:-

Without compensation:

AC

VR

QR

VS

QS

T.LQ1=demand of the load

Q2=supply to load

Figure 9.1

Net reactive power at the bus bar. Reactive power demand by load. Reactive power supply to load.

Voltage at receiving end

Case 1:- If In this case

Case 2:- If In this case

Now to balance;

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Shunt capacitor bank is connected, which supply

With shunt capacitor []:

C.B

QCQ2

Q1

Load

P1

VR=VS

Figure 9.2

[ ]

Where;

Active power input to the load

Where;

Power factor angle before compensation.

Power factor angle after compensation. Reactive power supply by capacitor bank.Shunt reactor compensation:

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C.B

Lsh

QLQ2

Q1

VR

QR

Figure 9.3

This means the reactive power consumed by load is very less and hence there is over voltage

at the receiving end.

Now to keep a shunt reactor (Lsh) is connected in parallel to load which absorb orconsume reactive power as

Q2

Q1

QLS2

S1

P1=P2

Figure 9.4

Therefore,

[ ]Therefore, []

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We know

From equation 1 and 2, we get [ ]

[ ]Relation between Q,V and f :

Therefore

Series capacitor [Cse] coponsation:

AC

LOAD

cosR

I

VRVS

Figure 9.5

Without Cse

[ ]With Cse

| | [ ]

Voltage regulation is minimum with series capacitor composition but it is normallypreferred for improving the transient stability.

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In case of fault condition it discharge the energy store in capacitor so that high voltageis developed which damage the insulation hence practically not preferred.

Tap changing transformer (OLTC):

V1

4

3

2

1

4

3

2

1

R JXL

VS

VR

V2

tr:11:ts

Sending end X-merReceiving end x-mer

HV/LVHV/LV

LOAD

Figure 9.6

[ ]

Now

Therefore

By changing the transformer number of turns the voltage is controlled. The tapes are normally provided on high voltage side because current is minimum on

H.V side. During tap changing arc is developed, arc intensity is lower if current is

low.

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Synchronous condensers:

A synchronous condenser is a synchronous machine running without a prime mover or a

mechanical load. By controlling the field excitation, it can be made to either generate or

absorb reactive power. With a voltage regulator, it can automatically adjust reactive power

output to maintain constant terminal voltage. It draws a small amount of active power from

the power system to supply losses.

Synchronous condensers have been used since the 1930s for voltage and reactive power

control at both transmission and sub transmission levels. They are often connected to the

tertiary winding of the transformers. They fall into category of active compensators.

Because of their high purchase and operating costs, they have been largely suppressed by

static var compensators.

Synchronous condensers have several advantages over static compensators. Synchronouscompensators contribute to system short circuit capacity. Their reactive power production is

not affected by system voltage. During power swing (electromechanical oscillations) there is

an exchange of kinetic energy between a synchronous condenser and the power system.

During such power swings, a synchronous condenser can supply a large amount of shunt

compensation, perhaps twice its continuous rating. It has about 10 to 20% overload capability

for up to. 30 minutes. Unlike other forms of shunt compensation, it has an internal voltage

source and is better able to cope with low system voltage conditions.

Static Var Compensators

According to the IEEE terms and definition:

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 electrical

power system (typically bus voltage)

The term static is used to indicate that SVCs, unlike Synchronous compensators, have no

rotating or moving components. Thus an SVC consists of static var generator (SVG) or

absorber devices and a suitable control device.

Types of SVC

The following are the 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-switched reactor (TSR) Thyristor-controlled transformer (TCT) Self- or line-commutated converter (SCC/LCC)

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We consider an SVS composed of a controllable reactor and a fixes capacitor. The resulting

characteristics are sufficiently general and are applicable to a wide range of practical SVS

configurations.

The Composite characteristic is derived by adding the individual characteristics of the

components. The characteristics shown in figure () is representative of the characteristics ofpractical controllable reactors.

Figure (9.9) Composite characteristics of an SVS

Power system characteristics

The power system V/I characteristics may be determined by considering the Thevenin

equivalent circuit as viewed from the bus whose voltage is to be regulated by the SVS. The

Thevenin impedance in figure () is predominantly an inductive reactance. The correspondingvoltage versus reactive current characteristics is shown in figure (). The voltage increaseslinearly with capacitive load current and decreases linearly with inductive load current.For each network condition, an equivalent circuit such as that in shown in figure () can be

defined.

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Figure (9.10) Power system voltage versus reactive current characteristic

Composite SVSpower system characteristics:

The system characteristics may be expressed as

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The SVS characteristic, with in the control range defined by the slope reactance , is givenby

For further outside the control range, the ratio

is equal to the slopes of the two extreme

segments of figure. These are determined by the rating of the inductor and capacitor.

Three system characteristics are considered in the figure (), corresponding to three values of

the source voltage. The middle characteristic represents nominal system conditions, and is

assumed to intersect the SVS characteristics at point A where and .

Figure (9.11) Graphical solution of SVS operating point for given system conditions

If the system voltage increases by (for example, due to a decreases in system loadlevel), will increases to , without an SVS. With the SVS, however, the operating pointmoves to B; by absorbing inductive current , the SVS holds the voltage at . Similarly, ifthe source voltage decreases (due to increases in system load level), the SVS hold the voltage

at , instead of at without the SVS. If the slope of the SVS characteristics were zero,the voltage would have been held at for both cases considered above.Thyristor-controlled reactor (TCR)

According to IEEE definition

A shunt connected, thyristor-controlled inductor, whose effective reactance is varied to

provide a rapidly variable phase angle.

A TCR is one of the most important building blocks of thyristor-based SVCs. Although it can

be used alone, it is more often employed in conjunction with fixed or thyristor-switched

capacitors to provide rapid, continuous control of reactive power over the entire selected

lagging-to-leading range.

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The Single-Phase TCR

A basic single-phase TCR comprises an anti-parallelconnected pair of thyristor valves, T1

and T2, in series with a linear air-core reactor, as illustrated in Fig. (5.12). The anti-parallel

connected thyristor pair acts like a bidirectional switch, with thyristor valveT1 conducting in

positive half-cycles and thyristor valve T2 conducting in negative half-cycles of the supplyvoltage. The firing angle of the thyristors is measured from the zero crossing of the voltage

appearing across its terminals.

Principle of operation:

The controllable range of the TCR firing angle, , extends from to . A firing angleof results in full thyristor conduction with a continuous sinusoidal current flow in theTCR. As the firing angle is varied from to close to the current flows in the form ofdiscontinuous pulses symmetrically located in the positive and negative half-cycles, as

displayed in Fig.(5.13). Once the thyristor valves are fired, the cessation of current occurs at

its natural zero crossing, a process known as the line commutation. The current reduces tozero for a firing angle of . Thyristor firing at angles below introduces dccomponents in the current, disturbing the symmetrical operation of the two anti-parallel valve

branches.

Figure (9.12) A TCR

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( ) The variation of per-unit value of with firing angle is depicted in Fig. (9.14). The per-unit value of

is obtained with respect to its maximum value

as the base quantity.

Figure (9.14) Control Characteristics of the TCR susceptance, .The TCR thus acts like a variable susceptance. Variation of the firing angle changes the

susceptance and, consequently, the fundamental-current component, which leads to a

variation of reactive power absorbed by the reactor because the applied ac voltage is constant.

However, as the firing angle is increased beyond , the current becomes non-sinusoidal,and harmonics are generated. If the two thyristors are fired symmetrically in the positive and

negative half-cycles, then only odd-order harmonics are produced.

Operating Characteristics of TCR

(a) Operating Characteristics without Voltage Control

The simplest SVC configuration consists of a TCR connected to the power system as shown

in Fig.(). In the analysis of compensator performance, the fundamental frequency behavior is

generally considered. In practice, harmonics are filtered and reduced to very low values. The

approach shown in Fig () is convenient for the performance analysis because the whole TCR

branch is replaced by an equivalent continuously variable reactor.

Figure (9.15) A simple SVC circuit using a TCR

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For a general SVC, which can be considered as a black box with an unknown but purely

reactive circuit inside, the overall compensator susceptance can be defined with thefollowing equation:

In the simple case of a TCR, the compensator susceptance is

Usually, three kinds of characteristics are of interest while analyzing an SVC, as described in

the paragraphs that follow.

VoltageCurrent Characteristic or Operating Characteristic:

This shows the SVC current as a function of the system voltage for different firing angles, as

depicted in Fig. (9.16).This V-I characteristic is given in a very general sense. No control

system is assumed to vary the firing angle, and any operating point within the two limits ispossible depending on the system voltage and the setting of the firing angle (other currents

and voltages may be shown, too). This characteristic clearly illustrates the limits of the

operating range, and it may include the steady-state characteristics of the various possible

controls.

Figure (9.16): VoltageCurrent Characteristic or Operating Characteristic

SVC TCR Susceptance Characteristics:

These illustrate the change of the total SVC susceptance when the TCR susceptance is varied,

as shown in Fig. (9.17). The susceptance characteristic for this case is very simple

because . Note that the TCR susceptance is negative, indicating that the TCR isan absorbing reactive component. These characteristics are of most interest to control-system

analysis because the controls affect the TCR firing angle, whereas the total susceptance

influences the power system.

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Figure (9.17): SVC TCR Susceptance Characteristics

(b) Operating Characteristic with Voltage Control

The operating range of Fig () can be reduced to a single characteristic of operating points if

the effect of the voltage control is incorporated. Let us assume that the compensator is

equipped with the voltage control shown in Fig. ().The system voltage is measured, and the

feedback system varies to maintain on the system.This control action is represented in the operating characteristic in Figure () by the horizontal

branch marked as control range. This characteristic shows the hard-voltage control of the

compensator, which stabilizes the system voltage exactly to the set point

.

Figure (9.18): The operating characteristics of a TCR with voltage control: (a) an SVC

control system and (b) the V/I characteristic.

Two system characteristicssystem 1and system2are depicted in Fig. () that illustrate the

decline in system node voltage when the node is loaded inductively and reactive power is

absorbed. The corresponding operating points for the two system conditions are .

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If the system voltage of system 2 raises, a new characteristicsystem 2results. Operating

point A then moves to the right and reaches the absorption limit of the compensator. Any

further increase in system voltage cannot be compensated for by the control system, because

the TCR reactor is already fully conducting.

The operating point will, therefore, move upward on the characteristic, corresponding tothe fully on reactor connected to the system ( ). The compensator then operates in theoverload range, beyond which a current limit is imposed by the firing control to prevent

damage to the Thyristor valve from an over current. At the left-hand side, the compensator

will reach the production limit if the system voltage drops excessively; the operating point

will then lie on the characteristic of the under voltage range.

The Fixed-Capacitor-Thyristor-Controlled Reactor (FC-TCR):

Configuration:

The TCR provides continuously controllable reactive power only in the lagging power-factor

range. To extend the dynamic controllable range to the leading power factor domain, a fixed-

capacitor bank is connected in shunt with the TCR. The TCR MVA is rated larger than the

fixed capacitor to compensate the capacitive MVA and provide net inductive-reactive power

should a lagging power factor operation be desired. The fixed capacitor banks, usually

connected in a star configuration, are split into more than three phase groups. Each capacitor

contains a small tuning inductor that is connected in series and tunes the branch to act as a

filter for a specific harmonic order. For instance, one capacitor group is connected to 5 th

harmonic and another to 7th, whereas yet another is designed to act as a high pass filter. At

fundamental frequency, the tuning reactors slightly reduce the net MVA rating of the fixed

capacitors.

Figure (9.19) FC-TCR SVC

Operating Characteristics:

Without the step-down transformer:

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The fixed capacitor extends the operating control range of SVC to the leading side as

compared to the characteristics of TCR with voltage control. The SVC current can beexpressed as a function of system voltage and compensator susceptance as

Where, and Figure shows the operating characteristics and the susceptance characteristics of an FC-TCR

without a coupling transformer and both also show that var production as well as var

absorption is possible.

Figure (9.20) The operating characteristics of an FCTCR without a coupling transformer

With the step-down transformer:

An FC-TCR SVC is usually connected to the high voltage power system by means of a step

down coupling transformer as shown in figure.

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( )

(

)

The susceptance characteristic is shown in figure. Both the exact characteristic from Eq. ( ) and thelinearized characteristic from Eq. ( ) are displayed. The errors from linearization are clearly visible.

Figure (9.22) Susceptance characteristics of an FC-TCR SVC with a step down transformer

Losses:A drawback of the FCTCR SVC is the circulation of large currents in the FCTCR loop

needed for cancellation of capacitive vars. This results in high steady-state losses, even when

the SVC is not exchanging any reactive power with the power system, as shown in Figure.Typical losses in an FCTCR scheme vary from 0.5% to 0.7% of the MVA rating. However,

these losses can be minimized by switching the fixed capacitors through mechanical breakers,

ensuring that the capacitors are inserted in the compensator circuit only when leading vars are

needed. Thus a smaller-sizeinterpolating TCR can be used, and consequently, the steady-

state operating losses can be reduced. Those FCTCRs having a 300-MVA inductive rating

have been already installed in the field.

Figure (9.23) Losses in an FC-TCR

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The Thyri stor-Switched Capacitor (TSC).

A single-phase thyristor switched capacitor (TSC) is shown in Figure 5.13(a). It consists of a

capacitor, a bidirectional thyristor valve, and a relatively small surge current limiting reactor.

This

reactor is needed primarily to limit the surge current in the thyristor valve under abnormaloperating conditions (e.g., control malfunction causing capacitor switching at a "wrong time,"

when transient free switching conditions are not satisfied); it may also be used to avoid

resonances with the ac system impedance at particular frequencies.

Under steady-state conditions, when the thyristor valve is closed and the TSC branch is

connected to a sinusoidal ac voltage source, v = V sin on, the current in the branch is given by

Where

VSW

VL

Vc

i

Figure 9.24. Basic thyristor-switched capacitor (a) and associated waveforms (b).

The amplitude of the voltage across the capacitor is

The TSC branch can be disconnected ("switched out") at any current zero by prior removal of

the gate drive to the thyristor valve. At the current zero crossing, the capacitor voltage is at its

peak value, . The disconnected capacitor stays charged to this voltageand, consequently, the voltage across the non-conducting thyristor valve varies between zero

and the peak-to-peak value of the applied ac voltage, as illustrated in Figure 5.13(b).If thevoltage across the disconnected capacitor remained unchanged, the TSC bank could be

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switched in again, without any transient, at the appropriate peak of the applied ac voltage, as

illustrated for a positively and negatively charged capacitor in Figure 2(a) and (b),

respectively. Normally, the capacitor bank is discharged after disconnection. Thus, the

reconnection of the capacitor may have to be executed at some residual capacitor voltage

between zero and Vn 2/(n 2 - 1). This can be accomplished with the minimum possible

transient disturbance if the thyristor valve is turned on at those instants at which the capacitorresidual voltage and the applied ac voltage are equal, that is, when the voltage across the

thyristor valve is zero. Figure 5.15(a) and (b) illustrate the switching transients obtained with

a fully and a partially discharged capacitor. These transients are caused by the nonzero dv /dt

at the instant of switching, which, without the series reactor, would result in an instantaneous

current of ic = Cdv/dt in the capacitor. (This current represents the instantaneous value of the

steady- state capacitor current at the time of the switching.) The interaction between the

capacitor and the current (and di/dt) limiting reactor, with the damping resistor, produces the

oscillatory transients visible on the current and voltage waveforms. (Note that the switching

transient is greater for the fully discharged than for the partially discharged capacitor because

the dv/dt of the applied (sinusoidal) voltage has its maximum at the zero crossing point.)

Figure 9.25 Waveforms illustrating transient free switching by a thyristor switched capacitor

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Figure 9.26 Waveforms illustrating transient with thyristor switched capacitor fully (a) or

partially discharged (b)

The conditions for "transient-free" switching of a capacitor are summarized in Figure 3. As

seen, two simple rules cover all possible cases: (1) if the residual capacitor voltage is lower

than the peak ac voltage (Vc < V), then the correct instant of switching is when theinstantaneous ac voltage becomes equal to the capacitor voltage; and (2) if the residual

capacitor voltage is equal to or higher than the peak ac voltage (VC V), then the correct

switching is at the peak of the ac voltage at which the thyristor valve voltage is minimum.

From the above, it follows that the maximum possible delay in switching in a capacitor bank

is one full cycle of the applied ac voltage, that is, the interval from one positive (negative)

peak to the next positive (negative) peak. It also follows that firing delay angle control is not

applicable to capacitors; the capacitor switching must take place at that specific instant in

each cycle at which the conditions for minimum transients are satisfied, that is, when the

voltage across the thyristor valve is zero or minimum. For this reason, a TSC branch can

provide only a step-like change in the reactive current it draws (maximum or zero). In otherwords, the TSC branch represents a single capacitive admittance which is either connected to,

or disconnected from the ac system. The current in the TSC branch varies linearly with the

applied voltage according to the admittance of the capacitor as illustrated by the V-Iplot in

Figure 5.17. The maximum applicable voltage and the corresponding current are limited by

the ratings of the TSC components (capacitor and thyristor valve).

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Figure 9.27 Conditions for transient-free switching for the thyrister-switched capacitor with

different residual voltages.

To approximate continuous current variation, several TSC branches in parallel (which would

increase in a step-like manner the capacitive admittance) may be employed, or, as is

explained later, the TSC branches have to be complemented with a TCR.

BC

VCmax

IC ICmax

V

CmaxI = current limit

VCmax= voltage limit

CB = admittance of capacitor

Figure 9.28 Operating V-I area of single TSC

Operating Characteristics:

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Controller

Dead band DV

Vref

V

VISVC

C1 C C2 3 . . . Cn Figure 9.29 TSC scheme

TheV/I characteristics of a TSC compensator is shown in Figure . We see that the voltage control

provided is discontinuous or stepwise. It is determined by the rating and number of parallel connected

units. In high voltage application, the number of shunt capacitor banks are limited because high cost

of thyristors. The power system V/Icharacteristics, as system condition change, intersect the TSC V/I

characteristics at discrete points. The bus voltage Vis controlled within the range , whereDV is the dead band. When the system is operating so that its characteristics is represented by line,then capacitor will be switched and operating point A prevails. If the system characteristicssuddenly changes to S2, the bus voltage drops initially to a value represented by operating point B.

The TSC control switches in bank C2 to change the operating point to C, bringing the voltage within

the desired range. Thus the compensator current can change in discrete steps. The time taken forexecuting a command from the controller ranges one half cycle to one cycle.

Thyristor-Switched Capacitor, Thyristor-Controlled Reactor Type Var Generator.

The thyristor-switched capacitor, thyristor-controlled reactor (TSC-TCR) type compensator was developedprimarily for dynamic compensation of power transmission systems with the intention of minimizingstandby losses and providing increased operating flexibility.

A basic single-phase TSC-TCR arrangement is shown in Figure 5.22(a). For a given capacitive output

range, it typically consists of n TSC branches and one TCR. The number of branches, n, is determined by

practical considerations that include the operating voltage level, maximum var output, current rating of thethyristor valves, bus work and installation cost, etc. Of course, the inductive range also can be expanded to

any maximum rating by employing additional TCR branches.

The operation of the basic TSC-TCR var generator shown in Figure 5.22(a) can be described as follows:

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Figure 9.30 Basic TSC-TCR type static var generator

The total capacitive output range is divided into n intervals. In the first interval, the output of the var

generator is controllable in the zero to range, where is the total rating provided by all TSCbranches. In this interval, one capacitor bank is switched in (by firing, for example, thyristor valve,)and, simultaneously,the current in the TCR is set by the appropriate firing delay angle so that the sum of the var output of theTSC (negative) and that of the TCR (positive) equals the capacitive output required.

In the second, third, and nth intervals, the output is controllable in the

,to

,

to ,. . . , and to range by switching in the second, third, ..., and nthcapacitor bank and using the TCR to absorb the surplus capacitive vars.By being able to switch the capacitor banks in and out within one cycle of the applied ac voltage, the

maximum surplus capacitive var in the total output range can be restricted to that produced by one

capacitor bank, and thus, theoretically, the TCR should have the same var rating as the TSC. However, toensure that the switching conditions at the endpoints of the intervals are not indeterminate, the var rating of

the TCR has to be somewhat larger in practice than that of one TSC in order to provide enough overlap

(hysteresis) between the "switching in" and "switching out" var levels.

In a way, this scheme could be considered as a special fixed capacitor, thyristor controlled reactorarrangement, in which the rating of the reactor is kept relatively small (

times the maximum capacitive

output), and the rating of the capacitor is changed in discrete steps so as to keep the operation of the TCRwithin its normal control range.

Susceptance Characteristic

The SVC susceptance in the TSCTCR scheme is as follows:

Where 1, 2. . . . is the number of TSC branches in operation and is the totalsusceptance of n TSC branches. With a linear approximation, Eq. () reduces to

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( ) ( )

Figure 9.32 The Susuptance characteristics of the TSC-TCR SVC.

Figure gives the total susceptance BSVC as a function of the susceptanceof the controlled

reactor BTCR for the example data. These characteristics are of importance for control

design, for the controls varyBTCR and the effect on the system is caused by.