Series and

Shunt Compensation

Series Compensation

Series compensation is basically a powerful tool to improve the performance of EHV lines. It consists of capacitors connected in series with the line at suitable locations.

Advantages of Series Compensation

1. Increase in transmission capacity– The power transfer capacity of a line is given by

sin.

X

VEP

where, E is sending end voltage

V is receiving end voltage

X is reactance of lineδ is phase angle between E and V

• Power transfer without and with compensation:

KXXXX

X

P

P

XX

VEP

X

VEP

LCCL

L

CL

L

1

1

)/1(

1

)(

sin)(

.

sin.

1

2

2

1

where K is degree of compensation.

The economic degree of compensation lies in the range of 40-70%

(K < 1, i.e. 0.4-0.7)

2. Improvement of System Stability

• For same amount of power transfer and same value of E and V, the δ in the case of series compensated line is less than that of uncompensated line.

L

CL

CL

L

X

XX

XX

VEP

X

VEP

)(

sin

sin

sin)(

.

sin.

1

2

2

1

• A lower δ means better system stability• Series compensation offers most economic solution for system

stability as compared to other methods (reducing generator, x-mer reactance, bundled conductors, increase no. of parallel circuits

3. Load Division between Parallel Circuits

• When a system is to be strengthen by the addition of a new line or when one of the existing circuit is to be adjusted for parallel operation in order to achieve maximum power transfer or minimize losses, series compensation can be used.

• It is observed in Sweden that the cost of the series compensation in the 420 kV system was entirely recovered due to decrease in losses in the 220 kV system operating in parallel with the 420 kV system.

4. Less installation Time

• The installation time of the series capacitor is smaller (2 years approx.) as compared to installation time of the parallel circuit line (5 years approx.)

• This reduces the risk factor.• Hence used to hit the current thermal limit.• The life of x-mission line and capacitor is

generally 20-25 years.

Disadvantages

1. Increase in fault current

2. Mal operation of distance relay- if the degree of compensation and location is not proper.

3. High recovery voltage of lines- across the circuit breaker contacts and is harmful.

4. Problems of Ferro-resonance

• When a unloaded or lightly loaded transformer is energized through a series compensated line, Ferro-resonance may occur.

• It is produced due to resonance occurred in between the iron-created inductance (i.e., due to iron parts in the transformer) and in the reactance of the compensated line.

• This will cause a flow of high current.• It rarely happens and may be suppressed by using shunt

resistors across the capacitors or by short circuiting the capacitor temporarily through an isolator or by-pass breaker.

5. Problems due to sub-synchronous resonance

• The series capacitors introduces a sub-synchronous frequency (proportional to the square-root of the compensation) in the system. In some case this frequency may interact with weak steam turbine generator shaft and give rise to high torsional stress.

• In hydro-turbine generators, the risk of sub-synchronous resonance is small because the torsional frequency is about 10 Hz or even less.

Sub-Synchronous Resonance

• We know that, with the series compensation used, the power handling capacity of line is

)1()(

sin.

KXXXXX

VEP

LCL

• where, K = XC/XL is degree of compensation and reactances are at power frequency ‘f0’

Sub-Synchronous Resonance (continued …)

XC = K. XL

1/ ωC = K ωL (1)

As at certain resonant frequency (fr) it is possible that

LCf

CfLf

LfXCf

X

r

rr

rfr

Lr

frC

2

1

2

12

2;2

1

Replacing LC = 1/K(2πf)2 from (1)

KffK

fr

2

)2( 2 As K is 0.4-0.7; fr < fHence called sub-synchronous frequency

Location of Series Capacitor

• The choice of the location of the series capacitor depends on many technical and economical consideration.

• In each case, a special system study concerning load flow, stability, transient overvoltage, protection requirements, system voltage profile etc. is necessary before the optimal location is chosen.

1. Location along the line

• In this method the capacitor bank is located at the middle of the line (if one bank) or at 1/3rd distance along the line (if two banks).

• This has advantage of better voltage profile along the line, lesser short circuit current through the capacitor in the event of fault and simpler protection of capacitor.

• The capacitor stations are generally unattended.

2. Location at one or both ends of line section on the line side in the switching

station• The main advantage of this location is that the

capacitor installation is near the manned sub-stations.

• However, requires more advanced line protection.

• For the same degree of compensation, more MVAr capacity is needed as compared to method 1.

3. Location within bus bars within Switching Stations

Shunt Compensation

• For high voltage transmission line the line capacitance is high and plays a significant role in voltage conditions of the receiving end.

• When the line is loaded then the reactive power demand of the load is partially met by the reactive power generated by the line capacitance and the remaining reactive power demand is met by the reactive power flow through the line from sending end to the receiving end.

Shunt Compensation (continued…)

• When load is high (more than SIL) then a large reactive power flows from sending end to the receiving end resulting in large voltage drop in the line.

• To improve the voltage at the receiving end shunt capacitors may be connected at the receiving end to generate and feed the reactive power to the load so that reactive power flow through the line and consequently the voltage drop in the line is reduced.

Shunt Compensation (continued…)

• To control the receiving end voltage a bank of capacitors (large number of capacitors connected in parallel) is installed at the receiving end and suitable number of capacitors are switched in during high load condition depending upon the load demand.

• Thus the capacitors provide leading VAr to partially meet reactive power demand of the load to control the voltage.

Shunt Compensation (continued…)

• If XC = 1/ωC be the reactance of the shunt capacitor then the reactive power generated of leading VAr supplied by the capacitor:

CVX

VQ

CC 2

2

2

2

• where, |V2| is the magnitude of receiving end voltage.

Shunt Compensation (continued…)

• When load is small (less than SIL) then the load reactive power demand may even be lesser than the reactive power generated by the line capacitor. Under these conditions the reactive power flow through the line becomes negative, i.e., the reactive power flows from receiving end to sending end, and the receiving end voltage is higher than sending end voltage (Ferranti effect).

• To control the voltage at the receiving end it is necessary to absorb or sink reactive power. This is achieved by connecting shunt reactors at the receiving end.

Shunt Compensation (continued…)

• If XL = ωL be the reactance of the shunt reactor (inductor) then the reactive VAr absorbed by the shunt rector:

LVX

VQ

LL /2

2

2

2

• where, |V2| is the magnitude of receiving end voltage.

Shunt Compensation (continued…)

• To control the receiving end voltage generally one shunt rector is installed and switched in during the light load condition.

• To meet the variable reactive power demands requisite number of shunt capacitors are switched in, in addition to the shunt reactor, which results in adjustable reactive power absorption by the combination.

Degree of series compensation

We know that the surge impedance

LC xxCj

Lj

C

LZ

Suppose Cse is the series capacitance per unit length for series compensation. Therefore total series reactance will be

seL

cse

se

sese

LjX

XLj

LCLj

Lj

Lj

C

jLj

C

jLjLj

111

1

.'

2

where γse is known as degree of series compensation. Therefore, virtual surge impedance

)1()1('

seCse

C ZCj

LjZ

Degree of shunt compensation

We know that the surge impedance

LLC xxCj

Lj

C

LZ

Suppose shunt inductance Lsh per unit length is used for shunt compensation. Therefore the net shunt susceptance will be

shLsh

c

sh

shsh

CjX

XCj

CLCj

C

C

L

jCj

LjCjCj

111

1

.1

'

2

where γsh is known as degree of shunt compensation. Therefore, virtual surge impedance

)1()1('

sh

C

shC

Z

Cj

LjZ

• Considering both series and shunt compensation simultaneously:

sh

seC Zc

Cj

LjZ

1

1

'

''

• Therefore, the virtual surge impedance loading

se

shC PcP

1

1'

• It is clear that a fixed degree of series compensation and capacitive shunt compensation decreases the virtual surge impedance of line.

• However, inductive shunt compensation increases the virtual surge impedance and decreases the virtual surge impedance loading of line. If inductive shunt comp. is 100%, the virtual surge impedance becomes infinite and loading zero.

• Suppose, we want flat voltage profile corresponding to 1.2 PC without series compensation, the shunt capacitance compensation required will be:

pu

PP

PcP

sh

shCC

shC

44.0

12.1

1'

• Now, assuming shunt compensation to be zero, the series compensation required corresponding to 1.2 PC :

pu

PP

PcP

se

seCC

seC

306.0

1/2.1

1/'

• However, because of lumped nature of series capacitor, voltage control using series capacitors is not recommended.

• Normally used for improving stability limits of the system.

Active Compensation

• Synchronous condensers are the active shunt compensators and have been used to improve the voltage profile and system stability.

• When machine is overexcited, it acts as shunt capacitor as it supplies lagging VAr to the system and when under excited it acts as a shunt coil as it absorbs reactive power to maintain terminal voltage.

• The synchronous condenser provides continuous (step less) adjustment of the reactive power in both under excited and overexcited mode.

Flexible AC Transmission System (FACTS)

• Using high speed thyristors for switching in or out transmission line components such as capacitors, reactors or phase shifting transformer for desirable performance of the systems.

• Power transfer between two systems interconnected through a tie-line is given as

sin.

X

VEP

• The FACTS devices can be used to control one or more of voltages at the two ends, the reactance of the tie-line and the difference of the voltage angles at the two ends.

FACTS Devices

• The various devices used are – Static VAr compensator (SVC)– Static Condensors (STATCON)– Advanced Thyristor Controlled Series

Compensation (ATCSC)– Thyristor Controlled Phase Shifting Transformer

Active Compensation using SVC

Static VAr Compensators (without rotating part)• An static VAr system consists of two elements in parallel (a

rector and a bank of capacitors).• Used for surge impedance compensation and for

compensation by sectioning a long transmission line.• Also for load compensation to maintain constant voltage

for – Slow variation of Load– Load rejection, outage of generator/line– Under rapid variation of Load

• Improves system pf and stability.

Static VAr Compensators (continued…)

• An ideal static reactive power compensator must be capable of step-less adjustment of reactive power over an unlimited range (lagging and leading) without any time delay.

• Some important compensators used in transmission and distribution networks are:– Thyristor controlled reactor (TCR)– Thyristor switched capacitor (TSC)– Saturated rectors (SR)

Common feature in Static compensators

• A fixed capacitor in parallel with controlled susceptance. The fixed capacitors are usually tuned with small reactors to harmonic frequencies to absorb harmonics generated by controlled susceptance.

Thyristor Controlled Reactor (TCR)

The controlled element is the reactor and the controlling element is the thyristor controller consisting of two opposite poled thyristors which conduct every half cycle of the supply frequency.

Currently available thyristors can block voltage upto 4000-9000 V and conduct current upto 3000-6000 A.

Practically 10-20 thyristors are connected in series to meet the required blocking voltage .

sinsin)(

cos)(

tL

Vti

tVtv

mL

m

• The current in the reactor can be controlled by the method of firing delay angle control. The closure of the thyristor valve is delayed wrt the peak of the applied voltage in each half-cycle. Let the firing delay angle is α, applied voltage is v.

• The amplitude ILF(α) of the fundamental reactor current iLF(α) can be expressed as:

2sin

121)(

L

VI m

LF

• The admittance as a function of angle α, can be written directly from the current equation.

2sin

121

1)(

LBL

• Evidently, the admittance BL(α) varies with α in the same manner as the fundamental current ILF(α).

• If the switching is restricted to a fixed delay angle, usually =0, then it becomes thyristor-switched reactor (TSR). The TSR provides fixed inductive admittance.

• As the SCR’s are fired then the distortion in the sine-wave is observed with the production of odd-harmonics.

• Arranging the TCR and coupling X’mer secondary in delta cancels the third harmonics and its multiple. But 5th, 7th, … harmonics are still present.

• Small reactors are usually included in the fixed capacitor branches, which tunes with these branches as filters for 5th and 7th harmonics.

Operating V-I area of the TCR (a) and of the TSR (b).

Thyristor Switched Capacitors (TSC)

• In this scheme TSC’s are used with TCR’s.

• The TCR’s and capacitance changed in discrete steps. The susceptance is adjusted by controlling the no. of parallel capacitors.

• The capacitors serve as filters for harmonics when only the reactor is switched.

• Advantage: Dynamic stability is better

• Disadvantages: more no. of SCRs, more cost

Basic TSC (a) and associated waveforms (b)

• Normally a relatively small surge current limiting reactor is used in series with the TSC branch. This is needed primarily to limit the surge current in the thyristor valve under abnormal condition (switching at wrong time).

• Transient free switching:

‘switching in’

• Case 1: vC <= V

– at vC =v or vsw = 0 (dv/dt should be 0) and

• Case 2: vC > V

– α = 0 and vsw = min.

‘switching out’ at i = 0.

Transient free switching

Transient free switching of TSC with different residual voltages

Operating V-I area of single TSC

TCR-FC

• The TCR-FC system provides continuously controllable lagging to leading VArs through thyristor control of reactor current.

• Leading VArs are supplied by two or more fixed capacitor banks. The TCR is generally rated larger than the total of fixed capacitance so that net lagging VArs can also be supplied.

• The variation of current through the reactor is obtained by phase angle control of back to back pair of thyristors connected in series with the reactor.

Basic TCR-FC and its VAr demand vs VAr output characteristics

Operating V-I area of TCR-FC

TSC-TCR

Basic TSC-TCR type static var generator and its VAr demand vs VAr output characteristic.

Operating V-I area of the TSC-TCR type VAr generator with two thyristor-switched capacitor banks

Mechanically Switched Capacitors (MSC)

• In this scheme MSC’s are also used with TCR’s.

• Uses conventional mechanical or SF6 switches instead of thyristors to switch the capacitors.

• More economical when there are a large no. of capacitors to be switched than using TSCs.

• The speed of switching is however longer and this may affect transient stability.

• This method is suitable for steady load conditions, where the reactive power requirements are predictable

Saturated Reactors (SR) Scheme

• In some schemes for compensation saturated reactors are used.

• Three-phase saturated reactor having a short circuited delta winding which eliminates third harmonic currents from the primary.

• Fixed capacitors are provided as usual.• A slope-correction capacitor is usually connected

in series with the saturated reactor to alter the B-H characteristics and hence the reactance.

• A three-phase saturated reactor having a short circuited delta winding which eliminates third harmonic currents from the primary winding.

• The SR compensator is maintenance free, it has no control flexibility and it may require costly damping circuits to avoid any possibility of sub harmonic instability.

• Has the overload capability which is useful in limiting overvoltage.

Static Condenser (STATCON) or Static Compensator (STATCOM)

The basic elements of a Voltage Source Inverter (VSI) based STATCON are an inverter, a DC capacitor and a transformer to match the line voltage When inverter fundamental output voltage is higher than the system line voltage the STATCON works as a capacitor and reactive VAr is generated.However, when the inverter voltage is lower than the system line voltage, the STATCON acts as an inductor thereby absorbing the reactive VArs from the system.

STATCON is a GTO (Gate Turn off) based compensation system.

For purely reactive power flow, the three-phase induced electromotive forces (EMFs), ea, eb and ec of the synchronous rotating machine are in phase with the system voltages, va, vb, and vc. The reactive current I drawn by the synchronous compensator is determined by the magnitude of the system voltage V, that of the internal voltage E, and the total circuit reactance (synchronous machine reactance plus transformer leakage reactance plus system short circuit reactance) X:

21

VX

VE

Q

X

EVI

By controlling the excitation of the machine, and hence the amplitude E of its internal voltage relative to the amplitude V of the system voltage, the reactive power flow can be controlled.

• From a DC input voltage source, provided by the charged capacitor CS, the converter produces a set of controllable three-phase output voltages with the frequency of the ac power system. Each output voltage is in phase with, and coupled to the corresponding ac system voltage via a relatively small (0.1-0.15 p.u.) tie reactance (which in practice is provided by the per phase leakage inductance of the coupling transformer).

• By varying the amplitude of the output voltages produced, the reactive power exchange between the converter and the ac system can be controlled in a manner similar to that of the rotating synchronous machine.

• That is, if the amplitude of the output voltage is increased above that of the ac system voltage, then the current flows through the tie reactance from the converter to the ac system, and the converter generates reactive (capacitive) power for the ac system.

• If the amplitude of the output voltage is decreased below that of the ac system, then the reactive current flows from the ac system to the converter, and the converter absorbs reactive (inductive) power. If the amplitude of the output voltage is equal to that of the ac system voltage, the reactive power exchange is zero.

• Hence, also known as Static Synchronous Generator (SSG).

• The main difference between the SVC and STATCON is that in case of SVC the current injected into the system depends upon the system voltage, but in case of STATCON it is independent of system voltage.

• STATCON generate or absorb reactive power without the use of capacitor or reactors.

• The STATCON current I is made perpendicular to the system voltage V. The STATCON coordinators adjust the phase of I so that it leads or lags wrt to V.

Advantages:• The steady state load ability of the line is improved.• The voltage rises due to capacitor switching is substantially reduced

both in magnitude and duration.• Voltage variation due to customer’s loading is reduced.

STATCON is more expensive than switched capacitors or static VAr compensation on a per unit steady-state MVA basis, however, the performance of the STATCON outweights the increase in cost.

GTO Thyristor-Controlled Series Capacitor (GCSC)

It consists of a fixed capacitor in parallel with a GTO thyristor (or equivalent) valve (or switch) that has the capability to turn on and off upon command.

Fig. (a) Basic GTO-Controlled Series Capacitor, (b) principle of turn-off delay angle control, and (c) attainable compensating voltage waveform

• The objective of the GCSC scheme is to control the ac voltage vc across the capacitor at a given line current i. Evidently, when the GTO valve, sw, is closed, the voltage across the capacitor is zero, and when the valve is open, it is maximum. For controlling the capacitor voltage, the closing and opening of the valve is carried out in each half-cycle in synchronism with the ac system frequency.

• The GTO valve is stipulated to close (through appropriate control action) whenever the capacitor voltage crosses zero. (Recall that the thyristor valve of the TCR opens whenever the current crosses zero.)

• When the valve sw is opened at the crest of the (constant) line current (γ = 0), the resultant capacitor voltage vc will be the same as that obtained in steady state with a permanently open switch. When the opening of the valve is delayed by the angle γ with respect to the crest of the line current, the capacitor voltage can be expressed with a defined line current, i(t) = I cos ωt, as follows:

sinsin

1)(

1)( t

Cdtti

Ctv

t

C

The amplitude of fundamental capacitor voltage can be expressed as a function of γ

)2sin12

1()(

C

IVCF

where γ is the amplitude of the line current, C is the capacitance of the GTO thyristor controlled capacitor, and ω is the angular frequency of the ac system.

Fundamental component of the series capacitor voltage vs. the turn-off delay angle γ.

This impedance can be written as )2sin12

1(1

)(

C

X C

In a practical application the GCSC can be operated either to control the compensating voltage, VCF(γ), or the compensating reactance, XC(γ). In the voltage compensation mode, the GCSC is to maintain the rated compensating voltage in face of decreasing line current over a defined interval Imin<= I <=Imax as illustrated in Figure (a1).In this compensation mode the capacitive reactance XC, is selected so as to produce the rated compensating voltage with I= Imin, i.e., VCmax = XC Imin. As current Imin is increased toward Imax, the turn-off delay angle γ is increased to reduce the duration of the capacitor injection and thereby maintain the compensating voltage with increasing line current.

• In the impedance compensation mode, the GCSC is to maintain the maximum rated compensating reactance at any line current up to the rated maximum. In this compensation mode the capacitive impedance is chosen so as to provide the maximum series compensation at rated current, XC = Vcmax/Imax, that the GCSC can vary in the 0 <= XC(γ) <= XC range by controlling the effective capacitor voltage VCF(γ), i.e., XC(γ) = VCF(γ)/I.

Thyristor-Switched Series Capacitor (TSSC)

• The operating principle: the degree of series compensation is controlled in a step-like manner by increasing or decreasing the number of series capacitors inserted. A capacitor is inserted by turning off, and it is bypassed by turning on the corresponding thyristor valve.

• A thyristor valve commutates "naturally," that is, it turns off when the current crosses zero. Thus a capacitor can be inserted into the line by the thyristor valve only at the zero crossings of the line current.

• Since the insertion takes place at line current zero, a full half-cycle of the line current will charge the capacitor from zero to maximum and the successive, opposite polarity half-cycle of the line current will discharge it from this maximum to zero.

• As can be seen, the capacitor insertion at line current zero, necessitated by the switching limitation of the thyristor valve, results in a dc offset voltage which is equal to the amplitude of the ac capacitor voltage. In order to minimize the initial surge current in the valve, and the corresponding circuit transient, the thyristor valve should be turned on for bypass only when the capacitor voltage is zero. With the prevailing dc offset, this requirement can cause a delay of up to one full cycle, which would set the theoretical limit for the attainable response time of the TSSC.

Thyristor-Controlled Series Capacitor (TCSC)

It consists of the series compensating capacitor shunted by a TCR. In a practical TCSC implementation, several such basic compensators may be connected in series to obtain the desired voltage rating and operating characteristics. This arrangement is similar in structure to the TSSC and, if the impedance of the reactor, X1, is sufficiently smaller than that of the capacitor, XC, it can be operated in an on/off manner like the TSSC. However, the basic idea behind the TCSC scheme is to provide a continuously variable capacitor by means of partially canceling the effective compensating capacitance by the TCR.

sin2)(

)(

)()(

LL

CL

LCTCSC

XX

XX

XXX

Thyristor-Controlled Phase Shifting Transformer (TCPST)

In general, phase shifting is obtained by adding a perpendicular voltage vector in series with a phase. This vector is derived from the other two phases via shunt connected transformers. The perpendicular series voltage is made variable with a variety of power electronics topologies. A circuit concept that can handle voltage reversal can provide phase shift in either direction. This Controller is also referred to as Thyristor-Controlled Phase Angle Regulator (TCPAR).