Static Var Compensator (SVC)

Shunt connected variable capacitor and reactor

components

Controlled by power electronics

Number of possible configurations, typical

Thyristor controlled reactor (TCR)

Thyristor -switched capacitor (TSC)

1

SVC

2

TCR

Allows continuous control of the reactive power absorbed

Thyristors in anti parallel connection act as a bidirectional switch

Delay firing of thyristors to reduce time that current flows through inductor

Thereby reducing fundamental magnitude of current and reducing reactive power absorbed

3

Firing Signals

Firing Angle ( ), Conduction angle ( )

Firing Angle is the delay time

Conduction angle is the time conducting

90 <

Example

A 3 phase FC -TCR with L=37.887mH, C=133.7uF is required to absorb reactive power in the range Q to +Q kVAr corresponding to conduction angles of 0 & 180 degrees respectively, when connected to a 690V 3 phase supply.

Find Q

Estimate conduction angle to give zero reactive power

SVC Firing Circuitry

Recall operation of SVC

Timing of firing angle determines magnitude of fundamental current by pulsing thyristors at appropriate time

( )Lr

SinB

X

7

TCR Firing Circuitry

A phase - locked loop or phase lock loop (PLL) generates a signal that has a fixed relation to the phase of a "reference" signal

In this case a sawtooth signal is generated of set frequency

DC comparison voltage adjusted up and down to give required positive crossover point i.e. required firing angle

9

TCR Firing Circuitry

Total Harmonic Distortion

THD is a measure of the harmonic content of a waveform. It can be calculated for V or I.

It is a measure of the effective value of the harmonic components of a distorted wave.

max

2

2

1

h

h

h

M

THDM

RMS

The rms value of the distorted waveform is not the sum of the individual harmonics but the square root of the sum of the squares

Relationship between RMS and THD is,

max

2

2

h

h

h

RMS M

2

1 1RMS M THD

Power and Power Factor

Harmonic distortion complicates the computation of power and power factor

Apparent Power (S) is product of RMS voltage and current

P is average rate of delivery of energy

Q is component of S in quadrature with P

Power and Power Factor

At fundamental freq.

If just the fundamental is considered then P has its usual form

P S Cos

Q S Sin

1 1 1

1 is displacement power factor (DPF)

P V I Cos

Cos

Power and Power Factor

If large current distortion will be small

=> PF small

1 1 1 1

1

V I Cos IPPF DPF

S V I I

1I

I

THD Example

n=1 n=3 n=5

| Vh| 0.707 0.212 0.071

| I h| 0.707 0.141 0.035

Cos 0.866 0.342 0.999

Sin 0.5 0.94 0.052

|P h| 0.433 0.01 0.0025

| Qh| 0.25 0.0282 0.0001

| Sh| 0.5 0.03 0.0025

( ) 0.3 (3 10 ) 0.1 (5 13 )

( ) ( 30 ) 0.2 (3 60 ) 0.05 (5 10 )

v t Sin t Sin t Sin t

i t Sin t Sin t Sin t

THD Example

2

0

0

1sin

1(1 2 )

1

2

V d

V Cos d

V

SVC

Filters for 5 th and 7 th and a HPF for higher order harmonics

Tripplen harmonics circulate in delta only under balanced conditions

During major disturbances, may get asymmetrical firing of thyristors

Give rise to DC components

Can saturate trafo and lead to further harmonics

Sometimes a 3 rd harmonic filter is used

Thyristor Switched Capacitor (TSC)

Allows control of supply of vars in discrete steps

Use a number of parallel connected capacitors

Each has thyristor pair bi -directional switch

Controller selects how many capacitors to switch in

21

TSC

Susceptance is adjusted by controlling # of capacitors (0,1,2....k)

Stepwise changes in susceptance

Could binary weight the capacitor to increase the number of steps for a given #

Usually all but one have susceptance B, the last one has B/2 giving 2k levels