Flexible AC Transmission Systems FACTS 8

7/29/2019 Flexible AC Transmission Systems FACTS 8

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Distributed Power and FACTS

Static Series Compensators: GCSC, TCSC and SSSC

Impedance compensation with a series connected device

Shunt compensation can provide a degree of power control,however, the power flow is ultimately limited by the impedance of the transmission line. Series compensation is far more effective atproviding control of the transmitted power. On long transmissionlines it is well established that improved power transfer can beaccomplished by compensating the transmission impedance withseries capacitors. The optimum location is in the centre of atransmission line but often this is inaccessible so a series capacitorat each line end or in each substation is employed as depicted in

Figure 1. For capacitors at each end typical values of 35% of theline inductances are used reducing the total line impedance to 30%of the line inductance. Series capacitors are popular for long lines of the order of 150 km particularly in the higher latitudes where theyreduce the effects of geomagnetic storms.

The transmission system will have a total reactance X T given by

(1 )T l c l

X X X k X (1)

and the power flow is then given by2 2

sin sin(1 )

T l

V V P

X k X

(2)

The phasor diagram for the arrangement given in figure 1a can begiven by Figure 2. From this it can be seen that the line current isgiven by

2sin

(1 ) 2l

V I

k X

(3)

and hence the reactive power provided by each series capacitor isgiven by

22

2(1 cos )

2 (1 )

cc

l

X V k Q I

X k

(4)

The relationship between the real and reactive power and load angleis shown in figure 3 for the uncompensated case and thecompensated case k=0.7.



Figure 1. Two popular arrangements for series capacitors

installed on long lines

Figure 2. Phasor diagram for the system given in figure 1a

a)

X c /2=35% X

X c /2=35% X

X l

X l /2 X l /2

X c=50% X l

b)

ImaginaryVS

jX l I

I

δ /2

δ /2

VR

Real

θ = π/2

VM

jX c /2 I

jX c /2 I



P uncompensated

P compensated

Q

Figure 3 Real and reactive power characteristics for thecompensated line impedance given in figure 1a comparedwith the uncompensated real power

As the series compensation reduces the line impedance it will

naturally lead to improved voltage stability (see figures 3 an 4 inOpportunities of Flexible AC Transmission Systems FACTS lecture).The improved electrical power flow on the transmission line will alsoimprove the power stability margin. Switching the seriescompensation can also provide power oscillation damping as shownin figure 5. However, the series capacitance can lead tosubsynchronous power oscillations due to resonance with the lineinductance that can damage large generators as the resonacefrequency is

1

1

2

c

res l

X

f LC f X

(5)



undamped

damped

An

gle

0

1

2

time

Q

compensato

r

undamped

damped

time

Power

Figure 5 Control of the series capacitance to reduce poweroscillations



GTO Thyristor-Controlled Series Capacitor (GCSC)

Full control of the series capacitance can be provided by the GTOThyristor-Controlled Series Capacitor arrangement shown infigure 6. This arrangement is comparable to the TCR. The effective

reactance is a function of turn-off delay angel γ given by

1 2 1( ) 1 sin(2 )

c X

C

(6)

But the harmonic voltages induced are given by

2

4 sin cos( ) cos sin( )( )

( 1)n

I n n nV

C n n

(7)

These characteristics can be seen in figure 7. Note that 3rd and 9th

harmonics are absent in three phase balanced connections (star ordelta) and a 12 puls arrangement eliminates 5th and 7th harmonics.

Figure 6 Fundamental element of GTO thyristor –controlledseries capacitor (GCSC).



0 20 40 60 80 100

0.05

0.1

0.15

3rd

5th

7th

9th

11th

13thFundamental/10

Delay angle (deg)

p u

Figure 7 Amplitude of the harmonic voltages drawn by aGCSC against GTO turn-off delay angle.

From figure 7 it can be seen that the relative current harmonics can

be quiet high and as the system impedance usually increases withfrequency (inductive) then the higher harmonics may createsignificant system voltage distortion. Filters can be added if necessary but this will increase the losses.

The fundamental Voltage produced by a GCSC is limited by themaximum GCSC impedance (γ=0) so that the V-I operating region is

as given in Figure 8.



Figure 8 Operating region for a GCSC

Another possible arrangement is a Thyristor-controlled seriescapacitor which is very similar to the FC-TCR but for a serieselement.

Switched converter type series compensators

A series compensator arrangement which can inject reactive poweris known as a static synchronous series compensator (SSSC) andthe usual arrangement capacitive sourced convertors is as shown inFigure 9.

Figure 9 Realisation of a static synchronous seriescompensator (SSSC) using a voltage sourced converter

V GCSC

I GCSC

X cmax

controller

settings

Vcomp

Voltage

sourced

converter 2

Series transformer



Converter 2 can provide Vcomp fully controlled in phase angle andamplitude and is limited by the converter VA rating and the VArating of the series transformer. This arrangement has is verycontrollable with good harmonic performance and rapid response.

The VI characteristics are then as given in figure 10.

Figure 10 Operating region for a SSSC

V SSSC

I SSSC

V max

I max

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Flexible AC Transmission Systems FACTS 8