flexible ac transmission systems facts 9

7/29/2019 Flexible AC Transmission Systems FACTS 9

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Distributed Power and FACTS

Unified Power Flow Controller (UPFC)

Phasor analysis of general series compensation

The usual arrangement for series compensation is as depicted inFigure 1. The compensator applies a series voltage Vpq as depictedin Figure 1. The phasor diagram for this arrangement is given inFigure 2

Figure 1 Series compensator arrangement for a power

transmission system

Figure 2. Phasor diagram for the transmission system

depicted in Figure 5. with series compensation

VMVS

jX/2

VR

jX/2

VG Vpq

Pr+jQr

ImaginaryVS

jXI

I

/2

/2

VR

Real

= /2

VM

VG

0

Vcomp


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For the uncompensated case (Vcomp =0, =0) the power transmitted

can be given by:*

r rP jQjX

G r

r

V VV (1)

For the compensated case we have*

r r rP jQjX

G comp rV - V - VV (2)

which gives*

r rP jQjX jX

*r compG r

r

V VV - VV (3)

Using0

2j

compV e

compV as shown in Figure 2 then

0 0 0 0sin sin sinr comp r compr G

r

V V V V V VP P

X X X (4)

0 0 0 0cos 1 cos cosr comp r compr G

r

V V V V V VQ Q

X X X (5)

The uncompensated real and reactive power is given by P0 and Q0

The real and reactive power is therefore fully controllable in the VA

rangemaxr compV V

X . If we normalise such that2

/ 1V X

for thetransmission line then the uncompensated real and reactive powerare related by

2 2

0 0( ) 1 ( ) 1Q P (6)

which can be plotted as given in Figure 3. The controllable regionfor a UPFC for a given load angle 0 can be given as shown in

Figure 4. This can be compared with the controllable locus for aGCSC or SSSC. Notice that the GCSC controllable range is alsolimited be the line current. If there is no current then the GCSC

output can only be zero. Also shown is the controllable loci of aphase angle regulator which is normally realised by thyristorcontrolled phase shift transformer as shown in Figure 5.

A UPFC offers far more controllability than other devices and isusually a lot faster than thyristor controlled devices.


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0 0.5 1

2

1.5

1

0.5

Real P ower

ReactivePower

Figure 3 The relationship between real power and reactive

power for an uncompensated line


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0 0.2 0.4 0.6 0.8 1 1.2 1.4

1.4

1.2

1

0.8

0.6

0.4

0.2

Real P ower

ReactivePower

Figure 4 PQ relationship for received power on atransmission line showing also the controllable region of aUPFC and the loci of the controllable SSSC, GCSC and phaseangle regulator

UPFC

controllableregion

controllable

loci of phase

angleregulator

controllableloci of SSSC

controllable

loci of GCSC


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Figure 5 Thyristor controlled phase shift transformer.

Unified Power Flow Controller (UPFC)

A series compensator which can inject real and reactive power isknown as a Unified Power Flow Controller and the usualarrangement using 2 back to back convertors is as shown inFigure 6. Converter 2 can provide V

comp fully controlled in phase

angle and amplitude and is limited by the converter VA rating andthe VA rating of the series transformer. Converter 1 is to absorb orprovide real power for converter 2. Both converter 1 and converter2 can provide/control line reactive power independently

Figure 5 Realisation of a Unified Power Flow Controller usingtwo back to back voltage sourced converters..

Phase-a

Phase-b

Phase-c

c)

controller Voltagesourced

converter 1

settings

busbar

Step down

transformer

Vcomp

Voltage

sourced

converter 2

Series transformer

flexible ac transmission systems facts 9

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