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UNIT-II Shunt Compensation

STATCOM


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Switching converter type VAR Generators

Voltage source converters (or current source converters)

can produce reactive power without the use of reactive energy

storage components by circulating alternating current among

the phases of the ac system. (Similar to a synchronous m/c,

which is over-excited or under-excited to provide lagging orleading VAR to the system).

Hence the name STATIC CONDENSER (STATCON) or

STATIC COMPENSATOR (STATCOM)


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Reactive Power

Command

Three-phase

Transformer

Three-phase IGBT

Inverter

Firing Pulses

Control Circuit

Receiving end

Vr

I

Vs V

Sending end

Basic configuration of STATCOM for reactive compensation


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Simplified representation of STATCOM

SolidState DC-AC

Converter

System Busbar, B

VbIdc

Xe

Ve

I

Vdc

Devices used in the converter are self commutating type.(IGBT)

The capacitor is used as the DC source. It is charged from the line

itself.

A transformer is used to raise the voltage to the bus level


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The compensator supplies only the reactive power but not

active power.

Therefore there is no need for the DC source

As for as the compensator losses are concerned, they can be

drawn from the line itself


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Why is a capacitor used instead of DC source?

The converters produce the required reactive current by establishing

circulating currents among the phases.

These reactive currents produce the VAR required

The circulating currents are established by the switching ON and OFF of the

devices at appropriate instants.


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Principle of operation

Vb is constant

Ve is variable

Vb

and Ve

are in phase

Depending on the sign of (Vb-Ve) there

can be two modes

--Inductive operation

--Capacitive operation

Vb

Ve

C

transformer

converter

Line

jXVVI eb )(

I


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Inductive operation If Vb > Ve , and the Ve is in phase

with Vb then

and Ilags Vb

The converter draws current

from the line The current is in quadrature

with Vb (lag)

The VAR produced is

inductive in nature

Vb

Ve

C

transformer

converter

Line

I

I

Vb

jXeIVe

jX

VVI eb

)(


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Inductive operation

Phasor representation of

inductive VAR generation.

When Vb > Ve, the current is

inductive and the

compensator absorbs

inductive VAR (QL)

Vb

Ve

I

inductive

Vb

Ve

I


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Capacitive operation

If Ve> Vb , and the Ve is in

phase with Vb then

and Ileads Vb

The converter

supplies current to

the line

The current is inquadrature with Vb

The VAR produced is

capacitive in nature

Vb

Ve

I

VeVb

I

I

Vb

j XeI

Ve

jX

VVI eb

)(


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Capacitive operation

Phasor representation of

capacitive VAR generation

If Ve > Vb, the current is

capacitive and the VAR

generated is capacitive.(QC)

Vb

Ve

I

Vb

I

Ve


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I

Vb

jXeIVe

IVb j XeI

Ve

PRINCIPLE OF OPERATION - no loss in VSC

Capacitive operation without loss

Inductive operation without loss


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jXeI

Capacitive operation with loss component

Vb

Ve

I

Inductive operation with loss component

jXeI

Vb

VeI

PRINCIPLE OF OPERATION considering

loss in VSC


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Power exchange

Steady state power exchangeonly reactive power (ideal)

At AC terminal

Ve>Vb

SupliesQ

Ve


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V-I OPERATING AREA (ideal)

VV

MAX

ICMAXILMAXIC

IL


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Real appearance of STATCOM in

power system


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Typical STATCOM applications: Utilities with weak grid knots or fluctuating reactive loads.

Unbalanced loads

Arc furnaces

Wind farms Wood chippers

Welding operations

Car crushers & shredders

Industrial mills

Mining shovels & hoists Harbor cranes


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Basic 6-Pulse, 2-level, Voltage-Source Converter

cc

Ta2 a2D Tb2

a

b

ai a1T

ib

i

a1 b1D T

D Tb2 c2 Dc2

dc

2

V

b1 c1D T c1D

dc+Cs

V

dci

dcV

Hypotheticalneu tral po int

2

ab

bc

ca

a

b

c

[a ]

Vdc

Vdc

Vdc

Converter configurations for STATCOM


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2, 3, 5-level, VSC Waveforms

dc

dc

+

vdc2

vdc2

eout

vdc+

eutralid-) poi nt

dc+

v

eout- v

dc

+ vdc

+vdc

dc+

v

eutralid-) point

+vdc

dc+

v

1

2

eout

2 dcv

dcv

Converter configurations for STATCOM


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STATCOM- Internal Control approaches

By controlling the output voltage of the STATCOM , we cancontrol the reactive power flow.

To control the output voltage two methods are commonly used.

Direct control & Indirect control


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Direct control

Direct control The dc bus voltage is explicitly controlled and

is maintained constant and the ac output voltage is regulated

by employing pulse width modulation

Uses PWM techniques with an exclusive voltage control loopfor maintaining constant Vdc

Standard PWM techniques

Direct control scheme


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Indirect control

The magnitude of the ac output voltage is directly

proportional to the capacitor voltage.

Because of this proportionality, the reactive power or current

can be controlled indirectly via controlling the capacitor

voltage and thereby the ac output voltage.

Indirect control the dc bus voltage is varied by drawing

appropriate charging/ discharging current (by advancing/

delaying the phase of Ve wrt Vb). Uses square/ quasi-square

wave output (no PWM)

Indirect control scheme


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Indirect control scheme


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Indirect control- Output voltage control

of a two-level VSC

v

oFv (

o( )o( )

*=

=) V(+ sint)

V=oF( ) sin(+ )

=

*

v sinV t

t

=

=

dc

tvdc

dci

CC

t(v+v)dc

dc nominalv

dcv)(v-

tv

1dci dt

f Cdci

t (V=vo o )

0

io

Vv =


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Many applications call for a different VAR generation and absorption

range . This can be achieved by combining the converter with either

fixed and/or thyristor switched capacitor and/or reactors.

For fast dynamic response, STATCOM or thyristor switched capacitor/reactor or thyristor controlled reactors can be opted for whereas for

slower, long term steady state operation, manually switched units can

be utilised. Thus distribution of the net VAr burden is considered cost

effective.


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The converter based var generator can generate or absorb the

same amount of maximum reactive power ; in other words it hassame control range for capacitive and inductive var output.

V

ILIc IScmax ISLmax

a) STATCOM

VSYSTEM BUS

STATCOM


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b) STATCOM WITH FIXED CAPACITOR

SYSTEMBUS

V

COUPLING

TRANSFORMER

STATCOM

C

The combination of converter

based var generator with a fixedcapacitor can generate VARs in

excess of the rating of the

converter.

Shifting the operating range into

the capacitive region as illustratedby the associated V-I

Characteristics


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b) STATCOM WITH FIXED CAPACITOR..

V

ILIc IScmax ISLmax IcmaxIc

V

Vm

ax

=

V

IScmax+Icmax IScmax ISLmax-Icmax ISLmax

+


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The combination of converter based

var generator with a fixed reactor cangenerate VARs in excess of the rating

of the converter.

Shifting the operating range into the

inductive region as illustrated by the

associated V-I Characteristics as shown

STATCOM

Coupling Transformer

c) STATCOM WITH FIXED REACTOR


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V

ILIc IScmax ISLmax

+ILmaxIc

V

Vmax

=

V

IScmax-ILmaxIScmax ISLmax+ILmaxISLmax

c) STATCOM WITH FIXED REACTOR


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Thus by combining STATCOM with Fixed Capacitor or Fixed

Reactor we can shift the operating range of converter basedvar generator into more capacitive or more inductive region

without changing the amount of controllable MVARs .


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The combination of converter based var generatorwith a TCR and TSC is shown in figure.

By using the combination of converter based var

generator with a TCR and TSC we can increase the

total control range of VAR output.

The change in V-I Characteristics are clearly shownin the figure.

Coupling Transformer

STATCOM

ISTATCOM ITSC ITCR


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V

ILIc IScmax ISLmaxILmaxIc

V

Vm

ax

ICmax

V

IScmax+ICmax IScmax ISLmax+ILmaxISLmax

Vm

+

=


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Faster Response since the converter can immediately provide

capacitive output before the TSCs could be switched in.

Reduced harmonics generation and the possible elimination of

filters since the converter can be designed to have a very low

harmonics generation. (TCR is the harmonic source in the SVC ).

Greater flexibility to optimize for loss evaluation criteria since thesurplus or shortage var (generation/absorption) of the converter

can be supplied by the TCR /TSC.

The generalized hybrid var generator scheme employing a

switching converter with TSCs ,TCRs and possibly fixed or

mechanically switched capacitor provides optimum loss versus var

output characteristics, performance and cost.


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Extended V-I Characteristics with droop

Ic ILILmax0ICmax

Absorbing

(Lagging)Supplying(

Leading)

STATCOM

voltage

overload

overload

C

urrentlimit

Curre

ntlimit

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