Flexible AC Transmission Systems(FACTS)

Static Var Compensator

Dr. Avik Bhattacharya

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 Contents
• Fixed capacitor-Thyristor control reactor (FC-TCR) static Var
compensator
• Mechanical switched capacitor-Thyristor control reactor
(MSC-TCR) static Var compensator
• Thyristor switched capacitor-Thyristor control reactor (TSC-
TCR) static Var compensator

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 Introduction
• Static Var Compensator (SVC), one of most common FACTS device used for the
reactive power compensation.
• It is a variable impedance device where the current through a reactor is
controlled using back to back connected thyristor valves.
• The thyristor valves used in SVC are rated for lower voltages ( connected by
step down transformer or connected to the tertiary winding of a power
transformer).
• The application of SVC was initially for load compensation of fast changing
loads such as steel mills and arc furnaces.
• Now the objective is to provide dynamic reactive power injection and also
balance the currents on the source side whenever required.

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 Static Var compensator (SVC)
• Application of SVC for transmission line
1. Increase power transfer in long lines
2. Improve stability with fast acting voltage regulation
3. Damp low frequency oscillations due to swing (rotor) modes
4. Damp subsynchronous frequency oscillations due to torsional modes
5. Control dynamic over voltages
A SVC has no inertia compared to synchronous condensers and can be
extremely fast in response (2-3 cycles). This enables the fast control of
reactive power in the control range.

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 Static Var compensator (SVC)
• The location of SVC is important in determining its effectiveness.
• Ideally, it should be located at the electrical center of the system or
midpoint of a transmission line.
• Due to voltage variation in the line (due to variation in δ) is maximum at
the midpoint.

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 Static Var compensator (SVC)
• There are three types of SVC:
1. Fixed Capacitor-Thyristor Controlled Reactor (FC-TCR)
2. Mechanically switched capacitor-thyristor controlled reactor
(MSC-TCR)
3. Thyristor Switched Capacitor - Thyristor Controlled Reactor
(TSC-TCR).
• The third type is more common and flexible and it requires smaller
rating of the reactor and consequently generates less harmonics

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Fixed Capacitor-Thyristor Controlled Reactor (FC-TCR)
• The Var generator arrangement using a fixed
(permanently connected) capacitor with a
thyristor-controlled reactor (TCR).
• The current in the reactor is varied by the method
of firing delay angle control.
• The capacitor always injects generate the fixed
amount of reactive power.
• So TCR will absorb the excess reactive power
controlled manner by varying firing angle
• The trade of between the fixed capacitor and TCR
will provide smooth control of SVC
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 FC-TCR(Cont…)
• During light load condition the transmission system, it self have the
higher reactive power (leading PF).
• So to make unity p.f or better operation of SVC, the TCR rating
should be higher than fixed capacitor.
• Normally the capacitors are connected in star format.
• The fixed capacitors having a small inductance in series, hence it
acting as passive LC filter
• LC filter provide capacitive impedance at fundamental frequency
and generate reactive power in to the system

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 FC-TCR(Cont…)
• The LC filter provide lower impedance to selected harmonics
(dominant harmonics produced by the TCR) such as 5th ,7th ,11th ….
• Each capacitors tuned for different frequency.
• additional to this LC filter an LC high pass filter also connected with
the system as shunt format.
• The fixed capacitor, thyristor-controlled reactor type Var generator
may be considered essentially to consist of a variable reactor
(controlled by delay angle α) and a fixed capacitor

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 FC-TCR(Cont…)
• The constant capacitive Var generation (Qc)
of the fixed capacitor
• Variable Var absorption (QL) of the thyristor-
controlled reactor
• Total Var output (Q) (SVC Var generation ).
• At the maximum capacitive Var output, the
thyristor-controlled reactor is off (α=90
(w.r.t voltage peek ))
• To decrease the capacitive output, the
current in the reactor is increased by
decreasing delay angle α FC-TCR characteristics

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 FC-TCR(Cont…)
• At zero Var output, the capacitive and inductive
currents are equal (capacitive and inductive Var is
cancel out).
• To make net out put is inductive Var(inductive current
becomes larger than the capacitive current) by further
decrease of angle α (assuming that the rating of the
reactor is greater than that of the capacitor).
• At zero delay angle, the TCR conducts current over the
full 180 degree interval, resulting in maximum
inductive Var output
• It is equal to the difference between the Vars
generated by the capacitor and those absorbed reactor

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 FC-TCR(Cont…)
Control of the thyristor-controlled reactor in the FC-TCR type var generator
It have to provide four basic functions
1. Synchronous timing
2. Reactive current (or admittance) to firing angle conversion
3. Computation of the required fundamental reactor current
4. Thyristor firing pulse generation

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FC-TCR(Cont…)

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 FC-TCR(Cont…)
2. Reactive current (or admittance) to firing angle conversion (cont…)
• Several circuit are available literary for implementing,
• one of the method is analog function generator, It producing in each half-
cycle a scaled electrical signal that represent the lLF(α) versus α
relationship.
• Another is a digital "look-up table" for the normalized lLF(α) versus α
function which is read at regular intervals starting from α =0(peak of the
voltage) until the requested value is found, at which instant a firing pulse
is initiated.

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Reactive current (or admittance) to firing angle conversion by analog function generator

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 FC-TCR(Cont…)
3. Computation of the required fundamental reactor current (lLF(α))
• From the requested total output current IQ (sum of the fixed capacitor and the
TCR currents) defined by the amplitude reference input IQref to the var
generator control.
• This is simply done by subtracting the (scaled) amplitude of the capacitor
current Ic from IQref .
4. Thyristor firing pulse generation
• This is accomplished by the firing pulse generator (or gate drive) circuit which
produces the necessary gate current pulse for the thyristors to turn on in
response to the output signal provided by the reactive current to firing angle
converter.

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 FC-TCR(Cont…)

VLmax=Voltage limit for TCR

The V-l operating area of the FC-TCR Var generator

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 FC-TCR loss
• In practical application additional to dynamic performance, loss also have
the important
• In the FC-TCR type Var generator, there are three major elements of the
losses encountered
1. Capacitor (or capacitive filter) losses (these are relatively small but constant).
2. Reactor losses (these increase with the square of the current)
3. Thyristor losses (these increase almost linearly with the current).

• Thus, the total losses increase with increasing TCR current and
consequently, decrease with increasing capacitive Var output.

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 FC-TCR loss
• This type of loss characteristic is
advantageous when the average
capacitive var output is relatively
high as, for example, in industrial
applications requiring power factor
correction,
• it is disadvantageous when the
average var output is low, as for
example, in the case of dynamic
compensation of power
transmission systems

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Thank you

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