Static VAR compensator:
The static VAR
compensator is also known as SVC. The SVC is a shunt compensator FACTS
device. The SVC is a generator or absorber which is used to exchange the
capacitive and inductive current from the power system. By this way, it can
control and maintain a specific parameter
of the system. The SVCs can draw reactive power from the line and improve
stability, control over voltage, reduce voltage flickering and regulate
voltage. It has capacitor bank fixed or controlled (switched) or fixed
capacitor bank and switched reactor bank in parallel. The thyristors are used to
control and switching (on & off) of capacitor and inductor. So, it is also
known as static VAR switches because it uses switching for VAR control.
For step-wise control
of the capacitor and reactor, thyristors in anti-parallel connections are used. In
this devices capacitor and reactor unit act as continuously variable in power
system, this can be done by controlling of firing angle of thyristors.
There are four
schemes for static compensation;
1) Saturated reactor
2) Thyristor controlled reactor
(TCR)
3) Thyristor-switched capacitor (TSC)
4) Combined TCR and TSC
compensator
SVC is placed to
solve below problems in power system;
* Voltage
flickering caused by varying high reactance load like arc furnace
* Transient
stability limit
* Power transfer
capacity of the transmission line
* Temporary over-voltage in the transmission system
* Regulate the
voltage
* To improve
power quality
The SVCs are placed near high
and rapidly varying load in industries like arc furnace.
1) Saturated reactor:
The saturated reactor is a
constant voltage reactive source. This is not very flexible with respect to the
operating characteristic. This is nothing but a multi-core
reactor and the phase winding is arranged to cancel the principal harmonics.
2) Thyristor controlled reactor (TCR):
It consists of a six-pulse or twelve pulse thyristor-controlled
reactors and a fixed shunt capacitor bank. By adjusting the firing angle of the
thyristor, the reactive power can control.
The control system consists of a voltage
and current measuring devices. Filters are used to provide fixed reactive power
and absorb the harmonic current generated by TCR. The TCR is used to continuous
control of the electrical power system. 3) Thyristor-switched capacitor (TSC):
It consists of a thyristor-switched capacitor bank. This capacitor
bank is split into a number of units to
achieve a step-wise control. All the units have an equal rating. By full or zero conduction operation of thyristor
valve, the effective resistance of TSC can be varied in a step-wise manner. For effective reactance calculation,
we have to consider fully connected or fully disconnected capacitor.
4) Combined
TCR and TSC compensator:
A continuously variable reactive
power can be achieved by using a TCR in combination with TSC. A continuous change
in the control order from fully lagging to fully leading current is obtained by
this combination. By using a different combination,
an SVC can get various requirements to absorb/supply reactive power from/to the
power system.
Advantages of SVCs:
1) It gives fast
response to change in power system
2) Less costly,
high capacity, faster and more reliable
3) It reduces
harmonics and voltage fluctuations
4) It provides
load balancing function
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