Static Var Compensator (SVC) is a
power quality device, which employs power electronics to control the reactive
power flow of the system where it is connected. As a result, it is able to provide
fast-acting reactive power compensation on electrical systems. In other
words, static var compensators have their output adjusted to exchange inductive
or capacitive current in order to control a power system variable such as the
bus voltage.
Moreover, the term static is used to
distinguish the SVC from its rotating counterparts like the synchronous
generators and/or motors.
Static var compensators are primarily
used to mitigate voltage fluctuations, as well as the resulting flicker since
the 1970s. Nowadays, large industries, particularly the steel-making plants
typically apply SVCs for flicker compensation in electric arc furnace
installations.
In addition, static var compensators
are installed at suitable points in the electric power system to augment its
transfer capability by improving voltage stability, while keeping a smooth
voltage profile under different system conditions. SVCs can also mitigate
active power oscillations through voltage amplitude modulation. Moreover, as an
automated impedance matching device, they have the added benefit of bringing
the system power factor close to unity.
Therefore, SVC is usually installed
near high and rapidly varying loads, such as electric arc furnaces, welding
plants and other industries prone to voltage fluctuations and flicker.
Furthermore, other benefits of static var compensators include:
Effect of Static Var Compensation |
·
Maximized power compensation
·
Near-instantaneous
response to system voltage variations
·
Increased
customer’s economic benefits
·
Eliminate
harmonics and reduce voltage distortion with appropriate shunt filters
·
Load balancing on three-phase systems
Basic Operation
The static var compensator regulates
the voltage by controlling the amount of reactive power absorbed from or
injected into the power system. For example, it generates reactive power by
switching capacitor banks when the system voltage is low or loads are
inductive. Consequently, the reactive power demand of the lagging load is
supplied by the SVC – relieving the distributing lines from delivering it.
Thus, the voltage drop decreases and the voltage at the load terminals shall
improve.
Likewise, the static var compensator
absorbs reactive power when the system voltage is high or loads are capacitive.
In this case, the SVC uses the reactors to consume the VARs from the system,
thereby lowering the system voltage.
Design and Configurations
There are three common configurations
of static var compensators and each will be described below.
1. Thyristor-controlled Reactors with
Fixed Capacitors (TCR/FC)
This SVC design consists of two
parallel branches connected on the secondary side of a coupling transformer.
One of the branches is composed of reactors that are controlled by AC thyristor
switches. In addition, the reactors are connected in delta for three-phase
applications. The other branch could either be fixed capacitor banks or shunt
filters.
Thyrsitor-controlled Reactors with Fixed Capacitors (or Shunt Filters) |
Thyristor-switched Capacitors (TSC) |
Basically, this is the combination of TCR
and TSC. In this configuration, the control of the static var compensator is
based on measuring the reactive component of load current at the instant of
voltage zero. Then, the measured current is used to determine the firing angle
so that the SVC absorbs or injects the amount of reactive power required for
compensation.
However, there is a time interval
between the instant of measuring the reactive component (in one half-cycle) and
the firing instant (the next half-cycle). This inherent delay of its operation
mode is one of its major limitations.
References:
Baggini, A. (2008). Handbook of Power
Quality
Hanzelka, Z. and Bien, A. (2006).
Voltage Disturbances - Flicker
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