STATIC VAR COMPENSATORS (SVC) IN THE POWER SYSTEM

Friday, September 23, 2011

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 Compensator
Static Var Compensator (SVC)
Applications and Benefits

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.
Effect of Static Var Compensation
Effect of Static Var Compensation
Furthermore, other benefits of static var compensators include:

·         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
Thyrsitor-controlled Reactors with Fixed Capacitors (or Shunt Filters)
The variation of reactive power is accomplished by controlling the thyristor’s firing instants and, accordingly, the current that flows by the reactance.

2. Thyristor switched capacitors (TSC)

Thyristor-switched Capacitors (TSC)
Thyristor-switched Capacitors (TSC)
In this static var compensator design, the capacitor banks are connected phase-to-phase, with each section switched by thyristors. Therefore, a discrete variation of the reactive power can be attained, but not a continuous change similar to that of a TCR. Nonetheless, by providing a suitably large number of small sections, the required resolution of reactive power variation for a single step can be achieved. Synchronization of switching and initial pre-charging of the capacitors limits the transients typically associated with capacitor switching. Generally, the reaction time for symmetrical operation does not exceed 20 ms.

3. Thyristor-controlled Reactors and Thyristor switched capacitors (TCR/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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I am an Electrical Engineer with a Masters Degree in Business Administration. My interest is in Power Quality and Protective Relaying. I have been working in an electric distribution utility for about seven years now as a Planning & Design Engineer. I handle PQ studies, power system analysis and capital budgeting for company projects.