What is Static VAR Compensator : Design & Its Working

The most crucial device used in the control system is the Compensator which is operated for the regulation of other systems. In many of the cases, this is operated by regulating either the output or input to the control system. There are essentially three kinds of compensators which are lead, lag, and lag-lead. In order to enhance the execution, adjusting the control system might pose damage to the performance like feeble stability or unbalanced stability. So, to make the system function as expected, it is more recommended to restructure the system and include a compensator where this tool counteracts the inadequate efficiency of the actual system. This article gives a detailed explanation of one of the most prominent types of compensators Static Var Compensator.

What is Static VAR Compensator?

This is a parallelly connected static type of VAR absorber or generator where the output is modified so as to substitute inductive or capacitive current where this regulates or manages corresponding factors of the current mainly the bus voltage factor. A static VAR compensator is dependent on thyristors having no gate switching off ability. The functionality and features of the thyristors understand the SVC adaptable reactive impedance. The crucial equipment which is included in this device is TCR and TSR which are a thyristor-controlled capacitor and thyristor-controlled reactor.

Static VAR Compensator
Static VAR Compensator

The device also provides quick functional reactive power in the case of extreme voltage electrical transmission systems. SVC’s come under the classification of adaptable AC transmission networks, voltage control, and system stabilization. The basic static VAR compensator circuit diagram is shown as follows:

Static VAR compensator basics can be explained as follows:

The assemblage of thyristor switch in the device regulates the reactor and the firing angle is used for the regulation of the voltage and current values that flow through the inductor. In correspondence to this, the inductor’s reactive power can be regulated.

This device holds the ability to reduce the regulation of reactive power even across extended ranges showing a zero-time delay. It enhances the constancy of the system and the power factor. Few of the schemes followed by SVC devices are:

  • Thyristor regulated capacitor
  • Thyristor regulated reactor
  • Self-reactor
  • Thyristor regulated reactor having a constant capacitor
  • Thyristor regulated capacitor with thyristor regulated reactor


In the one-line configuration of the SVC, through the PAM type of modulation by the thyristors, the reactor might be shifter internal to the circuit and this shows a constantly variable type of VAR to the electrical system. In this mode, extended levels of voltages are regulated by the capacitors and this is mostly known for providing efficient control. So, the TCR mode provides good control and enhanced reliability. And the thyristors can be regulated in an electronic way.

In the same way as semiconductors, thyristors also deliver heat and for cooling purposes, deionized water is used. Here, when the slicing of reactive load into the circuit takes place, brings in unwanted kind of harmonics, and in order to restrict this, a high range of filters are generally used to smooth out the wave. As there is capacitive functionality in the filters, they also will spread out MVAR to the power circuit. The block diagram is shown as below:

Static VAR Compensator Block Diagram
Static VAR Compensator Block Diagram

The device has a control system and it is included with:

  • A distribution section which defines the thyristor switched capacitors and reactors those need to be switched internally and externally and calculates the firing angle
  • A synchronizing section including a phase-locked loop which is synchronized on the pulse generator and the secondary level of voltages where those transmit a required number of pulses to the thyristors
  • A calculating section measures the positive voltage that has to be regulated.
  • A voltage controlling system that determines the variation in between the calculated and reference voltage levels.

The static VAR compensator device needs to be operated in a phasor simulation technique which is simulated using a powerful section. It can also be utilized in 3-phase power networks along with the synchronous type of generators, dynamic loads for the execution, and observation of the device on electromechanical variations.

High-end designs of static VAR compensators can also be designed where the exact level of voltage control is necessary. Voltage controlling can be done through a closed-loop controller. This is the static VAR compensator design.

Static VAR Compensator Operation

In general, SVC devices cannot be operated at the line voltage levels, some transformers are required to step down the transmission voltage levels. This decreases the equipment and the size of the device necessary for the compensator even though the conductors be required to manage the extended levels of currents related to the minimum voltage.

Whereas in few of the static VAR compensators used in commercial purposes like electric furnaces, where there might be prevailing mid-range of bus bars are present. Here, a static VAR compensator will have a direct connection so as to conserve the transformer price. The other general point for connection in this compensator is for the delta tertiary winding of Y-type autotransformers which are used for the connection of transmission voltages to the other kinds of voltages.

The dynamic behavior of the compensator will be in the format that how thyristors are series-connected. The disc type of SC’s will have a high range of diameters and these are usually placed in the valve houses.

Static VAR Compensator VI Characteristics

A static VAR compensator can be operated in two approaches:

  • As voltage controlling mode where there is regulation for voltage within the threshold values
  • As var regulation mode which means susceptance value of the device is maintained at a constant level

For the voltage controlling mode, the VI characteristics are shown as below:

As far as the susceptance value stays at constant within the less and high threshold limits levied by the entire reactive power of the capacitors and reactors, then the voltage value is controlled at the equilibrium point which is termed as a reference voltage.

Though voltage decrease generally takes place and this ranges in between the values of 1 and 4 % when there is extreme reactive power at the output. The VI characteristic and the equations for this condition are shown below:

SVC VI Characteristics
SVC VI Characteristics

V = Vref + Xs.I ( When the susceptance lies in between high and low ranges of capacitor and reactor banks)

V = -(I/Bcmax) at the condition (B = Bcmax)

V = (I/Bcmax) at the condition (B = Blmax)

Advantages and Disadvantages

Few of the advantages of static VAR compensator are

  • The power transmission ability for the transmission lines can be enhanced through these SVC devices
  • The system’s transient strength can also be increased through the implementation of SVC’s
  • In the case of a high range of voltages and for controlling steady states, SVC is generally used which is one of the foremost advantages
  • SVC increases the load power rating and so the line losses will be decreased and system efficiency enhances.

The disadvantages of the static VAR compensator are:

  • As the device has no revolutionary parts, for the implementation of surge impedance compensation, additional equipment is needed
  • The size of the device is heavy
  • Deliberate dynamic response
  • The device is not suitable to employ for the regulation of voltage up and downs because of furnace loads

And this all about the concept of SVC. This article focused on explaining static VAR compensator working, design, operation, benefits, limitations, and characteristics. In addition, also know about what are the crucial applications of static VAR compensator?