Methods of Voltage Control in Power System

The voltage of the power system may vary with the change in load. The voltage is normally high at light load and low at the heavy-load condition. For keeping the voltage of the system in limits, some additional equipment requires which increase the system voltage when it is low and reduces the voltage when it is too high. The following are the methods used in the power system for controlling the voltage.

1. On – Load Tap Changing Transformer
2. Off – Load Tap Changing transformer
3. Shunt Reactors
4. Synchronous Phase Modifiers
5. Shunt Capacitor
6. Static VAR System (SVS)

Controlling the system voltage by the help of shunt inductive element is known as shunt compensation. The shunt compensation is of two types, i.e., the static shunt compensation and the synchronous compensation. In static shunt compensation, the shunt reactor, shunt capacitor and static VAR system are used, whereas the shunt compensation uses the synchronous phase modifier. The methods used for controlling the voltage are explained below in details.

1. Off – Load Tap Changing Transformer – In this method, the voltage is controlled by changing the turn ratio of the transformer. The transformer is disconnected from the supply before changing the tap. The tap changing of the transformer mostly done manually.

2. On – Load Tap Changing Transformer – This arrangement is used for changing the turn ratio of the transformer for regulating the system voltage when the transformer delivers the load. Most of the power transformer is provided with on-load tap changer.

3. Shunt Reactor – The shunt reactor is the inductive current element which is connected between the line and neutral. The shunt reactor compensates the inductive current from the transmission line or underground cables. It is mainly used in the long distance EHV and UHV transmission lines for reactive power control.

The shunt reactors are used in the sending end substation, receiving end substation and in the intermediate substation of long EHV and UHV line. In the long transmission line, the shunt reactor is connected at the distance of 300 Km to limit the voltage at an intermediate point.

4. Shunt Capacitors – The shunt capacitors are the capacitors connected in parallel with the line. It is installed at the receiving end substation, distribution substations and in the switching substations. The shunt capacitor injected the reactive volt-ampere to the line. It is placed in the three phase bank.

5. Synchronous Phase Modifier – The synchronous phase modifier is the synchronous motor running without a mechanical load. It is connected with the load at receiving the end of the line. The synchronous phase modifier absorbs or generates the reactive power by varying the excitation of the field winding. It keeps the voltage constant at any condition of the load and also improves the power factor.

6. Series Var Systems (SVS) – The static VAR compensator inject or absorb the inductive VAR to the system when the voltage becomes higher or lower than the reference value. In static VAR compensator, the thyristor is used as switching device in place of circuit breakers. Nowadays, the thyristor switching is used in the system in place of mechanical switching because thyristor switching is faster and provides transient free operation by controlling the switching.