Voltage control in power system

Voltage control in power system

The power system voltage may vary with the load change. The voltage is usually high on light load and low on heavy load. To keep the system voltage within limits, some additional equipment requires increasing the system voltage when it is low and decreasing the voltage when it is too high. The methods used in the power system for voltage control are given below.

Run - Download Adapter Change Tap

Off - Load Adapter Change Tap

shunt reactors

Synchronous phase modifiers

shunt capacitor

Fixed VAR System (SVS)

The system voltage control with the help of a shunt inductive element is known as shunt compensation. Shunt compensation is of two types, i.e., static shunt compensation and synchronous compensation. In the static shunt compensation, the shunt reactor, the shunt capacitor and the static VAR system are used, while the shunt compensation uses the synchronous phase rate. The methods used to control the voltage are explained below in detail.

1. Off - Load Tap Changing Transformer - In this method, the voltage is controlled by changing the rotational ratio of the transformer. The transformer is disconnected from the supply before the tap is changed. Adapters are often changed manually.

2. Run - Tap Change Transformer - This arrangement is used to change the turn ratio of the transformer to regulate the system voltage when the transformer delivers the load. Most power inverters are provided with an on-load tap changer.

3. Shunt reactance - The shunt reactance is the inductive current component connected between the line and the neutral. The shunt reactor compensates for the inductive current from the transmission line or underground cables. Mainly used in long-distance EHV and UHV transmission lines for reactive power control.

Shift reactors are used in the sending substation, the receiving terminal and in the intermediate substation of the EHV and UHV long line. In the long transmission line, the shunt reactor is connected at a distance of 300 km to reduce the voltage at an intermediate point.

4. Shunt Capacitors - The shunt capacitors are the capacitors connected in parallel with the line. It is installed in receiving substation, distribution substation and in switching substation. The shunt capacitor injected reactive volt-amperes into the line. It is placed in a three-phase bank.

5. Synchronous phase rectifier – Synchronous phase rectifier is a synchronous motor that operates without mechanical load. It is connected to the load when receiving the end of the line. The synchronous phase modulator absorbs or generates reactive power by changing the excitation of the field coil. It keeps the voltage constant in any load condition and also improves the power factor.

6. Sequential Var Systems (SVS) - The static VAR compensator injects or absorbs the inductive VAR of the system when the voltage becomes higher or lower than the reference value. In a static VAR compensator, a thyristor is used as a switching device in place of a circuit breaker. Nowadays, thyristor switching is used in the system instead of mechanical switching because thyristor switching is faster and provides transient free operation by switching control.