Shunt Reactor Function in High Voltage Transmission Systems

Shunt reactors are crucial components installed in parallel (shunt) with high voltage (HV) AC transmission lines. They essentially act as large inductors connected to the system bus or line.

Understanding Over-voltages on Lightly Loaded Lines

A significant challenge encountered in the operation of long high voltage transmission lines is the phenomenon known as the Ferranti effect. This effect manifests as an increase in the voltage at the receiving end (load side) compared to the sending end voltage, particularly noticeable when the line is lightly loaded or operating under no-load conditions. The reason behind this voltage rise is the line's inherent capacitance. Transmission lines behave like distributed capacitance, and this capacitance draws a charging current even without any load connected. This charging current, leading to a net leading power factor at the receiving end, causes the voltage to rise above the nominal level.

Shunt Reactors Mitigate Over-voltages

Shunt reactors are specifically employed to counteract the Ferranti effect and control these undesirable over-voltages. Their mechanism of action is based on absorbing reactive power:

• Being inductive elements, shunt reactors consume reactive power ($Q_L$) from the power system.
• The capacitance inherent in long transmission lines generates a substantial amount of capacitive reactive power ($Q_C$) when the line is lightly loaded.
• By connecting a shunt reactor in parallel with the transmission line, it effectively absorbs a portion of this excessive capacitive reactive power.
• The amount of reactive power absorbed by the reactor is determined by its inductance ($L$) and the system voltage ($V$), calculated as $Q_L = \frac{V^2}{X_L}$, where $X_L$ is the inductive reactance of the reactor.
• By carefully selecting the reactor's rating (i.e., its inductance), engineers can ensure that the reactive power absorbed ($Q_L$) offsets the reactive power generated by the line capacitance ($Q_C$), thereby keeping the net reactive power ($Q_{net} = Q_C - Q_L$) within limits.
• This controlled absorption of reactive power prevents the voltage at the receiving end from rising to excessive levels.
• Consequently, the primary purpose of using shunt reactors is to limit over – voltages at the load side, especially under lightly loaded condition.

Analysis of Incorrect Options

• Option 1: Limit the short circuit current through the line. This function is typically handled by series reactors, which are connected in series with the line to increase the circuit's total impedance and thereby limit the magnitude of current during a fault (short circuit).
• Option 2: Compensate for the series reactance of the line under heavily loaded condition. The compensation for the line's series inductive reactance, particularly to improve power transfer capability and stability during heavy loads, is achieved using series capacitors.
• Option 4: Compensate for the voltage drop in the line under heavily condition. Voltage drops under heavy load conditions are usually addressed by supplying reactive power to the system, often using devices like shunt capacitors or synchronous condensers located at the receiving end. Shunt reactors, conversely, absorb reactive power.