reactors

by compensating for voltage drop due to reactive current at any voltage transmission.

It is well known that overhead lines generate reactive power due to their shunt capacitance. Although necessary to support voltage and to transmit power, excess reactive power at light loads can have the following undesirable effects:

1. Voltage rise at the receiving end due to capacitive current flowing through the line inductance.

2. Voltage rise at sending end due to capacitive current flowing through the source impedance.

Voltage rises of 20% are not unusual for systems with long lines and low short-circuit power. If not controlled, line overvoltages will reduce the lifespan of insulation materials and will eventually result in system faults. For that reason, and in order to keep voltages within acceptable limits, the excess of reactive power generated needs to be compensated. The desired voltage variation under normal conditions usually ranges between +5% and +10%, generally higher during heavy loading than under light loading.

Shunt reactors are used in transmission lines longer than 200km (or even shorter if supplied by a very weak system) for reactive power compensation, voltage control and,sometimes, for synchronous stability improvement.

what do you mean by synchronous stability improvement

Synchronous machines may become self-excited when a sufficiently heavy capacitive load is present. The capacitive current affects the excitation and may cause serious overvoltage and torque imbalances which will cause synchronous instability. Because of the inherent capacitance of transmission lines, the problem may arise when synchronous generators are energizing long unloaded or lightly loaded lines.

The use of shunt reactors at the sending end of the line usually at substation switchyards to compensate the capacitive current is therefore necessary.