Fixed Series Capacitor (FSC) Compensation

• The AC transmission lines are primarily limited by the inductive reactive impedance (XL). The Series capacitive compensation is adding the capacitive reactance (XC) in series with the transmission line to reduce the overall effective reactance (Xeff) of the line (from the sending end to the receiving end).

• The series capacitors provide fixed series compensation and it can be used to increase the power transmission capacity by reducing the overall effective reactance (Xeff) of the line. It is the most widely used compensation since it is a cost-effective technology.

• Fig. 1 shows the simple transmission line with series capacitor compensation. The effective reactance of the line (Xeff) can be derived as follows,

Where, k = XC/XL is the degree of series compensation. The value of k is in the range of 0 to 1. In practice, the degree of compensation is in the range of 25 % to 70 %.

Limitations of Series Capacitors

• The series capacitors can provide fixed series compensation. The series-compensated FACTS devices (TCSC and SSSC) can provide controllable (variable) series line compensation.

• Sub-synchronous oscillations can be produced when there is an interaction between series capacitor compensated line and generator-shaft torsional systems.

• Low-frequency oscillations (5-10 Hz) can be produced when there is an interaction between series capacitors and shunt reactors, especially during line switching and faults.

References

• Narain G. Hingorani, Laszlo Gyugyi, “Understanding FACTS concepts and technology of flexible AC transmission systems”, 1999.

• Eremia, Mircea, and Stig Nilsson. "Series capacitive compensation." Advanced Solutions in Power Systems: HVDC, FACTS, and Artificial Intelligence: HVDC, FACTS, and Artificial Intelligence, 2016.