FACTS: Series Compensation

The main purpose of series compensation in power systems is to decrease the reactive impedance of the transmission line to reduce voltage drop over long distances and to reduce the Ferranti effect.  By adding series capacitors to the line, engineers can compensate for the physical inductance inherent in the transmission line.  The voltage drop across the line is reduced with more compensation, allowing more power to be received by the load for any given sending power.  Two main types of series compensation are fixed series compensation, and thyristor controlled series compensation, each with their own advantages.

Fixed Series Compensation

Fixed series compensation (FSC) of a line is desirable for power transmission due to the effects of line reactance modification.  By adding series capacitance, the reactive impedance of the line decreases, thus lowering the voltage drop across the transmission line.  This effect can be seen through the simplified power flow equation (see the post about Power Transfer) obtained by neglecting line resistance and line charging capacitance.

Line reactance is counteracted by a series capacitance, resulting in overall lower line impedance and a lower voltage drop across the line.

By adding the series capacitance, it can be seen that the receiving line end voltage will be closer to the sending line end voltage.  This decrease in voltage drop across the line allows more power to be transferred over the line for any given sending line end voltage.

The advantage to using FSC compared to thyristor controlled series compensation is price.  Usually FSC allows for a majority of compensation for a lower cost when compared to thyristor controlled series compensation. The following phasor diagram demonstrates the effect of series compensation:

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This article was taken from the introduction of a report which was written by a partner and I, submitted to ECE3333: Power Systems I, taught by Professor Rajiv Varma at the University of Western Ontario.