Fixed series compensation – getting the most out of long AC transmission lines

Series compensation is a well established technology that is primarily used to reduce transfer reactances, most notably in bulk transmission corridors. The result is a significant increase in power transfer capacity and improvement of voltage and angular stability in transmission systems.

Fixed series compensation has long been the preferred solution for optimizing performance in very large bulk transmission corridors.

In a transmission system, the maximum active power transferable over a certain power line is inversely proportional to the series reactance of the line. Thus, by compensating the series reactance to a certain degree, using a series capacitor, an electrically shorter line is realized and higher active power transfer is achieved.

Installing a capacitive reactance in series in a long (typically more than 200 km) transmission line reduces both the angular deviation and the voltage drop, which increases the loadability and stability of the line. Since the current through the transmission line directly "drives" the MVAr output from the capacitor, the compensation concept is "self-regulating”, and this straightforward principle ensures that series compensation is an extremely cost effective solution.

Series compensation also improves the voltage profile along the power corridor and optimizes power sharing between parallel circuits.

Series compensation technology

Because series capacitors are installed in series on a transmission line, the equipment must be  elevated on a platform at system voltage,  fully insulated from ground . The capacitor bank together with the overvoltage protection circuits are located on this steel platform. Overvoltage protection is a key design factor, as the capacitor bank must be able to withstand the throughput fault current, even at a severe nearby fault. The primary overvoltage protection typically involves metal-oxide varistors, a Fast Protective Device/ FPD (CapThor) and a fast bypass switch. The digital control and protection system, designed to  act very quickly  on signals from optical current transducers in the high-voltage circuit, is located on ground and communicates with the primary equipment on the platform and measurements via fiber optics.