Fixed series compensation has long been the preferred solution for optimizing performance in very large bulk transmission corridors. 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 provides increased transient (angular) stability of a power corridor and increased voltage stability of the grid. It 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 housed on a platform that is fully insulated for the system voltage. The capacitor 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 non-linear metal-oxide varistors, a Fast Protective Device/ FPD (CapThor) and a fast bypass switch. Secondary protection is achieved with ground-mounted electronics acting on signals from optical current transducers in the high-voltage circuit.
Series compensation principle
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. Since the series capacitor is self-regulated, i.e. its output is directly (without control) proportional to the line current itself, it will also partly balance the voltage drop caused by the transfer reactance. Consequently, the voltage stability of the transmission system is increased.