Studying the operation of series reactive power sources in emergency modes
Flexible AC transmission systems (FACTS) are used around the world for decreasing the current load of individual power transmission lines. One of the ways in which the above-mentioned problem can be solved consists in using series reactive power sources (RPSs). The operation of single-phase and three-phase series RPSs used in flexible AC transmission systems under the conditions of three types of short circuit faults is considered. The possibility in principle of limiting short circuit currents by means of series RPSs is shown. A generic calculation of short circuit currents is carried out, and criteria for the worst-case operating modes are determined. An analysis has shown that the amplitude of current through the reactor and thyristors depends on the reactor inductance and on the short-circuit voltage amplitude and phase. The highest amplitude of current through the thyristors is observed in the modes when the short-circuit fault instant coincides with the beginning of the short-circuit voltage’s positive half-wave. Timing diagrams of the characteristic transients are obtained by simulation, and the maximum values of RPS parameters are indicated at different values of reactor inductance. A procedure for determining the reactor inductance is suggested. It is shown that the device always limits the fault currents through the semiconductor switches, and that in a number of cases the transmission line current can be limited in the line section containing the RPS to an extent that depends on the circuit impedance at the resonance frequency. At the given reactor inductance, the highest currents in the worst-case mode will flow under the conditions of a three-phase short-circuit fault in the system equipped with a three-phase RPS in case of both ground and line-to-line short-circuit faults. The maximal currents flowing through single-phase RPSs in case of three-and single-phase short-circuit faults are equal to each other and are characterized by smaller values.