Delayed Current Zeros in Synchronous Compensator Plants

With increasing penetration of renewables and HV/MV-DC transmission, synchronous compensators have regained importance in the transmission system to improve the short-circuit capacity, provide dynamic voltage support and the inertial response for frequency regulation. However, the short-circuit current characteristic of a synchronous compensator after a 3-phase terminal fault shows delayed current zeros (DCZ) which means prospective current has no natural crossing through zero for several cycles. This imposes high technical requirements on the generator circuit-breaker (GCB) employed in such application regarding the interruption of fault currents. It is essential to assess the possible occurrence of DCZ and select appropriate GCB to ensure high availability of the plant. In the beginning, this paper identifies requirements from a GCB in case of DCZ. Then, the possible cases of fault currents which show DCZ in a typical synchronous compensator plant are investigated using simulation model in the electro-magnetic transient program EMTP-ATP. The DCZ associated with synchronous compensator terminal faults and out-of-phase synchronizing faults have been analyzed. Furthermore, a third case is studied which can occur in synchronous compensator plant with 3-winding step-up transformers. The capability of GCB to interrupt those fault currents is specifically investigated by means of simulations which considers the effect of circuit-breaker arc-voltage on the prospective fault current. Comparison between SF6 and vacuum extinguishing technologies is presented.