A fault ride through strategy of unified power flow controller and its coordination with protection

When a fault occurs on the line equipped with Unified Power Flow Controller (UPFC) based on Modular Multilevel Converter (MMC), it may cause the blocking of MMCs and then make UPFC out of operation. To avoid the non-function of UPFC under the condition of external fault, a UPFC fault ride through (FRT) strategy, which can convert UPFC to Static Synchronous Compensator (STATCOM), is proposed. The FRT strategy is realized by connecting the Current Limiting Reactor (CLR) on the DC bus to limit the fault current flowing through the DC bus, which is good for the shunt side MMC being unblocked. On the other hand, the FRT strategy may cause the series side MMC be unblocked under some fault cases, which has adverse effects on the distance protection due to the vibration of the measured impedance. To solve this problem, the coordination of the FRT strategy of UPFC with the distance protection is provided, the High Voltage Breaker (HVB) across the series transformer of UPFC is forced to close if the series side MMC is unblocked during FRT, thus the performance of the distance relay is guaranteed. Simulation results based on PSCAD/EMTDC verify the proposed FRT strategy and coordinationi scheme.