Dynamic performance of series multiterminal HVDC during AC faults at inverter stations

Multiterminal high voltage direct current (MTDC) system can be either a series type or a parallel type. Series MTDC concept is essentially developed from the two-terminal HVDC but with some unique characteristics. In this paper, the AC fault ride through capability of series MTDC is discussed. AC faults at inverter stations, which are connected to electrically separated AC systems, will be highlighted. For an AC fault that happens at one inverter station, the DC voltage of that station may decrease to zero due to commutation failure, while the other inverter stations (the healthy inverters) can still retain power transmission capability to some extent, which will increase the availability of the series MTDC system. However, the current controller design, the voltage dependent current order limiter (VDCOL) logic and the AC system strength will impact on the dynamic performance of series MTDC during AC faults. In this paper, a + 800kV/3.2GW 4-terminal line commutated converter (LCC) based series MTDC model is established to study the dynamic performance when AC faults occur at an inverter. The controller of the series MTDC is designed; the VDCOL logic for series MTDC is developed; simulation results of the dynamic performance during AC faults at the inverter stations are presented.