A novel station online re-commssioning strategy for MMC-MTDC systems

A multi-terminal VSC-HVDC system based on the MMC technology (MMC-MTDC) has great application prospects for city center in-feed, grid connection and offshore wind energy integration because of its flexible power transmission. However, how to re-commission a converter station (after maintenance) online to the operating system without expensive dc Circuit Breakers(CBs) presents a huge challenge. By employing the structural characteristics of MMC, followed by certain sequential operations of the disconnectors (DSs) at dc side are proposed to implement the station online re-commissioning instead of using dc CBs. The charging process of a MMC station with a combination of ac and dc sources is studied firstly, and then a one-order circuit has been assumed to simplify the complicated charging process to get the potential maximum surge current and its decay constant. In order to decrease the surge current after closing the DSs at the positive and negative polarities separately, the valve should be blocked and the dc voltage of the operating system should be decreased properly. At last, the valve will be deblocked with increased dc voltage at the rated value. Besides easy operation, the strategy doesn´t increase any cost. The simulation of a real three-terminal VSC-HVDC system, which is currently under construction in Shantou, China, has been performed in PSCAD/EMTDC to verify the strategy.