Active Energy Control for Enhancing AC Fault Ride-Through Capability of MMC-HVDC Connected With Offshore Wind Farms

Modular Multilevel Converter based High Voltage Direct Current (MMC-HVDC) connected with offshore wind farms could suffer serious challenge: surplus power of MMC-HVDC caused by AC fault in onshore power grid. This paper proposes an active energy control (AEC) scheme for converter stations utilizing MMC sub-module capacitance. It can be used to actively absorb the surplus power of the MMC-HVDC in the stage of grid-side AC fault and enhance the AC fault ride-through (ACFRT) ability. First, the energy decoupling principle of MMC is analyzed, and a steady-state control scheme is proposed for MMC-HVDC with energy decoupling capability. Based on this strategy, an AEC scheme between onshore and offshore converter stations is proposed for ACFRT. It is divided into the following four stages: passive energy recovery, active energy recovery, active energy maintenance, and active energy release. Then, a set of coordinated control schemes for AEC and active power reduction of offshore wind farms are designed. This is used as an example to illustrate that the proposed AEC can effectively enhance the fault ride-through ability of the existing strategies, such as active power reduction of offshore wind farms. Finally, the correctness and effectiveness of the proposed control schemes are verified via the PSCAD/EMTDC platform.