Research on negative sequnence current coordinated control strategy for severe asymmetric offshore ac faults of wind power transmission system based on mmchvdc

When there is a severe asymmetric offshore ac fault in the grid-connected system for full-scale type-4 wind turbines via the high voltage direct current (HVDC) transmission system based on modular multilevel converters (MMC), there will be transient overcurrent in the MMC and the grid side converters (GSC) of wind turbines. In order to prevent transient overcurrent, a negative sequence coordinated current control strategy is proposed in this paper. The method of decoupling circuit characteristics and control characteristics is adopted to study the asymmetric fault characteristics of this system. The levels of the negative sequence current during severe asymmetric ac faults are analysed. The key influencing factor of the negative sequence currents is extracted. And the feature that the negative sequence currents of the MMC and the GSC cannot be suppressed to 0 simultaneously is revealed. Based on the above reasons, the control strategy that the negative sequence current of the MMC is suppressed to zero and the GSC cooperate to reduce the positive sequence current thus reducing the current stress of the bridge arm is proposed. Finally, an electromagnetic transient model was built on PSCAD to verify the effectiveness of the control strategy.