A Power Allocation Strategy for DC Line Fault in Serial Hybrid LCC-MMC HVDC System

The serial hybrid line commutated converter-modular multilevel converter (LCC-MMC) high voltage direct current (HVDC) system has structural features that cause the inverter side three MMCs to form a closed-loop after a DC line fault. If the steady-state control strategy of the MMCs is still used during the phase shift, there will be a severe overcurrent problem. In addition, during the power recovery process, the rectifier side and the main converter MMC simultaneously deliver active power to the constant active power converter, resulting in the loss of control of the constant active power converter. Based on this, an MMC active power allocation strategy is designed in this paper. In this strategy, during DC line faults, the constant DC voltage converter is switched to constant active power control, and the other two converters are switched to voltage droop control. During the phase shift, the active powers of the two droop converters are allocated according to the droop coefficients, and the active powers of the MMCs are dynamically and smoothly reduced to solve the uncontrol problem of the two converters during the power recovery process. Finally, a serial hybrid LCC-MMC HVDC system model is built in PSCAD/ETMDC based on engineering design parameters to verify the accuracy and validity of the proposed control strategy. Comprehensive simulations show that the proposed strategy can reasonably allocate the powers of the MMCs in case of DC line fault. Besides, it still shows great adaptability with different power distribution, fault locations, and fault resistances.