Low DC voltage control strategy of bipolar LCC-MMC hybrid HVDC transmission system

This paper investigates a bipolar hybrid high voltage direct current (HVDC) transmission system, the rectifier adopts line commutated converter (LCC) and the inverter adopts hybrid modular multilevel converter (MMC). The hybrid HVDC transmission system not only combines the advantages of LCC and MMC, but also has a strong capability to deal with AC or DC faults. This paper mainly studies the control strategy of the bipolar LCC-MMC hybrid HVDC transmission system when running at low DC voltage. For the traditional symmetric hybrid MMC, an improved control strategy is proposed to balance the voltages of the two seed modules, which significantly expands the operating DC voltage range of the system. Moreover, the topological structure of bipolar asymmetric hybrid MMC is designed and the control strategy when running at low DC voltage is presented. It is found that the DC voltage of the bipolar asymmetric hybrid MMC remains symmetric and can be adjusted freely. At last, the simulations based on PSCAD/EMTDC show the effectiveness of the proposed control strategy.