Stability analysis of cascaded LCC-MMC hybrid HVDC system based on impedance method

In recent years, the cascaded LCC-MMC (line commutated converter- modular multilevel converter) inverter has been suggested as a viable option for High Voltage Direct Current (HVDC) transmission systems due to its capability to combine the advantages of LCC and MMC inverters, e.g., a reduced susceptibility to commutation failures and cost-effectiveness. However, ensuring the stability of this inverter when connected to a weak grid has become a significant area of focus, particularly with the growing integration of renewable energy sources into the power grid. This paper addresses this concern by developing a small-signal admittance model for the hybrid LCC-MMC inverter. Using this model and the general Nyquist criterion, the study investigates how the conditions on the AC side of the rectifier and the DC circuit affect the stability of the cascaded LCC-MMC inverter when connected to a weak grid. This aspect, which is often overlooked, is examined in detail. In order to enhance the stability, this paper proposed an improved control strategy for managing circulating currents. This strategy is intended to reshape the DC equivalent impedance, thereby minimizing the effects of conditions on the AC side of the rectifier and the DC circuit on the LCC-MMC inverter. To validate the model's accuracy and assess the effectiveness of the proposed control strategy, time domain simulations are conducted.