Multi-terminal Heterogeneous Generalized Impedance Modeling and Analysis of MMC-MTDC Transmission Systems

The prominent risks of broadband oscillation among renewable energy, modular multilevel converter-based multi-terminal high voltage direct current (MMC-MTDC) and AC grid severely constrain the safe and stable operation and efficient accommodation of renewable energy. The control strategies of different converter stations of MMC-MTDC are various such as constant AC voltage, constant DC voltage and constant power control, and there is a strong nonlinear coupling between the multi-terminal converter stations, which brings challenges to the modeling and analysis of the system oscillation. Therefore, a modular modeling method of MMC-MTDC control system considering positive- sequence and negative-sequence control is firstly proposed in this paper. And then, aiming to the complex AC and DC grid structures, a generalized impedance model of different sections of MMC-MTDC is established with the multi-terminal nonlinear coupling considered. Finally, taking the three-terminal MMC-MTDC ring network as an example, simulation is conducted to verify the accuracy and validity of the generalized impedance model. The coupling effects of the multi-terminal interconnection system and the positive-sequence control and negative-sequence control on the impedance characteristics are analyzed, which will provide a basis for the oscillation analysis of renewable energy base connected into MMC-MTDC interconnection system.