Electro-mechanical transient modeling of MMC based multi-terminal HVDC system with DC faults considered

Modeling different types of DC faults in modular multilevel converter based multi-terminal HVDC (MMC-MTDC) systems for transient stability analyses has not been well studied. In this paper, an improved electro-mechanical model of MMC-MTDC system which is feasible for a variety of DC fault simulations is proposed. Firstly, the improved MMC electro-mechanical model with a second-order DC side circuit is derived theoretically. Then a method based on preset DC fault information for studying the impacts of DC faults on the stability of large-scale AC/DC power systems is proposed, with which the DC faults can be handled efficiently without reconstructing the DC topology. Theoretical and simulation studies show that the DC-side equivalent circuit of the MMC should be established as a second-order circuit when DC faults are considered for transient stability studies. Simulations of various types of DC faults in the modified IEEE 39-bus system incorporating a four-terminal MMC-HVDC system are carried out on PSS/E for validating the proposed method.