Modelling and simulation analysis of MMC-HVDC system based on transient energy flow

In the high voltage direct current (HVDC) transmission system composed of modular multilevel converters (MMC), considering the distribution of transient energy flow (TEF) contributes to the transient modelling and fault characteristic analysis when dc short-circuit fault occurs. For single MMC, based on transient energy conservation, the capacitors of half bridge sub-modular (HBSM) can be equivalent to time-varying capacitor. And the MMC transient model is established, in which dynamic switching of HBSM and ac import are considered. By solving state equations of the model, the fault and TEF characteristics of MMC are obtained. For MMC-HVDC grid, a TEF analysis method based on electromagnetic transient simulation results is proposed, which can not only reveal the evolution of dc fault, but also realize quantitative analysis of fault current through recursive calculation. Based on characteristics of TEF, an energy limitation strategy considering the fluctuation range of capacitor voltage in HBSM is proposed, which can delay the blocking time effectively. According to the simulation results in PSCAD, the correctness of the transient model and the energy limitation strategy is verified, meanwhile, the TEF analysis method is applied to the four-terminal MMC-HVDC grid to obtain its fault characteristics.