Analytical Calculation Models for Bipolar MMC-HVDC Systems under Valve-side Single-phase-to-ground Faults

This paper proposes two analytical valve-side single-phase-to-ground (SPG) fault calculation models for bipolar modular multilevel converter-based high-voltage direct current (MMC-HVDC) system. The first model is applicable to the half-bridge submodule (HBSM) configuration, and the second is suitable for the full-bridge submodules (FBSMs) or hybrid SMs with different FBSM ratios. In each calculation model, two post-fault equivalent MMC circuits are established following converter blocking for the independent study of the upper and lower arms. The detailed expression of the post-fault voltages and currents in each arm, valve-side, and grid-side are obtained from the proposed calculation models. Moreover, the applicability of the calculation models for solid, inductive, and resistive dc-grounding methods is also demonstrated, along with a further discussion on the influence of MMC arm/grid-side resistance as well as varying fault-grounding impedance. Multiple bipolar MMC-HVDC systems, incorporating HBSM, FBSM, and hybrid SM configurations, are developed in PSCAD/EMTDC to validate the accuracy of the proposed analytical calculation models.