Valve-Side Single-Phase-to-Ground Fault Clearance in Bipolar Hybrid-MMC HVDC Systems Utilizing Thyristor Commutation Branches and Mechanical Interrupter

The Hybrid Modular Multilevel Converter (HMMC) topology—comprising full-bridge (FB) and half-bridge (HB) submodules—is effective for DC fault clearance in HVDC systems. However, to prevent overvoltage in lower-arm full-bridge submodules (FBSMs) during valve-side single-phase-to-ground (SPG) fault mitigation—an uncommon yet critical internal fault in bipolar HVDC systems—the proportion of FBSMs used must account for approximately 86.6% of the arm voltage, compared to 43.3% for DC-side fault blocking. This diminishes the economic advantage of HMMC over fully full-bridge MMC topology. This paper proposes an HMMC SPG fault mitigation method using thyristor-based current commutation branches on the lower arms and a ground-side mechanical interrupter, while maintaining the minimum number of FBSMs required for conventional DC fault clearance. The method mitigates SPG-resultant lower-arm overvoltage by commutating current into the thyristor branches, after which the mechanical interrupter clear lower arms current, and subsequently thyristor branches are turned-off to clear the SPG-valve side fault. Additionally, the pole current is cleared using a mechanical breaker or passive oscillation DC circuit breaker (CB), effectively suppressing upper-arm submodule (SM) overvoltage under SPG valve-side fault in both short- and long-distance HMMC bipolar links. Although additional equipment is required, it incurs no on-state losses during steady-state operation. The proposed method is compared with existing SPG fault clearance approaches for HMMC bipolar systems. Its effectiveness is validated through PSCAD/EMTDC simulations and benchmarked against prior SPG fault clearance methods for HMMC systems.