Design Implications of DC Grounding in Bipolar VSC-HVDC Systems

This paper investigates the combined impact of DC neutral grounding resistance and neutral surge arrester coordination in bipolar VSC-HVDC systems with DMR. A parametric electromagnetic transient (EMT) study is performed using a state-of-the-art EMT simulation tool to evaluate system response under critical internal faults such as single phase-to-ground fault between converter and transformer and DC pole-to-ground faults. The analysis focuses on the delayed zero crossing of AC circuit breaker currents, transient overvoltages across converter valve arm, on healthy pole and cable, and the metallic return conductor. The results demonstrate a strong interdependence between DC grounding resistance and neutral surge arrester protective level. Higher grounding resistance improves damping and supports restoration of AC current zero crossing during internal ground faults but can lead to increased healthy pole overvoltages and reduce fault detection sensitivity. Inadequate surge arrester coordination may also lead to excessive energy absorption or insufficient protection capability. For configurations with DMR, DC faults can produce severe transient overvoltages on the return conductor, depending on the DC grounding resistance and the neutral surge arrester protective level.

The paper highlights the importance of system-level coordination of DC grounding and surge arrester design and discusses the implications for bipolar point-to-point HVDC schemes and potential future multi-terminal VSC-HVDC systems.