Ground fault protection for DC-filter high voltage capacitors based on virtual capacitance

Abstract

DC filter is an important electrical apparatus to restrain harmonics in a line-commutated converter based HVDC (LCC-HVDC) system. The used differential protection for DC filters has poor selectivity and insufficient sensitivity to identify ground faults of DC-filter high voltage capacitors (DFHVCs). Based on virtual capacitance, a novel ground-fault protection for DFHVCs is proposed in this paper. According to the equivalent circuit of DC filter in a LCC-HVDC system, virtual capacitance characteristics are analyzed under different ground faults of DFHVC. Then, these characteristics are used for constructing protection scheme to identify DFHVC ground faults from external faults. A ±500 kV HVDC system built in PSCAD/EMTDC is used to verify the performance of the proposed protection. Comprehensive simulations show that the proposed protection can reliably identify DFHVC ground fault in time domain and is immune to fault resistance, fault location, measurement error and DC filter structure. Moreover, it is noted that the protection can identify a ground fault of DFHVC during the whole fault period and is of high reliability, making it valuable and prospective for applications in LCC-HVDC system.