Failure-Mode Analysis and Protection of Three-Level Neutral-Point-Clamped PWM Voltage Source Converters

This paper presents the detection and protection methods uniquely associated with three-level neutral-point-clamped voltage source converters. The line-to-ground fault can cause significant dc-link neutral voltage and can therefore be effectively detected and located through monitoring the dc-link midpoint voltage without extra hardware. The inner device short in the three-level phase leg can lead to dc-link voltage imbalance and doubling the stress on capacitors and devices, which must be suppressed for the safe operation of the converter. Two overvoltage-limiting methods are proposed to limit the overvoltage. The design of both ground fault detection and overvoltage limiting has been covered in the paper. The schemes are verified through simulation and experiments.