Analysis of Common Mode Voltage in Fuel Cell Power Conditioners Connected to Electric Utility

Fuel cell stacks produce a DC output with a 2:1 variation in output voltage from no load to full load. A power conditioner consisting of DC-DC and DC-AC converters is required for utility interface. The DC-DC and DC-AC stages are operated in high frequency employing PWM control for voltage and current regulation. The switch mode nature of the power converter generates common mode voltage with respect to ground. The presence of high frequency common mode voltage with respect to ground contributes to circulating ground current which can interfere with ground fault protection system and also contribute to neutral shift and electro magnetic interference (EMI). This paper presents an analysis of common mode voltage in fuel cell powered converters connected to electric utility and discusses several mitigation methods. Several possible fuel cell power converter topologies are considered for utility scale generation.