Analysis and mitigation of common mode voltages in photovoltaic power systems

A typical photovoltaic (PV) power system is composed of multiple strings of PV arrays with one central inverter or a combination of PV modules with micro-inverters. Per-module DC-DC converter and series connected multilevel inverter topologies are possible candidates. In this paper, the analyses of common mode voltages and currents in the afore-mentioned PV topologies are detailed. The grid integration of PV power employs a combination of pulse width modulation (PWM) DC-DC converters and inverters. Due to their fast switching nature a common mode voltage is generated with respect to the ground, inducing a circulating current through the ground capacitance. Common mode voltages lead to increased voltage stress, electromagnetic interference and malfunctioning of ground fault protection systems. This paper analyzes the common mode voltages and current present in high and low power PV systems with different topologies. Mitigation strategies such as common mode filter and transformer shielding are proposed to minimize common mode voltages and currents. A practical rooftop PV system rated at 2.76 kW per string is used in the analysis.