A modulation strategy for single-phase HB-CMI to reduce leakage ground current in transformer-less PV applications

Abstract

Single-phase cascade multilevel inverters are able to synthesize a sinusoidal AC voltage by means of multiple voltage levels at the output. This feature makes them appealing for photovoltaic (PV) applications given their low harmonic distortion and therefore, the small size of the filter used. However, the cascade multilevel inverter (CMI) is not directly functional in grid-connected PV transformer-less systems since the variable voltage across the stray capacitances embodied between the PV panel and the ground, may generate large leakage ground currents. In order to address this problem, it is proposed a modulation strategy to control the HB-CMI. This modulation strategy allows to operate the converter with very low leakage ground currents. Among the advantages of this CMI topology is that the amplitude of the output voltage, compared with a single HB topology, is higher and makes possible the power injection into the grid without a voltage boost stage. Numerical results show that the proposed modulation strategy reduces the leakage current and make the topology suitable for transformer-less PV applications.