Multilevel transformerless topologies for single-phase grid-connected converters

Multilevel voltage source inverters synthesize the AC output terminal voltage from several levels of voltages, stepped waveforms can be produced, which approach the reference waveform with low harmonic distortion, thus reducing filter requirements. The need of several sources on the DC side of the converter makes multilevel technology attractive for photovoltaic and fuel cells applications. For low-power grid connected applications a single phase converter can be used and it is possible to remove the transformer in the inverter in order to reduce losses, costs and size. Galvanic connection of the grid and the DC sources in transformerless systems can introduce additional leakage currents due to the earth capacitance. This currents increase conducted and radiated electromagnetic emissions, harmonics injected in the utility grid and looses. Amplitude and spectrum of leakage current depends not only on converter topology, it depends also on switching strategy and resonant circuit formed by the ground capacitance, the converter, the AC filter and the grid. In this paper several topologies are analyzed and compared focusing on leakage current potential.