Leakage current reduction of transformerless three-phase cascaded multilevel PV inverter

Multilevel inverter is an attractive solution for the high power transformerless PV system. In contrast to the neutral-point clamped (NPC) or flying capacitor (FC) multilevel inverter, the dc-link voltage rating is much lower for the cascade H-bridge multilevel inverter. Therefore, the voltage stresses of the semiconductor switches are reduced. However, more than one PV array is needed for providing the separate dc-link voltage, and thus there are more paths of the leakage currents. Consequently, it is much more challenging to deal with the leakage currents than the existing scenarios. In this paper, the common mode model for the conventional cascade H4 PV inverter is analyzed. The relationship between the leakage currents and the common/differential mode voltages are mathematically expressed. And the reason why the conventional cascade H4 inverter fails to reduce the leakage current is comprehensively clarified. In order to solve the problem, a new cascade H5 inverter is proposed to deal with the leakage current. Theoretical analysis and test results are presented to verify the effectiveness of the proposed new solution, which provide a new insight for the leakage current reduction of the transformerless three-phase cascade multilevel PV inverter.