Improved full-bridge neutral point clamped transformerless inverter for photovoltaic grid-connected system

Abstract

Characterized by high efficiency and low cost, single-phase transformerless inverters are suitable for grid-connected photovoltaic (PV) generation especially at low power (<;5kW). Since galvanic isolation is omitted, leakage current can flow through stray capacitance between the PV array terminals and the ground. Therefore, the leakage current generated by transformerless inverter has to be regulated in order to meet the safety requirements. Full-bridge neutral point clamped (NPC) inverter topologies feature reduced input voltage and low leakage current. Hence, they are preferred in the applications like transformerless grid-connected PV systems. In this paper an improved full-bridge NPC transformerless inverter, which eliminates leakage current is presented. Unipolar sinusoidal pulse width modulation (SPWM) is used to achieve high efficiency and three-level output voltage. The detailed operating principle and the effect of parasitic capacitance of switches on common-mode voltage are described. Finally, the improved leakage current performance of full-bridge NPC inverter is verified by simulation studies and results are presented for peak power rating of 1kW.