Operation of a transformerless three-phase PV inverter using multiple bidirectional choppers

A 50-kW class three-phase photovoltaic (PV) inverter typically applies a transformerless circuit configuration due to its high efficiency and small volume/size. On the other hand, the conventional PV inverter may suffer from the deterioration of solar panels due to a dc leakage current flowing through the stray capacitance of the solar panels. To solve this problem, this paper presents a transformerless three-phase PV inverter consisting of multiple bidirectional choppers, which prevents the dc leakage current by maintaining a negative PV array voltage with respect to the ground potential to a dc voltage that is zero or higher. In addition, the size/volume of an ac filter can be reduced by applying the so-called phase-shifted PWM to the inverter. In this paper, the operating principles and control method of the inverter are discussed and the validity of the control method is verified by performing computer simulation using the "PSCAD/EMTDC" software package and conducting experiments using a 150-V downscaled model.