A Single Stage Common Ground Three-Level PV Inverter With Integrated Power Decoupling

Abstract

In this paper, a T-type common ground transformer-less single phase inverter with dynamic swing of the dc-link voltage is presented for photovoltaic (PV) application. The topology is a combination of a bi-directional, partial-power-processing boost stage and an asymmetric half-bridge inverter stage along with a T-branch. It takes advantage of the three-level switching states with reduced voltage stress on the main switches to achieve lower switching loss and almost one-half the inductor current ripple w.r.t. two level implementation. The double line frequency power decoupling is addressed by a dynamic dc-link approach, which allows a large swing of the dc-link to reduce the decoupling capacitor requirement, enabling an all-film capacitor implementation. This topology also significantly reduces the high-frequency capacitive coupled ground current by directly connecting the PV negative terminal to the grid neutral. Moreover, an adaptive dc-link voltage control scheme that optimally changes the average value of the dc-link voltage as the operating conditions (load and power factor) vary, has been proposed and thoroughly investigated from the perspective of better utilization of passive components and further reduction of switching losses. A SiC MOSFETs-based 1 kVA laboratory prototype has been built to validate the converter's operation at 200 V dc nominal input and 120 V/60 Hz ac nominal output with a wide range of power factor and load operations. Extensive experimental results validate the superior performance of the topology with the adaptive dc-link voltage control implementation showing a peak efficiency of 98.22% and a CEC efficiency of 98.03% at 50 kHz switching frequency.

Keywords

• DC-AC power converters
• leakage currents
• wide band gap semiconductors