Power quality and efficiency improvements for transformerless grid connected PV inverters

Abstract

There is currently significant growth in the number of residential scale grid connected photovoltaic (PV) systems. Generally, if environmental costs are not accounted for, the cost of generation from PV sources remains high compared to conventionally generated electricity. There is, therefore, a strong incentive to reduce the cost of PV systems and improve their efficiency while satisfying injected power quality standards. The focus has been on the power electronic single phase converter bridge which is typically used as interface between the DC output of the PV panels and the terminals of the AC network. One of the two main objectives was to investigate the possibility of operating an inverter connected to the AC grid without an interfacing power transformer, while keeping DC injection into the grid below levels specified by Australian and International Standards. A low cost method of controlling the DC offset content of the current injected into the AC mains has been proposed. A mathematical model of the DC offset controller has been developed and experimentally validated. A design procedure for the controller has also been developed. The second objective was to investigate the dependence of efficiency on inverter switching strategies such as bipolar and unipolar switching. It was confirmed by specially designed tests that unipolar switching meant significantly lower switching losses when compared with bipolar switching. However, the quality of current injected into the AC mains in the case of unipolar switching was not considered acceptable because of significant current distortion near the AC mains voltage zero crossing. A new method of inverter switching has been proposed which exploits the efficiency advantage of unipolar switching while avoiding the problem of current distortion. The main outcomes of this project are a DC offset controller which allow transformerless operation and an inverter switching strategy that results in significantly reduced switching losses while maintaining the quality of injected current. Implementation of the proposed DC offset controller and inverter switching strategy will result in both capital cost savings and higher conversion efficiency.