DC Injection Suppression in Transformer-Less Grid-Connected Inverter using a DC-Link Current Sensing and Active Control Approach

There is increasing interest in the use of transformer-less systems for grid-connected photovoltaic applications. Compared to transformer-coupled solutions, transformer-less systems offer a typical efficiency increase of 1-2%, reduced system size and weight, and a reduction in cost. However, the removal of the transformer has technical implications. In addition to the loss of galvanic isolation, dc current injection into the grid is a potential risk. While desirable, complete mitigation of dc current injection via convention current control methods is known to be particularly challenging. For this reason, this paper proposes an active dc suppression method, in which the dc current injection is accurately determined by extracting the line-frequency component from inverter dc-link current measurements, and then, mitigated with an active closed-loop controller. Experimental results from a laboratory grid-connected inverter system are presented to demonstrate the high performance of the proposed technique.