Mitigation of DC Current Injection in Transformer-Less Grid-Connected Inverters Using a Voltage Filtering DC Extraction Approach

Transformer-less grid-connected PV inverters are an attractive technology choice due to their high-efficiency performance and relatively small volume. However, in the absence of galvanic isolation, dc current can potentially flow into the grid and adversely affect the grid performance. In addition, as the sinusoidal ac output is significantly larger than the dc injection component, accurate dc current sensing at the inverter output remains a technical challenge. As a result, this paper proposes a two-stage dc voltage detection technique that employs an isolated RC attenuation circuit and a software dc-component filtering algorithm. The principle of the proposed technique can be applied in many switch-mode power converter topologies. The simple and cost-effective combination facilitates dc component extraction within the accuracy of 1 mV; moreover, this can be further used as digital dc feedback for active current control purposes. Experimental results from a laboratory prototype demonstrate the high performance of the proposed dc extraction and suppression approach.