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.

I. Introduction

Pv INVERTER technology is primarily driven by cost, efficiency and reliability. For this reason, a diverse range of grid connected PV inverter topologies and control methods have been developed and researched in industry and academia. Among these innovative topologies, transformer-less grid connected inverters have drawn increasing attention in low-power PV applications [1], [2]. Compared to alternative inverter systems which typically insert a bulky transformer between the PV panel and the utility grid, transformer-less topologies generally offer improved efficiency with lower cost and volume [2]–[6]. However, the lack of galvanic isolation has technical and safety implications. Among these implications, dc current injection into the grid continues to be a major technical challenge.