DC current determination in grid-connected transformerless inverter systems using a DC link sensing technique

Due to the advantages of cost, volume and efficiency, transformerless grid-connected power converters have drawn an increasing attention, especially in low-power PV applications. However, the lack of isolation between inverter and grid brings in some potential risks to the distribution network. Among them, the dc injection flows into the grid remains a significant concern. Owing to the offset and nonlinearity of the current sensor and signal condition circuitry, the complete removal of this dc current is difficult via conventional current controllers. In this paper, an active dc determination is investigated based on a dc link current sensing technique. Through a software current reconstruction process, this study provides a cost-effective solution which includes robust current control and accurate dc detection. Both simulation and experimental results are presented in the paper to validate the effectiveness of the proposed technique.