Minimisation of DC current component in transformerless Grid-connected PV inverter application

Single-phase current controlled, voltage source inverters are commonly used to feed unity power factor sinusoidal current into the distribution network from PV sources. A large 50 Hz transformer is often used at the inverter output for galvanic isolation and to prevent dc current injection into the distribution network. This transformer increases cost, size and overall power losses of the system. For this reason, transformerless inverters are of increasing interest. When an output transformer is not present it is important that the output current is controlled accurately in order to minimise the DC component. The accuracy of the DC current sensor then becomes important to achieving this. A scheme is proposed in which DC link current sensing and current control are used to minimise the output DC current component. Current controllers are affected by errors associated with nonlinearity and offset in the current sensors. This paper applies an auto-calibrating DC link current sensing technique to minimise the errors produced by current sensors in the single-phase, 3-level half-bridge inverter. Characteristics of the proposed method are illustrated using simulation and experimental results with a resistive load.