A new approach of prevention of DC current component in transformerless grid-connected PV inverter application

In grid-connected photovoltaic applications, a supply-frequency output transformer is normally used to isolate the inverter from the supply. This transformer is heavy, costly and adds to the overall power loss. However removal of the output transformer can result in unwanted DC components appearing in the inverter output current. Excessive DC current injection into the distribution network can affect distribution components as well as other loads connected to the network. There are various circuits which can be used to grid connection without the use of an output transformer. These include the 2-level half-bridge and the H-bridge inverters. These circuits have the disadvantage of the requirement for higher rated power devices or increased EMI problems due to high frequency switching of the DC link relative to earth. To overcome these problems, a three-level half-bridge inverter circuit is used, where the DC-link voltage can be twice the device voltage rating allowing the use low rated switching devices. The neutral conductor is connected to the mid-point of a split rail supply from PV array, and therefore the DC link voltage is not switching relative to earth. This paper applies an auto-calibrating DC link current sensing and control technique to monitor and calibrate the DC link current sensors used in the single-phase, 3-level half-bridge inverter. Characteristics of the proposed method are illustrated using simulation and experimental results.