Minimisation of output DC current component in grid-connected inverters for solar power applications

Abstract

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 for 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. The aim of this research is to minimise the DC current component in the output of a grid-connected inverter when a supply-frequency output transformer is not used. A three-level diode-clamped half-bridge inverter is proposed to interface the PV panel directly to the utility grid. The main contribution of this research lies in the development of an auto-calibration technique for the DC-link current sensors in the multi-level inverter. Combined with a current feedback control scheme this technique allows the minimisation of DC current offset drift in the Hall-Effect current sensors. Auto-calibrated DC-link current sensors in turn allow the inverter output current controller to minimise the output DC current component in spite of sensor drift and other disturbances. A comprehensive review on the different types of grid-connected PV systems, the problems caused by DC current injection into the grid, and up-to-date techniques to overcome this problem is included. The performance of the auto-calibration technique is investigated using both computer simulation and an experimental test rig.