DC current offset compensation technique in grid connected inverters

This paper proposes dc current offset compensation technique of grid connected inverters which may applicable to medium-scale solar farms, or any group of grid connected inverters confined within a relatively small area. DC current injection into ac grid severely affects distribution system components such as isolation transformers, measurement units and protective relays. The proposed dc current offset compensation technique exploits a simple proportional-resonance (PR) controller that aims to suppress dc offsets to zero in the output currents of grid-connected inverters before entering the main 50/60Hz interfacing transformer. By employing this technique, the capital cost of a medium-scale solar farm is reduced since it enables transformerless operation of all inverters within the farm, and uses only one power transformer at point of common coupling. Additionally, this technique does not rely on high-precision current measurement devices nor uses any form of complex transformer. Simulation and experimentation are used to demonstrate the practicability of the proposed system.