Removal of DC offset current from transformerless PV inverters connected to utility

Abstract

Some electricity supply authorities in Australia allow PV inverters of up to 10 kW output to be directly connected to their grid network. The benefits of avoiding the use of power transformers in such PV applications are higher conversion efficiency of the PV system and lower costs. These benefits however, can only be realised if the level of DC offset current injected from such transformerless inverter systems is maintained within the stringent limits governed by Australian Standard AS4777.2. Currently, there is a lack of evidence that the level DC offset current can be economically and efficiently maintained within these legal limits. In this paper, it is shown that irrespective of its source, the DC offset current can be maintained within these limits without sacrificing the overall conversion efficiency of the inverter system. Simulation and experimental results are presented to confirm that this simple, cost effective technique can be used to measure and separate small magnitudes of DC offset currents from large values of AC currents without compromising the dynamic response of the current feedback loop. This new technique can also be used to improve the quality of the power supplied by PV inverters that use power transformers.