DC offset elimination in a single-phase grid-connected photovoltaic system

Abstract

[Abstract]: Australian Standard AS 4777.2-2005, section 4.9 imposes limits on DC injection into the AC network by grid connected inverters. One way to ensure that this requirement is met is to use a power transformer as interface between the output of the inverter and the AC network. But this adds costs, mass, volume and power losses. It is, therefore, an advantage to design the inverter system so that zero DC offset is guaranteed at its output. Ideally no DC would be expected at the output of the inverter. In practice, however, in the absence of special measures, a small amount of DC is present because of circuit component imperfections. Techniques that have been proposed so far for the elimination of the DC offset current are based on the sensing of the DC offset voltage at the output of the inverter. The output of the sensor is used to drive a feedback system designed to control operation of the inverter so that the DC offset is eliminated. The focus of this paper is on the mathematical modeling of a recently proposed dc offset sensor and dc offset control system. Experimental validation of the model is presented. It is demonstrated that while satisfactory performance is achievable the technique has some serious disadvantages.