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

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

[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.

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