Limitation of direct current (DC) injection into the alternating current (AC) network is an importantudoperational requirement for grid-connected photovoltaic (PV) systems. One way to ensure that this requirementudis met is to use a power transformer as interface to the AC network. But this adds costs, mass, volume and power losses. In a transformerless system, the inverter forming part of the PV system has to be operated so that the DC content in its output current is below the specified limits. Ideally, there should be no DC at the output ofudthe inverter, but in practice, in the absence of special measures, a small amount of DC current is present. Audtechnique for elimination of the DC-offset is proposed. It is based on the sensing of the DC-offset voltage at theudoutput of the inverter. A sensor output is used to drive a feedback system designed to control the operation of the inverter so that the DC-offset is forced to stay within the acceptable limits. A mathematical model for the DC-offset controller is derived. A design procedure, based on the model, is proposed for the controller. The results of tests performed on a system of 1 kW nominal rating provide validation for the mathematical model.
展开▼
