Synchronous reference frame grid current control for single-phase photovoltaic converters

This paper proposes a high performance control in a d-q reference frame for a single-phase grid connected converter. The proposed control allows to obtain easily an infinite control loop gain at fundamental frequency and a superior disturbance rejection: this assures only active current injection thus maximum conversion efficiency and decreases current distortion. The control has been implemented in a single-stage converter for single-phase PV systems. The converter is a standard full-bridge three-level voltage source inverter that delivers a sinusoidal current to the grid through an inductive filter and a line frequency isolation transformer. The transformer blocks out the DC components injected in the grid but the current feedback, closed around the output of the transformer, can lead to core saturation. To avoid saturation a compensation strategy has been developed relying on output current distortion in correspondence of voltage zero crossings. Simulation and experimental results confirm the effectiveness of the proposed control architecture and of the strategy adopted to avoid transformer core saturation.