Interface control and islanding protection of grid-connected inverters based on unified constant-frequency integration control

In this paper, a new control approach based on unified constant-frequency integration control (UCI) for current tracing is presented for photovoltaic grid-connected inverters. The working theory is analyzed and the control equation is deduced, and also the islanding protection is studied. The control approach is realized by a digital controller DSP. Different from the normal SPWM and PLL(phase locked loop) control approach in grid-connected inverters interface control, this control approach has the merit of automatic phase tracking capability, simplified control and high output power factor. And it can regulate the output in accordance with the dc voltage and grid voltage automatically.