Smooth mode transition of A DC bus voltage controlled PV inverter using a novel phase locked loop method

This paper describes a control algorithm for smooth transition from standalone (SA) to grid connected (GC) mode of a PV connected two stage inverter. High voltage DC bus is obtained by using a frontend boost converter. The DC bus voltage is kept regulated during all the modes of operation. A new phase reference generation method is also described which does not require any lookup table. The PLL synchronization method is also described and verified with experiments. The system runs in voltage control mode during standalone mode of operation keeping output voltage constant irrespective of change in load. If grid becomes available, it performs a smooth transition from stand alone to grid-connected mode. If grid fault occurs grid voltage falls bellow allowable level. In this situation system does a mode transition and goes to SA mode. In GC mode system runs in current control where current reference comes from maximum power point tracking algorithm. The strategy has been verified with experiments, and results are presented in this paper.