Modified SRF-PLL to operate under unbalance grid for grid synchronization of DVR

Abstract

Dynamic Voltage Restorer (DVR) injects a compensated voltage to mitigate undesirable impacts on critical loads due to voltage sag/swell. In this work the controllers for DVR is proposed in stationary reference frame and injects a voltage which is in phase with the positive sequence of grid voltage. The method for load reference generation for stationary reference frame control is highly determinantal in the performance and operation DVR. Unlike other FACTS devices, DVR deals with unbalanced voltages (grid side) as well as balanced voltages (load side). This paper deals with the selection of appropriate method for load reference generation for proper operation of DVR. Synchronization issue faced due to load reference generation from load side is elaborated. Hence the load reference is generated from unit vectors extracted from the utility grid voltage. During sag/swell, the grid voltage will be unbalanced and the unit vector generation method should be able to accept/operate in the unbalanced condition. A modified synchronous reference frame PLL is proposed as appropriate method for unit vector construction for proper operation of DVR.