Unified Grid Integration Algorithm for Synchronization and Power Control of Doubly-Fed Induction Generator

Grid integration of a doubly-fed induction generator (DFIG) system requires synchronization of stator induced voltages with grid voltages before grid connection, and active and reactive power (PQ) control after grid connection. This paper proposes a unified control algorithm for both synchronization and power control of a speed sensorless DFIG. Closed form expressions are derived for rotor current references, which are valid for both synchronization and PQ control. Synchronization of stator induced voltages with the grid voltages requires rotor position information which, in turn, needs sensing of stator voltages. This requirement of stator voltage sensors is eliminated by parking the rotor at a known initial position, prior to initiation of control. A method for parking the rotor at a desired position through rotor side current controllers is explained and is demonstrated experimentally. Experimental results show that the transients are negligible, when the breaker between the DFIG and grid is closed. The experimental platform is a 10 hp DFIG coupled to a squirrel cage induction machine in the laboratory.