Efficiency Optimization Control of Doubly-Fed Induction Generator Transitioning into Shorted-Stator Mode for Extended Low Wind Speed Application

Transitioning a conventional doubly-fed induction generator (DFIG) into a shorted-stator mode extends its operation for low wind speed power application. Since the stator of the shorted-stator mode disconnects from grid, the grid's voltages no longer contribute to building up of stator flux linkage. Stability boundaries for both modes are predicted using eigenvalue computations from steady-state models obtained from both electrical and mechanical dynamics. Propensity for instability at sub-synchronous stator frequency is investigated for situations such as sub-synchronous resonance. An effective control scheme that transitions the conventional DFIG to and from the shorted-stator mode is developed for loss minimization and flux weakening. Robustness of the controller to system parameter mismatch is investigated. Simulation and experimental results are presented for a 5Hp machine.