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 subsynchronous stator frequency is investigated for situations such as subsynchronous 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 5 Hp machine.