Efficiency optimization of doubly-fed induction generator transitioning into shorted-stator mode for extended low wind speed application

The rotor voltage of a doubly-fed induction generator (DFIG) increases with decrease in wind speed. Hence, much decrease in wind speed can result in the rotor voltage exceeding its rated value. In order to extend the operation of the DFIG for low wind speed application, the stator terminals of DFIG are shorted to one another whereby the stator loses its synchronous angular frequency characteristics. Transitioning into a shorted-stator mode ensures an operation whereby the rotor voltage decreases with decrease in wind speed. Since the stator of the shorted-stator mode is disconnected from the grid, the voltage of the grid no longer contributes to the development of the stator flux linkage. Therefore, for an effective transitioning, a control scheme is developed to ensure that the resulting transients and stator flux linkage are regulated within permissible limits with the objective of minimizing copper and magnetization losses. Some results are presented for a 5Hp machine.