Direct Voltage Control With Slip Angle Estimation to Extend the Range of Supported Asymmetric Loads for Stand-Alone DFIG

The control system for stand-alone doubly fed induction generator (DFIG) is commonly based on the "sensorless" direct voltage control. This paper adopts the direct voltage control and employs negative-sequence compensation through rotor-side converter to support asymmetric "unbalanced" loads. This paper demonstrates the limitation of the conventional direct voltage control to obtain the slip angle, and the consequent limitation of the negative-sequence compensation control to support full range of asymmetric loads. To overcome these limitations, this paper proposes a new estimator of the angle of the rotor current in the synchronous reference frame. The proposed estimator requires one DFIG parameter only which is the stator inductance, which can be measured allowing using real machine parameter. This paper proposes a simple method to integrate the proposed estimator into the direct voltage control, which enables it to obtain the slip angle, and it enables the negative-sequence compensation to support full range of asymmetric loads. Experimental results for a 1.1-kW prototype DFIG are provided to validate the proposed method and to demonstrate its advantages over the conventional direct voltage control.