Sensorless Closed-Loop Voltage and Frequency Control of Stand-Alone DFIGs Introducing Direct Flux-Vector Control

In this article, an innovative control method, termed direct flux-vector control, is proposed for the stand-alone doubly fed induction generators (DFIGs) feeding local ac loads. In these systems, both the amplitude and frequency of the stator voltage must be precisely controlled. The proposed method, instead of controlling the rotor currents or voltages, directly controls the rotor flux vector, including magnitude and angle. To achieve an accurate control, two separate closed-loop hysteresis controllers are employed in which the stator-voltage amplitude and frequency are adjusted through the rotor-flux magnitude and angle, respectively. The proposed control method is performed in the rotor reference frame, thus it does not require the rotor speed/position sensors or any reference-frame transformation. As an outstanding feature, the method works for both sub and supersynchronous speed modes without changing the switching table. Hence, no need to detect the operation mode of DFIG. Besides, the method consists of a simple implementation and is almost parameter independent, as it only requires the rotor resistance. The proposed method is implemented on a 3-kW laboratory scale DFIG and its performance, effectiveness, robustness, and correctness are evaluated and discussed for a series of experimental tests.