Sensorless direct voltage control method for stand-alone slip-ring induction generator

A control method for stand alone mode operation of doubly fed induction generator is presented. The method bases on the stator voltage vector control represented in the synchronously rotated polar coordinates. The generated stator voltage is controlled by rotor currents. Amplitude of stator voltage and its frequency are controlled independently. The output signals from the voltage and the frequency regulators are the reference signals for the rotor current amplitude and frequency. This way, in spite of the variable generator speed, the fixed stator voltage amplitude and frequency, and controlled voltage phase are achieved. Developed direct voltage control method permits the DFIG the stand alone and the grid connected operation. After the mains outage the DFIG instantly supplies loads. Methods of supply grid voltage faults detection are developed. One method bases on the generated voltage vector amplitude, whereas a second bases on the voltage frequency. Both methods allows soft transients of generator disconnection from the grid. The controlled voltage phase permits for soft synchronization and grid connection after the mains reappearing. This way the load is protected from voltage phase rapid change. Applied methods allows the load to be uninterruptible supplied. Topology of power system, controller operation and results from laboratory tests of the DFIG are presented in the paper