Variable Frequency Operation of Stand-Alone Doubly Fed Induction Generator Connected to the DC system
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This research focuses on the stand-alone operation of doubly fed induction generator (DFIG) with variable stator frequency. In stand-alone configuration, the stator side is connected to a three phase diode rectifier and a stand-alone electrical load. The dc output voltage regulation is achieved by applying stator flux oriented control scheme in synchronous reference frame. The losses that occur in the power electronics components depend on the current flowing through the switches and so in the power rating of the converter. WECS based on DFIG are designed for variable speed operation. So, the main objective of the DFIG control strategy is to regulate the generator speed so that maximum amount of wind energy is extracted for the given speed of the wind. But the generator cannot be run in full speed range if the converter rating used is smaller than the rated power of the turbine. Thus, the speed range of the DFIG is determined by the converter rating and hence the slip is limited using variable frequency operation of DFIG to reduce the converter rating and extend the speed range of operation. The dc-link capacitor voltage is regulated to a constant rated value and so the stator voltage amplitude is also constant. The flux-frequency product being constant, a variation in stator flux magnitude results in a frequency variation. So, the stator frequency can be scheduled in such a way that the output power is maximized. Some simulation tests have been run during this work to find the optimal stator frequency for a fixed range of speed and used as a look up table to improve the power conversion efficiency of the whole generator system.