Time-frequency based power quality analysis of variable speed wind turbine generators

Wind energy is an important contributor to the evolution of the smart grid, but the power electronics necessary to interface a wind turbine generator (WTG) to the grid can have a negative impact on the power quality (PQ) of the grid if the effects are not well understood. In this paper, the wind energy conversion system is reviewed with models of the two most common types of WTGs: a doubly-fed induction generator (DFIG) and a synchronous generator (SG). Time-frequency analysis is introduced as a unique, advantageous way of quantifying and comparing the time-varying disturbances introduced by these generators, which can not be accomplished from the existing PQ indices published by International Electrotechnical Commission (IEC). Time-frequency distributions and metrics are employed to analyze the PQ effects of the DFIG and SG under two case studies: a three-phase-to-ground fault and a generator trip and recovery. The analysis of these studies reveals time-varying frequency information that can be used to properly compensate for such events in the future depending upon the type of generator employed and improve the quality of power these types of WTGs inject onto the grid.