Modeling of Extremely Short-Time Power Variations of Wound Rotor Induction Machines Wind Farms for Flicker Studies

Time-varying nature of wind farms is one of their major obstacles in providing a constant and reliable power output. They can be considered as a time-varying source of power considering different timeframes, from long to extremely short time periods. The focus of this study is modeling the wind farms for power quality studies by focusing on voltage flicker caused by the extremely fast wind farm output power variations. Despite there being several models developed for modeling the variations over longer time periods, there are few models that consider the extremely short time power variation, i.e., those in the range of 5-15 milliseconds. Our research started with the acquisition of a large data set of actual instantaneous voltage and current signals, recorded at a wind farm under different weather and operating conditions. The data set is utilized to develop practical models for the individual wind turbines and for the whole wind farm suitable for the mentioned extremely short time variations for the case of wind farms with the wound rotor induction generators (WRIG). The proposed model can be used for voltage flicker studies in power systems with WRIGs. The equivalent model of the WRIG is represented by a current source which its magnitude and phase change every half-cycle. It is observed that the variations of active and reactive powers follow a non-stationary seasonal time series where the seasonal part is not a simple single frequency. The seasonal term contains several frequencies which are modeled by 10 frequency components between 0.1 Hz to 1 Hz plus a DC component. The remaining component is modeled by autoregressive moving average (ARMA) models. The accuracy of the proposed equivalent model is assessed by several tests based on actual data and their corresponding simulated time series.