Rotor Speed Fluctuation Analysis for Rapid De-Loading of Variable Speed Wind Turbines

During rapid de-loadings, the unbalanced power of a variable speed wind turbine (VSWT) will accelerate the rotor and may cause over speed problems. In this paper, we specifically analyze the rapid de-loading rotor speed response of a typical VSWT, and divide the process into four stages. Among them, we show that the third stage, when the rotor accelerates over the rated speed and the pitch control gets into action, is the most critical in terms of the maximum rotor speed during the transient phase. Then, based on the characteristics of the response trajectory, we propose an estimation method to calculate the maximum de-loading amplitude to ensure that the transient rotor speed does not exceed the limit. Extensive simulations under different wind speeds are provided to illustrate the deloading response and the effectiveness of the proposed estimation methodology.