Optimal Gearbox Ratio to Minimize the Power Converter Ratings in DFIG-based Type-3 Wind Turbines

Type-3 wind turbines based on the doubly-fed induction generator with a partial-scale power converter are commonly implemented in wind energy conversion systems. This paper investigates the effects of the rated slip and thus the gearbox ratio of the wind turbine on the ratings of the partial-scale power converter. Three design methods are proposed to select the optimal rated slip and gearbox ratio. The first design method is aimed at minimizing the converter's rated power compared to the system's rated power. The analysis showed that a rated slip of -11.84% will minimize the converter ratings. The second design method is based on selecting the rated current of the converter while the third design method is aimed to maximize the power produced at low wind speeds. The three proposed design methods are analyzed and compared to the conventional design method for a 2 MW wind energy conversion system. Simulation results for the complete system were performed where the dynamics in the wind speed and the thermal model of the Silicon-Carbide MOSFET power converter are considered. The simulation results verified the effectiveness of the proposed methods. Using the first proposed method, the power converter rating is only around 10% of the rated power. The third proposed method can be used to maximize the power produced at low wind speeds to maximize the annual energy production.