Reactive power control in doubly-fed induction generators for wind turbines

High penetration level of wind power generation in the interconnected electrical network and the increasing rated power of the wind energy converter units imposes new technical challenges to engineers. The first one is to guarantee grid stability despite the stochastic nature of wind while the second one is to limit the power losses since electrical machines efficiency do not augment proportionally to the rated power increase in the MW-class. Attending to the new requirements of the power companies on wind turbines concerning the contribution to network stability through reactive power exchange with the mains supply, this work proposes a reactive power control scheme for a doubly-fed induction generator drive that minimises the system power losses. This drive topology enables besides the control of active power the independent control of reactive power with both inverters. This degree of freedom is used in order to share the reactive currents resulting on the reactive power required on the network and increasing the efficiency. The steady-state power losses are modeled and the optimal reactive power share working points are found by an iterative procedure. The control structure design is presented and discussed as well as implementation issues. Experimental results are presented in order to prove the theoretical assumptions.