A new controller design and analysis of DFIG wind turbine systems for MPP operation

In this paper, a simplified and easily implemented MPP control scheme is proposed for DFIG wind turbine systems. In particular, taking into account the complete nonlinear model of the system in the synchronously rotating dq reference frame operating at the voltage oriented control mode, a cascaded q-axis controller scheme and a simple proportional d-axis controller are proposed. The inner-loop of the cascaded q-axis controller does not require decoupling networks and is used to achieve the desired q-axis current damping. The outer-loop PI controller enforces the MPP operation of the system by continuously tracking the rotating speed to the optimum. Its structure is slightly modified in a way that permits a second order linear system analysis. Stability and convergence to the equilibrium is proven by taking into account the inner-loop current controllers in the nonlinear analysis of the complete system. Extensive simulation results on a 2MW DFIG wind turbine system illustrate the enhanced system performance and verify the effectiveness of the controller.