Minimum copper loss control of doubly-fed induction generator for wind turbines

This paper proposes the control algorithms aimed at minimizing the copper loss of a doubly-fed induction generator in wind energy conversion system. In the system, the stator is directly connected to the grid while the rotor is connected to the grid through the back-to-back converter. Typically, both the stator and grid-side converter currents are controlled to be in-phase with the grid voltage for unity power factor in grid. However, it is not the optimal solution, considering the energy efficiency. The losses can be reduced under the condition of the minimum copper loss (MCL) operation. In this paper, the optimal set of rotor current commands is calculated from the analysis of torque and power properties. In this case, the grid-side converter takes the role of the reactive power compensation for unity power factor in grid. So, the required amount of the converter current is also estimated. For the MCL operation, the proposed method manipulates the d-q rotor currents and d-axis converter current while the grid power factor, speed, and dc link voltage are simultaneously controlled. The feasibility and MCL tracking performance of the proposed control system are verified by simulations under various operating conditions.