Control Strategies for Variable-speed Fixed-pitch Wind Turbines

Abstract

This chapter is emphasized on control strategies for a variable speed fixed-pitch wind turbine with the main objective to optimize energy capture in below- and above-rated wind speed regions. Two main laboratory experiments were conducted: 1) three maximum peak power tracking (MPPT) algorithms and active stall regulation with rotational speed control, and 2) an MPPT algorithm with a grid-connected converter. All the MPPT algorithms were implemented on a low cost DSC board and tested with a developed wind turbine simulator. The first algorithm tracks the maximum power using a torque reference obtained from the wind turbine characteristic. The second method is based on the observation that the power versus rotational speed curve has a single well defined peak. Therefore, a necessary condition for the speed being at the maximum power point is that the first derivative of the power respect to the rotational speed is zero. The third algorithm employs a fuzzy logic as the key controller. It can be concluded from the first experimental results that the MPPT with torque reference offers fastest tracking time in the below-rated wind speed region and the MPPT with fuzzy logic is favored in terms of power fluctuation and tracking time in the above-rated wind speed region. Although the second MPPT algorithm has the slowest tracking time and the highest rotational speed fluctuation, it is attractive for a small amount of computational resource and therefore low cost for implementation.