Optimal Control Structure For Variable Speed Wind Power System

Abstract

This paper presents an optimal control structure for variable speed, fixed pitch wind turbine. The control objective results from the optimization criterion that includes two contradictory demands: maximization of the energy captured from the wind and minimization of the damage caused by mechanical fatigue. We admit, as a modeling assumption, that wind speed has two components: a slowly varying component, named seasonal and a rapidly varying component, named turbulence. Hence, two control structures, which should function simultaneously, are identified in the optimal control problem.
The first optimization structure aims to maximize the wind turbine energetic efficiency by maintaining operational point on optimal regime characteristics (ORC). According to the slowly varying seasonal component of wind speed, a control loop adjusts system operating point in order to maintain it on ORC. In the second optimization structure, the system is considered to be operating on “static” optimal point assured by the above mentioned control loop, with the turbulence component being the input variable. The second control loop aims dynamic optimization that implies the minimization of the tip speed ratio variations around its optimal value, while minimizing the torque variations, thus the mechanical stress. Mathematically, this objective is defined as an integral criterion, belonging to linear quadratic optimization.