Control Strategies for Variable-speed Fixed-pitch Wind Turbines

This chapter deals with a design of controllers for variable-speed fixed-pitch wind turbines. The chapter begins with an introduction to wind turbine control and the detail of nonlinear models of the wind energy conversion system (WECS). Linearization around a set of equilibrium points is presented to obtain a linear parameter variable model from a nonlinear system. Open-loop characteristics for small signals are then described to analyze dynamic behaviors under different operating conditions. The control objectives for variable-speed fixed-pitch wind turbines detailed in this chapter consists of three operating conditions: 1. Maximization of extracted energy: The wind turbine should extract wind energy at the highest efficiency to obtain the highest energy conversion ratio. Three alternative maximum peak power tracking (MPPT)-based algorithms for fixed pitch wind turbines are introduced. The first algorithm is guided by a torque reference (Mirecki et at., 2004). The second method searches an optimal operating point from the slope of the powerrotational speed curve. The last method is based on the control objective derived from a fuzzy rule base. 2. Limitation of extracted energy with active stall with rotational speed control: The aim of this control is to limit stresses on the turbine while minimizing the power fluctuations around a constant value, normally around the nominal power. 3. Control of MPPT and stall regulation at the overlapping region: This operating condition is effective with constant rotational speed control. The controller objectives, controller schemes and controller designs are discussed in detail. The developed controllers for fixed-pitch wind turbines are based on a speed and torquefeedback control scheme. The proper design of the reference signal allows accurate tracking of each control strategy along the entire operating range. Hardware and software implementation for the control algorithms are explained. The case studies were carried out with two laboratory experiments with a developed wind turbine simulator: 1) three MPPT algorithms and active stall regulation with rotational speed control, and 2) an MPPT algorithm with a grid-connected converter. The chapter is concluded in the last section. The references are also provided for further research and studies. Source: Wind Power, Book edited by: S. M. Muyeen, ISBN 978-953-7619-81-7, pp. 558, June 2010, INTECH, Croatia, downloaded from SCIYO.COM