Maximum power point tracking for variable-speed fixed-pitch small wind turbines

Variable-speed, fixed-pitch wind turbines are required to optimize power output performance without the aerodynamic controls. A wind turbine generator system is operated such that the optimum points of wind rotor curve and electrical generator curve coincide. In order to obtain maximum power output of a wind turbine generator system, it is necessary to drive the wind turbine at an optimal rotor speed for a particular wind speed. In fixed-pitch variablespeed wind turbines, wind-rotor performance is fixed and the restoring torque of the generator needs to be adjusted to maintain optimum rotor speed at a particular wind speed for maximum aerodynamic power output. In turbulent wind environment, control of wind turbine systems to continuously operate at the maximum power points becomes difficult due to fluctuation of wind speeds. Therefore, special emphasis is given to operating at maximum aerodynamic power points of wind rotor. In this paper, the performance of a Fuzzy Logic Maximum Power Point Tracking (MPPT) controller is investigated for applications on variable-speed fixed-pitch smallscale wind turbines. INTRODUCTION Variable-speed wind turbines are generally characterized as having higher efficiency than fixed-speed wind turbines and hence are becoming more popular, particularly for small wind turbines. Typically, variable-speed wind turbines are aerodynamically controlled, usually by using power electronics, to regulate the torque and speed of the turbine in order to maximize the output power. Variable-pitch aerodynamically controlled wind turbines are more costly and complex. Therefore, variable-speed fixed-pitch approach is becoming more popular for low cost construction and is the most common scheme for small wind turbines. In this scheme, a MPPT control mechanism is used to control the restoring torque of the electrical generator for optimum operation of the wind turbine system [1]. The performance of variable-speed fixed-pitch wind turbines could be optimized without the need for a complex aerodynamic control. These turbines are usually operated such that the relevant optimum points of wind rotor curve and electrical generator curve coincide, as shown in Figure 1. Therefore, in order to obtain maximum output power from the turbine, it is necessary to drive it at an optimal rotor speed for a particular wind speed. CONTROL STRATEGIES Wind speed, turbine rotational speed and turbine rotor characteristics are the main factors that determine the maximum power points. The electrical-generator characteristics may be used in order to control the restoring torque to track the optimum operation points. If wind speed is varied from V1 to V2, the rotor speed should be changed from ω1 to ω2 for optimum operation of the wind turbine (see Figure 1). However, rotational speed of the wind turbine cannot be changed instantaneously. Usually, a controller that employ wind speed sensor (or in some cases, sensor-less control) is used to control the wind turbine. In systems that employ wind speed sensors, the sensor provides the reference signal to the MPPT controller. This reference is compared with the power extracted from the wind energy converter. In sensor-less control technique, no anemometer is used to provide the wind speed information; hence, it is essential to estimate the wind speed. The generator output frequency and power or torque mapping techniques are used to track the MPP [2]. Another way for MPP tracking is the use of “searching” method, which is a suitable strategy for small wind turbines [3, 4]. The latter operates without knowledge of system parameters. The output power is used as feedback signal for the perturbation & observation algorithm, which is used to find the maximum power point of the system. Figure 1Operating points of wind power system V1 V2 Wind rotor curves Restoring power curve of generator Maximum aerodynamic power points of wind rotor Rotational speed Po w er