Advanced and Comprehensive Control Methods in Wind Energy Systems
Renewable energy solutions like solar power access from photovoltaic modules, energy harness from wind turbines etc. conform to sustainable means of reliable and clean electrical energy across the world. The integration of wind energy conversion systems (WECSs) into the electrical grid is increasing rapidly as they are economically feasible, environmentally clean, and are safe renewable power sources compared to conventional coal and nuclear power plants. However, WECSs are considered as fluctuating power sources due to uncertain nature of the w'ind, wind shear, and tower shadow effect. Due to intermittent nature of the WECSs, they introduce mechanical oscillations and large torque ripples that have negative impacts on the grid. Therefore, it is necessary to control the WECSs to minimize the power fluctuations and capture the maximum power out of various wind speeds. Regarding this, a comprehensive discussion of various control techniques such as pitch control method, inertial control, direct current vector control, sliding mode control, model predictive control system, and coordinated control of WECSs is presented in this chapter.
State-of-the-art WECSs can be realized as fixed-speed and variable-speed types. In a fixed-speed WECS, wind turbine always spins at the same rotor/generator speed during the entire operation disregarding the wind speed (or any change in wind speed). Therefore, the tip-speed ratio changes with wind speed and the rotor aerodynamic behavior is optimal at a referenced speed. In case of variable-speed WECS, wind turbine spins in proportion wfith the changes in w'ind speed, thus the rotor/ generator speed is allowed to vary accordingly. As a result, a constant tip-speed ratio is maintained and optimal aerodynamic behavior is obtained for the complete range of wind speed variation - between cut-in and rated (nominal) speed during operation. However, above the nominal speed the rotor/generator speed is kept constant. Variable-speed WECS needs to be controlled actively for effective pow'er and torque generation and maintenance.
As an active control method designed for variable-speed WECS, pitch control implies regulation of the blade angle in regard to execute controlled aerodynamic power extraction from wind. Inertial control method takes into account the physical properties and locomotion characteristics of the w ind turbine blade to utilize its kinetic energy and inertia for power regulation. Direct current vector control method is a more developed and effective technique than the indirect current control to have improved regulation of maximum wind power extraction, system reactive pow'er and grid voltage support. Nevertheless, more advanced and complex control algorithms are designed and implemented in WECS frameworks w'hich are capable of dealing with complex performance optimization constraints, highly nonlinear perturbations, dynamic uncertainties during wind turbine operation, and overall system disturbances. There are a number of novel control strategies proposed and applied by the researchers and practitioners such as sliding mode control, model reference adaptive controller, fuzzy-logic based mechanism, modal analysis of dynamics, feedback linearization control, genetic algorithms, particle swarm optimizer, model predictive control, multiple model predictive controller, combined active and reactive pow'er control, coordinated control etc. Among these, in this chapter, fundamental concepts of sliding mode, model predictive and coordinated control schemes are articulated.
WIND ENERGY CONVERSION SYSTEMS
The mechanical power output from the wind turbine can be expressed as follows:
where, PK is the mechanical power extraction from the wind turbine, p is the air density (kg/m3), V№ is the wind speed (m/s), R is the radius of the wind turbine blade (m), CP (A, p) is the power coefficient which is a function of the tip speed ratio A and blade pitch angle p (°) and is defined as:
where, com is the rotational speed of the wind turbine blade (rad/s).
The plot of wind power vs. rotational speed is incorporated in Figure 6.1 for different wind speed. Figure 6.1 is obtained using power coefficient curve for pitch angle p= 8° in Figure 6.2. Figure 6.1 demonstrates that the maximum power point changes owing to the change in rotational speed at various wind speed. Therefore, it is necessary to track the maximum power point at variable wind speed. This is achieved by the wind turbine controller that decreases the pitch angle to extract more aerodynamic power if wind speed is below the rated speed and increases the pitch angle above rated speed to reduce aerodynamic power as illustrated in Figure 6.2. Thus, regulating the pitch angle wind turbine controller minimizes the power fluctuations caused by the variable wind speed. The characteristics of wind power with respect to wind speed are demonstrated in Figure 6.3. As shown in the figure, the output power of WECS is approximately zero below cut-in wind speed (5 m/s), and below rated wind speed, wind turbine controller tracks the maximum power point shown in Figure 6.1. The wind turbine operates at rated power above the rated wind speed, however, once wind speed reaches cut-out wind speed (25 m/s), WECS is disconnected form the system to prevent mechanical damage.