Variable speed constant frequency wind turbine rated constant speed above constant power control
As an inexhaustible, clean and pollution-free renewable energy source, wind energy has been widely used by countries all over the world. As one of the countries with abundant wind resources, China has made rapid progress in the localization of wind turbines. During the “Ninth Five-Year Plan” period, it achieved 96% localization rate of 600kW wind turbines and successfully developed 600kW stall winds. The key technology of the genset control system. At present, we have undertaken the research and development of the national 863 "megawatt-class variable-speed constant-frequency wind turbine electrical control system", and the research and development work is being carried out actively and effectively.
One of the great advantages of variable speed constant frequency wind turbines compared to stalled wind turbines is the stable output power above the rated wind speed. When the variable-speed constant-frequency wind turbine is operated above the rated wind speed, it is necessary to make the output power above the rated power point stable, avoid fluctuations, and make the generator set transmission system have good flexibility, and also consider effective protection for the wind turbine. At present, the MW-class variable-speed constant-frequency wind turbines we have developed mainly use pitch control technology. The pitch control technology is to change the aerodynamic torque obtained by the wind turbine by adjusting the pitch of the blade and adjusting the aerodynamic torque obtained by the wind turbine when the wind speed is too high, so that the power output of the unit is stable. This control strategy uses a power feedback closed-loop control system to achieve the control target above the rated wind speed of the variable speed constant frequency unit. Pitch Mechanism Introduction The pitch actuator consists of a mechanical and hydraulic system that adjusts the blades of the fan along the longitudinal axis of the fan. Because the inertia of the blade is large and the pitch actuator should not consume a lot of power, the actuator has the limiting ability, and its dynamic characteristics are nonlinear dynamics with saturation limits at both the pitch angle and the pitch rate. When the pitch angle and the pitch rate are less than the saturation limit, the pitch dynamics are linear. The pitch actuator is shown in Figure 1.
The actuator model describes the dynamics between the pitch angle command from the controller and the excitation of the command. The mathematical model can be described as the following first-order system
The given value in the actual control system is the control voltage from the pitch angle deviation to the proportional valve -DC10V ~ +DC10V.
Controller design
The basic purpose of this controller is to adjust the constant power output by adjusting the pitch angle. As shown in FIG. 2, the current generator output power P is measured by power collection. The power error ΔP is calculated compared to the given power P*. The deviation of the power is used as the input of the PID controller, and the controller commands the current pitch angle error Δβ=β*-β (the current pitch angle β) according to the command of the household to issue the β* of the blade reference pitch angle. Then, the pitch change rate is determined according to the parameters of the variable pitch mechanism. The reference pitch angle is limited to the range of 0 to 92°, within which the controller adjusts the wind turbine blades to the new pitch angle requirements.
In the box of Fig. 2 is the PID controller. The stable value range of the proportional, integral, and differential gains Kp, Ki, and Kd is determined by the Rolls stability criterion of the closed-loop transfer function shown in the figure. The gains of proportional, integral, and derivative are obtained by simulation. The principle is to maintain the fan power output at the rated output power.
Simulation results
1) The rate of change of the pitch angle varies within the range of -5°/s to +5°/s allowed by the hydraulic system.
2) The change of the pitch angle β is the same as the change trend of the wind speed v, the wind speed v increases, and the average pitch angle β increases; on the contrary, the wind speed v decreases, and the average pitch angle β decreases.
3) The power Pmech and the wind energy utilization coefficient Cp of the instantaneous absorption of the fan blades show that the change of the pitch angle β limits the power Pmech of the blade instantaneous absorption, and the blade works at a lower efficiency.
4) The generator output power Pe can be smoothly changed around the rated power by the change of the pitch angle β to maintain constant power.
5) The fluctuation of the generator speed is affected by (Pmech-Pe) and the inertia of the unit.
