Wind energy, as an inexhaustible, clean, and pollution-free renewable energy source, has received widespread attention worldwide for its development and utilization. As one of the countries rich in wind resources, my country has made rapid progress in the localization of wind turbine generator sets. During the Ninth Five-Year Plan period, it achieved a 96% localization rate for 600kW wind turbine generator sets and successfully developed the key technology of a 600kW stall-type wind turbine generator set control system. Currently, we are undertaking the national 863 project, "Electrical Control System for Megawatt-Class Variable Speed Constant Frequency Wind Turbine Generator Sets," and the research and development work is progressing actively and effectively.
Compared to stall-prone wind turbines, a significant advantage of variable-speed constant-frequency wind turbines is their stable power output above rated wind speed. When operating above rated wind speed, variable-speed constant-frequency wind turbines must maintain stable power output above the rated power point, avoiding fluctuations, while also ensuring good flexibility in the transmission system and effective protection for the turbine. Currently, our megawatt-class variable-speed constant-frequency wind turbines primarily employ variable pitch control technology. This technology adjusts the blade pitch at high wind speeds, changing the airflow's angle of attack on the blades, thereby altering the aerodynamic torque obtained by the turbine and maintaining stable power output. This control strategy utilizes a power feedback closed-loop control system to achieve the control target above rated wind speed. The variable pitch mechanism, composed of mechanical and hydraulic systems, adjusts the turbine blades along the longitudinal axis of the wind turbine. Because the blades have a large moment of inertia, and the pitch actuator should not consume a large amount of power, the actuator has a limiting capability. Its dynamic characteristics are nonlinear dynamics with saturation limits on both the pitch angle and pitch rate. When the pitch angle and pitch rate are less than the saturation limits, 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 that command. Its mathematical model can be described as a first-order system as follows:
The setpoint in the actual control system is the control voltage from the pitch angle deviation to the proportional valve, which is -DC10V to +DC10V.
Controller Design
The basic purpose of this controller is to regulate constant power output by adjusting the pitch angle. As shown in Figure 2, the current generator output power P is measured through power acquisition. Compared with the given power P*, the power error ΔP is calculated. The power deviation is used as the input to the PID controller. The controller receives a command from the user for the blade reference pitch angle β*, and then calculates the current pitch angle error Δβ = β* - β (current pitch angle β). The pitch change rate is then determined based on the parameters of the pitch mechanism. The reference pitch angle is limited to the range of 0–92°. Within this range, the controller adjusts the wind turbine blades according to the new pitch angle requirements.
The box in Figure 2 shows the PID controller. The stable value ranges of the proportional, integral, and derivative gains Kp, Ki, and Kd are determined by the Routh stability criterion of the closed-loop transfer function shown in the figure. The specific values of the proportional, integral, and derivative gains are obtained through simulation, with the principle of maintaining 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 in pitch angle β follows the same trend as the change in wind speed v. As wind speed v increases, the average pitch angle β increases; conversely, as wind speed v decreases, the average pitch angle β decreases.
3) The changing trends of the instantaneous power absorbed by the wind turbine blades, Pmech and the wind energy utilization coefficient Cp, show that the change in the pitch angle β limits the instantaneous power absorbed by the blades, Pmech, and the blades operate at a lower efficiency.
4) The generator output power Pe can be smoothly varied around the rated power by the change of the pitch angle β, thus maintaining a constant power.
5) The fluctuation of generator speed is affected by (Pmech-Pe) and the inertia of the unit.
Since the wind turbine generator's electrical control system operates in harsh natural environments and under strong electromagnetic interference, the requirements for the control system's reliability and anti-interference capabilities are very high. Therefore, we selected the Siemens S7-300 series programmable logic controller (PLC) (CPU: 315-2DP) as the core of the entire electrical control system. The S7-300 controller has a built-in continuous PID controller function "CONT_C". In actual use, we only need to call "CONT_C" and set the relevant parameters. A partial usage program for "CONT_C" is as follows:
I_ITL_ON：=
D_SEL：=
CYCLE:=
SP_INT:="DB9"． an_power_set∥Power setpoint
PV_IN:="DB6"． an_power∥Generator Output Power
PV_PER :=
MAN:=
GAIN: =="DB9"． GAINl∥Proportional Constant
TI:="DB9"． TI1∥Integral Constant
TD:="DB9"． TD1∥Differential Constant
TM_LAG :=
DEADB_W: =="DB9". an_deadband1∥Power Regulation Dead Zone
LMN_HLM:="DB9" ∥ Output upper limit of propeller pitch angle
LMN_LLM:="DB9" ∥ Output pitch angle lower limit value
PV_FAC :=
PV_OFF:=
LMN_FAC:=
LMN_OFF:=
I_TLVAL：=
DISV:=
LMN:="DB9" . out_pitch ∥ Output Reference Pitch Angle
LMN_PER:=
QLMN_HLM:=
