The control system of the wind turbine is an integrated control system. Take the grid-connected wind power generation system as an example. It monitors the grid, wind conditions, and operating data to control the grid-connected and disconnected wind turbines to ensure safety and reliability. Under this premise, the control system must not only optimize the control of the wind turbine according to changes in wind speed and direction to improve wind energy conversion efficiency and power generation quality, but also restrain dynamic loads, reduce mechanical fatigue, and ensure the operating life of the wind turbine. . Figure 1 shows the ideal power curve of a typical wind turbine, and its operating range is limited by the cut-in wind speed and the cut-out wind speed. When the wind speed is less than the cut-in wind speed, the available wind power is too small to compensate for operating costs and losses, so the wind turbine does not start; when the wind speed is greater than the cut-out wind speed, the wind turbine must also be shut down to protect the wind turbine from damage due to overload.
Generally, people are accustomed to dividing the ideal power curve shown in Fig. 1 into three intervals, and the control target of each interval is different. In the low wind speed area (section I), the available wind power is less than the rated power, so the wind power should be absorbed as much as possible, so the wind turbine should be operated at the maximum wind energy utilization coefficient Cpmax; in the high wind speed area (section Ⅲ), the control objective is to limit the power of the wind turbine below the rated power to avoid excess. Because the available wind power in this section is greater than the rated power, the wind turbine must be operated with a wind energy utilization factor less than Cpmax. Interval Ⅱ belongs to the transition interval, at this time. The control goal is to keep the noise of the wind turbine at an acceptable level by controlling the rotor speed of the wind turbine, and to ensure that the centrifugal force experienced by the wind turbine is within the allowable value. Therefore, Section II is a constant speed zone.
It should be particularly emphasized that when designing a control system, one should not only consider the wind turbine to track the ideal power curve, but also the mechanical load on the wind turbine. The two are often contradictory. The tighter the wind turbine power tracking ideal power curve is, the smaller the mechanical load may be. Therefore, the control of the wind power generation system is a multi-objective optimal control.
In summary, the main objectives and functions of the wind turbine control system are as follows:
(1) Ensure the stable and reliable operation of the wind generator set within the normal wind speed range.
(2) Ensure that the speed of the wind turbine is below the allowable speed, and suppress the noise of the wind turbine and the centrifugal force of the wind turbine.
(3) In the low wind speed area, the best blade tip speed ratio is tracked, maximum power point tracking (MPPT) is realized, and the maximum wind energy is captured.
(4) In the high wind speed area, restrict the capture of wind energy and keep the output power at the rated value.
(5) Maintain the stability of the output voltage and frequency of the wind turbine to ensure the quality of power.
(6) Reduce the mechanical load of the transmission chain to ensure the service life of the wind turbine.
(7) Suppress torque fluctuations caused by wind gusts, and reduce the mechanical stress and output power fluctuations of the wind turbine.
The above control content is only part of the basic functions of the control system. In order to better achieve all control objectives and functions, the steady-state operating point of the wind turbine must be accurately controlled. For different types of wind turbines, their control strategies and control contents are different. In specific practice, it is also necessary to formulate their own operating processes and control strategies for constant-pitch and constant-speed wind turbines, variable-pitch constant-speed wind turbines, and variable-pitch variable-speed wind turbines.