3 motor side converter control strategy
In this paper, the maximum wind energy tracking is realized by controlling the speed of the generator set, so that the generator speed can follow the changing wind speed and obtain more energy from the wind: when the wind speed is below the rated wind speed, the purpose of the system speed control is to ensure the unit It is operated under the maximum wind power tracking state; when the actual wind speed is higher than the rated wind speed, it is limited by the mechanical strength, generator capacity and inverter capacity, so the energy captured by the wind wheel must be reduced, so that the power is kept near the rated value. The pitch angle control needs to function to keep the unit near the rated power.
3.1 Maximum power tracking algorithm for wind turbines below rated wind speed (mppt)
The amount of power output by the fan varies with the speed. For any wind speed, there is an optimum speed for maximum power. Therefore, the goal of fan control is to control the speed so that the fan is always running at the maximum point of output power. When the pitch angle is constant, there is an optimum tip speed ratio λ such that the wind energy utilization coefficient cp is the largest, that is, the output power is maximized. According to the formula, to achieve the maximum power tracking of wind energy, the generator speed must be adjusted according to the wind speed to maintain the optimal tip speed ratio.
The electromagnetic torque of the permanent magnet synchronous generator depends on the stator current of the motor. For the direct drive wind power generation system, the permanent magnet synchronous generator is used, and there is no speed increasing mechanism. Therefore, the speed of the wind turbine under various wind speeds corresponds to the corresponding generator. The speed of rotation, ie ω=ωg, (ω is the fan speed, ωg is the generator speed), so the wind speed of the wind turbine should be kept at the optimal speed at the wind speed, that is, the rotor speed of the generator follows the wind speed. And maintain the optimal speed at a certain wind speed. The generator speed control mode needs to detect the wind speed signal first, and then find the optimal speed through the relationship between the wind speed and the optimal speed. The optimal speed is input into the motor driver as the reference speed, and the generator reaches the maximum through the speed closed loop system. Excellent working point. Since the speed of the generator has a direct relationship with the electromagnetic torque, the torque link can be designed as the inner ring of the speed link. For the permanent magnet motor, the excitation current is not required, and the stator current only generates torque. In the rotating coordinate system, the electromagnetic torque of the permanent magnet motor te=1.5pψfiq is only related to the q-axis current, and is independent of the d-axis current, so the torque link The control can be converted into the control of the current link. Therefore, the control of the generator torque speed can be realized only by controlling the q-axis current. The speed control mode is a closed loop control system in which the current is controlled as an inner loop and the speed is controlled as an outer loop. The main function of the generator-side converter is to adjust the output voltage signal ug and the electrical frequency fe according to the actual wind speed change. According to the vector control principle of the permanent magnet motor, the speed adjustment can be achieved by controlling the phase and amplitude of the generator rotor current vector. It can be seen from the torque formula of the permanent magnet motor that when the excitation flux and the orthogonal axis inductance of the permanent magnet are determined, the torque of the generator depends on the space vector ig of the stator current, and the magnitude and phase of the ig depend on Id and iq, the torque of the generator can be controlled by controlling these two currents. A certain speed and a certain torque correspond to a certain id and iq. By controlling the two currents, the actual id and iq tracking command values i*d and i*q realize the control of the generator and speed.
