Generally, there are the following three methods for grid connection of asynchronous wind turbines.
(1) Directly connected to the grid. The conditions for direct grid connection of asynchronous wind turbines are as follows:
①The rotation of the generator rotor is consistent with the direction of the rotating magnetic field, that is, the phase sequence of the generator is the same as that of the power grid. This method must be strictly followed, otherwise, the generator will be in the state of electromagnetic braking after connecting to the grid, and the phase sequence should be adjusted when wiring.
②The speed of the generator is as close to the synchronous speed as possible. The requirements of this method are not very strict, but the smaller the error between the generator’s rotational speed and the synchronous rotational speed when the grid is connected, the smaller the inrush current generated when the grid is connected, and the shorter the decay time.
The direct parallel connection between the asynchronous wind turbine and the grid is shown in Figure 1. When the wind turbine is started under the drive of the wind, when the rotor of the asynchronous generator is brought to the vicinity of the synchronous speed (usually 98%~100%) through the speed-increasing gear box, the speed measuring device gives an automatic grid connection signal, and the closing and grid connection process is completed through the circuit breaker. This grid connection method is simpler than the quasi-synchronous grid connection of synchronous generators, but because the generator itself has no voltage before the grid connection, an inrush current of 5 to 6 times the rated current will be generated during the grid connection process, causing the grid voltage to drop. Therefore, this grid connection method can only be used in occasions where the capacity of the asynchronous generator is below 100 kilowatts and the capacity of the grid is relatively large.
(2) Step-down and grid connection. This method of grid connection is to connect a resistor or reactor in series between the asynchronous motor and the grid, or connect an autotransformer to reduce the instantaneous impact current amplitude and grid voltage drop when the grid is closed.
(3) Soft grid connection through thyristor. This grid connection method is to connect a bidirectional thyristor in series between the stator of the asynchronous generator and the power grid, and the three phases are controlled by thyristors. Both ends of the bidirectional thyristor are connected in parallel with the moving contact of the grid-connected automatic switch K2 (see Figure 2). The purpose of connecting the triac is to control the inrush current at the moment when the generator is connected to the grid within the allowable limit. The grid connection process is: when the wind turbine generator receives the start command issued by the microprocessor in the control system, it firstly checks whether the phase sequence of the generator is consistent with the phase sequence of the power grid. If the phase sequence is correct, a release command is issued, and the wind turbine starts to start. When the generator speed is close to the synchronous speed (99% to 100% of the synchronous protection speed), the control angle of the triac is gradually opened synchronously from 180° to 0°; at the same time, the conduction angle of the bidirectional thyristor increases gradually from 0 to 180°. At this time, the grid-connected automatic switch K2 does not act, and the moving contact is not closed, and the asynchronous generator is smoothly integrated into the grid through the thyristor; as the generator speed continues to increase. The slip rate of the motor tends to zero gradually. When the slip rate is zero, the grid-connected automatic switching action, the moving contact is closed, the bidirectional thyristor is short-circuited, and the output current of the asynchronous generator will no longer flow through the bidirectional thyristor, but flow into the grid through the closed automatic switching contact. After the generator is connected to the grid, a compensation capacitor should be incorporated at the generator end immediately to increase the power factor (cosφ) of the generator to more than 0.95.