Grid-connected conditions of synchronous wind turbines
The circuit of a synchronous wind generator set running in parallel with the power grid is shown in Figure 1. In the figure, the stator winding of the synchronous generator is connected to the power grid through a circuit breaker, and the rotor field winding is controlled by an excitation regulator.
When a synchronous wind turbine is connected to the grid in parallel, in order to prevent excessive current and torque shocks, the instantaneous value of the voltage at each phase terminal output by the wind turbine must be exactly the same as the instantaneous value of the corresponding phase voltage at the grid terminal. The specific conditions are as follows:
(1) The phase sequence of the generator is consistent with the phase sequence of the grid.
(2) The voltage waveform of the generator is the same as the grid voltage waveform.
(3) The frequency of the generator is the same as the grid frequency.
(4) The size and phase of the generator no-load voltage are the same as the grid voltage.
When connecting to the grid, because the direction of rotation of the wind turbine remains unchanged, as long as the output ends of each phase winding of the generator correspond to the grid, the condition (1) can be satisfied; condition (2) can be guaranteed by generator design, manufacturing and installation; therefore, when connecting to the grid, it is mainly the detection and control of the other two conditions, of which condition (3) must be met.
Grid-connected method of synchronous wind turbines、
For wind turbines with constant speed and constant frequency control using synchronous generators, there are two main ways to put them into grid parallel operation:
(1) Accurate synchronization method. In order to meet the conditions for parallel operation of the power grid, a synchronization indicator is required. The simplest synchronization indicator consists of 3 synchronization indicator lights, which are respectively connected across the parallel switch (contactor or oil circuit breaker) between the grid side and the corresponding phase of the motor side, as shown in Figure 2. If the three lights turn on and off, it means that the generator phase sequence is different from the grid phase sequence, and the generator should be shut down immediately and the generator phase sequence should be changed. If the three lights flash at the same time (that is, they are on and off at the same time), it means that the phase sequence is correct but the frequency is different. At this time, the speed of the generator set should be adjusted until the three lights no longer flash, indicating that the frequency conditions for parallel operation have been met. If the three lights may maintain the same brightness, it indicates that the condition (4) for parallel operation has not been met. At this time, the excitation current of the synchronous motor should be adjusted to make the indications of the voltmeters V1 and V2 in Figure 2 equal. If the three lights still maintain a certain brightness, it means that although the generator voltage is equal to the grid voltage, there is still a phase difference. At this time, the instantaneous speed of the generator set can be adjusted to adjust the phase of the generator voltage, so that all 3 indicator lights are extinguished. At the same time, the indication of the voltmeter V1 should also be 0, indicating that the synchronous generators all meet the 4 conditions for putting into the power grid, and the switch can be closed immediately to put the synchronous generators into the power grid for parallel operation. It is important to note that the grid connection process of the accurate synchronization method is generally completed automatically under the control of a computer. The advantage of using the accurate synchronization method to enter the grid is that there is no obvious current impact. However, due to the randomness of wind speed, it is difficult to switch on and switch on the synchronous generator after all the conditions for putting the synchronous generator into the power grid are met (that is, accurately synchronized). Therefore, the self-synchronizing grid-connected method is often used.
(2) Self-synchronization and grid connection method. Self-synchronous grid-connected means that the synchronous generator is driven by the prime mover when the rotor is not excited and the excitation winding is short-circuited by the current-limiting resistor, and when the rotor speed of the synchronous generator rises to close to the synchronous speed (80%~90% of the synchronous speed), the generator is put into the grid, and then immediately put into the excitation, relying on the electromagnetic force between the stator and the rotor, the generator automatically enters synchronous operation. Since the synchronous generator is not excited when it is put into the grid, there is no whole-step process of adjusting and calibrating the voltage and phase angle of the generator when quasi-synchronous grid-connected, and fundamentally rule out the possibility of asynchronous closing. After the grid fails and returns to normal, if the generator needs to be quickly put into parallel operation, this method of grid connection is often used.