In a doubly-fed asynchronous wind turbine, transient rotor overcurrent and overvoltage caused by voltage sag will damage power electronic devices, and the attenuation of electromagnetic torque will also lead to an increase in rotational speed. As shown in Figure 1, it is the structure of a doubly-fed asynchronous wind turbine with a rotor Crowbar(the Crowbar switch draws on the advanced technology in the international high-voltage testing field, and uses the instantaneous discharge of the capacitor to discharge the large inductance. When the current reaches the peak value, the switch is triggered, so that the current continues to pass through the product, so as to achieve a long discharge time). In general, a Crowbar circuit is connected to the rotor, which is a commonly used and effective low-voltage ride-through technology for doubly-fed asynchronous wind turbines. When the grid voltage sags, a path is provided for inrush currents generated on the rotor side during voltage sags through resistors connected to the rotor windings. It is suitable for two common circuits of double-fed asynchronous wind turbines. The control of the various rotor-side Crowbars is basically similar. That is, when the rotor-side current or the DC bus voltage increases to a predetermined threshold, the switching device is turned on, and all the switching devices in the machine-side power converter are turned off at the same time, so that the rotor fault current flows through the Crowbar.
The selection of the resistance value in the Crowbar is more important. When the Crowbar is connected in series with the rotor, the DFIG can be simply regarded as a wound rotor asynchronous generator. The larger the resistance of the Crowbar, the faster the rotor current decays, and the smaller the current and torque oscillation amplitudes. However, if the resistance value is too large, it will bring overvoltage to the rotor-side power converter, which cannot protect the rotor power converter. It should be noted that the Crowbar circuit is connected to the rotor of the doubly-fed asynchronous wind turbine. Although the power converter is protected, the current and torque characteristics of the generator are not changed. Therefore, the torque fluctuation and mechanical stress are relatively large; after the rotor is short-circuited, as an asynchronous generator, it must absorb reactive power from the grid, which is not conducive to the recovery of grid faults. Therefore, it is often necessary to cooperate with other methods to obtain good results.
Because the DC bus will have overvoltage and undervoltage, in order to keep the DC bus voltage stable, it is necessary to connect an energy storage system (ESS) to the DC bus. As shown in Figure 2, a schematic structural diagram of a doubly-fed asynchronous wind turbine with an energy storage system on the rotor side is given. If there is a voltage drop in the power grid, the DC component and the negative sequence component will appear in the stator flux linkage, and then a large electromotive force can be induced in the rotor circuit. Due to the small leakage inductance and resistance value of the rotor circuit, a large electromotive force will inevitably generate a large current in the rotor circuit. In order to weaken the influence of the change of the stator flux linkage on the rotor circuit, a scheme of dynamic compensation control of the flux linkage can be adopted. That is, by controlling the rotor current, the direction of the rotor current is located in the opposite direction of the DC component of the stator flux linkage and the negative sequence component, so that the influence of the stator flux linkage on the rotor flux linkage can be weakened or even eliminated to a certain extent.
It should be noted that, in specific practice, introducing some new circuits on the stator side of the DFIG can also improve or improve the low voltage ride through capability of the DFIG, such as series passive impedance or dynamic voltage restorer on the stator side.
Further reading: low voltage ride through technology of permanent magnet synchronous wind turbine.