Practice has proved that a sudden drop in the voltage of the grid connection point will reduce the output power of the wind turbine, which in turn will cause an imbalance between the input power and the output power, resulting in adverse effects. For different types of wind turbines, the ability to resist low voltage is different.
1. The influence of low voltage on DFIG
In the doubly-fed asynchronous generator set, the stator side is directly connected to the grid, which makes the voltage drop of the grid directly reflected on the generator stator terminal voltage, resulting in an increase in the stator current;
And because the flux linkage cannot change abruptly at the moment of fault, there will be a DC component (a negative sequence component will also appear in the case of asymmetrical drop) in the stator flux linkage, which will induce a large electromotive force in the rotor and generate a large rotor current, resulting in a substantial increase in the voltage and current in the rotor circuit. The large fluctuations of the stator and rotor currents will cause dramatic changes in the electromagnetic torque of the doubly-fed asynchronous generator set, which will impact on mechanical components such as wind turbines and gearboxes, and affect the operation and life of the wind generator set.
The AC-DC-AC power converter is connected to the rotor side of the doubly-fed asynchronous generator set, and its power electronic devices have limited overvoltage and overcurrent capabilities. High transient rotor currents can pose a threat to vulnerable power electronics if no control measures are taken to limit fault currents from voltage sags; controlling the rotor current will increase the voltage of the power converter. Excessive voltage will also damage the power converter, and the mismatch of the input and output power of the power converter may cause the DC bus voltage to rise or fall (related to the super-synchronous speed or sub-synchronous speed of the generator at the time of the fault). Therefore, the realization of low voltage ride through of the DFIG is more complicated.
When the stator voltage of the doubly-fed asynchronous generator set drops, the output power of the generator decreases. If the captured power is not controlled, the speed of the generator will inevitably increase. When the wind speed is high, that is, the mechanical power torque is large, even if the fault is removed and the electromagnetic torque of the doubly-fed asynchronous generator increases, it is difficult to quickly suppress the increase of the generator speed. The speed of the doubly-fed asynchronous generator further increases, and the absorbed reactive power further increases, which makes the stator terminal voltage drop, which further hinders the recovery of the grid voltage. In severe cases, the grid voltage may not recover, resulting in system collapse. This condition is related to generator inertia, rating and fault duration. As shown in Figure 1, the effect of voltage sag on the DFIG asynchronous generator set is given.
2. The impact of low voltage on permanent magnet synchronous generator sets
In a permanent magnet synchronous generator set (PMSG), the stator is connected to the grid via an AC-DC-AC power converter. The instantaneous drop of the grid voltage will lead to the reduction of the output power, while the output power of the generator will remain unchanged instantaneously. Obviously, the power mismatch will cause the DC bus voltage to rise, which will inevitably threaten the safety of power electronic devices. If control measures are taken to stabilize the DC bus voltage, the current output to the grid will inevitably increase, and the excessive current will also threaten the safety of the power converter. When the DC side voltage of the power converter fluctuates within a certain range, the power converter on the generator side can generally maintain controllability. During the grid voltage drop, the generator can still maintain good electromagnetic control.