1 Introduction
Generally, since DFIG wind turbines account for a small proportion of the power grid, when a fault occurs in the power grid, in order to ensure the stability of the power grid voltage, the strategy of directly disconnecting the wind turbines is usually adopted. However, as the proportion of DFIG wind turbine installed capacity in the power system continues to increase, when the power grid voltage drops sharply, directly disconnecting it from the power grid will lead to violent fluctuations in the power flow of the power grid, and even cause large-scale power outages. These seriously affect the stability of the power system and the recovery of the power grid [1]. In view of the control objectives and specifications required for DFIG to achieve low voltage ride-through, experts and scholars from various countries have proposed a series of different technical methods in a large number of documents. At present, there are two main implementation strategies: one is to improve the control method of the frequency converter [2]; the other is to add hardware protection circuits to change the topology of DFIG [3]. The former is suitable for situations where the power grid drop is not obvious, while the latter is suitable for large power grid drops. Both methods have their own application scope and advantages and disadvantages, so they should be reasonably selected when used. In the case of small power grid voltage drops, this paper adopts the stator flux orientation control (SFO) strategy.
2DFIG mathematical model
Figure 1. Structure of a doubly-fed induction wind turbine system
As shown in Figure 1, the DFIG system mainly consists of a wind turbine, a variable speed gearbox, a doubly fed generator, a dual PWM inverter, a DC-side capacitor, and a transformer. In the figure, the stator side of the DFIG is directly connected to the power grid through a transformer, and the rotor side is connected to a dual PWM inverter that can adjust the frequency, phase, and amplitude of the rotor current. A bidirectional reversible dedicated inverter is used to realize the bidirectional flow of excitation and slip power. In addition, the grid-side PWM can maintain the DC bus voltage stability, and the rotor-side PWM can indirectly control the active and reactive power on the stator side [4][5]. However, this structure also makes the DFIG quite sensitive to grid voltage disturbances, and the small capacity of the inverter also makes it less able to cope with grid faults. Therefore, when the voltage drop is small, it is necessary to adopt corresponding control strategies to overcome the shortcomings of the DFIG.
Using the voltage and flux linkage equations of the DFIG in the dp coordinate system under two-phase arbitrary speed rotation, and taking the stator coordinate axis as the reference frame, according to the principle of coordinate transformation, the vector equations of the stator and rotor voltages and flux linkages of the DFIG in the dp coordinate system under synchronous speed rotation can be derived (according to motor convention):
