## 2 Analysis of Influence of SSO on DFIG System Operation
### Analysis of Influence of SSO Frequency Variation on DFIG System  
By analyzing the mathematical model of the RSC under SSO, we can understand the divergence process in the sub-synchronous current in the DFIG system. However, the establishment of the corresponding mathematical model in Section 2.1 was based on the assumption that the oscillation frequency within the input DFIG system is a fixed value. The change in the oscillation frequency may have an impact on the output of the DFIG system and the control effect of the SSO suppression strategy. In order to verify the influence of the SSO frequency variation on the designed oscillation suppression strategy for the DFIG system, as done in [15], simulation and analysis of an SSO suppression strategy for a DFIG system based on a quasi-resonant controller were carried out.  
Firstly, the output of the DFIG with the grid SSO was observed, and the simulation results are shown in Figure 2.  
Figure 2 shows a comparison of the DFIG output stator current waves before and after the generation of the power grid SSO. The simulated condition was that, at 0.5 s, the grid frequency was 10 Hz and the oscillation amplitude was 20% of the grid base wave voltage. It can be seen from Figure 2 that the DFIG output stator current was a three-phase sinusoidal alternating current before 0.5 s. After 0.5 s, due to the influence of the SSO of the power grid, an oscillating current with a frequency of 10 Hz was generated in the three-phase current of the DFIG stator, which is consistent with the aforementioned oscillation mechanism.  
Next, the influence of the SSO frequency change on the corresponding oscillation suppression strategy was analyzed. In this study, the effects of the SSO suppression strategy on a DFIG based on a quasi-resonant controller were investigated with different frequency oscillations. The simulation results are shown in Figure 3.  
Figure 3 shows the waveforms of the DFIG stator A-phase output current with oscillation frequencies of 10 Hz and 15 Hz, respectively. Suppose that the oscillation suppression frequency of the resonant controller is 10 Hz, and a quasi-resonant suppression link is added to the RSC control system at 0.7 s. According to Figure 3a, when the grid SSO frequency is 10 Hz, an oscillation component with a frequency of 10 Hz appears in the DFIG stator current. When the simulation was run 0.7 s, a quasi-resonant suppression ring was added into the RSC control system to suppress the internal oscillation of the DFIG system. At this point, the oscillation component of the DFIG output stator current was greatly weakened, and the quasi-resonant controller had a significant effect on SSO suppression. In Figure 3b, the SSO frequency has been changed to 15 Hz, and the oscillation suppression frequency of the quasi-resonant controller is still 10 Hz. In this simulation, an  
Figure 2: The stator current waveform output by the DFIG.  
oscillation component with a frequency of 15 Hz is generated in the DFIG output stator current, and the quasi-resonant controller is also used to suppress the SSO at the same time. As the SSO frequency is beyond the control range of the quasi-resonant controller, the oscillation component of the DFIG stator current is basically not controlled, and the quasi-resonant controller's suppression effect on the SSO is obviously reduced.  
In conclusion, the generation of a stator sub-synchronous current can disturb the output of the DFIG system, while the change in the oscillation frequency may cause the failure of the SSO suppression strategy within the DFIG system, such that suppression of the SSO at different frequencies is required during SSO suppression.