## 5 Simulation and Experiment
### Experimental Verification  
In order to verify the actual effect of the SSO suppression strategy, experimental verification of the strategy proposed in this paper was carried out using a model platform based on a 15 kVA doubly-fed wind power system. The layout of the experimental platform is shown in Figure 13 and a detailed illustration of each part of the experimental platform is shown in Figure 14.  
Figure 12: Comparison of stator power and capacitance voltage waveforms before and after SSO suppression: (**a**) power before the SSO inhibition; (**b**) power after the SSO inhibition; (**c**) \(V_{\text{dc}}\) before the SSO inhibition; (**d**) \(V_{\text{dc}}\) after the SSO inhibition. Figure 13: Layout diagram of experimental platform for a DFIG system. The DFIG experimental platform was composed of a GSC, an RSC, a simulated power grid and an SSO simulation system. The RSC and GSC control systems both used DSPF28335 as the control chip. The permanent magnet synchronous motor was driven by the prime mover to generate sub-synchronous voltage, which was merged into the simulated power grid through the transformer, thereby simulating the experimental environment for the grid SSO. The experimental parameters are shown in Table 1.  
Before the experiment, it was necessary to check whether the DFIG met the grid connection requirements. The DFIG stator A-phase voltage and the simulated grid A-phase voltage waveforms before the grid connection are shown in Figure 15. As shown in the figure, the DFIG stator voltage and the grid voltage basically had the same amplitude, phase, frequency and phase sequence, so the DFIG system could be connected to the grid. Figure 16 shows the DFIG stator and rotor current waveforms at different speeds. The rotor speeds were set to 900 r/min and 1200 r/min, respectively; that is, the motor ran in sub-synchronous and super-synchronous states. By observing the rotor A-phase current and stator A-phase current, it could be seen that, at different speeds, the rotor current frequency of the DFIG also changed correspondingly, while the stator current was always kept at 50 Hz. The experiment proved that the DFIG system controlled by the back-stepping controller and the proportional controller could achieve constant-frequency variable speed control. Figure 14: Experimental platform for the oscillatory inhibition of the DFIG system: (a) GSC experiment platform; (b) RSC experiment platform; (c) DFIG power generation device; (d) simulation power grid platform. Figure 15: Output voltage waveforms for the grid and DFIG before grid connection. The results for the SSO suppression experiments are shown in Figures 17-19. In the experiments, the prime mover was selected to drive the PMSG to simulate the generation of the grid oscillation voltage. The amplitude of the sub-synchronous voltage was 10% of the simulated grid voltage, and the frequencies were 30 Hz, 20 Hz and 10 Hz, respectively. The rotor speed was fixed at 900 r/min, and the motor was run in a sub-synchronous state. The motor speed, the simulated SSO voltage waveform, the simulated grid voltage waveform when the oscillation occurred and the DFIG stator and rotor current waveforms before and after the oscillation suppression were obtained by the sampling circuit. By comparing the experimental results for the grid SSO frequencies of 30 Hz, 20 Hz and 10 Hz, it can be seen that, when the grid voltage SSO was generated, the stator and rotor currents in the DFIG were disturbed accordingly, and the DFIG output also oscillated. With the use of the adaptive quasi-resonant controller to control the SSO, the oscillatory components in the stator and rotor currents were clearly suppressed, thus facilitating the sinusoidal output of the stator current and maintaining the stable operation of the DFIG.