## 4 Simulation Analysis
### Performance Comparison with Another Method  
The reference method [16] is taken as the comparison object because it is the same as the research object of the method proposed in this paper, both of which are 300 MW variable-speed pumped-storage units. The method in [16] is different from the method proposed in the paper in terms of implementation principle, required measurement data, and calculation burden. In terms of implementation principle, the harmonic characteristics of the circulating current between stator branches are used to judge whether a rotor winding short-circuit fault occurs [16]. In essence, the fractional harmonic magnetic field distortion inside the motor after the fault is taken as the fault characteristic. In the proposed method, the negative-sequence current of the rotor and the \(1\pm 2\) s component currents of the stator are mainly taken as the fault characteristics. In terms of required measurement data, the method in [16] needs to measure the three-phase branch circulation of the stator (i.e., the split-phase transverse differential current), while the proposed method needs to measure the three-phase phase current of the stator and rotor. The data sources of the two methods are different, but neither needs to install additional measuring devices. In terms of computational burden, the fractional harmonic frequencies of the method in [16] are related to the slip ratio; that is, the harmonic characteristic frequencies will change accordingly when the rotor speed changes. Therefore, it is necessary to recalculate the harmonic characteristic frequencies according to the measured rotor speed. This leads to a large computational burden on the method in [16]. However, no special treatment is required for the rotor speed in the proposed method, which makes the computational burden of the proposed method smaller. The above differences will inevitably lead to differences in the performance of the two methods. In terms of protection action speed, the proposed method has obvious advantages over the method in [16]. The FFT algorithm is used to calculate the fractional harmonic components of the stator branch circulation in [16]. The frequency resolution of FFT is inversely proportional to the length of the time window. To overcome the influence of rotor speed on harmonic characteristic frequencies, the time window selected is longer, usually 1-5 s, and the corresponding frequency resolution is 1-0.2 Hz. However, the longer time window means that for the protection of poor speed, the action time reaches the second level. The action time of the proposed method is analyzed in Section 4.1. When the slip ratios are 0.1, 0.05, and 0.01 respectively, the protection action times of the example fault are 47.5 ms, 91.7 ms, and 365.0 ms, respectively. Its protection speed is obviously better than that of the method in [16]. Especially when the slip rate is large, the protection action time can be increased more than 10-fold. In terms of protection coverage, the proposed method is less than the method in [16]. For the same VSPSGM, when the slip rate is 0.1, the protection action number of the method in [16] is 11,875, and the protection coverage is 98.52%. The protection action number of the proposed method is 11,209, and the protection coverage is 92.98%. Therefore, it can be said that the above two methods have their advantages and disadvantages. Considering that the rotor short-circuit fault is a serious destructive fault, it is necessary to configure two different protection methods to realize the dual configuration scheme of the rotor's main protection. When both methods are configured, the actionable protection number of the proposed method is 11,954, as shown in Table 5. The actionable protection number is increased by 79 and 745, respectively, compared with the method in [16] and the