*4.3. PPF Criterion*

Through the analysis in Section 2.2, in the initial stage of a PPF, there is a phenomenon of SM discharge. The fault current contains a large DC component. Through spectrum analysis, the fault current contains different frequency bands components with a large amplitude. The amplitude of the 0 Hz component can clearly distinguish the PPF. The fault criterion is set as

$$\text{A0} > 2.7 \text{ kA}\_{\prime} \tag{7}$$

where A0 is the amplitude of the 0 Hz component. Through a large number of simulation experiments, the setting value is 2.7 kA. Therefore, if A0 > 2.7 kA, it is considered as a PPF. Otherwise, it is considered as another fault.

#### *4.4. Recognition Method for PGF, LD, and LF*

Combined with the analysis in Section 3, the spectrum distributions of PGF, LD, and LF have clear differences from 6250 Hz (200 kHz/24+1) to 25,000 Hz (200 kHz/22+1). Therefore, the wavelet entropy values of the 3rd to 4th levels have distinguished differences accordingly. This paper takes the value of the 4th level of wavelet entropy as the criterion of PGF. The fault criterion is set as

$$\text{W4} = 0,\tag{8}$$

where W4 represents the wavelet entropy value of the 4th level of the current line mode component during the fault. Therefore, if W4 = 0, it is considered as a PGF. Otherwise, it is considered as a lightning strike.

According to the previous analysis, this paper adopts the value of the 3rd level of wavelet entropy as the criterion for the LD. The fault criterion is set as,

$$\mathbb{W3} = 0,\tag{9}$$

where W3 represents the wavelet entropy value of the 3rd level of the current line mode component during the fault. If W3 = 0, it is considered as an LD. Otherwise, it is considered as an LF.

## *4.5. Flow Chart of Protection Plan*

Combined with the above analysis, the protection scheme is designed as shown in Figure 15. Firstly, the current value is detected to determine whether the protection system is activated or not. Then, the wavelet entropy of the line mode component is calculated and used to determine its fault zone based on the internal and external fault criterion. If it is an internal fault, the fault will be judged according to the PPF criterion, PGF criterion, and LD criterion in turn. Finally, the fault type is given.

**Figure 15.** Flowchart of the protection plan.
