*4.1. Starting Criterion*

The starting element contains one transient criterion and one substitute criterion. The specific starting criterion is

$$|\Delta i| > 0.1 I\_{\text{ll}} \text{ or } i > 1.1 I\_{\text{ll}} \tag{5}$$

where Δ*i* is the change in positive current. It is calculated by subtracting the value of the instantaneous current 1 ms before. *i* is the instantaneous current. *In* is the current rating.

The purpose of setting the starting element is to enhance the reliability of the protection.

#### *4.2. Internal and External Fault Criterion*

According to the wavelet entropy distribution in Figure 13, the high-frequency energy of external transients is filtered and attenuated due to the existence of boundaries or discontinuous boundaries, such as current-limiting reactors, SM capacitances, bridge arm reactors, etc. According to the analysis in Section 3.3, the value of the 5th wavelet entropy can be used as a criterion for distinguishing internal and external faults. The fault criterion is set as

$$\mathsf{W}\mathfrak{F}=\mathsf{O},\tag{6}$$

where W5 represents the wavelet entropy value of the 5th level of the current line mode component during the fault. Therefore, if W5 = 0, it is considered as an external fault. Otherwise, it is determined as an internal fault.
