*3.5. Effect of Ground Resistor*

The ground resistor affects the sensitivity of the protection. An excessively large ground resistor may make the transient characteristics of the fault unobvious and cause the protection to fail. Figure 14 shows the distribution of wavelet entropy when the internal fault occurs with different ground resistors. The resistances of the ground resistor studied in this section are 0.001, 1, and 50 Ω. They are located at 100 km of the overhead lines. Figure 14a shows entropies of PGF; the values of the 2nd to 4th levels are 0. The trend of wavelet entropy changes is predominantly the same, and excessive resistor has little effect on the entropy. Figure 14b shows entropies of PPF; the values of the 5th level are larger than 0, are different from the external faults, and all values fluctuates slightly. Figure 14c shows entropies of LF; the wavelet entropy of each level fluctuates slightly, but the trend is the same. The values of the 3rd level are larger than 0 as well, and different from the LDs. Although the wavelet entropy has small numerical changes in some levels, the value has a small difference, and its overall distribution is the same.

**Figure 14.** Effect of ground resistor. (**a**) PGF; (**b**) PPF; (**c**) LF.

From the analysis, it can be seen that wavelet entropy is not affected by distance and a certain range of ground resistor.

As discussed in this section, different types of faults have different distributions of wavelet entropies. The 5th level wavelet entropies of the external faults such as SMFs and AG-ACs are 0, while it is not zero for internal faults. The 4th level wavelet entropy of PGFs is 0, which is different from that of LDs and LFs. The 3rd level wavelet entropy of LDs is 0. It is different from the 3rd level wavelet entropy of LFs. Therefore, through the comparison of wavelet entropies in certain decomposition levels, it is possible to identify internal faults, external faults, and the fault types.

#### **4. Methods of Transmission Line Protection**
