## 4 Tree Flash Fault Traveling Wave Model and Attenuation Analysis  
Using PSCAD/EMTDC, a three-section hybrid overhead-underground line model with a voltage level of 220 kV was constructed to investigate the frequency characteristics of tree flash fault current traveling wave attenuation. The topology of this hybrid overhead-underground line model is illustrated in Figure 9, and the specific parameters of the overhead lines and cables are listed in Table 2. The diagram of geometric parameters for the cable route is depicted in Figure 10. The cable type is \"XLPE YJLV\({}_{22}\) 220 kV 3 \(\times\) 250 mm\({}^{2}\)\". Insulator 1, Insulator 2, and Insulator 3 are, respectively, XLPE insulation, non-woven tape, and flame-retardant PVC outer sheath. The \"220 kV hybrid line\" refers to a type of power transmission line with a voltage level of 220 kV. It includes both overhead sections and underground or buried cable sections. This hybrid configuration combines the characteristics of overhead lines and underground \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Line Type** & **R\({}_{0}\)/(\(\Gamma\)/km)** & **R\({}_{1}\)/(\(\Gamma\)/km)** & **L\({}_{1}\)/(mH/km)** & **L\({}_{0}\)/(mH/km)** & **C\({}_{1}\)/(\(\mu\)F/km)** & **C\({}_{0}\)/(\(\mu\)F/km)** \\ \hline Overhead line & 0.009 & 0.149 & 0.837 & 3.438 & 0.014 & 0.008 \\ Power cable & 0.162 & 0.162 & 0.141 & 0.141 & 0.328 & 0.328 \\ \hline \end{tabular} \(\overline{\text{R}}_{0}\), \(\text{R}_{1}\), \(\text{L}_{1}\), \(\text{L}_{0}\), \(\text{C}_{1}\), and \(\text{C}_{0}\) represent the zero-sequence resistance, positive-sequence resistance, positive-sequence inductance, zero-sequence inductance, positive-sequence capacitance, and zero-sequence capacitance of the line, respectively.  
\end{table} 
Table 2: Parameters of the hybrid transmission line. Figure 8: Tree flash fault discharge waveform spectrum. Figure 9: Topological structure of hybrid overhead–underground line model. cables. It is typically used in situations where the line needs to traverse certain areas and cables need to be buried underground in specific regions. The cross-section areas of the conductors for the overhead and underground sections are 500 mm\({}^{2}\) and 250 mm\({}^{2}\), respectively. The hybrid line MN, with a length of 50 km, consists of a 20 km long overhead line ML\({}_{1}\), a 10 km long cable line L\({}_{1}\)L\({}_{2}\), and a 20 km long overhead line L\({}_{2}\)N.  
The actual magnitude of tree flash fault currents depends on various factors, including the operating voltage of the power system, line design parameters, type and height of trees, and environmental humidity. Moreover, tree flash faults are complex phenomena, and different circumstances may result in different fault currents. For the tree itself, when a tree flash fault occurs, an arc forms on the branches, leading to the gradual carbonization of the tree. As the degree of carbonization deepens, the electrical resistance of the tree body decreases accordingly, and the transition resistance reduces, eventually falling below 1 k\(\Omega\). For a 220 kV overhead-underground hybrid line, the tree flash fault discharge current typically exceeds 220 A. This is because the arc discharge phenomenon caused by the tree flash fault increases the current to a much higher level, far exceeding the rated current of the line. As the measured current waveforms of tree flash faults have relatively low amplitudes and milliampere-level fault currents cannot significantly affect the line, the amplitude of the tree flash fault current wave is amplified to simulate the actual current magnitude when a tree flash fault occurs in the line. In PSCAD, a tree flash fault current source module is established, as shown in Figure 11, with the waveform data using the tree flash discharge waveform collected in the above experiments (Figure 7b), with an amplitude of 242.64 A.  
Figure 11: The tree flash fault current simulation component in PSCAD. Figure 10: Presentation of the 220 kV underground cable line.