## 2 Methodology
### Voltage Uprating for the Typical Tower of 132 kV Transmission Line Processes and Its Analyses
#### 2.3.1 Step 1: Increasing Insulation Length and Their Creepage Distance  
Figure 4a shows the 132 kV steel lattice tower with 275 kV suspension string of toughened glass discs insulator type. The typical 275 kV suspension insulator string in Malaysia has 16 toughened glass discs and 2849 mm overall string length, including fittings at the top and bottom of the string. The available phase-to-phase clearances in the existing 132 kV transmission line system in still air condition show adequate electrical clearances between the phase-to-phase to uprate to a 275 kV transmission line system. In addition, it can be seen that the available minimum phase-to-ground at the bottom-section is 15.83 m which exceeds the required ground clearance by 7.0 m, as shown in Table 3. This margin should be sufficient for maximum load conditions. However, expectedly the phase-to-earth clearance does not satisfy the electrical clearance requirement in still air.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{3}{c}{**(a)**} \\ \hline \multicolumn{4}{c}{**Normal Wind Condition**} \\ \hline
**BIL for 275 kV Line** & \multicolumn{3}{c}{**Minimum Clearance (m)**} \\ \cline{2-4}
**(Highest System Voltage 300 kV)** & **Phase-to-Earth** & **Phase-to-Phase** & **Phase-to-Ground** \\ \hline Power frequency (300 kV) & 0.51 & 0.83 & \\ Switching impulse (850 kV) & 1.80 & 2.60 & 7.00 \\ Lightning impulse (1050 kV) & 1.90 & 2.10 & \\ \hline \multicolumn{4}{c}{**(b)**} \\ \hline \multicolumn{4}{c}{**Extreme Wind Condition**} \\ \hline
**BIL for 275 kV Line** & \multicolumn{3}{c}{**Minimum Clearance (m)**} \\ \cline{2-4}
**(Highest System Voltage 300 kV)** & **Phase-to-Earth** & **Phase-to-Phase** & **Phase-to-Ground** \\ \hline Power frequency (300 kV) & 0.51 & 0.83 & 7.00 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Requirement of electrical clearance under **(a)** normal, and **(b)** extreme wind conditions [33; 34; 35].  
Figure 4: The 132 kV steel lattice tower geometry; **(a)** with 275 kV glass discs insulator, and **(b)** adjusting insulator length and its creepage distance.  
In order to fulfill the requirement of electrical clearance, the 132 kV transmission line towers were modified by adjusting insulator lengths and their creepage distance. As summarised in Table 4, the existing suspension string insulator length is 2240 mm, while the proposed suspension string insulator length is 1640 mm. However, its creepage distance is slightly greater than the existing suspension string insulator. The existing and proposed suspension string insulator had similar mechanical characteristics, whereas the electrical characteristic of the proposed suspension string insulator had excellent compared to the existing one. Meanwhile, the proposed suspension insulator string has 12 toughened glass insulator discs, and its overall string length, including fittings at the top and bottom of the string, was 2289 mm. Figure 4a shows the existing 132 kV transmission line with a typical 275 kV suspension string insulator.  
Figure 4b shows the available minimum phase-to-earth clearances of 1.24 m which are significantly not meeting the required phase-to-earth clearances of 1.80 m and 1.90 m in switching and lightning overvoltages for voltage uprating the 275 kV transmission line system. Table 5 summarised the minimum electrical clearances for the proposed model under still air conditions.