## 3 Results and Discussion
### Effect of Voltage Urparting for 132 kV Transmission Tower on the Voltage Profile under Still Air  
Figure 12 shows the voltage magnitude at the L1, L2 and L3 with the different voltage stresses under backflashover and shielding failure events. For the backflashover event, it was found the voltage stresses, i.e., BIL and BIL + 10% at L1, L2 and L3, remained lower than the estimated CFO of 1089 kV, as can be seen in Figure 12a. However, at 1.8 m of electrical clearance, the voltage stress at L1 and L2 for BIL + 25% remained higher than the estimated CFO for negative lightning. The voltage stress, i.e., BIL + 25% at the L3, was relatively close to the estimated CFO. The percentage of the difference on estimated CFO between simulations of voltage stresses, i.e., BIL, and BIL + 10%, was between 3.8% to 23.9%. It is believed that the insulation strength under lightning impulse is sufficient to operate of 275 kV transmission line throughout the requirement of the electrical clearance. For the shielding failure under the case study, all L1, L2 and L3 experience significant insulation strength to withstand switching and lightning overvoltages, as shown in Figure 12b. The percentage difference of estimated CFO for negative lightning between different BIL was between 30.7% and 71.3%, as summarised in Table 10.  
Effect of Voltage Urparting for 132 kV Transmission Tower on the Voltage Profile under Normal Wind Condition  
The normal wind condition demonstrates a slight movement of the suspension string insulator due to environmental factors that could directly or indirectly influence the insula \begin{table}
\begin{tabular}{c c c} \hline \hline
**Types of Overvoltages** & **Applied BIL** & \
\begin{tabular}{c} **Phase-to-Earth Minimum** \\ **Clearance at Crossarm Section** \\ \end{tabular} &
\begin{tabular}{c} **Percentage Difference between** \\ **Estimated CFO and Simulations (\%)** \\ \end{tabular} \\ \hline Backflashover & BIL + 10\% & L1 = L2 & 17.7 \\  & BIL + 10\% & L3 & 23.9 \\  & BIL + 25\% & L1 = L2 & 10.0 \\  & BIL + 25\% & L3 & 17.1 \\  & BIL + 25\% & L1 = L2 & \(-\)2.11 \\  & BIL & L3 & 3.8 \\  & BIL + 12\% & L1 = L2 & 47.8 \\  & BIL + 10\% & L3 & 71.3 \\  & BIL + 20\% & L1 = L2 & 38.6 \\  & BIL + 10\% & L3 & 64.4 \\  & BIL + 25\% & L1 = L2 & 30.7 \\  & L3 & 59.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: The voltage magnitude and the percentage difference between CFO and simulation results. Figure 12: Voltage magnitude due to (**a**) backflashover, and (**b**) shielding failure activities. tion strength. Malaysia's electric utility operator has considered the 15\({}^{\circ}\) swing angles of the suspension string insulator as the normal wind condition. In this study, the minimum availability of electrical clearances for 15\({}^{\circ}\) swing angles was 2.07 m of phase-to-earth. There was a slight reduction of phase-to-earth electrical clearance between still air and normal wind condition, and its percentage difference was 13.8%. For backfl flashover under the case study, the voltage magnitude at L4, L5 and L6 under, i.e., BIL and BIL + 10% of voltage stresses are significantly lesser than the estimated CFO. At 1.8 m, the minimum electrical clearances, the BIL + 25% at L4, L5 and L6 shows exceeded the estimated CFO, as shown in Figure 13a. The percentage of increment between the estimated CFO and L4, L5 and L6 under BIL + 25% are 4.7 and 6.9%, as summarised in Table 11.  
The voltage magnitude shows sufficient electrical clearances for shielding failure events even the L4, L5, and L6 stresses with BIL, BIL +10%, and BIL +25%. Figure 13b shows clearly at 2.0 m of electrical clearances at phase conductor to structure steadily remains lower than estimated CFO. It is believed the reduction of electrical clearances is due to normal wind conditions, which lead slight decrease in insulation strength. However, it is noticed that the insulation strength could withstand almost 2.0 m of electrical clearances even voltage stresses up to BIL + 25% were applied.