Overview of typhoon “Mangkhut”
On the afternoon of 7 September 2018, No. 22 Typhoon “Mangkhut” was generated in the Northwest Pacific Ocean. After shifting into the northeastern corner of the South China Sea on the morning of 15 September, it suddenly shifted westward, hitting the Pearl River Delta head-on and seriously affecting Guangdong province. “Mangkhut” had the widest impact area on land, the longest impact time of strong wind and the maximum gust speed among all typhoons that have landed in Guangdong province in history[29]. This case studied selected the 110-kV THJ line, a typical line with a running time of 12 years. Figure 2 shows the typhoon path and the location of the THJ line.
The typhoon caused 56 trips on 500-kV lines, 132 trips on 220-kV lines and 121 power outages on 110-kV lines, totalling 390 trips. There were 69,000 courts and 4.591 million users affected. After preliminary investigation and verification, the economic loss was 347 million yuan[30].
Prediction result
When calculating wind load, the uneven coefficient of wind pressure takes 0.75; the adjustment coefficient of wind load on towers is 1; the windward area of towers is 5 m2; the outer diameter of lines is 0.0096 m; the span is 250 m. The height variation coefficient of wind pressure is 1; and the shape coefficient of wind load is 1.2. Both two-step reliability calculations use the 3-s average instantaneous wind speed, which makes the predicted results more conservative and is conducive to more comprehensive prevention work.
The mean value of the uniform corrosion depth distribution is 2.7143 mm and the standard deviation is 2.0267 mm. The design wind speed of the equipment after guy wire reinforcement increases from 35 to 40 m/s. The aging parameters are α = 48.002 and β = 3.458.
Table 1 shows the damage probability under 10 conditions, including a basic damage probability P1 of tower-only and nine kinds of multi-effect combinations. We define that the comprehensive damage probability is calculated under the effects of corrosion and aging. The tower-line coupling damage probability is P2. The damage probability of the tower after guy wire reinforcement is P3. The tower-line coupling damage probability after guy wire reinforcement is P4. The aging failure probability of equipment is P5. The corrosion failure probability of equipment is P6. The comprehensive damage probability of the tower is P7. The comprehensive tower-line coupling damage probability is P8. The comprehensive damage probability of the tower after guy wire reinforcement is P9. The comprehensive tower-line coupling damage probability after guy wire reinforcement is P10.
Figure 3 shows the trend of predicted damage probability from 61 towers of THJ line under typhoon “Mangkhut”, while Supplementary Table 2 shows the specific damage probability value.
Supplementary Table 2 shows that, when tower-line coupling, corrosion and aging effects are not considered, the maximum damage probability of towers is only P1max = 0.363. When tower-line coupling effect is added, the maximum is P2max = 0.979, indicating that tower-line coupling effect makes a significant contribution to the comprehensive damage probability. When only the guy wire reinforcement is considered, the maximum damage probability of towers is reduced to P3max = 0.217, which is significantly lower than the case without guy wire reinforcement. After guy wire reinforcement, the maximum tower-line coupling damage probability is reduced to P4max = 0.975. Therefore, guy wire reinforcement also has a certain effect, but it is not significant. The tower-line coupling effect plays an important role in the damage of transmission equipment under typhoon disasters.
The failure probability of equipment aging is only P5 = 0.006, indicating that the 12-year operating time has little contribution to the damage probability of the equipment. The failure probability of equipment corrosion is P6 = 0.228, indicating that the corrosion effect has a greater contribution to equipment damage. Considering the effects of aging and corrosion, the maximum comprehensive damage probability of towers is P7max = 0.511, which is significantly higher than P1max = 0.363. Thus, the aging and corrosion effects should be considered in actual calculations.
Considering the tower-line coupling, aging and corrosion effects, the maximum comprehensive damage probability is P8max = 0.984. It is the highest among all the probabilities, indicating that all the considered effects contribute to the damage probability. The maximum comprehensive damage probability of towers after guy wire reinforcement is reduced to P9max = 0.399. It is lower than P7max = 0.511 when towers are not reinforced. However, the maximum comprehensive tower-line coupling damage probability after guy wire reinforcement is P10max = 0.981, which is not significantly lower than P8max = 0.984 without guy wire reinforcement. Meanwhile, Figure 3 shows that the calculation results of these conditions have roughly the same trend. Towers 10-36 have a relatively high damage probability. This feature is more advantageous for the damage analysis of the same line. However, when the prediction results are applied to the entire network, we need a unified damage probability threshold to judge the damage state, so we should comprehensively consider the tower-line coupling, aging and corrosion effects and the influence of guy wire reinforcement.
Figure 4 shows the four cases with the highest damage probability, namely the tower-line coupling damage probability P2, the tower-line coupling damage probability P4 after guy wire reinforcement, the comprehensive tower-line coupling damage probability P8 and the comprehensive tower-line coupling damage probability P10 after guy wire reinforcement. The tower-line coupling effect significantly increases the damage probability of transmission lines. At the same time, P4 is lower than P2, while P10 is lower than P8. Thus, the guy wire reinforcement has a certain effect on restraining the coupling effect.
Figure 5 shows the damage probability without considering the tower-line coupling effect. It can be seen that the comprehensive damage probability P7 of the tower is the highest, indicating that aging and corrosion contribute significantly to the tower damage. At the same time, the failure probability P5 of equipment aging and the failure probability P6 of equipment corrosion are almost constant. The reason is that both are related to time and the operating years are not much different. The damage probability P3 of towers after guy wire reinforcement is lower than P1 without guy wire reinforcement, and the comprehensive damage probability P9 of towers after guy wire reinforcement is also lower than P7 without guy wire reinforcement, indicating that guy wire reinforcement has an effect on reducing the damage probability.
In summary, the tower-line coupling effect occupies a dominant position in the contribution of tower damage and should be considered in engineering practice.
As shown in Figure 3, we found that the damage probability of Towers 17-27 is significantly higher than the others. The main reasons are the high wind speed acting on the tower and the significant tower-line coupling effect, followed by the corrosion and aging effects. The towers actually damaged during the typhoon were Towers 19-21, and the damage reason for Tower 19 was the corrosion of the anchor rod, which broke down under the wind load. Therefore, the prediction result can reflect the real damage situation, which proves the scientific nature of the method in this paper.
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