In Japan, more than 40% of the outages of transmission lines are caused by lightning strikes. An outage means the tripping of a circuit breaker, which eliminates flashovers across insulator strings. The outage caused by lightning is referred to as a lightning outage.

Over the past 30 years, application of line surge arresters (LSAs) has increased rapidly as lightning protection for overhead transmission lines in Japan. Here, almost all LSAs have the external gaps without any mechanical cutout and considered to be applied the LSAs to the earth side of the insulator strings. External gapped line arresters are typically referred to as EGLAs.

EGLAs for transmission lines rapidly cut off the flashover of insulator strings due to the lightning stroke and reduces the lightning outage, and the application phase number of the EGLAs using the zinc oxide (ZnO) element approached about 430,000 phases in Japan as of March 2021. In addition, sum for application phase number of EGLAs including the ZnO type and the fault current interrupting arcing horn (FCIAH), which is not a EGLAs using a metal-oxide varistors (categorized as current limited gaps: CLGs), approached about 480,000 phases as of March 2021.

This edited contribution to INMR by Toru Miki and Megumu Miki at the Central Research Institute of Electric Power Industry (CRIEPI) as well as Shinji Watanabe and Ryuji Yamada of Chubu Electric Power Co. discussed experience and lessons learned.

It is necessary to understand the characteristics of the lightning outages of transmission lines to implement appropriate lightning protection for transmission lines. Data on lightning outages have been studied with the aim of protecting transmission lines from lightning. However, lightning is a natural phenomenon. Thus, the variations on the data on lightning outages are very large. It is necessary to collect data on lightning outages in a large area over an extended period to obtain accurate information.

The Central Research Institute of Electric Power Industry and 11 transmission system operator companies (TSO) in Japan (the Japanese electric power utilities were separated the power generation and power transmission divisions in 2020) have organized an investigative committee to study lightning protection design. The committee has collected data on lightning outages of transmission lines since 1980 throughout Japan. Here, we report data on lightning outages in Japan and discuss the effects of transmission line surge arresters and lightning performances on the viewpoint form the lightning outages of transmission lines. In this report, the lightning outage rate is defined as the number of lightning outages of transmission lines over a line length of 100 km in a year. The yearly variations of the lightning outage rates of transmission lines for all voltage classes from 1980 to 2020 are presented.

Transmission Lines in Japan

There are mainly 11 voltage classes of transmission lines [1000 kV, 500 kV, 275 kV, 220 kV, 187 kV, 154 kV, 110 kV, 66 kV, 77 kV, 33 kV, 22 kV] in Japan. Fig. 1 shows the line lengths of transmission lines for each voltage class in Japan, where the line length is the distance between two points along a transmission line. Here, voltage classes indicate operation voltages of transmission lines.

Fig. 2 shows examples of transmission lines of 500 kV and 77 kV. The transmission lines with a voltage of greater than or equal to 187 kV are used for the bulk power system. These transmission lines have a direct earth system and 90 % of them are double circuit transmission lines, which have more than one ground wire. These transmission lines with a voltage of less than 187 kV have basically been used as subsystems. Many of these transmission lines have a resistive earth system or a non-earth system and most of them have ground wires except for some of the transmission lines with a voltage of less than 66 kV.

Definition of Outage Rate & Fault Rate

Here, the lightning outage rate is defined as the number of lightning outages of transmission lines over a line length of 100 km in one year. A double circuit fault means the tripping of circuit breakers of both circuits in a double circuit transmission line. The double circuit fault rate is defined as the number of trips of both circuits in transmission lines over a line length of 100 km in one year.

Lightning Outages of Transmission Lines in Japan

Fig. 3 shows the yearly variation of the lightning outage rates of transmission lines for all voltage classes from 1980 to 2020. The lightning outage rates of 66-77 kV transmission lines are similar to those of less than 66 kV transmission lines. The yearly variation of the lightning outage rates falls and rises alternately with a periodicity of 2-6 years. The difference between the maximum and minimum lightning outage rates is very large. In the voltage classes of less than 187 kV, the maximum difference in the lightning outage rates between years is more than 4 [cases /year every 100 km]. In the voltage classes of greater than or equal to 187 kV, the maximum difference in the lightning outage rates between years is about 1 [cases /year every 100 km].

Fig. 4 shows the periodic average of the lightning outage rates. The periodic average is the average of lightning outage rates over periods of almost 10 years and indicates the long-term trend. The variations of the averages in the periods of 1980-1990 (11 years), 1991-2000, 2001-2010 and 2001-2020 are less than about 1.1-1.4 [cases /year every 100 km] in the transmission lines of greater than or equal to 187 kV. The variations are very small compared with the yearly variations of the lightning outage rates. This suggests that there have been little long-term trends of the lightning outage rate. On the other hand, the variation of the averages between the periods of 1980-1990 (11 years), 1991-2000, 2001-2010 and 2001-2020 in the voltage classes of less than or equal to 154 kV are varies about 2-5 [cases /year every 100 km], which is significant compared with the yearly variation in the lightning outage rate. This suggests that the lightning outage rate in the transmission lines with voltage classes of less than or equal to 154 kV have decreased over the long term.

Detailed Data on Circuit Faults Caused by Lightning Strikes: Features of Ground Faults (Voltage Classes Greater than or Equal to 187 kV)

Here, data on the ground fault phases in transmission lines of greater than or equal to 187 kV is presented. Fig. 5 shows the percentages of different numbers and positions of ground fault phases in transmission lines. In transmission lines of greater than or equal to 187 kV, 80% of the ground faults caused by lightning strikes occurred at one phase. Furthermore, the percentage that occurred at the middle phase is the highest among the three phases. The percentage that occurred at upper phase is slightly less than that for the middle phase. Thus, the upper and middle phases account for more than 80% of the ground faults.

Discussion

Relationship Between Number of Lightning Flashes & Lightning Outage Rate
Fig. 6 shows the yearly variation of the number of lightning flashes in Japan. The number of lightning flashes varies with a periodicity of 2–6 years and shows a long-term trend of increase. This is likely to be due to the improvement of the instruments used to measure the number of lightning flashes.

As shown in Fig. 6, the periodicity of the yearly variation of the lightning outage rates is similar to that of number of lightning flashes. The years with peaks in the yearly variation of the lightning outage rates coincide with those in the yearly variation of the number of lightning flashes. This suggests that the periodicity of the yearly variation of the lightning outage rates is due to that of the number of lightning flashes. The correlation coefficients between the lightning outage rates and the number of lightning flashes are about 0.3-0.6. This weak correlation is caused by the instruments used to measure the number of the lightning flashes. In transmission lines of less than 187 kV, the periodic variation of the double circuit 7 fault rates coincides with that of the lightning outage rate. Thus, the periodic variation of the double circuit fault rates in transmission lines of less than 187 kV is likely to be caused by that of the number of lightning flashes.

Effect of Transmission Line Surge Arresters on Lightning Outage Rate

Below, effect of LSAs on the lightning outage rate is discussed. Fig. 7 shows photographs of typical LSAs used in Japan. LSAs using metal-oxide varistors are referred to as metal-oxide surge arresters. The number of metal-oxide surge arresters is the total number of LSAs using metal-oxide varistors. The metal-oxide surge arresters prevent the tripping of the circuit breakers owing to the nonlinear resistance of metal-oxide varistors. Fig. 7a shows a metal-oxide surge arrester with external gap. Fig. 7b shows an arcing horn with metal-oxide surge arrester, which is a compact type of metal-oxide surge arrester. Fig. 7c shows a fault current interrupting arcing horn (FCIAH), which is not an arrester using a metal-oxide varistors. It interrupts the fault current to prevent the tripping of the circuit breakers before the system protection relays sense the earthing.

The aim of application LSAs are mainly shown as follows:

(1) Installation LSAs in a single circuit side to avoid the double circuit fault in a double circuit transmission line. (It allows the single circuit fault in worst condition.)

(2) Installation LSAs in both circuits to eliminate the any lightning outage unless the arrester failure occurs due to the quite large current or large transferred charges of lightning.

Fig. 8 shows the yearly variation of the cumulative number of LSAs installed in transmission lines for all voltage classes. Fig. 9 shows the number of LSAs for each voltage class of the transmission lines [1], which shows that most of the LSAs are installed in the 66-77 kV transmission lines. The number of metal-oxide surge arresters installed starts to increase from 2000.

Cumulative number of LSAs and the long-term trend of the double circuit fault rates in transmission lines of less than 187 kV were compared. However, it is difficult to make such a comparison because of the periodic variation in double circuit fault rates. Thus, the moving average of 9-year (before/after 4 years) is used to remove the effect of such periodic variation. Fig. 10 shows the 9-year moving average of the double circuit fault rates and the cumulative numbers of LSAs, those are classified with operation voltage classes. From Fig. 10, it is found that the moving average of the double circuit fault rate in 66-77 kV transmission lines decreases with increasing cumulative number of metal-oxide surge arresters or fault current interrupting arcing horn. In particular, the cumulative number of LSAs increases rapidly from 2003 and the double circuit fault rate in 66-77 kV transmission lines starts to decrease rapidly from 2003. Fig. 10 also shows that the double circuit fault rate of 110-154 kV transmission lines also decrease from about 2005, and those of less than 66 kV transmission lines gently decrease rather than in 66-77 kV and 110-154 kV transmission lines.

Furthermore, the correlation between the cumulative number of LSAs installed in the transmission lines and the double circuit fault rate was estimated in each voltage classes 110-154 kV, 66-77 kV and less than 66 kV, respectively. The correlation coefficients are -0.97 in 110-154 kV, -0.99 in 66-77 kV and -0.90 in less than 66 kV, which mean a strong correlation in each voltage class. This shows that the moving average of the double circuit fault rate has a strong correlation with the cumulative number of LSAs. Thus, it is inferred that the long-term trend of the double circuit fault rate in transmission lines of 110-154 kV, 66-77 kV and less than 66 kV are due to effect of the LSAs.

Conclusions

Data on the lightning outages of transmission lines from 11 transmission system operator companies (TSO) in Japan since 1980 were collected. This article presents data on lightning outage rates and double circuit fault rates in transmission lines. In addition, data on lightning flashes and LSAs with data on lightning outages and double circuit faults in transmission lines were compared. From these comparisons, the following conclusions are obtained:

1. Lightning outage rates vary with a periodicity of 2-6 years. The periodicity of the double circuit fault rates in transmission lines of less than 187 kV coincides with that of the lightning outage rates. The periodic variation is likely to be due to the periodic variation of the number of lightning flashes. The double circuit fault rates in transmission lines of greater than or equal to 187 kV vary with a periodicity of 2-3 years.

2. Although there is no positive proof to confirm the long-term trends of the double circuit fault rates of each line, there is a long-term negative trend in the double circuit fault rate in transmission lines of 66-77 kV, 110-154 kV and less than 66 kV. It is inferred that this is due to effect of LSAs.

3. On transmission lines of greater than or equal to 187 kV, 80% of the ground faults occurred at one phase in the circuit of the transmission lines and 80% of one-phase ground faults occurred at the upper or middle phase.

Bibliography
[1] Study committee on lightning risk, Subcommittee for transmission lines, “Guide to lightning protection design for transmission lines (revised in 2022)” (CRIEPI: Grid Innovation Research Laboratory Report No. GD21026, 2023). (in Japanese)
[2] Study committee on lightning risk, Subcommittee for transmission lines, “Application guide for transmission line surge arresters” (CRIEPI: Electric Power Engineering Research Laboratory Report No. H07, 2012). (in Japanese)
[3] T. Shindo, H. Motoyama, T. Miki, M. Saito, A. Matsueda, N. Honma, A. Matsumoto, K. Shinjo, K. Hayashi, H. Amazu, K. Makabe, M. Fujikawa, S. Kurihara, and M. Sato, “Lightning occurrence data observed with lightning location systems of electric power companies in Japan: 2009-2013” (Asia Electromagnetic Conference (ASIAEM 2015), No. 45, Jeju, 2015, 8).
[4] T. Chino, M. Iwata, S. Imoto, M. Nakayama, H. Sakamoto, and R. Matsushita, “Development of arcing horn device for interrupting ground fault current of 77 kV overhead lines” (IEEE Transactions on Power Delivery, Vol. 20, No. 4, 2005, pages 2570-2575).
[5] T. Ohtaka, M. Iwata, F. Minoura, N. Tsukiyama, K. Kamimura, “Development of arcing horns interrupting fault current for overhead transmission line”, (Proceedings on 4th IEEJ-EIT Joint symposium on advanced technology in power and energy system, 2009, pages 151-156).
[6] T. Ohtaka, M. Iwata, S. Tanaka, Y. Goda, “Development of an EMTP simulation model of arcing horns interrupting fault current”, (IEEE Transactions on Power Delivery, Vol.12, No.3, 2010, pages 2017-2024).
[7] T. Ohtaka, M. Iwata, H. Misaka, H. Awazu, E. Nishikawa, T. Nakanishi, K. Kamimura, M. Uehara, “Development of Low-Cost High-Strength Fault Current Interrupting Arcing Horns For 77 kV Overhead Transmission Lines (Part 2) – Development of Demonstration Device – “, (CRIEPI: Electric Power Engineering Research Laboratory Report No. H17001, 2018). (in Japanese)