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Ground Flash Density and Lightning Exposure of Power Transmission Lines G. Diendorfer, Member, IEEE, and W. Schulz
Abstract—Lightning is one of the major causes for outages in electric distribution and transmission networks. Data provided by a lightning location system in Austria allows detailed correlation of reported outages and the corresponding lightning discharges. Even within the small region of the Austrian territory the lightning flash exposure of HV transmission lines varies in a range from 0,6 to 2,6 km-2yr-1. A comparison of the actual observed lightning related line outages with calculated estimates based on the IEEE flash program show an underestimation of the outage rate by the flash program by a factor of about 2.
Index Terms—Lightning, Transmission lines, shielding failures
L
I. INTRODUCTION
IGHTNING is a major source for interruptions of electric power systems, both on the distribution and transmission line level. Ground flash density Ng is defined as the number of lightning flashes per km2 and year. When no data from a lightning location system are available, various equations exist for the estimation of Ng from the number of observed thunderstorm days Td. One of the most widely used relations is [1] Ng = 0,04 . Td1,25.
(1)
When data from a lightning location system (LLS) are available, the average ground flash density can be derived from statistical analysis of the data collected over a period of several years. In Austria operation of a nationwide LLS started in 1992 and today a dataset covering a 10 year period exists for detailed analysis of the local ground flash density in the different regions of Austria [2]. Location accuracy of the Austrian LLS is in the range of 400-500 meters. This accuracy has been confirmed by a comparison of the LLS output data with ground truth data measured on an instrumented tower [3]. Evaluation of the ground flash density from LLS data is highly affected by the selected grid size for the statistical G.Diendorfer is with Austrian Electrotechnical Association, OVE-ALDIS, Kahlenberger Str. 2A, A-1190 Vienna (e-mail:
[email protected]). W.Schulz is with Austrian Electrotechnical Association, OVE-ALDIS, Kahlenberger Str. 2A, A-1190 Vienna (e-mail:
[email protected]).
analysis. Depending on the application of the ground flash density selection of an appropriate grid size for the statistical analysis is required. The smallest grid size is determined by the location accuracy of the LLS. For densely meshed distribution lines of lengths of up to a few kilometers only, the lightning exposure of the line can be assumed nearly uniform along the line. II. LIGHTNING EXPOSURE OF TRANSMISSION LINES For transmission lines with a typical length of several tens of kilometers the lightning exposure can exhibit a significant variation along the line. For the 220 kV and 400 kV transmission lines operated by Verbund Austrian Power Grid (APG) we have determined values for the six year (1996-2001) average ground flash density ranging from 0,5 to 3,1 (see Table I), even within the relatively small country of Austria. Lightning flash density for each line is calculated from the number of flashes located by ALDIS within a corridor of ±1km along the line route (see Fig.1). Extension of the corridor width up to ±5km showed very similar results TABLE I GROUND FLASH DENSITY ALONG SOME INDIVIDUAL HV TRANSMISSION LINES OF VERBUND IN AUSTRIA
System
vg_471d
Length
72 km
1996 1997 1998 1999 2000 2001 1996 1997 1998 1999 2000 2001
vg_473d
vg_433
vg_451d
vg_421d
33,7 km 37,5 km 86 km 91,9 km Number of Flashes within ± 1km 210 114 133 139 150 321 321 118 158 120 411 294 203 223 93 342 171 286 125 30 511 260 243 291 129 230 139 271 176 94 -2 -1 Annual Flash Density [flashes km yr ] 1,4 1,6 1,7 0,8 0,8 2,2 4,6 1,5 0,9 0,6 2,8 4,2 2,6 1,3 0,5 2,3 2,4 3,7 0,7 0,2 3,5 3,7 3,1 1,7 0,7 1,6 2,0 3,5 1,0 0,5 Ground Flash Density (1996-2001) [flashes km-2 yr-1] 2,3 3,1 2,7 1,1 0,5
2
IV. COMPARISON OF ESTIMATED OUTAGE RATE AND OBSERVED OUTAGE RATE
IEEE provides an engineering method to estimate the lightning caused backflashover- and shielding failure rate of HVlines [4]. The calculation is based on the following input parameters: - Distribution of lightning peak currents - Ground flash density Ng along the line - Tower geometry (position of phase/ground wires) - Tower surge impedance - Footing resistances
Fig.1. Detected lightning flashes within a corridor of ±1km along transmission line 471d
III. CORRELATION OF OUTAGES WITH LIGHTNING LOCATION
We have calculated the estimated outage rate of the different HV-lines based on the actual ground flash density of the individual lines using the IEEE FLASH 1.7 program [4,5]. The original program uses as input the number of thunderstorm days Td. Eq.(1) is than applied to calculate the ground flash density Ng. We have modified the program to allow a direct input of Ng given in Table I.
DATA
Since 1996 Verbund APG can provide outage records with a timing accuracy of better than a second. Before 1996 timing accuracy of outage reports was up to several seconds and therefore we have used only data from 1996 to 2001 for the following analysis. We have searched in the ALDIS lightning database for time correlated lightning - outage events for power line outages of “unknown” reason or when lightning was already assumed to be the reason. Maximum time window was set to +/5 seconds and a maximum spatial distance of 5 km between the flash and the line was allowed. The numbers of correlated events and the time differences between outage records and lightning events are summarized in Table II. TABLE II
Year 1996 1997 1998 1999 2000 2001 total
-4
1
1
Time Difference in [s] -2 -1 0 1 1 20 2 3 14 2 1 3 4 27 4 1 1 3 17 3 1 5 2 20 2 1 1 19
-3
4
11
14
117
11
2
1
1
Table II shows that 90 % from the total of 159 correlated events are within a time window of ± 1 second.
In previous statistical analysis of the ALDIS data we have determined a mean value of about -14 kA for the negative peak current distribution in Austria [1], whereas in the IEEE FLASH program the distribution defined in [6], the so called CIGRE distribution with a mean value of -31 kA, is assumed. For comparison we have performed for each HV line the calculations using a log-normal distribution for the lightning peak currents of a mean of -31 kA (CIGRE) and -14 kA (ALDIS), respectively. The footing resistances for all the towers of the HV lines were available and applied for the calculations. The input to the FLASH program for the footing resistances is in form of a cumulative distribution as given in Table II for line 471d. TABLE III : FOOTING RESISTANCES FOR LINE 471d
Number of Towers Mean Maximum Minimum Standard deviation
244 12,1 Ω 66,7 Ω 0,6 Ω 16,4 Ω
10 % 20 % 30 % 40 % 50 % 60 % 70 % 80 % 90 %
< 0,9 Ω < 1,6 Ω < 2,1 Ω
