Phase-Ground Switching Overvoltages, Transmission Lines
13.4 Estimating the Strike Distance
In this chapter, the estimate of the strike distance was performed using Brown's method, which was placed in terms of KG and K f . The IEC method may also be placed in terms of these quantities. That is,
Where Ze is from Table 6. To use this information, the value of ofn must be known, which is given by Eq. 102. To illustrate, assume the desired SSFOR is 1/100 and from Table 6, Ze = 2.0542. Also assume the data in the last example, E-, = 808 kV, oo/E2 = 0.10, of/CFO = 0.05, and Es/ER = 1.00. For this example let n = 200.
To obtain a solution, the equation for KG must be iterated, since the value of oh depends on the CFO. First assume that ofn = 0, then iterate per the following table.
For this case,
Zf = -2.6137; Kf = 0.9778; and of,, = 0.34657 of (116)
Another method is to set Ze of Eq. 112 to the value given in Table 6, i.e., 2.0542.
Thus
From the last example, (TO = 80.8, and [in = 642. Thus CFOn = 0.8693 CFO
Chapter 3
First assume that q n is zero, then
"f n CFO,, CFO "f n
For either method, the CFO is 933. For kg = 1.2, S = 2.37m. Brown's method gives 2.38m, and the computer program gives 2.40m. Thus both approximation methods are viable.
14 REFERENCES
1. A. R. Hileman, P. R. LeBlanc, and G. W. Brown, "Estimating the Switching Surge Performance of Transmission Lines," IEEE Trans. on PACES, Sept./Oct. 1970, pp. 1455-
1456.
2. T. H. Frick, J. R. Stewart, A. R. Hileman, C. R. Chowaniec, and T. E. McDermott,
"Transmission Line Design at High Altitude," IEEE Trans. on PACES, Dec. 1984, pp. 3672-3680.
3. K. J. Lloyd, and L. E. Zaffanella, "Insulation for Switching Surges," Chapter 11 of Transmission Line Reference Book-345 kV and Above, 2d ed. Palo Alto, CA: Electric Power Research Institute, 1982.
4. G. W. Brown, "Designing EHV Lines to a Given Outage Rate-Simplified Techniques,"
IEEE Trans. on PA&S, Mar./Apr. 1978, pp. 379-383.
5. A. R. Hileman, "Weather and Its Effects on Air Insulation Specifications," IEEE Trans.
on PA&S, Oct. 1984, pp. 3104-31 16.
6. Charles Lipson and Narendra J. Sheth, Statistical Design and Analysis on Engineering Experiments, McGraw-Hill, 1973.
7. Roy Billington, Power System Reliability Evaluation, Gordon and Breach, 1970.
8. E. J. Yasuda and F. B. Dewey, "BPA's New Generation of 500 kV Lines," IEEE Trans.
on PA&S, Mar./Apr. 1980, pp. 616-624.
9. G. N. Alexandrov, "Methods of Choosing Insulation for Distribution Networks with the View to Reliable Operation in the Presence of Internal Voltage Surges," Energetika, No. 7, 1962, pp. 16-24.
10. J. Elovaara, "Risk of Failure Determination of Overhead Line Phase-to-Earth Insulation under Switching Surges," ELECTRA, Jan. 1978, pp. 69-87.
11. IEC Standard 71-2, "Insulation Coordination - Part 2: Application guide," 1996-12.
12. CIGRE Working Group 13.02, "Switching Overvoltages in EHV and UHV Systems with Special Reference to Closing and Reclosing Transmission Lines," ELECTRA, Oct.
1973, pp. 70-122.
15 PROBLEMS
1. Using the simplified method, determine the strike distances and insulator length for the center and outside phases of a 500-kV, 550-kV maximum transmission line for the following conditions (note: 1 per unit = 450 kV). Use the estimating method as described in this chapter. Following this, check the answer with the appropriate computer program.
Phase-Ground SOVs, Transmission Lines
V-string insulators on all phases.
Switching surge wave fronts are equal to the critical wave front.
Line length is 200 km with three towers per km.
Line altitude is 1000m.
Gaussian SOV distribution with E2 of 1.8 per unit, o0/E2 of 0.07, and Es/ER = y = 0.88. the maximum switching overvoltage occurs at E2[1
+
2(oo/E2)1.of/CFO = 0.05, and assume wet conditions, decrease the CFO by 4%.
The line is to be designed for an SSFOR of one flashover per 100 switching operations.
Height of the phase conductor is 20m, and the tower width is 1.8 m.
2. Assume a line has been designed for a SSFOR of 1 flashover per 100 breaker reclose operations. Also assume that the line has lightning flashover rate of 0.5 flashovers per 100 km-years and that the line length is 200 km. Find the storm outage rate SOR (1) if only a single breaker reclose operation is permitted and (2) if two reclosures are used.
3. The SOV distribution at the line entrance to a 230kV, 242 kV maximum station may be approximated by an extreme value positive skew distribution with an E2 of 2.6 per unit and a PIE2 of 0.09. Between the line entrance tower and the opened breaker there exists an equivalent of 10 post insulators. Determine the SSFOR for these 10 post insulators assuming that the CFO of a single post insulator is 644 kV and of/CFO is 0.07. Assume the SOVs are equal on all insulators and that 1 per unit is 198 kV (Note: the CFO of 644 kV is an estimate for a 900 kV BIL post insulator). Use the estimating method as described in this chapter, then check the answer with the appropriate computer program.
4. Determine the crossarm lengths Ai and the crossarm separation distance Y for the 345 kV, 362 kV maximum tower shown in Fig. 35. Design the line for a SSFOR of 1 flashover per 100 breaker operations. The arm length for phase C must be 1.0m longer than the arm length for phase A. The insulator string must contain a minimum of 18 insulators (5: x loinches or 146 x 254mrn). Use the following data:
PHASE B
Figure 35 A 345-kV tower of Problem 4.
134 Chapter 3
1. Gaussian SOV distribution with E2 of 2.8 pu, ao/E2 of 0.07, and Es/ER of 0.90.
Assume all wave fronts are equal to the critical wave front.
2. Altitude = 2000 m, tower width = 1.3 m, I-string insulators on all phases, of/CFO = 0.05, 100-hour wind speed = 60 km/h, for wind = 1.9, D / W = 1.3, V / H = 1 .O; assume all conductor heights = 15 m, number of towers = 500.
Use the estimating method as presented in this chapter, then check the answer with the appropriate computer program.
5. Determine the SSFOR of a 500 kV line, 550-kV maximum, for the following conditions (assume 1 pu = 450 kV). Use the estimating method as described in this chapter, then check this with the appropriate computer program.
1. Single-circuit, horizontal-phase configuration, having a strike distance of 2.6 m for both the center and outside phases. Altitude is 1000m. Phase conductor height is 20m, and the tower width is 1.5m. Number of towers = 100.
o f / C F 0 = 0.05.
2. Gaussian SOV distribution, E2 = 2.0 pu, o& = 0.10 at the opened end of the line. SOV profile: Es/ER = 0.90.
6. Calculate the SSFOR for a 500-kV line (1 pu = 450 kV) for (1) 1 tower and (2) 200 towers for the following conditions:
1. CFO = 900 kV, of/CFO = 0.05, flat SOV profile: Ev/Ev = 1.00.
2. SOV distribution given by the following table.
No. of No of
v,
pu observationsv,
Pu observations1.2 1 1.6 21
1.3 15 1.7 10
1.4 20 1.8 5
1.5 27 1.9 1
Use the above table directly. Do not use the data to approximate or determine the SOV continuous SOV distribution. That is, assume that the probability of occur- rence of 1.2 pu is 0.0 1.