Phase-Phase Switching Overvoltages, Transmission Lines
6.4 Sensitivity
The sensitivity of the SSFOR to the strength-to-stress ratio is presented in Fig. 16.
The strength is V30 where
and the stress E2z is
These curves are constructed for n = 500 towers or spans and y+ = y- = yz = 1.00, i.e., the voltage is constant along the line. The parameter of the curves is the per unit standard deviation of Vz or crz/E2z. As noted, the three curves cross at a SSFOR of l.O/lOO for which the ratio V30/E2z is 1.00. In the more practical case7 where the voltage profiles are less than 1 .OO, the ratio V30/E2z required for a SSFOR of 1 .O/lOO will decrease. Thus7 conservatively7 as for the phase-ground case, for a SSFOR of
l.O/lOO, set V30 equal to EZz.
"30' â‚ z
Figure 16 Sensitivity of the phase-phase SSFOR to the stresslstrength ratio.
158 Chapter 4
For other desired values of SSFOR, Tables 5, 6, and 7 of Chapter 3 can be used to estimate the required ratio of V30/E2z. That is, from Table 5, the value of Kf is obtained using a of/CFO of 0.02. The value of KG or KE is then obtained from Table 6 or 7. As before, V30/E2z is equal to either KfKG or KfKE.
7 CALCULATING THE STRIKE DISTANCE-SIMPLIFIED METHOD The technique in calculating the strike distance given the desired SSFOR is exactly the same as for the phase-ground case as developed in Chapter 3. In fact, Table 5 of Chapter 3 contains the value of Kf for crf/CFO = 0.02, which is the value required for oFp/CFO0. Of course, yz should be used. Also the same weather or altitude correction factors as for the phase-ground case should be used. The use of the simplified method in calculating the strike distance is illustrated in a homework problem.
8 INTERNAL INSULATIONS
The presentation in this chapter has been directed primarily to self-restoring or external insulations such as exist on transmission lines and in portions of substa- tions. For internal insulations such as in transformers, cables, and GIs, the insula- tion strength is only a function of the phase-phase voltages and is not a function of the division of this voltage into positive and negative polarities. Thus the insulation coordination procedure is simplified to that shown in Fig. 17. The density function is that for phase-phase overvoltages. The strength of the insulation is usually not statistically known. That is, the strength is specified only by the conventional BSL. Since the statistical strength characteristic is unknown, the only viable assump- tion is that at the BSL, the probability of failure increases instantaneously form 0 to 100%. The SSFOR for this situation as illustrated in Fig. 17 is
BSL
Figure 17 Calculating the phase-phase SSFOR for internal insulation.
Phasephase SOVs, Transmission Lines 159
Because the strength is identical for both the original and the reversed parameters7 the integral is not multiplied by 112. However, usually, the phase-phase voltage is modified by a surge arrester and will be considered in Chapter 5.
9 EXTERNAL INSULATIONS WITH KL = 1 . 0 0
As stated previously, for small gaps in the order of 3 meters or less, KL = 1.0.
Therefore oz = op, pz = pp, EZZ = EZP. Also CFOo = CFOp. Therefore, the SSFOR is
As for internal insulations, the integral is not multiplied by 112 since both the original and the reversed parameters result in the same SSFOR. To calculate the strike distance, the input value of the SSFOR must be equal to 112 of the desired value of the SSFOR. For example, if the desired SSFOR is 1.0/100, then in the calculation, a value of 0.5/100 is used.
1 0 IEC AND CIGRE
A short background may be helpful. In 1976, IEC published the insulation coordi- nation standards 7 1- 1 and 7 1-2. However, these standard publications did not include phase-phase insulation coordination. Therefore, in about 1977, IEC Technical Committee 28 began work on a new standard on phase-phase. At this time, there existed much confusion in the technical understanding of the process of phase-phase insulation coordination. In an attempt to mitigate the maelstrom, CIGRE Committee 33 on Insulation Coordination published four articles on the subject in ELECTRA [6]. This did little to relieve the confusion. Few engineers understood the concepts and methodology and few engineers had faith in the process.
Work continued within CIGRE Working Group 33.06, and in 1985 [9] a report was presented that suggested the method presented in this chapter. Recently, the IEC Technical Committee began revision of Publication 71. Both parts 71-1 and 71-2 are now complete and available. IEC 71-1, "Definitions, Principles, and Rules,'' was written primarily by Gianguido Carrara, while IEC 71-2> "Application Guide," was written primarily by Karl Weck.
The IEC application guide only considers insulation coordination of sub- stations, and therefore the comparison of the techniques here presented with those presented in the IEC application guide will await Chapter 5.
1 1 REFERENCES
1. G. Gallet, B. Hutzler, and J. Riu, "Analysis of the Switching Impulse Strength of Phase- to-Phase Air Gaps," IEEE Trans. on PA&S, Mar./Apr., 1978, pp. 485494.
2. K. J. Lloyd and L. E. Zaffanella, "Insulation for Switching Surges," Chapter 11 of Transmission Line Reference Book, 2d ed, Pa10 Alto, CA: Electric Power Research Institute, 1982.
160 Chapter 4
M. Miyake, Y. Watanabe, and E. Ohasaki, "Effects of Parameters on the Phase-to- Phase Flashover Characteristics of UHV Transmission Lines," IEEE Trans. on Power Delivery, Oct. 1987, pp. 1285-1291.
I. S. Grant and A. S. Paulson, "Phase-to-Phase Switching Surge Design," EPRI EL- 3147, Project 1492, Jun. 1983.
IEC Standard 71-2-"Insulation Coordination, Part 2: Application Guide," 1996.
CIGRE Study Committee 33, Insulation Coordination, ELECTRA, May 1979, pp. 138- 230.
6.1. F. Crespo, K. F. Foreman, G. LeRoy, R. Gert, 0 . Volcker, and R. Eriksson of CIGRE Working Group 33.02, "Part I, Switching Overvoltages in Three-Phase Systems," ELECTRA, May 1979, pp. 138-157.
6.2. A. Pigini, L. Thione, R. Cortina, K. H. Weck, C. Menemenlis, G. N. Alexandrov, and Yu. A. Gerasimov of CIGRE Working Group 33.03, "Part 11-Switching Impulse Strength of Phase-to-Phase External Insulation," ELECTRA, May 1979, pp. 158-181.
6.3. K. H. Weck and G. Carrara of CIGRE Working Group 33.06, "Part 111-Design and Testing of Phase-to-Phase External Insulation," ELECTRA, May 1979, pp. 182- 210.
6.4. A. Pigini, E. Gabagnati, B. Hutzler, J. P. Riu, H. Studinger, K. H. Weck, G. N.
Alexandrov, and A. Fisher of CIGRE Task Force 33.03.03, "Part IV-The Influence of Non Standard Conditions on the Switching Impulse Strength of Phase-to-Phase Insulation," ELECTRA, May 1979, pp. 21 1-230.
R. Cortina, P. Nicolini, A. Pigini, and L. Thione, "Space Occupation of EHV and UHV Transmission Lines as Affected by the Switching Impulse Strength of Phase-Phase Insulation," Stockholm: CIGRE Symposium on Transmission Lines and the Environment, 198 1.
R. Cortina, M. Sforzini, and A. Taschini, "Strength Characteristics of Air Gaps Subjected to Interphase Switching Surges," IEEE Trans. on PA&$ Mar. 1976, pp.
448452.
A. R. Hileman, "Phase-Phase Switching Overvoltage Insulation Coordination,"
CIGRE Working Group paper, CIGRE SC33-85(WG06)6lWD3 1985.
T. Udo, "Minimum Phase-to-Phase Electrical Clearances for Substations Based on Switching Surges and Lightning Surges," IEEE Trans. on PA&$ Aug. 1966, pp. 838- 845.
PROBLEMS
1. Using the simplified method with pp+ = 1.00, calculate the phase-phase strike distance for a 625 tower, 500 kV line (1 pu = 449 kV) at an altitude of 100Ometers for a phasephase SSFOR of 1.0 flashover per 100 breaker operations for the following conditions:
Stress
For phase-ground: f (v') is Gaussian having the parameters
For phaseephase: f (Vp) is normal having the parameters
Phase-Phase SOVs, Transmission Lines
Strength:
2. For a 500 kV line (1 pu = 449 kV) the phase-phase SOV distribution of Vz approximates an extreme value positive skew distribution with the parameters E2z = 2.8 pu and Pz/EZz = 0.10. The phase-to-ground SOV distribution is also an extreme value positive skew with E: = 1.8pu and = 0.10. The voltage profiles are y+ = yz = 0.90. The phasephase strength parameters are KL = 0.68 KGp = 1.26, and O ~ ~ / C F O ~ = 0.02. The phase-ground strength parameters are kg = 1.2 (no decrease for wet conditions) and q / C F O = 0.05. The line is composed of 500 towers. Estimate the phase-phase and phase-ground strike distance for a phase- phase and phaseground SSFOR of 1 flashover per 100 breaker operations. Assume sea level conditions, i.e., the altitude is zero.
3. For a 230 kV line (242 kV maximum, l.Opu = 198 kV), estimate the phase- phase strike distance for a SSFOR of 1.0/100. The line has 625 towers and is at sea level. Assume that KGp = 1.30 and K = 1.00. Assume that the correlation coeffi- cient is 1.00. Also for a Gaussian SOV distribution