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370 Dielectric Gases

TABLE III Arc Interruption Capabilities of Gases and Gas thus allowing the transmission of power at high voltages
Mixtures a,b straight into urban centers for distribution.
Z 0 = 450 Ω Z 0 = 225 Ω
Gas/mixture I c (kA) SF 6 = 100 I c (kA) SF 6 = 100 D. Compressed-Gas Insulated Cables

SF 6 21.0 100 26.3 100 Transmission voltage cables using a compressed gas as the
SF 6 /N 2 (75/25) 17.8 85 20.4 78 electrical insulation medium have been in commercial use
CH 4 /CClF 2 CF 3 (50/50) 17.8 85 20.2 77 since 1971. The voltage rating ranges from 138 to 800 kV.
CF 2 CFCF 3 /SF 6 (75/25) 17.0 81 20.0 76 Such cables are used as getaways from substations to over-
CF 3 SO 2 F/SF 6 (50/50) 16.5 79 18.3 70 head transmission lines, in crossings of two or more trans-
SF 6 /He (75/25) 15.4 73 20.4 78 mission lines, in tunnels from hydroelectric and pumped
SF 6 /N 2 (50/50) 14.9 71 17.2 65 storage generator plants, in river crossings, and so on.
CF 2 CFCF 3 14.8 70 17.8 68 They are especially attractive when high-voltage and high-
SF 6 /He (50/50) 14.7 70 19.7 75 power ratings are required in limited space, when metal-
CClF 2 CF 3 /SF 6 (75/30) 14.0 67 17.6 67 clad substations are connected directly to gas-insulated
CHClF 2 /SF 6 (75/25) 13.8 66 14.7 56 cables, or when a reduction in fire hazard is required, as
CBrF 3 /SF 6 (75/25) 11.6 55 14.5 55 in tunnels.
CF 3 SO 2 F/SF 6 (75/25) 11.4 54 13.8 52 The most common design of compressed-gas insulated
CF 4 11.1 53 14.6 56
CBrF 3 11.1 53 16.8 64 cables consists of an aluminum center conductor and
CClF 2 CF 3 10.8 51 15.4 59 a concentrically located outer aluminum enclosure. The
conductor is supported by epoxy insulators in the enclo-
a Total pressure = 0.62 Mpa.
b Data from Electric Power Research Institute. (1982). “Gases su- sure. SF 6 is the only dielectric gas used for gas-insulated
perior to SF 6 for insulation and interruption,” EPRI · EL-2620, Electric substations and cables. Besides being an excellent dielec-
Power Research Institute, Palo Alto, CA [prepared for Westinghouse tric, SF 6 also has good heat transfer characteristics for
Electric Corporation.] removing the heat generated by the resistive losses of
the conductor to the enclosure. Usually the SF 6 pressure
metalclad substations are free from industrial and coastal ranges from 3 to 5 bar. A schematic of a typical design of a
pollution problems and are esthetically more pleasing. The gas-insulatedcableisshowninFig.14,andatypicalinstal-
compactness of gas-insulated substations also offers the lation is shown in Fig. 13. Some designs include built-in
option of locating them in the basements of buildings, particle traps to improve the reliability of the system.

FIGURE 13 Totally enclosed metalclad substation using SF 6 gas as the insulating medium. (Courtesy of Westing-
house Electric Corporation.)