Dielectric Losses and Stresses in Relation to Cable Failures

When transmission cables were first operated at potentials exceeding about 7500 volts, it was noted that cable failures occurred in service with loads materially below those which had theretofore been found to be permissible with low-voltage cables, and this reduction in carrying capacity increased with increase of the normal working potential. For example the author has previously reported that No. 0 A. W. G. four conductor cables operating on a four-wire three-phase system with a maximum normal potential of about 4000 volts between phases carry 200 amperes on each of three conductors without damage due to the overheating, whereas a 250,000-cir. mil cable operated at 20,000 volts was found to have excessive burn-outs if the load exceeded 175 amperes per conductor. For a number of years it has been recognized that this reduction in carrying capacity of high-voltage cables was due to the dielectric losses and a number of papers have been presented to the Institute on this subject. A temperature survey of the 20-kv. cable above mentioned showed that nearly all of the burn-outs occurred in a portion of the conduit near the substation, which conduit contained a large nuumber of heavily loaded cables, and in which the temperature was 10 deg. to 15 deg. cent. higher than the rest of the conduit. This portion of the 20-ky. line was replaced over two years ago with cable having a low dielectric loss, since which time no further cable failures have occurred.