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 number 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-kv. line was replaced over two years ago with cable having a low dielectric loss, since which time no further cable failures have occurred. The method of analysis first suggested by Bang and Louis and later extended by Clark and Shanklin was applied to this particular case, and the carrying capacity for the cable as determined in this manner was found to agree closely with the results of experience. The method was therefore extended so as to determine the law connecting the size of conductor, the dielectric loss and the carrying capacity. Curves and charts are presented showing the carrying capacity of all sizes of three-conductor cables above 100,000 cir. mils and of the entire commerical range of dielectric losses. These results were then compared with the operating records of a transmission system having cables ranging in size from No. 00 A. W. G. to 500,000 cir. mils and with operating voltages of 9, 12, 20 and 22 kv. The results of this comparison appear to indicate that practically all failures on these transmission lines, which were not due to external damage to the lead sheath, were due to the cables being loaded beyond their safe carrying capacity, and that the dielectric losses had not been given proper consideration in determining the carrying capacity of these cables. During the year 1921 the number of cable failures on the 20,000-volt lines was about one per hundred miles. This was about the same as the record on the 9000-volt cables, and this result leads to the conclusion that when the transmission cables are operated at safe loads, in the determination of which dielectric losses have been given proper consideration, the resulting failures are of the order of one per hundred miles per year. This result indicates that very few of the cable failures which have occurred in the past are due to dielectric stresses and that most of the failures occur due to the reduction in dielectric strength caused by the heating of the cables above their critical temperature. Foreign cable manufacturers and operating engineers have apparently appreciated the reduction in carrying capacity due to dielectric losses as they ordinarily limit their maximum conductor temperature to a point well below the critical temperature as determined by these investigations. Their publications also appear to indicate that they fully appreciate that the quality of the insulation is of prime importance and that increased security cannot be obtained by increasing the thickness of insulation without improving the quality. The tests on cables made at the factory indicate that with the thicknesses of insulation that are commonly used in this country, and with the insulation of the first quality, the cable will pass the test required by the Standards of the A. I. E. E. with a wide margin of safety. Dielectric strength tests made on foreign cables and reported in the technical press indicate that the dielectric strength is generally materially above the corresponding figures obtained from tests on cables made in this country by a number of leading manufacturers. Apparently some material reductions in the thicknesses of insulation now used in this country for the transmission voltages can be made if the insulation is of first quality and has a low dielectric loss.