AmnpacityofDirect-Buried EHV Cables Insulated withSF6Gas

Three modelsof275-kVpipe-type cables insulated withSF6gashavebeenexperimentally manufactured, andvarious tests onthemweremadebytheauthor andhisassociates from 1965to1967. Someproblems fortherealization ofsuchgas-insulated cables remain unsolved, andamongthese itmaybefairly important toassess thermal characteristics ofthecables whenthey areburied underground. Inchecking theheattransfer behavior ofSF6-insulated cable installed beneath thesurface oftheground, thethird cable model usedforshort-circuit currenttesting was buriedunderground, andsomeheattransfer characteristics ofthemodelwereinvesti- gated inthecaseofsteady current flowofupto3500amperes. Tokeepthemaximumtemperature oftheinnerconductors at lessthan900C, current flowofupto3000amperesmightbeal- lowed, givensoil offairly goodcondition witha specific thermal resistivity g remaining within65°C.cm/W. Meanwhile, when values ofgare80and1000C.cm/W, 2800-and2500-ampere cur- rents, respectively, areabletoflowtomaintain theconductor tem- perature of900C. Fromtheresults oftheaboveexperimental data, thermal charac- teristics ofgas-insulated cables canbe deducedbycalculation, anditmaybepossible todesign EHV SF6cables sothattheir heat transfer characteristics aremostfeasibly accounted for.