Theoretical Calculation and Experimental Research on Thermal Time Constant of Single-core Cables

Cable conductor temperature in operation is the key factor influencing insulation life,and cable operation state can be monitored through conductor temperature.Not only the value of temperature rise but also the change rate of conductor temperature is concerned.Generally the change rate of cable conductor temperature is represented by its thermal time constant.Consequently,the thermal time constant of single-core cable was introduced,transient thermal circuit models of cable body and its ambient medium were established,and the simplified equivalent ones were given.Moreover,thermal time constants of single-core cable buried in soil and in air were calculated,the on-site temperature rise test under step current was carried out,then the actual thermal time constants were obtained by fitting of the conductor temperature detected in tests.The validity of theoretical calculation was verified by test results.Thermal time constant of cables can be used to roughly estimate the transient response of conductor temperature and to determine the needed time of achieving highest allowable conductor temperature of cables when step current is applied,which provides theoretical support for cable condition monitoring and fault early warming in cable operation.