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

Abstract: Owing to the existence of cable heat capacity, the temperature of cable alters gradually as the time passes when applying the step current. The temperature achieves the steady state after a certain time. The speed of temperature change of cable conductor is reflected by thermal time constant. Hence, the single-cable cable is taken as an object of study and thermal time constant of cable is introduced. The transient thermal circuit models of cable body and ambient media are established and simplified; thermal time constant of single-core cables in air and buried in ground is calculated; the temperature rise test of single-core cable is conducted on site at step current. The actual thermal time constant of cables is obtained by curve fitting of real transient process of conductor temperature, which proves the correctness of theoretical calculation. The thermal time constant of cable can be used to 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. That provides theoretical support for cable condition monitoring and early fault warning during cable operation.   

Key words: Conductor temperature, thermal time constant, single-core cable, transient thermal circuit model, calculation, test, temperature rise time     

Introduction

The conductor temperature of cables in operation is the decisive factor in affecting the lifespan of insulation material. The status of cables in operation can be monitored via the temperature of cable conductor. However, due to technical restrictions, it is difficult to measure the temperature of operating cable conductor accurately. Hence, it is common practice to use theoretical calculation to obtain the conductor temperature. When the single-core cable works continuously at rated carrying capacity, its components including conductor, insulation and metal sheath will produce losses, forming the steady-state temperature field. The temperature of conductor can be calculated via steady-state thermal circuit and its calculation method is given by IEC60287 standard.