Analysis on the longitudinal temperature and computation of ampacity of 10kV cables in pipes crossing unfavorable thermal regions

When power cables cross regions with unfavorable thermal conditions, the conductor temperatures higher than the design value can occur; the ampacity of the rated cable is significantly affected for high thermal resistivity in thermally unfavorable regions. This paper presents a three-dimensional heat network model with consideration the heat flow in two directions-radial and longitudinal. The algorithm of the permissible current-carrying capability of power cables is derived from the Fourier´s law, energy balance equation and equation proposed by IEC 60287. This algorithm is implemented in MATLAB and used to obtain ampacity solutions for cables in pipes crossing thermally unfavorable regions. A practical numerical example is considered with 10kV pipe-type cable distribution circuit, the results reveal that the ampacity of the pipe-type rated cable should be reduced up to almost 31% accordingly. This program is of great use to reasonably calculate the permissible current-carrying capability of power cables and to operate underground power systems stably and reliably.