Thermal assessment of sheathed medium voltage power cables under non-sinusoidal current and daily load cycle

In this paper thermal analysis of power cables in the presence of harmonic currents and daily load cycle is performed. A derating factor is proposed for underground power cable ampacity due to the harmonic currents and daily load cycle. Derating factor is defined based on the ac to dc resistance ratio of cable in fundamental and harmonic current conditions which are the main parameters in power cable analysis. Ac to dc resistance ratios are calculated based on the finite element simulation with consideration of skin and proximity effects. For consideration of daily load cycle, the cyclic rating factor is determined using the IEC60853 relations and also the thermal transient analysis. Possibility of cable overloading in daily load cycle of harmonic current is studied based on Arrhenius life model and also failure rate of cables. Medium voltage power cables with solid bonded metallic sheaths and circulating current losses are considered. Simulation results show the accuracy of new derating factor. Also it is shown that defined cyclic rating factor by IEC60853 is conservative and cables can be loaded higher than rated ampacity according to current condition and daily load diagram.