Improved thermal analysis for three-core cable under unbalanced three-phase loads

For the three-core cable widely used in medium voltage system, the phenomenon of three-phase unequal load is common. However, the accepted thermal rating method for three-core cable, i.e. IEC 60,287, was proposed based on the assumption of three-phase equal load. Especially for the calculation of thermal resistance of irregular filler layer in IEC 60,287, it was conducted on the basis of the symmetric cable temperature distribution which was not tenable in the case of three-phase unequal load. The applicability of method for the case of three-phase unequal load was not specialized in IEC 60,287. Moreover, this issue has not been sufficiently addressed in the current literatures either. In this paper, the error analysis for the conventional three-core cable thermal rating method under three-phase unequal load was conducted. It was found that the existing calculation method of thermal resistance of filler layer was no longer applicable. Then, based on the discussion for the internal temperature distribution characteristics of three-core cable under three-phase unequal load, an improved thermal rating method for three-core cables was proposed. In the improved method, the modified calculation for the thermal resistance of filler layer considering the asymmetry of temperature distribution was included. The precision of the improved method was verified by the real cable experiments. The results show that the improved method can well reflect the inherent temperature difference of the three-phase cable's conductors under three-phase unequal load. The improved method is instructive for increasing the utilization of the three-phase cables, which is useful for the management and operating of the medium voltage cable lines.