FEM modelling of faults occurred in buried power cables due to the removal of tree roots

Abstract

SUMMARYAn engineering approach to numerical modelling of power cable heating due to a line-to-earth short-circuit caused by previous mechanical damages of a cable sheath and insulation is proposed in this paper. Nonlinearity of the finite element model is based on the soil thermal conductivity and heat sources as functions of temperature and time. The nonlinear transient analysis is applied to an authentic example of line-to-earth shortcircuit caused by a mechanical damage of the cable sheath and insulation that has arisen during the previous removal of tree roots. The represented fault occurred on 14 November 2007 in a buried 0.4 kV cable line installed in the city of Bijelo Polje, Republic of Montenegro. Also, this paper outlines a proposition of a technical report prescribing procedures for the removal of tree roots from the soil surrounding cables and for laying power cables nearby high vegetation. The proposed engineering approach is applicable to power cables of all voltage levels laid in soils with changeable moisture content.

Insulation modelling for thermal FEM analysis of PVC and XLPE cables under fault conditions

“…According to [1], the process of thermal degradation of the insulation of XLPE cables starts approximately in the second range, during which its electrical breakdown also occurs, whereas according to [5], the complete loss of mass of the XLPE insulation occurs within the third range. Based on the aforementioned, it can be concluded with certainty that expression (2) in the case of PVC insulation is valid for the temperature range of 0-200 C, whereas in the case of XLPE insulation, it is valid for the temperature range of 0-400 C. In other words, PVC and XLPE insulations do not lose their electrical properties in the given temperature ranges. Based on the aforementioned, it can be concluded with certainty that expression (2) in the case of PVC insulation is valid for the temperature range of 0-200 C, whereas in the case of XLPE insulation, it is valid for the temperature range of 0-400 C. In other words, PVC and XLPE insulations do not lose their electrical properties in the given temperature ranges.…”

“…Hence, the introduction of the electrical resistance and thermal conductivity models to the FEM model of fault enabled performing of simulations in accordance with the real cable heating scenarios during the fault. These models provide the most credible simulations of heating of the considered cables under fault conditions, as opposed to the ones shown in [2]. These models provide the most credible simulations of heating of the considered cables under fault conditions, as opposed to the ones shown in [2].…”

“…The real fault scenario most certainly differs from the scenario considered in [2]. Therefore, modelling of electrical resistance of the insulation at the fault location is most significant for carrying out a proper simulation of fault thermodynamics in terms of determination of the fault duration and the damage degree of insulation after the fault.…”

“…The real fault scenario most certainly differs from the scenario considered in [2]. A common fault scenario and assumptions identical to those in [2] are taken for the XLPE cable, which is similar to the NAYY 4Â120 SM 0.6/1-kV cable in terms of its construction. Just like in [2], in this paper, it is also necessary to start from the assumption that the single phase-to-earth fault current instantly flowed through zero-resistance at the fault location at its full magnitude.…”

“…However, Reference [6] should be singled out among them as the most relevant one for the model suggested herein, where the idea for modelling of electrical conductivity of the insulation between two cable conductors was similarly exploited; and it was also the case with References [1][2][3][4][5], from which the FEM model of fault, fault scenario and thermophysical PVC and XLPE property measurements were utilized for the purpose of determining a thermal/electrical breakdown in the mentioned insulation materials. However, Reference [6] should be singled out among them as the most relevant one for the model suggested herein, where the idea for modelling of electrical conductivity of the insulation between two cable conductors was similarly exploited; and it was also the case with References [1][2][3][4][5], from which the FEM model of fault, fault scenario and thermophysical PVC and XLPE property measurements were utilized for the purpose of determining a thermal/electrical breakdown in the mentioned insulation materials.…”