511 / 2022-03-25 10:36:24

Analysis for the Grounding System of 110 kV and the Above HV Single Core XLPE Cable

High voltage cable,Aluminum sheath,Suspended potential,Grounding system

Purpose/Aim

High voltage single core cable is more and more widely used in urban transmission network, but when its grounding system is abnormal, it will produce high suspension voltage and lead to serious consequences. Single-core XLPE cable is a typical multi-layer cylindrical capacitor structure. Considering the outermost graphite semi-conductive layer, single-core cable is a typical multi-stage capacitor series structure.

Experimental/Modeling methods

Based on the theoretical analysis of the three-stage capacitance of the cable "conductor core-aluminum sheath-graphite layer-grounding body", this paper calculates and analyzes the high suspension voltage of the cable line caused by the abnormality of the grounding system,according to the two working conditions of single terminal grounding system and cross interconnection grounding system.Through the case study, puts forward the preventive measures to solve the high suspension voltage of the cable line in this paper.

Conclusions

When the reliable grounding of the aluminum sheath disappears, a suspension voltage occurs on the cable aluminum sheath.  The suspension voltage is proportional to the line voltage level and is related to the grounding state of the cable graphite (semi-conductive) layer.  When the graphite layer is reliably grounded, the cable has a two-stage capacitance structure.  When the graphite (semi-conductive) layer is not grounded, the cable has a three-stage capacitance structure.  The single-phase voltage of the cable line is inverted by the capacitor in series. The smaller the capacitor value is, the larger the partial voltage is.  For single-end grounding cable line, when there is confusion between protection box and grounding box, grounding wire stealing and falling off, the aluminum sheath is not effectively grounded, and the suspended voltage generated on the aluminum sheath has nothing to do with the cable length.  When the graphite layer is not effectively grounded, it will produce continuous high-frequency charging and discharging, which may lead to different types of serious consequences such as fire of outer sheath, ablation of aluminum sheath and breakdown of main insulation, and endanger the safety of operation personnel.