Research On Circulation Characteristics And Suppression Measures Of Metal Sheath Of XLPE Cable

With the increase in the proportion of high-voltage cross-linked polyethylene（XLPE）cables in power grid construction,the problem of abnormal grounding current of the metal sheath is becoming more and more serious.Excessive grounding current of the metal sheath not only increases the power loss and reduces the current carrying capacity of the cable,but also affects the cable operation life,and even cause major accidents such as cable line fires.With the rapid growth of the cable laying quantity,the investment in the operation and maintenance of the cable line is inevitably increased.Based on the in-depth analysis of the grounding current characteristics of the cable metal sheath,a systematic and in-depth study of the suppression measures has practical engineering value.The grounding current of the cable sheath consists of three parts: induced current,capacitive current and leakage current,of which the induced current has the largest proportion.Therefore,the accurate calculation of the induced voltage of the cable sheath is an important prerequisite for analyzing the grounding current of the sheath.In this paper,based on the principle of electromagnetic induction,the distance proportional method is used to calculate the sheath induced voltage of single-circuit and double-circuit.The sheath induced current is solved according to the loop current equations,and the accurate analytical formula of the metal sheath grounding current is obtained by superposing it with the capacitive current vector.This paper analyzes the effects of cable arrangement,three-section length,number of loops,phase spacing,loop spacing and cable core phase sequence on the grounding current of the metal sheath.The calculation results show that factors such as inconsistent arrangement,uneven three-section,increasing number of loops,increasing phase spacing and decreasing loop spacing can cause the grounding current of the cable sheath to increase to varying degrees.Under the single arrangement,the sheath grounding current is the smallest,followed by the right angle,and the horizontal arrangement is the largest;in the mixed arrangement,selecting the appropriate sheath segment ratio can minimize the overall grounding current and loss of the sheath.In the case of multi-circuit laying of cables,the grounding current of the intermediate circuit sheath is smaller than the grounding current of the outer cable sheath.Changing the phase sequence of the cable cores can reduce the sheath grounding current as a whole,but there is no obvious rule,and it is necessary to combine the number of cable circuits and the detailed analysis of the cable arrangement,and other specific situations.On this basis,this paper further analyzes the suppression effect on the grounding current of the cable sheath when resistors and inductors with different impedance values are connected in series.In order to facilitate the analysis and research of the calculation results,metal sheath of the single-loop cables in the two layout modes of parallel and equilateral triangle are respectively set as unequal lengths in the cross-connection unit.In addition,in order to more fully reflect the benefits of the suppression measures,the cable sheath grounding current,sheath loss,series resistance loss and cable current carrying capacity were calculated and analyzed simultaneously.Under the same suppression measures,the variation laws of the sheath grounding current and cable core ampacity are basically the same for the two layouts of the single-circuit cable in parallel and in the equilateral triangle.As the impedance values of the resistors and inductors increase,the metal sheath ground current and losses are greatly reduced.Compared with tunnel laying,the effect of improving the current carrying capacity of cables is more obvious when laying directly.However,with the continuous increase of the series resistance value,the loss brought by itself cannot be ignored.In the case of the same impedance value,the effect of series resistance and inductance on the jacket grounding current,jacket loss and cable core ampacity is negligible.The work done in this paper has deepened the understanding of the generation mechanism of the metal sheath grounding current of high-voltage cables,and further deepened the systematic understanding and research of its electrical characteristics.The analysis results have high guidance for the design and operation and maintenance of high-voltage cable lines.