| The proportion of cable lines in urban power supply is getting higher and higher.With the increase of cable voltage level and transmission capacity,the sheath current induced by the core increases.Excessive sheath current not only affects the safe operation of the cable,but also causes increasing the cable reactive power.In this paper,starting from the existing equivalent calculation model of cable sheath current,the defects in the calculation process are pointed out and a new distributed equivalent circuit model is proposed to study the factors affecting the cable sheath current and reactive power.The thesis focuses on the following aspects:Based on the theoretical analysis of the existing single-core cable equivalent circuit model,the problems of the existing model are pointed out;the current of the sheath is calculated using the existing model in the case of three-phase cable cross-connect,and the calculation results and simulation and experiment The results are completely different;the reasons for the problems in the existing model are explained in terms of the size and phase change of the sheath current,and the influence of the distributed capacitance;an equivalent model of the cable based on the distributed circuit is proposed,and the formula for calculating the sheath current is given And verified the correctness of the proposed model through simulation.According to the established cable distributed circuit model,the influence of the current of the sheath on the length of the cable segment when the cross-connected segments of the cable are uniform and uneven is studied,and the current change when the ground fault of different parts of the sheath is grounded.The results show that the sheath current in L_a-L-L mode with two equal lengths of cable cross-connect cycle is larger than that in L-L_b-L mode.L_a and L_b can be 30%and 40%longer than the reasonable equal length section.In the case of unequal lengths of the three sections,the maximum length difference of the section length should not exceed 20%of the original reasonable length;when the ground fault of the cable sheath occurs at the phase commutation,the sheath current increases the most and the segment is uneven The fault ground current is greater.The current changes of the double-layer cable sheath are simulated and studied.By changing the phase combination mode,laying distance and grounding resistance of the double-circuit cable,the specific requirements for cable laying are proposed.The results show that the sheath current of the double-circuit cable is laid horizontally and horizontally<double right-angled and laid horizontally,and the combined phase of the minimum sheath current is determined.As the phase spacing increases,the sheath current gradually increases;as the loop spacing increases,the longitudinal horizontal laying sheath current gradually increases,and the horizontal horizontal sheath current gradually decreases;the sheath current increases as the grounding resistance Decrease.According to the distributed capacitance and cable structure in the model,the influencing factors of the cable reactive power are studied,the reactive power variation under moisture and eccentricity inside the cable is calculated,and the cable maximum transmission distance caused by the cable reactive power is calculated.The results show that the internal capacitive reactive power of the cable is greater than the external capacitive reactive power;after the cable enters into the water,the reactive power increases by 10%～30%according to the cable voltage level.The percentage of reactive power incrementΔQ%after the cable enters the water is proportional to the square of the eccentricity of the cable(D~2);when the cable voltage level is higher,the transmission distance of a certain power is shorter,and the difference is given.The limit transmission distance of voltage grade cable. |
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