Research On The On-line Monitoring Technology Of Cross-bonded Cable Insulation Based On Resistive And Sheath Current

The cable is an important power equipment in the transmission link of power system.On line monitoring of its insulation condition is an important means to ensure the safe and stable operation of power grid.Long distance high-voltage cable usually adopts the grounding mode of cross interconnection of metal sheath to reduce the induced voltage and current of sheath.Because the sheath is cross connected through transposition box,the insulation on-line monitoring of long-distance cable is very different from that of short-distance cable.Aiming at this problem,this paper analyzes the cross interconnection structure of metal sheath,proposes an on-line insulation monitoring method based on the analysis of resistive and sheath current,expounds its monitoring principle,gives the implementation scheme,and carries out relevant research.According to the operation characteristics of cross interconnected three-phase cable,the distributed parameter model is established,the relationship between the instantaneous values of voltage and current at both ends of the cable and the cable parameters is constructed,the selection method of reference voltage for online monitoring system is given,and a calculation method of cable parameters is proposed.Through theoretical analysis and formula derivation,the expressions of insulation monitoring parameters,such as resistive current and dielectric loss factor,are obtained.The research on on-line monitoring of insulation condition of each section of cable is carried out,the overall implementation scheme is designed,the sheath current is analyzed,the leakage current and sheath loop current are separated,and the calculation method of each phase resistive current in a single cross interconnection unit is obtained.The calculation of any phase resistive current in any cross interconnection unit is realized.Through the change relationship between the resistive current and sheath loop current,the on-line insulation monitoring of a small section of cable is realized.The feasibility and sensitivity of the resistive current separation method are tested by simulation of the monitoring system.Considering the actual operation conditions of cables,the influence of synchronous measurement error,different ground potential,electrical parameter change,operating temperature change and grid frequency fluctuation on insulation monitoring parameters is studied and analyzed.The common cable faults are simulated,including the insulation fault of the whole cable and each section,the open circuit fault of the sheath in the grounding box and transposition box,the short circuit grounding of the sheath caused by the immersion of transposition box,and the breakdown short circuit of the sheath in the intermediate joint.The variation rules of each monitoring measurement are obtained,and the discrimination methods of various faults are given.Taking the physical simulation experiment as the verification means,the test system is built in the laboratory,the hardware and software of the lower computer are designed,and the hardware circuit and the overall test circuit are debugged and verified.The test results are compared with the calculated values to verify the accuracy of the test system and the feasibility of the monitoring method.