Research On Generation Mechanism And Distribution Laws Of AC Stray Current Of Mining Cable

The research content in this dissertation is the main part of“Research on Distribution Laws and Prevention Technology of AC Stray Current in Mine Power Supply System”（No.201701D221240）which is the Youth Natural Science Foundation of Basic Research Programs of Shanxi Province,and is the main part of“Study on Stray Current Distribution and Hazard Prevention in Mine Power Supply System”（No.JMJS-JS-2012050）which is sponsored by Shanxi Jincheng Anthracite Mining Group Co.,Ltd..It is also an important part of“Study on Insulation Fault Mechanism and Residual Life Estimation Methods of High-voltage Mining Cable”（No.51377113）,which is the National Natural Science Foundation of China.With the popularization of fully mechanized coal mining technology and the increase of load capacity and power supply distance,the power supply voltage becomes higher greatly.At the same time,AC stray current,the potential safety hazard,has become more and more severe.As a consequence,it is of great importance to research the generation mechanism and distribution laws of AC stray current of mining cable,and to determine its influencing factors and influencing regularity.The research objects are the Cross-Linked Polyethylene（XLPE）and Ethylene Propylene Rubber（EPR）insulation shielded cable adopted commonly in coal mine.Through theoretical analysis,aging experiment and modeling simulation,the AC stray current generation mechanism and distribution laws of mining cable were studied,and the influencing factors of AC stray current and their influencing mechanism were explored,which lays a theoretical foundation for effectively preventing and controlling AC stray current in coal mine.The specific contents of this paper are as follows.According to the structural characteristics and operating environment of the mining cable,the generation mechanism of AC leakage current,the main source of AC stray current,was theoretically analyzed.It was clarified that AC leakage current of cable insulation is mainly composed of electrical conduction current,instantaneous charging current and absorption current.Taking the complex permittivity,complex electric modulus and AC leakage current as the main parameters,the XLPE samples under different thermal aging stages at 120°C-140°C and 150°C,and different electro-thermal aging stages at 10kV/120°C were measured respectively.The relationship between relaxation peak and frequency and temperature was obtained.The activation energy was calculated and the effects of temperature,frequency,thermal aging and electro-thermal aging on the AC leakage current of XLPE insulation were studied.The results show that the active part of AC leakage current of XLPE will increase sharply with the deepening of aging degree,increase with the rise of temperature and frequency,and the effect of temperature is mainly reflected in the low frequency region.In the range of 10^-2-10⁵Hz,the reactive part of the AC leakage current shows a strong dependence on frequency,which is relatively less affected by temperature and aging.Compared with thermal aging,electro-thermal aging significantly accelerates the aging process of XLPE insulation,increases the insulation conductance and dielectric loss factor,and also significantly increases the AC leakage current of the insulation.Comparing the variation of the dielectric characteristics and AC leakage current of EPR and XLPE insulation under different electro-thermal aging stages,it can be seen that,the active part of AC leakage current of the two insulation materials have a relatively large range of variation（several times,dozens of times or even hundreds of times）with the deepening of the aging degree,while the reactive part changes relatively smaller（1-2 times）and have strong frequency dependent characteristics.In order to acquire the influence rules of cable insulation aging on AC stray current,the distributed parameter model and the theoretical calculation model of AC stray current for mining shielded cable were established.Taking the shielding layer stray current I_P,the absolute value of shielding layer stray current distribution slope K_IP,the metal mesh stray current I_W,the absolute value of metal mesh stray current distribution slope K_IW and the sheath layer voltage U_h as the main parameters,the influence of cable insulation parameters unbalance,local insulation degradation and whole insulation aging on AC stray current were investigated through theoretical calculation in MATLAB and simulation analysis in RTDS.The results show that,the more serious the cable insulation parameters unbalance,local insulation degradation and whole insulation aging,the greater the amplitude of those above parameters.Meanwhile,the influence of local insulation degradation is the most significant.In the case of single-phase local insulation degradation,I_P,U_h and K_IP are related to the degradation degree,degradation length and degradation position,while I_W and K_IW are only related to the degradation degree and degradation length.Meanwhile,the K_IP can be used to diagnose and locate the cable insulation degradation section accurately.In terms of the absolute variation of AC stray current and voltage,the influence of cable insulation resistance is less than the influence of distributed capacitance,however,the relative variation of AC stray current and voltage can reach several hundred times while the cable insulation resistance decreases to the value corresponding to the human safety current,much larger than the influence of cable distributed capacitance on AC stray current and voltage（1-2 times）.Therefore,the cable insulation degradation can be diagnosed by AC stray current,and the active current extraction and utilization are the key.In order to ascertain the influence laws of the structural characteristics of the mine power supply system and cables on AC stray current,the influence of cable grounding situation,cable shielding layer,neutral point grounding mode,the junction box,load unbalance on AC stray current were studied based on the aforementioned models and research methods.The results show that,when other conditions are constant,the greater the grounding resistance is,the larger the I_W、U_h and K_IW are,but the sum of the I_P at both ends of the cable is basically unchanged,and the I_P is smaller on this side with larger grounding resistance.The metal shielding layer can collect most of the AC stray current and drain into the ground,and limit the magnitude of I_W and U_h effectively at the same time.Neutral point arc suppression coil grounding operation mode and the cable junction box can reduce AC stray current and stray voltage effectively.The greater the load unbalance degree is,the larger the I_P、I_W and U_h are,but the increase is extremely small.As long as the negative sequence voltage unbalance factor is less than 2%,its effect on AC stray current will be effectively limited.