Multi-factor Aging And Life Evaluation Method Of Insulation For EPR Mining Cables

The dissertation is supported by the National Natural Science Foundation of China,namely “Study on Insulation Fault Mechanism and Residual Life Estimation Methods of High-voltage Mining Cable”(No.51377113).That is the research of the surface electrical tracking of 10 kV ethylene propylene rubber insulated cables and its fault diagnosis method.Ethylene Propylene Rubber(EPR),which consists of ethylene,propylene and non-conjugated diolefins,is of excellent moisture resistance,corrosion resistance and flexibility,so EPR has been the unique choice of insulation for cables in some special areas,especially in coal mine.It is well-known that the environment of coal mine is harsh,which will affect the life of cables.For example,cables used in the shearer for coal mine generally suffer electrical,thermal and mechanical stress together.For coal mining cables,electrical stress is generally less than about 1.1 kV/mm so single electrical stress hardly affects the life of cables.Thermal stress comes from high current when the shearer frequently starts and stops under varied load.Although the big majority of mine power cable is designed with proper armoring and/or supporting wires to avoid mechanical stress issues,cables,which are dragged by the shearer moving along the coal face,are subjected to pressure,pulling force and are bended frequently,which will result in the mechanical damage of EPR cables.Therefore,the life of EPR cables depends strongly on the electrical stress,thermal stress and mechanical stress in coal mine.The cable is subjected to stress,such as electrical stress,thermal stress and mechanical stress,for a long time,which causes the insulation to accelerate to the end of the service life,eventually forming insulation damage and causing cable failure.A serious failure will not only cause a shutdown,but will also trigger a gas explosion,resulting in a more serious safety incident.Therefore,according to the actual operation experience of the cable in the coal mine and considering the problems brought by the increase of service life,the monitoring and evaluation of the aging status of the mine EPDM cable urgently needs further study.In view of the above problems in the insulation of 10 kV ethylene-propylene rubber cable for mining,this paper takes the research object of ethylene-propylene rubber cable,and conducts the combined electric,thermal and mechanical aging test of constant temperature and variable temperature in the laboratory.The relevant parameters are measured,and the characteristic quantities are extracted while analyzing the aging mechanism.The specific research contents are as follows:Short cables intercepted from long mine cables are taken as research objects.A method of grading ring and silicone rubber infusion is used to treat the partial discharge problem that occurs when a high voltage is applied to the cable.The ageing test scheme was determined,and an electrical,thermal and mechanical combined ageing test platform was set up in the high-pressure laboratory to carry out the ageing test.Based on the analysis of the physical,chemical and mechanical properties of cable insulation in different aging cycles at constant temperature,combined with the single aging mechanism of electrical stress,thermal stress and mechanical stress,the combined aging mechanism of electrical,thermal and mechanical was obtained.The aging characteristics were extracted by tensile test,infrared spectroscopy,dielectric spectroscopy and PDC test,and the feature quantity with clear function relationship with aging time was determined.Based on the analysis of the physical,chemical and mechanical properties of cable insulation with the same aging period at variable temperature,combined with the electrical,thermal and mechanical aging mechanism at constant temperature and space charge theory,the effect of temperature on the aging process of electrical,thermal and mechanical was analyzed.The aging characteristic parameters related to temperature were extracted by tensile test,infrared spectroscopy and dielectric spectroscopy.Characteristic quantities which have definite function relationship with aging temperature are determined.Evaluation of aging state of mine ethylene-propylene rubber cable based on ultra-low frequency tanδ(0.1Hz)method and isothermal relaxation current method.The function between the elongation at break and aging time under the constant temperature was determined.Based on the variable temperature test to calculate the influence of temperature on the elongation at break,the life evaluation equation of the ethylene-propylene rubber cable for mining is obtained based on the Arrhenius equation.