Generation Mechanism And Aging Characteristics Of AC Space Charge In XLPE Insulated Cable

Crosslinked polyethylene(XLPE)insulated cable has been widely employed in power transmission and distribution system,for its good electrical properties and aging resistance.The reliability of cable has a strong relationship with safety and stability of the power system.Modern power enterprises pay much attention to insulation defect detection and fault prognosis in cables.Space charge is an important factor both in dielectric aging and breakdown.According to present research results,space charge distorts partial electric field,aggravates breakage of functional groups and initiates electrical tree growth;charge injection and extraction,charge trapping and detrapping,which are accompanied energy transfer and release,could destroy microstructures of insulation;charge trapping often causes partial disturbance and stores electricmechanical energy,and fast release of electric-mechanical energy in charge detrapping leads to insulation breakdown.Under the aging effect of high electric field and high temperature,defects increase in cable insulation and contribute to space charge accumulation,which would accelerates aging speed of insulation consequently.Most cables in power system are ac voltage cables,so it is very important to research on space charge characteristics under ac voltage.However,current research on ac space charge is in the primary stage,and dynamic behavior,generation mechanism and aging characteristics of ac space charge have not been clearly revealed.In this paper,generation mechanism and aging characteristics of ac space charge in power cable were mainly studied,which had great research significance for aging mechanism of ac cable,discovering insulation defect and estimating cable life.In this paper,a testing device of space charge in power cable under ac voltage was developed.Ac space charge behavior within the cable was measured,based on which a theoretical model was put forward to explain generation mechanism of ac space charge.Cable samples were aged under electrical-thermal stress,and the ac space charge behavior in cable peelings at each aging stage with aging time was analyzed,and the electrical and material properties were tested in different stages to assistantly analyze aging characteristics of cable insulation.The changing tendency of ac space charge behavior within cable insulation in the electrical-thermal aging was obtained by two heating strategies: heating in thermotank and under temperature gradient field.The principal innovative results are as follows;(1)The testing device for ac space charge measurement in cable was firstly developed by adopting new ac phase matching method.The heat flow loop to generate temperature gradient field was designed to achieve space charge measurement in cable insulation under the combined effect of temperature gradient field and ac electrical field.(2)It was discovered that ac space charge in cable can be divided into two types—“polarization charge” and “space charge”,and the characteristics of two charges were analyzed.“All-phase average method” was proposed to to analyze ac space charge waveform,which could provide more obvious profiles of stationary charge.Experimental results show that space charge increased gradually with aging time,and polarization charge presents few changes with aging time.(3)The theoretical model of trap energy level was proposed to explain generation mechanism of space charge.Based on the analyses of the dynamic behavior of electrons in each cycle,it was deduced that injection quantity of electrons in each negative half cycle was larger than extraction quantity of electrons in each positive half cycle.The asymmetry of migration between positive and negative period is the primary reason for accumulation of space charge.Polarization charge was suggested to be caused by dipole polarization in heterogeneous regions,according to the fast generation speed and short residence time.(4)The changing tendency of ac space charge in cable peeling samples with aging time was researched.It was discovered that trap density of cable insulation reduced in the early aging stage and increased in the middle-late aging stage.The mechanical strength and degree of crosslinking increased in the early aging stage and reduced in the middle-late aging stage,which were opposite to the changing tendency of trap density.Therefore the trap density variation was explained according to the results of mechanical strength and degree of crosslinking.The permittivity in high-frequency band(103Hz~106Hz)decreased with aging time,and the dielectric loss factor in lowfrequency band(10-2Hz~1Hz)increased with aging time.(5)Space charge accumulation was shown in the cable insulation position where the stress cone was installed after electrical-thermal aging.The changing tendency of space charge distribution in the stress cone installed position with aging time was researched.With the increasing aging time,the distribution range of space charge increased both in axial and radial direction,and the amplitude of space charge increased.Cable samples were aged under stress of ac electrical field and temperature gradient field,and the changing tendency of space charge with aging time was researched.The experimental result showed that in the aging process,space charge appeared from zero,and then grew gradually with aging time;stopped growing for a certain time,and then migrated to outward direction.Space charge diffused and the charge density decayed gradually in the migration process.