Effect Of Thermal Aging On Growth Characteristics And Partial Discharge Of Electrical Tree In EPDM

Ethylene Propylene Diene Monomer(EPDM)is widely used as the main insulation material of vehicle cable terminal because of its high temperature resistance,corrosion resistance and excellent insulation performance.However,due to the bad operating conditions and complex preparation process of the terminal,it is inevitable to introduce air gap,impurities and other insulation micro defects on the insulation layer,which will lead to the initiation of electrical tree and eventually lead to insulation failure.Considering that the main insulation layer is affected by high temperature for a long time during the operation of the terminal,which has a great impact on the initiation and growth of electrical tree.Therefore,through the accelerated thermal aging test,this paper studies the influence of thermal aging on the insulation structure and electrical tree characteristics of EPDM,and analyzes the partial discharge phenomenon in the process of electrical tree growth,so as to effectively solve the insulation fault of high voltage cable terminal It provides a theoretical reference.In this paper,a certain number of EPDM samples were prepared,and the accelerated thermal aging test of gradient was designed and carried out.The structural parameters of EPDM were measured.The results show that the crosslinking reaction is the main reaction in the early stage of thermal aging,the crosslinking density and crystallinity increase,the hole trap density and energy level decrease slightly,and the crosslinking structure is further improved;But,with the aging process going on,the chain segments are mainly degraded,the by-products increase,the crosslinking density and crystallinity decrease greatly,the trap density and energy level increase continuously,and the insulation performance deteriorates continuously.The morphology of electrical tree in unaged EPDM is branch like with few branches.With aging,the electrical tree change into clumps or clusters,and the structure becomes complex.In the early aging stage,the development of electrical tree can be divided into four stages: initiation,growth,hysteresis and breakdown,and in the late aging stage,it can be divided into two stages: rapid growth and breakdown.The growth rate,initiation voltage,initiation time,breakdown time and fractal dimension of electrical tree are significantly different with the increase of tree growth time and aging time.Moreover,the crystallinity and deep trap density have strong negative correlation with fractal dimension and breakdown time,respectively.The analysis shows that the increase of deep trap density and energy level leads to the difficulty of charge de trapping,the local electric field distortion and the accumulation of heteropolar space charges at the interface,which are the main reasons for the difference of electrical tree development under different aging time.At the same time,the decrease of crystallization region and the increase of deep traps lead to the formation of low-density region,the decrease of resistance to the growth of electrical tree,and the increase of free path for collision between charge particles and molecular chain,which promote the development of electrical tree.The results show that the discharge phases of the unaged EPDM are in the range of 0 ° to 90 ° and 150 ° to 270 ° and 330° to 360°,and the discharge in positive half cycle is more severe.The results show that the PRPD spectrum and the characteristic phase distribution spectrum ()of partial discharge also show a certain periodic variation with the growth of electrical tree,and the time characteristic parameters of partial discharge are quite different with the increase of aging time.The analysis shows that with the deepening of EPDM aging,the increase of trap density and energy level leads to the increase of charge trapping probability and difficulty for detrapping,so the increase of initial electron emission probability and the number of migrating particles in the electrical tree channel,which is conducive to the formation of electron avalanche and reduces the voltage amplitude required for partial discharge;moreover,a large number of heterosexual space charges accumulate at the dielectric interface,and with the aggravation of aging,the dissipation degree of accumulated charge decreases and the interface electric field increases,which promoted the partial discharge.