Effect Of Morphology On Space Charge Characteristics In Polyethylene Under Multi-physical Field Coupling Conditions

High voltage and extra-high voltage is the development trends of extruded-insulation direct current(DC)cable.Polyethylene is the most common and basic insulating polymers used in extruded power cables,which is characterized by high volume resistivity and low carrier mobility.However,these excellent performance lead to the accumulation of space charge under DC electric field,and thus the severe distortion of internal electric field,affecting the properties of material degradation and electrical breakdown.Except withstanding high DC field,extruded DC cable insulation also experience temperature field caused by the conductor core during the actual operation.Therefore,a further study on space charge characteristics under coupling conditions of high temperature and strong electric field is indispensable to overcome the key technique of extruded-insulation DC cable.Morphology includes surface morphology and crystal morphology,which affects its electrical properties.This paper is to analyze the effect of morphology on the steady and transit processes of space charge under multi-physical field coupling conditions through catching the evolution behavior of space charge in polyethylene under different temperature and electric field by pulsed electro-acoustic(PEA)method,providing the experimental and theoretical support for high-performance cable insulation materials developing and promoting.This paper obtained the space charge and DC breakdown characteristics of three LDPE sample groups with different surface morphology under multi-physical field coupling conditions,namely,LD-iPP(polypropylene as substrate),LD-PTFE(polytetrafluoroethylene as substrate)and LD-G(glass as substrate).As for LDPE-G,LDPE-PTFE and LDPE-iPP,the breakdown strength follows the order of most to least,the amount of the negative charge injected ranged from small to large and the positive charge injected ranged from large to small under-100 kV/mm.All groups appeared packet-like space charge with fast evolution behavior under-125 kV/mm at 60?,whose mobility increase by at least one order of magnitude compared with that at 25?.The amount of positive packet-like space charge arranged as LDPE-PTFE > LDPE-G > LDPE-iPP.Combined with the results of AFM,DSC,XRD,FTIR and dielectric constant test,these experimental phenomena above drawn a good explanation.Polyethylene samples with different crystal morphology were prepared,and the evolution behavior was studied under multi-physical field coupling conditions in this paper.The slower cooling rate results in the polyethylene with bigger size and higher level of uniformity unit cell.The faster cooling rate leads to higher conduction current density and higher trap density especially these deep traps.As the temperature rise,the maximum internal electric field value also increase(except the LD-A)and the time when internal electric field reaches the maximum trends to appear earlier under-100 kV/mm.Positive packet-like space charge was observed in all samples at 20?,and positive packet-like space charge in LDPE-I samples had the largest injection rate and mobility.When temperature rise to 40?,packet-like space charge only appeared in LD-I who has the maximum electric field distortion value.During the transient process under-100 kV/mm at 60?,LD-I appeared positive packet-like space charge,LD-S appeared the negative,LD-A appeared the both polarity,all of these packet-like space charges follow negative differential mobility.