Effect Of Coating Material On Space Charge Distribution And Conductivity Of Double-layered XLPE/EPDM

Currently,the High Voltage Direct Current(HVDC)transmission is occupying more and more important position in power grid construction and operation.Except overhead line,HVDC cable also plays irreplaceable role in renewable energy and marine economy.HVDC cable,as the same as Alternating Current(AC)cable,should also match with corresponding cable accessories for electrical field controlling.Generally speaking,DC cable always selects Cross-linked Polyethylene as primary insulation and cable accessory selects Ethylene Propylene Diene Monomer as main insulator body.Distinguishing with AC cable,it is easy to accumulate space charge in DC cable system,especially on the interface between cable insulation(XLPE)and accessory insulation(EPDM).It will lead to electrical field distortion and even break down failure case.In order to improve the interface status of XLPE/EPDM,different coating materials are daubed on the interface,such as silicone grease.But the effect of those coating materials on the space charge distribution is still uncertain by now.This paper will investigate the effect of coating material on double-layered XLPE/EPDM composite insulation structure,through measuring space charge distribution and conductivity.It also indicates the temperature and electrical field impact to space charge distribution and conductivity on the double-layered XLPE/EPDM.The research will help to provide theory and technical support for DC cable accessory design.The space charge test result indicates that three kinds of interface lead to different space charge distribution in double layered XLPE/EDPM.At the same temperature,space charge will increase according to electric field strength.The sample with polar grease has obviously charge accumulation on the interface between XLPE and EPDM.But even the same temperature condition,low or high temperature has different impaction on space charge distribution.At low temperature(30℃),direct touch interface sample turn the negative charge to positive charge accumulation following electric field strength enhancement.General grease and polar grease samples always appear to positive charge accumulation with electric field enhancement.At high temperature(50℃～90℃),three kinds of samples all have positive charge accumulation on the interface and all increase with the electric strength enhancement,only except that the sample with polar grease will appear space charge decline at the temperature of 90℃.At the same electric field strength,three kinds of double layered sample all have space charge accumulation and the account increase with temperature rising up.Sample with direct tough interface will turn negative charge to positive charge accumulation under low temperature.On the other side,the sample with general grease or polar grease has obviously positive charge accumulation on the interface.The conductivity test result indicates that three kinds of interface also lead to different conductivities in double layered XLPE/EDPM.At the same temperature,the current density and conductivity all rise up according to the electric field strength enhancement.The sample with direct tough interface increase slowly and the sample with general grease or polar grease increase rapidly.At the same temperature,the current density and conductivity all rise up according to the electric field strength.The sample with polar grease interface has most increasement than any other both.Through Maxwell-Wagner polar theory and Energy Band theory,analysis the working mechanism of different coating material on the interface of XLPE and EPDM under different temperature and different electric field.Combining with the test result to summarize the impaction of different coating material on the space charge and conductivity.It will help to select reasonable coating material to improve DC cable accessary application under different electrical field and temperature condition.