Research On The Effect Of Cooling Ways On Space Charge Behavior Of LDPE/MMT Nanocomposites

Polyethylene becomes the insulation materials of high-voltage direct currenttransmission, due to its high mechanical and excellent dielectric properties. Withthe rapid development of high voltage dc system (HVDC) in our country, theelectrical aging problem of insulation caused by space charge is more and moreserious. In order to inhibit generation and accumulation of space chargeaccumulation and improve the dielectric properties, people have tried a variety ofmethods. Cooling process is an important part of the preparation process ofpolyethylene cable. Microstructure of material is greatly influenced by thecooling medium and the space charge propertie of material is affected by themicrostructure of material.Low density polyethylene (LDPE) and organic montmorillonite (O-MMT)were used as raw material. Low density polyethylene/montmorillontenanocomposites (LDPE/MMT) were made by melting intercalation. LDPE andLDPE/MMT nanocomposite film samples were prepared by the plate vulcanizingpress and cooling ways. The crystallization morphology of samples was analysedby the polarizing microscope. Space charge accumulation and attenuation ofsamples was measured by pulsed electro acoustic method. In addition, otherdielectric properties were also tested, including conductivity and dielectricspectrum.The experimental result shows that the crystalline grain size of slow coolingLDPE sample is larger and scatters evemly. The crystalline grain size of watercooling and oil cooling in LDPE/MMT nanocomposites sample is smaller. Slowcooling and oil cooling LDPE sample appear the heterogeneous space chargenear the cathode under the20kV/mm field. Under the40kV/mm field, slowlcooling LDPE sample injects more charge and air cooling LDPE/MMTnanocomposites sample appears a number of the heterogeneous space charges near the cathode and many charges in sample. The average charge density is minin oil cooling sample. The residual space charges in slow cooling and air coolingsamples is more, the average charge density larger, the average charge rate decayfaster. The conductivity is the largest under8kV/mm field, but it is min under50kV/mm field in oil cooling LDPE sample. The conductivity in differentcooling LDPE/MMT nanocomposites is slow cooling> air cooling> watercooling> oil cooling. The relative dielectric constant in slow cooling sample islager than that of air cooling, water cooling, oil cooling sample. The loss factor ismin in slow cooling LDPE sample, but it is the largest in slow coolingnanocomposites sample.