Study On The Influence Of Preload On Thermal Aging Characteristics Of Transformer Oil-paper Insulation

The axial pre-tensioning force of the transformer winding must be set,and its existence and changes have an important impact on the operating state of the transformer.At present,most of the studies related to the effect of pretension force on transformers focus on the relationship between the pretension force and the natural frequency of the transformer windings.Few studies have been done on the aging characteristics of transformer oil paper insulation under the action of pretension force.In this paper,the change of the aging characteristics of the oilpaper insulation of transformers under different pre-tightening forces is studied.Based on crystallography and free volume theory,and considering the production process and structural characteristics of insulating paper,a mechanical structure model of insulating paper is established in this paper.The analysis and research are carried out from the changes of the crystal structure of the insulating paper,the change of the composite system of the insulating paper and the insulating oil,and the main factors affecting the aging of the oil paper insulation,such as the change of the moisture and the state of the small molecule acid.Using the Force module of Materials Studio software,under the PCFF force field and NPT ensemble,the molecular dynamics simulation of cellulose,oil molecules,water molecules,and small molecule acids under different preload forces is performed.Simulation results,such as the mean square displacement of oil-paper insulation,show that the increase in pre-tightening force will slow down the thermal aging of oil-paper insulation.In order to verify the simulation results,a radial unit test model of the insulation block between the cakes was designed according to the working condition of the transformer winding compaction,and a test platform was set up to perform an accelerated thermal aging test for oil-paper insulation at 130? for60 days.Three kinds of pre-tightening force were designed,and samples were taken every 15 days,and the samples were tested by scanning electron microscopy,degree of polymerization,and frequency domain dielectric spectroscopy.The results show that as the pretension force increases,the cellulose structure of the sample tends to become narrower,the degree of polymerization degree decreases,and the frequency-domain dielectric spectrum shifts to higher frequencies.It is concluded that the trend of slowing down in the early stage of thermal aging of oil-paper insulation with the increase of pretension force is consistent with the simulation results.