Research On Partial Discharge Characteristics Of Transformer Bushing Oil-paper Insulation With Different Aging Degrees

The converter transformer is in operation for a long time,and its bushing is affected by the thermal field and electric field for a long time,resulting in the gradual decline of the insulation performance of the bushing.In fact,the insulation of transformer bushing is the weakest link in transformer insulation.Therefore,it is necessary to study the influence of long-term thermal stress and AC stress on the partial discharge characteristics of oil paper insulation.Based on the scientific research cooperation between our unit and Xuzhou electric power test center,the partial discharge characteristics of the cardboard surface under the shielding electrode at the bushing end of converter transformer are preliminarily studied.Firstly,the insulation cardboard samples with different thermal aging degrees are prepared,and a hemispherical electrode is designed to simulate the structure of the bushing end shielding electrode,and then the partial discharge characteristics of the cardboard with different thermal aging degrees are analyzed The discharge characteristics of oil paper insulation were studied by using Fourier transform infrared spectroscopy and electric field simulation results.The results show that the pre discharge mainly occurs in the oil gap between the edge of hemispherical electrode and paperboard.In the initial stage of partial discharge,the aging of paperboard has little effect on the characteristics of partial discharge.With the development of partial discharge,the insulating oil in the pores of paperboard decomposes and produces bubbles,which overflow and adhere to the surface of paperboard.These bubbles have an important impact on the electric field distribution of hemispherical model.In the later stage of partial discharge,the aging of paperboard has a great influence on the characteristics of partial discharge.With the deepening of aging,the total discharge and maximum discharge increase,and the number of discharge increases,which accelerates the development of partial discharge.Comparing the FTIR spectra of different aging degree of paperboard,it is found that the absorptivity of O-H functional groups decreases with the increase of aging degree of paperboard.In order to further characterize the changes of surface morphology of paperboard with different aging degree under partial discharge condition,the scanning electron microscope(SEM)images of paperboard were processed by means of enhancement,denoising,segmentation,optimization and edge detection,and the fiber width,pore size and surface roughness of paperboard were extracted on this basis.The results show that the image processing method used in this paper can reveal the micro morphology changes of paperboard surface in the process of partial discharge,and can evaluate the thermal aging of paperboard by fiber width and surface roughness.When the polymerization degree decreased to 500,the fiber width of the insulation board was 91.1% of that of the non aging board,and the roughness was0.162.At this time,the board was in the middle of aging.When the polymerization degree decreased to 250,the fiber width of the insulation board was 79.2% of that of the non aging board,and the roughness was 0.225.At this time,the board was in the end of aging.