| Capacitance voltage transformer(CVT)is an important metering and protection device in power system,which is widely used in HV and EHV neutral directly grounded power system.At present,CVT capacitor mainly adopts film-paper insulation,which is composed of polypropylene(PP)film and capacitor paper overlapped in a certain proportion.In recent years,scholars are more interested in the development of high-performance dielectric composite materials,and the research on the partial discharge(PD)characteristics mainly focuses on such insulating materials as oil-impregnated paper and epoxy resin,while few people had conducted in-depth experimental research and detection analysis on the PD characteristics of the film-paper insulation in CVT capacitor.In this paper,the whole process from discharge to breakdown of the film-paper materials was detected and analyzed in detail.Firstly,the PD experimental platform for three-layer-film and two-layer-paper insulation model was built,and the phenomena and local discharge characteristics of the whole discharge process were recorded and analyzed.The PD stages were divided according to the analysis results,and the influence of bubbles on the PD process was analyzed by using finite element software COMSOL.The results showed that the whole process of PD can be roughly divided into four stages: initial,development,explosion and near breakdown.In the initial stage,the discharge was unstable and random,and the gas generation rate was small.With the increase of discharge time,the gas generation rate and discharge quantity both showed the characteristics of slow increase in the early stage and sharp increase in the later stage.Due to the high discharge frequency and discharge quantity near the breakdown stage,the heat generated by PD cannot be dissipated in time,resulting in thermal breakdown of the samples.The existence of bubbles can cause serious distortion of the electric field distribution,and the electric field intensity of its surrounding medium has far exceeded its rated working field.Meanwhile,the size and spacing of bubbles have a great effect on the electric field distribution.Then,the surface morphology of the sample was observed by scanning electron microscope(SEM)at different discharge stages.The 3D modeling of the image was carried out and its surface height curve was plotted.The surface roughness of the materials with different discharge time was characterized.The results showed that PD had a significant effect on the surface morphology of the sample.With the increase of the discharge time,the material surface defects gradually increased.In the late stage of the PD,the surface of PP film appeared a large number of "bumps",and surface white spots increased.The surface structure of capacitor paper had been damaged seriously and the cracks had appeared obviously.After the breakdown of the sample,some of the materials near the breakdown point were severely ablated,and obvious granular and massive white spots appeared on the surface of the sample.The surface roughness of the material also increases with the increase of the discharge time,and the surface roughness increases sharply in the discharge explosion stage.Finally,fourier transform infrared spectroscopy(FTIR)was used to analyze the changes of chemical components in the materials at different discharge stages.The results showed that both the PP film and capacitor paper were degraded to a certain extent during the PD process.Some macromolecular chains of polypropylene were degraded to small molecular chains.The O-H and glycosidic bonds of cellulose molecules in the capacitor paper were also broken to a certain extent.The reason for the material degradation is that the high-energy particles generated by PD break the molecular chains of the transformer oil and film-paper materials to generate new free radicals,which are connected with each other to generate small molecular chains. |
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