Research On Dielectric Frequency Response Of Transformer Complex Oil-Paper Insulation

With the development of the electrified railway, the equipments of traction power supply increase rapidly. The condition maintenance has become an inevitable trend for these numerous equipments, which demand urgently the accurate assessment of insulation condition. At the same time, the insulation of the traction power supply equipment is demand more reliable with the development of the high-speed passenger transportation line, and the heavy-loaded train transportation. The traction transformer used as the key equipment of traction power supply system, its operation conditions directly influence the safety and stability of the system. It has important theoretical significance and engineering value to realize the accurate assessment of the transformer and develop the new oil-paper insulation system which is more adapt to the traction load.Based on the characteristics of traction load, the dielectric frequency response testing system for oil-paper insulation was established. The dielectric frequency responses of oil-paper composite insulation with different moisture contents at different temperatures have been measured within the frequency from10-4Hz to106Hz. The relaxation model with two relaxation times has been established to analyzing the measured data. The results show that the variation rules of insulation condition can be indicated by the parameters of relaxation model. The relationships about the complex permittivity and the moisture content、temperature and frequency were built. The dielectric frequency response method provides a new method for the assessment of oil-paper insulation properties.The micro-models of paper-moisture/oil-paper-moisture systems were established using molecular simulation method. The motion trajectory and diffusion coefficient of H2O in paper/oil-paper systems under different temperature were calculated by molecular dynamics. The relationships between the diffusion coefficient and the free volume and the trajectory of H2O molecular were discussed. It was proposed that a theoretical model for dielectric medium with an arbitrary distribution of conductivity and dielectric constant. Based on the steady state results, the dielectric frequency response of oil-paper with transient moisture equilibrium was simulated using the proposed model. The computed response is validated against experimental data. According to the dielectric model of oil-paper insulation with transient moisture equilibrium and its frequency response characteristics, it was proposed that the dielectric response method for assessment the oil-paper with moisture imbalance distribution.The TiO2-oil and SiO2-oil nanofluids were prepared by solvothermal. The dielectric frequency responses of oil-paper composite insulation at different temperatures and moisture contents were measured before and after SiO2or TiO2nanoparticles were added. The relaxation models with three relaxation times were established to analyze the results. The nanoparticles enhance the oil-paper insulation dielectric properties. The SiO2is better than TiO2. The breakdown voltage level and the partial discharge initial voltage have been proven the dielectric frequency responses.The field test system based on dielectric frequency response method has been developed. The dielectric frequency responses of the insulation between high voltage terminals and low voltage terminals of110kV and220kV transformers were measured in Guangzhou. The relaxation model has been established to analyzing the results. The value of the parameters of the model can reflect the conditions of the transformers.