Research On Evaluation Of Oil-paper Insulation Based On Time Domain Dielectric Properties

UHVDC technologies and projects are developing rapidly in China, the input and safe operation of converter transformers as core devices is the important guarantee of HVDC transmission project obtaining effectiveness. Oil-paper insulation of converter transformers valve side winding requires to withstand composite voltage in normal operation, which has the strongest voltage tolerance, the most complex structure, the highest uniform field strength density at present. Therefore, this paper presents the subject "Research on evaluation of oil-paper insulation based on time domain dielectric properties."In this paper, dielectric response theory is the basis. Through Debye model of oil-paper insulation and electric field simulation research, revealing the influence rule of Debye model electrical characteristic parameters on polarization/depolarization current curve and fields-pictures distribution laws of different insulation medium; constructing the electrical aging test platform to carry out a series of different composite voltages aging test and morphology study of insulating paper to obtain the pertinence of polarization/depolarization current curve of oil-paper insulation and degree of polymerization, and then finding the condition assessment method of oil-paper insulation based on time-domain dielectric properties.Analysis of a large number of experimental data obtaining the following rules:If slip resistances of Debye model are bigger and the capacitances are smaller, the polarization depolarization current curves are lower, the insulating properties of oil-paper insulation is better; With the growth of the aging time, the polarization/depolarization current curves upward move overall; Micro-morphology changes of cellulose is macro-performance of insulation paper's degree of polymerization, as the percentage of the AC voltage is increasing, charry ring marks of the upper surface of the oil-paper insulation are diminishing, charry spots on both sides of oil-paper insulation are gradually deepening, the influence of insulation paper's degree of polymerization is aggravating from the top oil-paper insulation near the high voltage side to the middle layer of oil-paper insulation, and around 75% percentage of the AC voltage exists extreme value range, the top oil-paper insulation deteriorates seriously; Polarization/depolarization charge difference of oil-paper insulation and degree of polymerization can be used to fit by f(qCDM)= a×eb-b-qCDM, and use nondestructive method of polarization/depolarization charge difference to assess oil-paper insulation aging condition.Aging test results are consistent with the law of modeling and simulation, determinate the relating rule of polarization/depolarization charge difference and degree of polymerization, will provide theoretical basis and practical reference for the nondestructive condition assessment of converter transformers'oil-paper insulation based on time-domain dielectric properties.