Micro-moisture Content Evaluation Of Oil-paper Insulation Transformer Based On Characteristics Of Time Domain Dielectric Spectroscopy

Oil-paper insulation transformer is one of the key equipments in the electrical power system.It has been proved that the micro-moisture is "the first enemy" that makes the transformer out of work.Therefore,evaluating the damp state of transformer oil-paper insulation system effectively is crucial.Time domain dielectric method is a nondestructive,fast and effective electrical diagnostic method,and the spectral lines of which provide abundant information leading to being widely used in oil-paper insulation moisture evaluation.This research is supported by the Project of Chinese National Science Foundation named"The Damping Diagnosis Method of Oil-Paper Insulation Based on Circuit Analysis and Recovery Voltage Response"(No.61174117)targeting to use the time domain dielectric spectrum characteristics to diagnose the micro-moisture content of internal media of oil-paper insulation transformer.We plan to establish equivalent circuit model which can reflect the real dielectric relaxation response of oil-paper insulation based on time domain dielectric response theory.And analyzing the relationship between time domain dielectric spectroscopy characteristics and oil-paper insulation micro-moisture content by experimental data and simulation to provide effective methods for evaluating the micro-moisture content of oil paper insulation transformer.Main contributions of this thesis are listed as following:(1)Based on Extended Debye equivalent circuit model of oil-paper dielectric relaxation response equivalent circuit,a method is proposed to distinguish the Ended Debye equivalent circuit model using depolarization current differential spectrum according to the characteristic of time domain dielectric spectroscopy;Using the number of sub-line from the decomposition of depolarization current differential spectrum to confirm the number of polarized branch.And then identify the equivalent circuit model parameters accurately from the sub-line parameter of depolarization current differential spectrum.(2)Based on the polarization and depolarization current characteristics in the time domain dielectric spectroscopy,combined with identification method of Extended Debye equivalent circuit model parameters,two new characteristics of the peak value points of the depolarization current second differential lines and the depolarization current relaxation capacity were introduced to uncover the microscopic processes of dielectric polarization characteristic change due to moisture penetration to transformer oil-paper insulation systems.The results provide reliable new diagnostic feature for evaluation of micro-moisture content of oil-paper insulation transformer.(3)From the return voltage characteristics in the time domain dielectric spectroscopy,we analyzed the relationships among the polarization spectrum peak voltage,the main time constant and the micro-moisture content of transformer oil-paper insulation system.Simultaneously,choosing initial mean slope as initial slope improved feature number,a diagnostic criterion was proposed using the feature to evaluate the micro-moisture content in oil-paper insulation system.(4)Using the improved analytic hierarchy process,considering the time domain transformer dielectric spectrum feature variation of the micro-moisture content in the oil-paper insulation transformer,we established the transformer micro-moisture content diagnostic model based on the time domain dielectric spectroscopy characteristics and the improved analytic hierarchy process.This model provides a new insight for the comprehensive and scientific evaluation of the insulation system damp state and shows practical application prospect,thus deserves further exploration.