| Power transformers are the critical equipment to ensure the safe and reliableoperation of power grid. How to evaluate the exact aging condition of transformerinsulation and improve the safety operation level has become an urgent technicalproblem for power department. Frequency-domain spectroscopy (FDS) technology,based on the principle of dielectric response, which has the advantage ofnon-destructive measurement, user-friendly control and strong anti-interference abilityget the wide attention of scholars at home and abroad. However, most of the currentstudy was limited to qualitative describe FDS curve of oil-paper insulation withvariation of testing temperature、moisture content and aging state. Few studies involvedin quantitative description of FDS curves and in extraction of frequency-domaindielectric characteristic parameters. The quantitative relationship between frequency-domain dielectric characteristic parameters and the above factors had barely beenstudied. Furthermore，the field FDS measurement for transformer oil-paper insulationcould be carried out until transformer had been out of service. Its internal oil-paperinsulation system was in cooling process. The research about the impact of coolingprocess on frequency-domain dielectric properties of oil-paper insulation has rarelybeen reported.In this paper, a modified Cole-Cole model was firstly introduced to describedielectric polarization process. The parameters extracted from this model were proposedto be used as the frequency-domain dielectric characteristics parameters of oil-paperinsulation FDS measurement results. After then, the experiment of oil-impregnatedpressboard sample in different testing temperature、moisture content and aging statewere carried out via FDS measurement technology. The frequency-domain dielectriccharacteristics parameters of the above measurement results could be extracted. Theinfluence of the above factors on frequency-domain dielectric properties of oil-paperinsulation and its frequency-domain dielectric characteristics parameters werecomprehensive analyzed. Finally，a10kV transformer main insulation model wasaccelerated thermal aging different times. The FDS curves of the main insulation modelin three different test timing which were measurement at a constant temperature of90℃(Test timing M1)、measurement at the beginning of zero moment of natural coolingprocess (Test timing M2) and measurement after the start of natural cooling process 3h(Test timing M3) were measured at different thermal aging stages. On the basis offurther study of the influence of different test timing on frequency-domain dielectricproperty of oil-paper insulation, the frequency-domain dielectric characteristicparameters were extracted, and the variation of the frequency-domain dielectriccharacteristic parameters were analyzed. The main achievements made in the paper areas follows:(1) The modified Cole-Cole model containing one or two relaxation time constantscould better describe the frequency-domain dielectric properties of transformeroil-paper insulation sample in the range of10-4Hz to103Hz. The characteristicparameters of α and β-relaxation component、dc conductivity and hopping conductivitycomponent could quantitative describe the frequeny-domian dielectric properties ofoil-paper insulation.(2) The frequency-domain dielectric properties of oil-impregnated pressboardcould be significantly influenced by testing temperature、moisture content and agingstate, but the tanδ spectroscopy varies at different frequency ranges under the influenceof moisture content and aging state. With the increase of moisture content or aging state,the value of high-frequency dielectric constant εhf、χsαof α-relaxation component、dcconductivity σdcand increase，but the value of ταof α-relaxation component and χsβofβ-relaxation component decrease. The increase of moisture content could reinforce bothα and β-relaxation component, while the increase of aging state could only reinforceα-relaxation component. The relationship between frequency-domain dielectriccharacteristic parameters and moisture content、aging state were established，and itcould be used as empirical formula for quantitative assessment of moisture content andaging state.(3) The FDS measurement results of transformer main insulation model and itsfrequency-domain characteristics parameters are significantly influenced by test timing.When applying the regular monitoring results of FDS measurement technology to assessthe state of oil-paper insulation, if taking inconsistent test timing, it would lead toinaccurate assessment results. In order to improve accuracy of state assessment fortransformer oil-paper insulation based on frequency-domain dielectric spectroscopymeasurement technology, the appropriate and consistent test timing should be selected. |
PDF Full Text Request