| Oil-immersed transformer is an important part of the power system.If it fails during operation,it will cause significant economic losses to the power system,which in turn will affect social development.The insulation system of oil-immersed transformers is also affected by various stresses in the transformer to produce moisture,furfural and other substances,resulting in weakened mechanical properties and insulation strength.And the moisture generated during the operation will accelerate the speed of insulation aging,so accurately diagnosing the moisture content of transformer oil-paper insulation and evaluating the degree of moisture are the keys to prolong the service life of the transformer.In addition,the mineral insulating oil commonly used in oil-immersed transformers has the disadvantages of low ignition point and low biodegradation rate,while vegetable insulating oil has the advantages of high ignition point and high biodegradation rate,and they have gradually been used in transformer.Therefore,it is necessary to find a new assessment method of moisture content of oil-paper insulation suitable for vegetable oil transformers.Frequency Domain Spectroscopy（FDS）is a new dielectric response measurement technology.It has rich measurement data and strong anti-interference ability,which is conducive to the on-site diagnosis of transformers.And the measurement results of insulation complex dielectric constant and dielectric loss factors can reflect the moisture level of transformer oil-paper insulation structure.This paper first analyzes the dielectric response process of oil-paper insulation with dielectric theory,then introduces the conductivity and Havriliak–Negami dielectric model,and proposes to use an improved Havriliak–Negami dielectric model to assess the moisture content of vegetable oil-paper insulation.First,in order to investigate the influence of moisture on the FDS of oil-paper insulation system,a FDS test platform for oil-paper insulation was established in the laboratory.Then oil-paper insulation samples with different moisture conditions were prepared,and the complex permittivity curves of the samples were measured and analyzed.The results show that the change rate L of the real part FDS curve of the complex permittivity of oil-paper insulation at frequency of 10-4Hz was strongly affected by the polarization of moisture.Second,combined with the improved Havriliak–Negami dielectric model,the frequency domain relationship of the change rate L is established,and the relationship between L and oil-paper insulation dielectric response parameters is analyzed.Third,the dielectric parameters of the oil-paper insulation are identified with FDS data,and the parameter results and L are obtained to establish a functional relationship between L and moisture content w.Finally,The influence of the insulation paper aging on the characteristic quantity L is analyzed,and then a compensation factorθis proposed in combination with the interpolation function to eliminate the influence of the insulation paper aging on the characteristic quantity L.Furthermore,a method of using compensation factorθto improve the accuracy of the assessment of the moisture content of vegetable oil-paper insulation systems with different aging was proposed.This method mainly uses the relationship between the DP and the evaluation of moisture,and combines the compensation factor surface to modify the quantity L,and then obtains the revised feature quantity that can be used to assess the content of moisture.In addition,the temperature will also affect the FDS test results and reduce the accuracy of moisture assessment.Therefore,the compensation factorγof the characteristic quantity L was introduced to correct the measured characteristic quantity,so as to eliminate the interference of temperature on the assessment method of the vegetable oil-paper insulation.Then,combined with the compensation factorγ,a method to eliminate the influence of temperature on the assessment was proposed.In addition,the study found that water will affect the frequency normalization results of the FDS curves,so a method was proposed in this paper to eliminate the influence of moisture.Finally,the data of on-site transformer were used to verify the examples.The results show that both methods can eliminate the influence of temperature,and the results of moisture assessment are close to.This shows that the methods in this paper are reliable,which provides a certain help for studying the moisture state of transformer oil-paper insulation. |
