Studies On Methodology Of Insulation Status Measurement And Life Assessment Of Power Transformers

As the most important and expensive facility in power transmission and distribution systems, power transformer's operational conditions render tremendous significance for reliability and stability of the power grids. However, under complicated loading and operating conditions, the actual life expectancy of a power transformer is not equal to the rated longevity shown in the nameplate due to insulation aging of multiple mechanisms. Hence, extensive investigation on the monitoring and analyzing technology as to accurately evaluate life expectancy of the power transformers, will present unique importance in developing condition-oriented maintenance strategy for power equipments as well as guaranteeing secure, reliable and stable operation of the power systems.With detailed analysis on the physical degradation mechanisms of both solid insulation and oil, an integrated analyzing and evaluating scheme has been proposed by comprehensive utilization of three effective methodologies based on Measurement of the Degree of Polymerization, the Furfural Concentration Analysis as well as the Polarization and Depolarization Current Analysis.Based on elucidation of the physical process of cellulose decomposing, furfural producing and dissolving in oil, mathematical models to assess the remnant lifetime of a power transformer were established respectively through solid insulation degree of polymerization and furfural concentration in oil. A lot of site tests were carried out with viscosimetry for the degree of polymerization measurement and high performance liquid chromatograph for the furfural concentration analysis, and some concrete measures were presented for test improvements. Also, the effectiveness of above proposed mathematical models were in part verified by the test results.With reference to the dielectric polarization theory, the prevailing polarization and depolarization current formulae were re-deduced with amendment by introducing the transient displacement polarizing current component, which gave a more reasonable description of the physical nature of dielectric polarization. Experiments were done to measure the polarization and depolarization current, and the impact from temperature and moisture changes on the polarization and depolarization current curve were also accounted for. By dielectric response function, the polarization process in time domain was transformed into frequency domain, and an equivalent circuit was established to simulate the properties of the transformer's main insulation system, with the circuit parameters being calculated by means of least square method. With this equivalent circuit, the polarization and depolarization current curve could be simulated, while the dielectric dissipation factor of transformers'insulation system was also computed.With regards to the obtained data of the degree of polymerization, the furfural concentration as well as the polarization and depolarization current under different loading and operating conditions, statistical methods including correlation analysis, variance analysis and non parametric hypothesis verification were utilized as to obtain the changing behaviors of the solid insulation degree of polymerization, the furfural concentration in oil and the solid insulation DC conductance versus the years in-service, as well as their regional distribution feature under different loads. In addition, the mathematical relationship between the solid insulation degree of polymerization and the furfural concentration in the oil was also derived.With integration of the above several methods for transformer life prediction, as well as their mathematical relationship and applied performance, a comprehensive decision tree was developed to evaluate the life expectancy of transformers'insulation system, which could effectively improve the veracity of the predictions. Also, a novel combined scheme based on both lifetime prediction and failure rate analysis was also advised for practical applications so as to guarantee operational reliability of the power transformers.