Investigation Into Mechanism Of Micro-cavity Discharge In Solid Insulation Of Transformer Winding And Its Life Analysis

Power transformer is one of most important and expensive equipment, its running reliability is very important to over-all in-service electrical networks. The insulation defects of power transformer will cause severe failure to bring the huge power-failure loss and maintenance cost. Therefore, it is very significant to study the insulation fault diagnosis for power transformer.Based on the polyvinyl acetal enameled over coated bondable epoxy resin with paper insulation CTC, i.e. transformer winding specimen, this dissertation combines the gas discharge mechanism with the electromagnetic field simulation result of winding model to check partial discharge (PD) probability. The step-up breakdown experiment and the long-term aging experiment for transformer winding specimen with partial discharge (PD) measurements (including IEC60270 and ultra-high frequency (UHF) detection methods are set up to investigates into mechanism of generation, development and evolution for micro-cavity in epoxy resin of power transformer winding. Thus, the fundamental theory and engineering application for power transformer's insulation diagnosis and aging life estimation are established by Weibull distribution. According to on-site PD UHF detection characteristic of power transformer, the suitable PD UHF signal-denoising algorithm for the power transformer is studied. The above researches could provide the profound theoretical foundations to the localization and the quantitative investigation for the power transformer PD UHF detection. The main achievements are as follows:①Utilizing the micro-cavity discharge characteristics in epoxy resin of the power transformer windings, the new tool for aging life estimation for power transformer is presented for the first time. An inspection of winding strands showed that there are micro-cavities (or bubbles, voids) with different shapes and sizes. Then, utilizing the Maxwell 2D EM simulation software, electrical field concentration of cavity in Baklak was calculated. Based on the Paschen law, the breakdown voltage for different micro-cavity sizes and in different place of the epoxy resin was calculated to conclude that it is very well possible to have partial discharges in the micro-cavities in the epoxy insulation of power transformer windings under specific conditions.②A denoising algorithm based on complex wavelet transform (CWT) technique is presented for power transformer on-line PD monitoring system. According to the value of signal-to-noise ratio (SNR) for PD signal, integrating different soft and hard threshold value method and the hybrid evaluation the simple information and the compound information of CWT, the PD signal can extract effectively and accurately from the noise.③The step-up breakdown experiment with PD measurements for the specimen of power transformer winding is carried out for the first time. In addition, the apparent quantity qn could be separated into three stages and the patterns for phase-resolved partial discharge (PRPD) could be presented 6 kinds specific patterns. For long-term aging experiement, the apparent quantity qn can be separated into two stages and the responding patterns of PRPD are similar to the step-up breakdown results. Therefore, the PD characteristic of step-up breakdown experiment was utilized to predict the aging condition of on site power transformer.④According to the property of micro-cavity in the power transformer winding, there is no correspondent relationship between the apparent quantity by IEC60270 and the voltage level by UHF detection. During the step-up test, the sensibility of UHF detection method is lower than IEC60270, and with the increasing of voltage level, the higher frequency component is increasing. During the long-term test, the information of the phase pattern for UHF method is much more abundant than that of IEC60270. Therefore, the hybrid of two methods is recommended to the on-site PD detection for power transformer.⑤Based on the step-up and long-term test results analyzed by two parameter Weibull distribution, the combined estimation method of the Weibull parameters and the piecewise life estimation model is established for the specimen of power transformer windings. A 99% reliability running time ( T9 9%) could be utilized to validate the aging life under normal condition (supposed transfomer designed life as 40 years).