Research On Degradation Mechanism And Assessment Method Of Nomex Insulation For Mining Dry Type Transformer

The dissertation is funded by the National Natural Science Foundation of China, based on the project of “Study on Degradation Mechanism of Nomex Insulation for Mining Flameproof Dry-type Transformer”(51577123), which is proposed according to the aging problem of the main insulation for dry type transformers. Nomex paper insulation is widely used in high voltage motors and transformers for its good mechanical properties, electrical properties and thermal stability. The vast domestic and international mining dry type transformers are insulated by Nomex papers. The mining dry type transformer works underground in coal mines, where the temperature and humidity is much higher than that on the ground, and with much more interference. It usually undergoes overvoltage and overcurrent of the power supply system underground. Therefore, degradation of the transformer insulation is accelerated by the combined effect of these factors, which would lead to the decline of its electrical and mechanical properties, and in turn causes a variety of insulation faults, including mine fire and non-plan power cut. Efficient methods in the Nomex insulation assessment can be found by studying the effect process and mechanism for the stresses on the Nomex insulation of the mining dry type transformer. Therefore, it is very important to study degradation mechanism of the Nomex insulation to prevent coal mines from mine fire and gas explosion accidents.Much researches have been done on the aging mechanism and assessment methods for insulations used in oil-immersed transformers, such as the insulating oil, insulation paper and the oil-paper insulation. And many insulation assessment methods based on aging characteristics were proposed by researchers. However, the insulation degradation mechanisms of mining dry type transformers are not fully understood, so that the study of assessment methods can not be investigated thoroughly. Therefore, thermal and electrical aging mechanisms of the Nomex insulation was studied experimentally and theoretically in the dissertation. Then insulation assessment methods of the mining dry type transformer based on aging characteristics of the Nomex insulation were studied. The reliability assessment model and insulation condition assessment method were proposed, respectively. The studies are presented as follows:The thermal aging test of the Nomex insulation paper at temperature of 180℃ and moisture of RH 60% was conducted by establishing the thermal aging test system, determining the value of aging stress and the strategy of the aging test. Electrical and mechanical parameters of insulation specimens were measured periodically during the thermal aging test. Then the relationship between these aging parameters and the aging state of the insulation was analyzed. And the thermal aging process and mechanism of the Nomex insulation paper was studied. The research results show that the decomposition temperature of the Nomex decrease significantly in the presence of water. The DP(Degree of Polymerization) of the Nomex insulation paper declines with t aging of specimens. However, the electrical properties of the Nomex insulation paper do not decrease as faster as its mechanism properties. The transparency and surface smoothness of the Nomex fibers become worse with the aging time. And the fiber width of the Nomex insulation paper become narrowing with the aging time.The electrical aging test of Nomex paper has been conducted at the voltage of 1.6 kV by establishing the electrical aging test system and designing the turn insulation model. The electrical and mechanical parameters of Nomex insulation paper were measured periodically during the electrical aging test. Then the relationship between these aging parameters and the insulation aging state was analyzed. And the electrical aging mechanism of the Nomex insulation paper was studied. Compared with the thermal aging result, the DP has the similar trend in the electrical aging test. However, its descending speed is lower than in the thermal aging test. Besides, the degradation of the Nomex is generated from the regions where fibers interlace together or the pulps and local defects, and then develops along the fibers, which proves that the degradation of the Nomex begins from these weak links.Based on the degree of polymerization, partial discharge and the microstructure characteristic, the degradation model, the regression model and the expert evaluation model were established, respectively. Then the reliability assessment method was proposed combined with these models. Therefore, the parameters of the insulation can be comprehensively considered in the condition assessment of insulation, and the reliability accuracy was improved in this method through the fusion of multiple reliability parameters.The trend of PD signals and the PVT curves of the PD statistical characteristics was analyzed. Then the relationship between the PVT curves of the PD statistical characteristics and the aging period were found. The insulation assessment method was proposed based on the relationship analysis. The effectiveness of the method was eventually verified by evaluating the aging state of Nomex insulation paper specimens.