Research On Properties And Diagnosis Methods Of Aging Of Nomex Insulation In Mining Dry-type Transformer

The project in this thesis is a subtopic continuation of the project“Safety Status Monitoring and Fault Diagnosis Forecast of Mine Ventilation Power Supply System”(No: 2007BAK29B05),sponsored by the National Key Technology R&D Program of China during the Eleventh Five-year Plan Period.It is based on the current situation that there is no clear deterioration mechanism and aging factors of insulation in mining flameproof dry-type transformer,lack of research of aging characteristics and imperfect diagnosis methods of aging status.With the continuous development of coal mining technology of large mining height fully mechanized working face,the total installed power of coal cutter and scraper conveyor in working face has increased rapidly,and the capacity of dry-type transformer for coal mine has exceeded more than 5000 kVA.As the core of power supply system of coal mine,the overall performance and the stand or fall of running state of dry-type transformer for mining directly affects the reliability,security and continuity of power supply of coal mine.However,due to extremely harsh coal mine environment and the presence of many complex and uncertain factors in coal mining areas that accelerate the aging of dry-type transformers,incerasing the hidden faults.The aging mechanism of insulation in dry-type transformer is very complex,and physical and chemical aging process is not fully understood.In order to achieve the effective recognition of early fault of insulation,improve fault warning system of mining dry-type transformer,the degradation patterns and aging properties must be fully studied.It has important realistic signifcance and academic value to ensure the safety and reliability of coal mine power supply.Therefore,it is meaningful to accelerate aging of insulation,study the variation of each parameter change with time and establish diagnostic system of insulation state assessment,so as to timely detect the security risks of insulation in dry-type transformers and to promote coal mine power system planned maintenance to condition-based maintenance.In this paper,combing the special environment of the coal mine,the mechanism of aging characteristic influenced by temperature and moisture was explored through the deep analysis of the infulencing factors and degradation process of Nomex insulation in the mining flameproof dry-type transformer.The accelerated aging test in double factors of Nomex paper was carried out in laboratory and the changes of its properties before and after aging was studied which aimed to provided a highly reliable and precise diagnosis method of insulation status of mining dry-type transformer.The main research contents of this paper are as follows.According to the degradation mechanism of Nomex,the accelerated aging test model in different temperatures and moisture factors was designed,and the variations of morphology and mechanical properties of Nomex under different aging conditions were studied.Combined with the dielectric response theory,the physical processes of dielectric response of Nomex were studied.A three-electrode test system was builded to measure the frequency domain dielectric spectroscopy of Nomex.The results show that the frequency domain dielectric characteristics of Nomex are affected by aging,measuring temperature,moisture contents,but the bands are not the same.Combined with the actual measurements of dielectric spectroscopy and the model of Havriliak-Negami,a two-relaxation response model with DC electrical conductivity was eatablished.Through fitting the parameters of the model,the effects of frequency domain dielectric spectroscopy of Nomex casued by different degrees of aging,moisture contents and test temperature were quantitative analysised.The results show that the model can well describe the dielectric properties of the sample in the measurement frequency range,and the assessment of the situation of Nomex is achieved preliminarily.The test platform of partial diacharge was builded and simulated the turn-to-turn insulation of dry-type transformer the PD signals of Nomex in different aging state were measured.For white-noise include in PD signals,a method of suppressing white-noise interference in PD signals based on EEMD and higher order statistics was proposed.The PD signals in multiple frequency cycle were counted,and different two-dimensional spectrum of PD were ploted.For the characteristic difference of two-dimensional spectrum,the statistic characteristics parameters of PD were introduced to analysis the aging state of Nomex.The results show that the statistic characteristics parameters of PD can reflect the aging state of Nomex well.The diagnostic method based on SVM for aging state of Nomex of dry-type transformer was proposed.A multi-classification model of C-SVC was established in this paper.For the unknown parameters in the model,the K-CV method was used to optimize them.Using statistic characteristics parameters of PD as the input of the model,the aging state of Nomex was diagnosised.The results show that the method has high recognition rate,and can make a reasonable diagnosis about the aging state of Nomex.