| The dissertation is funded by the National Natural Science Foundation of China,namely “Study on Degradation Mechanism of Nomex Insulation for Mining Flameproof Dry-type Transformer”(No.51577123),which is aimed at the problem about the aging law and life prediction of Nomex insulation for the coal mining flameproof dry-type transformer of Group I used in explosive atmospheres.It is well-known that Nomex insulation paper is widely used as insulation material for transformers,motors and generators due to its excellent electrical properties,heat resistance and mechanical properties.Nowadays,Nomex insulation paper is used as insulation material for the most of flameproof dry-type transformers.However,due to the severe environment in coal mines,Nomex insulation paper can be subject to various aging factors(thermal,electrical,mechanical stress,and surrounding media)during its long-term operation,which could gradually result in the aging and degradation.Aging can not only affect the mechanical and electrical properties of Nomex insulation paper,but also its physico-chemical properties,which would lead to insulation failure and even cause transformer safety accidents.However,most of the studies focus on the insulation aging mechanism of oil-paper composite insulation in the oil-immersed transformer,and there are very few reports on the research of the insulation mechanism of mining dry-type transformers,especially the aging mechanism and aging law under the combined action of multi-factor are not yet clear.Therefore,it is of great theoretical and practical significance to study the rules of physico-chemical properties,electrical properties and mechanical properties of Nomex insulation paper under the combined action of multi-factor,analyze the aging mechanism of Nomex insulation paper under the action of different aging factors,and predict the life of Nomex insulation paper.In this paper,the Nomex T410 insulation paper,which is applied to the 10 k V mining dry-type transformer,was used as the experimental samples.Samples were aged via the multi-factor aging test in which temperature,moisture and voltage were controlled.The influence of various factors on its physico-chemical properties and electrical properties was analyzed.The kinetics law under multi-factor aging of Nomex insulation paper was studied based on the test results of the degree of polymerization.The research content is as follows:In order to accurately simulate the actual aging process of Nomex insulation in mining dry-type transformers,in this paper,the aging factors and their strength were firstly determined.A multi-factor aging test box was designed.The multi-factor aging test platform was built according to both the actual operating environment and structural characteristics of Nomex insulation paper.The multi-factor aging test of Nomex insulation paper under the combined action of temperature,moisture and voltage was completed.The results verified the feasibility of the test scheme.Physico-chemical property is one of the most basic properties for insulation materials,which can be helpful to reveal other macroscopic properties of insulation materials from the microscopic mechanism.In this paper,the physico-chemical properties of Nomex insulation paper after multi-factor(including temperature,moisture and voltage)aging were studied from three aspects: micro morphology,polymerization degree,and thermal stability.The microstructure of the Nomex insulation paper aged under different conditions was observed by scanning electron microscope(SEM)and optical microscope.The polymerization degree was measured through the capillary viscometer method.The thermal stability was tested by thermogravimetry and differential scanning calorimetry.The results shown that the higher the strength of each aging factor,the more serious the deterioration of the microstructure of Nomex insulation paper.The chopped fibers and pulp have been degraded to varying degrees,the degree of polymerization has decreased significantly,and the thermal stability has gradually deteriorated.The research conclusion of the physico-chemical properties of Nomex insulation paper can provide a reliable theoretical analysis basis for the study of electrical properties and mechanical properties.In order to clarify the influence of multi-factor on the electrical properties of Nomex insulation paper,in this paper,the frequency domain dielectric spectrum of Nomex insulation paper was measured by a wide-band dielectric spectrometer,and the variation of relative dielectric constant and dielectric loss factor with aging temperature,initial moisture content and aging voltage was analyzed.The experiment found that the change of relative dielectric constant and dielectric loss factor with various aging factors was closely related to the aging degree of Nomex insulation paper.The results of surface potential decay research showed that with the increase of aging temperature,initial moisture content and aging voltage,the surface potential decay rate of Nomex insulation paper gradually increased,which also confirmed the influence of micro-morphology changes on electrical properties.Combined with the discharge test for Nomex insulation paper,based on the PRPD spectrum and the analysis of the change rule of the average discharge amplitude and the number of discharges with various aging factors,it is concluded that affected by micro-defects and surface charge decay rate,under the same applied voltage,with the increase of aging temperature,initial moisture content and aging voltage,the average discharge amplitude gradually decreased,and the number of discharges showed the opposite trend.The research on the change of electrical performance of Nomex insulation paper under multi-factor aging can provide a basis for the analysis and design of the insulation structure of mining dry-type transformer.The mechanical properties of transformer insulation materials generally degrade faster than electrical properties,as a consequence,the mechanical properties is a powerful tool for evaluating the insulation status.In this paper,the tensile test and dynamic mechanical test of the aged Nomex insulation paper were conducted.The tensile strength and elongation at break of Nomex insulation paper were studied using an electronic universal material testing machine and found that the tensile strength and elongation at break of insulation paper decreased with the increase of aging temperature,initial moisture content and aging voltage.Based on dynamic thermomechanical analyzer,the analysis of storage modulus temperature spectrum,loss modulus temperature spectrum and loss factor temperature spectrum showed that with the increase of aging temperature,initial moisture content and aging voltage,the glass storage modulus of insulation paper gradually decreased,the peak loss factor increased and the glass transition temperature moved to a lower temperature.The research results of the mechanical properties of Nomex insulation paper help to understand the relationship between its microstructure and mechanical properties from multiple anglesThe life of Nomex insulation paper directly determines the service life of mining dry-type transformers.In this paper,based on the measurement results of polymerization degree,the relationship between the decomposition rate of Nomex insulation paper and the aging temperature,initial moisture content,and aging voltage was analyzed,and the aging dynamic model and life model of Nomex insulation paper under the action of multi-factor were established.The validity of the aging dynamic model and the life model was verified by comparing the calculated value with the measured value.The research results of the thesis will provide a basis for on-line monitoring and fault diagnosis of mining dry-type transformer,which is a key link to realize mine intelligentization. |
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