| Insulation defects of basin insulators have always been one of the principal causes of the GIS equipment failures.The presently availble insulation condition assessment methodology based on electrical features cannot provide sufficient accuracy and reliability,especially for reliable detection and residual life assessment of the latent and developmental insulation defects with GIS basin insulators.It is urgent and also a pending issue in the electrical power sector to explore more effective detection and characterization methods for this sake,which puts forward practical application requirements and new technical challenges for scientific research in the area.As an extension of the existing electrical detection technology,the activation energy research based on the intrinsic thermal dynamic properties of the insulating materials provides possible solutions and technical approaches as to address the above issues.However,the epoxy resin composites used in GIS basin insulators also render problems such as incomplete curing,complex pyrolysis reaction and the difficulty in solving multiple reaction mechanisms,which to some extent restricts practical application of the activation energy-based methodology in considtion assessment of the basin insulators.On the basis of clarifying the microscopic mechanism of aging and pyrolysis of epoxy resin materials in basin insulators,the calculation model of pyrolysis kinetic parameters should be constructed and the insulation failure criterion and life evaluation model should be established.In order to break through these technical bottlenecks,this thesis combines microscopic simulation and experimental research methods to systematically study the pyrolysis reaction mechanism of the epoxy resin materials in basin insulators,the solution method of thermodynamic parameters as well as the insulation life prediction model,which can provide theoretical basis and technical routes for condition assessment and residual life prediction of the GIS basin insulators based on pyrolysis kinetic parameters.The molecular system of cross-linked epoxy resin was constructed by Material Studio platform and reaction molecular dynamics simulation of the electrothermal pyrolysis was carried out based on the ReaxFF module of ADF(Amsterdam Density Function).The dynamic evolution process of pyrolysis products was studied and the formation path of small molecular products such as CO2,H2O and H2 was tracked in details.It was found that the thermal stress was the main factor affecting the formation of pyrolysis products during the pyrolysis process,and it showed a variety of multiple reaction processes.At the same time,the electrothermal aging experiments with epoxy resin samples was carried out by using the self-built electrothermal accelerated aging test rigs and the microstructure features were observed by scanning electron microscopy.The results showed that the epoxy resin composites manifested a variety of morphological characteristics such as bulges,holes and cracks.It is pointed out that there are many reaction mechanisms during the aging process.Furthermore,the thermokinetic characteristics of the epoxy resin samples were studied by thermogravimetric analysis(TGA)and mass spectrometry(MS),and the pyrolysis characteristic temperature and escape gases were analyzed accordingly.It was found that the pyrolysis process showed unique features of multiple reactions,complex by-products and prominent double pyrolysis steps.In order to quantitatively characterize the reaction mechanism of complex pyrolysis process of the epoxy resin samples,a universal four-parameter characterization model for pyrolysis mechanism function was proposed in this thesis,which was suitable for characterizing the complex pyrolysis process containing multiple reactions.With regards to the multiple reaction process and double-step effect of epoxy resin composites with basin insulators,a methodology for solving the pyrolysis activation energy was proposed.The initial reaction temperature in the pyrolysis step? is relatively low,hence,combined with the asymptotic properties of the integral median point,a median approximation function was etablished and then the influence of low temperature effect was taken into consideration.An activation energy calculation method based on integral median scheme for pyrolysis reaction step ? was proposed,which greatly reduced the calculation error of activation energy caused by low temperature approximation.The initial temperature of pyrolysis reaction step ?is relatively high and the low temperature approximation treatment is no longer effective.Therefore,an improved exponential activation energy calculation method was proposed.Based on the improved temperature integral approximation,an activation energy calculation method by exponential integral transformation was established.When this method is applied,it is unnecessary to assume the reaction mechanism function,and the calculation error of activation energy is less than 0.025%.Combining the activation energy of both step ? and step ?,the reaction mechanism function which can describe the whole pyrolysis process was deduced.A new form of rational integral transform was further proposed to facilitate an improved calculation method of activation energy by rational integral transformation.With the constructed error function equation,the effectiveness of the calculation method for activation energy and the reaction mechanism function was systematically verified.The calculation method provides a new scheme and a general form for the numerical integral approximation,which can further reduce the error of temperature integral approximation.The above method can be used to reliably solve the activation energy of the staged pyrolysis process of epoxy resin composites,and realize quantitative mathematical characterization of the pyrolysis reaction mechanism of epoxy resin.In order to obtain the correlation between pyrolysis characteristics,electrical parameters and material aging degree,accelerated aging experiments were carried out at 100?,130? and 160? for different types of epoxy resin composite samples by using the combined electrothermal aging test rigs.At the same time,the insulation breakdown and mechanical tensile strength experiments were also carried out for the epoxy resin samples with different aging states,and the correlation between the mass loss rate,breakdown strength,mechanical tensile strength versus aging temperature and time was obtained.The results illustrate that there may be a secondary curing phenomenon in the aging sample,making the breakdown strength and mechanical tensile strength increase locally at the early aging stage.If the aging temperature is higher than the glass transition temperature,the breakdown E-field strength and mechanical tensile strength of the sample decrease rapidly with the aging time,while the insulation breakdown field strength decreases the fastest.Therefore,50%of the initial breakdown E-field strength is taken as the termination criterion of the insulation life.Based on the correlation law between electrical and thermal dynamics parameters,the failure criterion of epoxy resin composite insulation was proposed with the mass loss rate of 4.3%as the threshold(corresponding to pyrolysis reaction conversion rate of 86%).Furthermore,the thermal weight loss experiments were carried out on the insulation samples for the whole aging cycle,and the activation energies of pyrolysis steps ? and ? were calculated.The quantitative relationship between activation energy and mass loss rate was thereby established,based on which the residual life prediction model of epoxy resin composite insulation of GIS basin insulator was proposed,and validity of the residual life assessment method was verified by referring to actual GIS insulators with already-known operating life.The above proposed research provides a theoretical basis and an effective methodology for life assessment of the operational GIS insulators. |
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