| The safe operation of power grid is the basic of stable and reliable power supply. With the development of economy, not only the demand for energy is increasing, but also security of power operation becomes more and more urgent. Breakdown of power network or black-out in large area will not only cause enormous economic loss, but also threaten the public security and result in serious social problem. It is statistically that the transformer failure caused by insulation aging accounts for an important part in the transformer accidents. Therefore, study on the aging characteristics of transformer insulation and its lifetime estimation method is the key technology to understand aging mechanism better, search for new fault diagnostic method and the way to extend the usful lifetime of transformer.It has been almost a half of century since the related aging research work was developped. However, the aging diagnostic methods for transformer oil-paper insulation are still based on thresholds of some single aging characteristc, such as furfural and dissolved gas in oil. There is not a synthetic aging diagnostic method by now to give an overall evaluation of transformer insulation. Moreover, with the widly application of new insulation materials, the validity and reliability of conventional aging characteristics and methods are not taken into consideration in the present diagnostic mathods. The present aging diagnostic technology is mainly based on the longterm operating experience and accelerated aging experiments; the aging mechanism was seldom concerned.In this paper, insulation paper used in transformers were aged in mineral oil and vegetable oil for thousands hours, serveral chamical aging characteristics and their varying tendency were studied. An impproved thermal lifetime model was obtained based on the experiments data. In order to give the feasible explanation for the results observed in accelerated aging experiment, some modern analysis equipments and molecular simulation method were used to investigate the aging mechanisms. In addition, taking into consideration of the hidden trouble caused by local defects on oil-paper insulation, partial discharge (PD) measurement is used to study the risk assessment method caused by defects development. Finally, according to the exist operation regulations and published research results, this paper presented a concept of "Health Index"for transformer's main insulation, based on which an assessment method to quantify and evaluate the aging degree of transformer insulation was investigated exploringly. The main innovative achievements of this paper are as following:①This paper systematically compared the difference of thermal aging characteristics between mineral oil-paper and vegetable oil-paper, and analyzed the reasons that caused such differnece. It is presented that vegetable oil can not only retard the degradation rate of insulation paper, but also increase the value of leveling-off degree of polymerization (LODP) and thus extend the useful life of paper significantly.②It is innovative to intrduce the time-temperature-superposition (TTSP) to improve the thermal lifetime model baed on two-order kinetic equation. The modified model makes good use of all the experimental data obtained in accelerated aging test and is able to predict the lifetime of oil-paper at arbitrary temperature in the range of extrapolating temperature through parallel shifting factor on the time axis.③Several modern analysis equipments and molecular simulation method were used in this paper for the first time to investigate the aging mechanisms. Integrating the macroscopical varying tendency of aging characteristics and microcosmic mechamisms, the aging factors and reasons that vegetable oil retarding aging rate are investigated systematically.④According to partial discharge measurement, a new system with detailed methods is proposed to evaluate the risk caused by local defects, which solved two problems: one is separation and recognition of mix PD source, the other is extracting method of PD features that closely related to paper aging degree and diagnostic method.⑤A concept of"Health Index (HI)"and evaluation method is presented to quantify the aging degree of transformer's main insulation. It is suggested to seperated HI into three individual parts: the whole aging index, the operating condition index and the partial defects risk index. For the first time, HI innovativly quantify the aging characeristics, and fully considered the exist operation regulations and published research results, based on which a innovative way is proposed, which can not only give a feasible evaluation for overall aging, but also make the judgement of the hidden risk that caused by local defects in oil-paper insulation.
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