Dissolved gas analysis (DGA) is considered as one of the most effective techniques to diagnose transformer incipient faults. With the increasing applications of alternative liquids in transformers, fault gas generation characteristics of the liquids were investigated in laboratory under simulated faults. Immersed heating method has been widely used to simulate thermal faults, of which the complete fault temperature range was hardly achieved. In this paper, pool boiling theory was introduced to explain the underlying reason that why the complete fault temperature range cannot be achieved using immersed heating method. There are three stages in pool boiling including natural convection region, nucleate boiling region and film boiling region. The Nukiyama temperature of a mineral oil was found to be 330ºC. This is the highest stable temperature achieved by the immersed heating method in T2 region, beyond which the heating element temperature is unstable and hence temperature measurement is unreliable. The Nukiyama temperature of an ester liquid was found to be 390ºC, which is higher than that of the mineral oil due to its higher boiling point. The findings imply that the previously published T2 DGA results obtained using immersed heating method at reported temperatures beyond the Nukiyama temperature have likely been in T3 region near to the melting temperature of the heating element (normally higher than 1000ºC).

In this paper we present a timetemperature superposition method for predicting the permanence of paper by extrapolating accelerated paper ageing data to ambient conditions. The presented method includes a test for the presence of shift factors to superpose all of the raw accelerated ageing data over the temperature range studied to obtain a master curve, a numerical fit of the master curve for producing a master equation representing the kinetics of paper degradation, a critical examination of applying Arrhenius equation for explaining the relationship between the empirically determined shift factors and the accelerated ageing temperature, and a verification of the Arrhenius activation energy extrapolation assumption. Unlike the usual approach that extrapolates the Arrhenius relationship between lifetime and temperature, without corroborating evidence, to ambient temperatures, we test the Arrhenius activation energy extrapolation assumption by determining the influence of acidity on cellulose hydrolysis reactions, and have found that detection and identification of the acid-sensitive linkages in cellulose substances is an ultra-sensitive and reliable method to measure degradation of cellulose and paper in what is normally the extrapolation region (ambient temperatures). Taking the examples of natural ageing data in literature from 18 bleached kraft dry-lap pulps for 22 years under ambient conditions and three handsheet samples for 22 years under controlled conditions, comparison of the predictions with natural ageing results has been addressed.

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