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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).

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In insulating liquids, a breakdown event is the consequence of streamer initiation and propagation. A number of parameters such as stopping length, propagation velocity, and current and light pulses can be used to characterize the streamer phenomenon. In addition, it is recognized that the geometric patterns of streamers in insulating liquids are with a fractal nature and the shape complexity of streamer patterns can be analyzed by using fractal index, i.e. fractal dimension. This paper described the fractal features of streamer patterns in a synthetic ester insulating liquid under lightning impulse voltages. The fractal dimensions of streamer patterns at different propagation times and voltage levels were calculated by using box-counting method. Relationships between fractal index and other parameters such as stopping length and streamer area were finally presented. Results indicate that fractal dimension with propagation time and applied voltage follows the similar trend to streamer length and area. All of them generally increase with the increase of propagation time and applied voltage level.