Dissolved gas analysis of thermal faults in transformer liquids simulated using immersed heating method

Abstract

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).

Analysis of Gas Generated in Mineral Oil, Synthetic Ester, and Natural Ester as a Consequence of Thermal Faults

“…-The heating element should achieve a temperature in the range from 150 • C to 450 • C and keep it for about two hours. The authors of the papers [10], [22], [23] reported that in this temperature range the generation of gases was observed, while the two-hour time allowed to generate measurable gas concentrations.…”

“…3) Heating the liquid flowing through the heated pipeclosed liquid circuit: the container with cold oil → furnace → the container with cold oil [20], [21]. 4) Local heating of the insulating liquid using a heating element immersed in it [10], [22], [23]. According to the authors, the local overheating of the insulating liquid by means of heating element immersed in it best suits the conditions of such a defect in the real transformer.…”

“…In the article [23] the dynamics of the increase in gas concentrations generated as a result of mineral oil (Gemini X) and synthetic ester (Midel 7131) overheating was compared. The heating element immersed in the oil was a resistance wire with a diameter of 0.6 mm, wound on a ceramic insulator in the shape of a pipe with a diameter of 5 mm and a length of 70 mm.…”

“…Taking into account the differences in the experiments described above, it was impossible to compare the gas amount generated during thermal faults. The measurement systems and procedures used in [10], [22], [23] differed significantly, for example, authors used differ: liquid volume, time of gas generation, heating element temperature, surface of the heating element. These differences prevented a quantitative comparison of generated gases.…”

“…However, in this case, the comparison of relative values is reasonable. Therefore, in section VII the gas ratios calculated on the basis of the data presented in [10], [22], [23] with ratios calculated by authors of this paper were compared.…”