Dissolved Gas Analysis (DGA) is regarded as one of the most effective methods to diagnose transformer incipient electrical and thermal faults. Recently, alternative insulating liquids have been developed as potential replacement of mineral oil for transformers. Due to the lack of field experiences, the fault gas generation characteristics of alternative insulating liquids need to be investigated under laboratory testing scenarios which represent transformer faults. There are three methods in laboratory to simulate transformer thermal faults which are immersed heating method, tube heating method and bottle heating method. For the immersed heating method, it is easy to control the input power but difficult to measure and adjust the heating element temperature. The tube heating method can simulate thermal faults and cover the whole fault temperature range but it requires a complicated experimental setup. The bottle heating method can only simulate thermal faults below the flash point or fire point of the oil for safety consideration. Immersed heating method and tube heating method were investigated in this PhD research. This PhD thesis summarised the gas generation characteristics under laboratory thermal faults of three types of transformer insulating liquids. For the experiments using immersed heating method, the pool boiling phenomenon was observed and reported for the first time in the investigated insulating liquids. The pool boiling curves of the three liquids were measured and reported. The Nukiyama temperature (the upper stable temperature limit of immersed heating element in nucleate boiling regime) of the investigated mineral oil was about 330 degree Celsius, beyond which the heating element temperature was unstable and its measurement was unreliable. The Nukiyama temperatures of the investigated alternative liquids were about 390 degree Celsius which were higher than that of the mineral oil due to their higher boiling points. Based on this finding, the immersed heating method was proved difficult to cover the whole range of thermal fault temperature. As a result, the DGA experimental results with immersed heating method reported in previous publications might need to be reinterpreted. As an alternative, a tube heating based DGA experimental system was developed in this PhD research work. The dissolved and free gas contents of three investigated liquids in the experimental system were measured, presented and compared under thermal faults from 250 to 750 degree Celsius. Fault gas pattern varied among different oil types and different fault levels. Different key gas indicators of the tested liquids were found and reported. The gas generation results could provide diagnostic information on gas generation characteristics of the tested insulating liquids. To calculate the gas in total value, the Ostwald coefficients were needed. Although the Ostwald coefficients for mineral oil and synthetic ester liquid could be obtained from literatures, those of gas to liquid (GTL) oil should still be determined in this PhD research. In addition, online DGA monitor and conventional laboratory DGA method have been used simultaneously to observe and quantify their difference.
Date of Award | 31 Dec 2017 |
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Original language | English |
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Awarding Institution | - The University of Manchester
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Supervisor | Zhongdong Wang (Supervisor) & Qiang Liu (Supervisor) |
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DISSOLVED GAS ANALYSIS (DGA) OF TRANSFORMER INSULATING LIQUIDS UNDER LABORATORY SIMULATED THERMAL FAULTS
Wang, X. (Author). 31 Dec 2017
Student thesis: Phd