Dissolved Gas Analysis of Transformer Liquid-Paper Insulations under Laboratory Simulated Thermal Faults by Using Tube-heating Method

Abstract

Dissolved gas analysis (DGA) has been widely used for detecting incipient faults in transformers. Previously published DGA experiments were mainly conducted in liquid insulation with less consideration of the presence of solid insulation like Kraft paper. The relationships between fault gas generation characteristics and paper ageing indicators like degree of polymerization (DP) and 2-FAL in oil are rarely studied. Meanwhile, the CO2/CO ratio used to identify paper related thermal faults in mineral oil defined in IEC/IEEE standards has evolved through revisions. In addition, technical challenges are faced by asset managers of ester liquid filled transformers since the condition monitoring features of ester liquid filled transformers might not be the same as those of mineral oil filled transformers due to the different chemical structures of the liquids. Therefore, this PhD study focuses on the experimental investigation on fault gas generation characteristics in different transformer oil-paper insulations under laboratory simulated thermal faults using the tube-heating method. A tube-heating experimental setup previously used for liquid only experiments was further developed to conduct DGA experiments in liquid-paper insulation systems. Temperature profile tests were conducted in mineral oil and synthetic ester liquid to help understanding temperature distribution inside the heating tube. During the experiments, a small amount of liquid or liquid-Kraft paper insulation inside the heating tube was heated using a furnace, with the generated fault gases detected using an online DGA monitor. The liquid only experiments were performed at various thermal fault temperatures ranging from 350 ºC to 750 ºC, and the liquid-Kraft paper experiments were performed at 250 ºC under various heating durations. The results in synthetic ester liquid only experiments reveal that CO2 and CO are generated in large amounts and hence are the dominant fault gases created during 350 ºC to 650 ºC thermal faults. Except for carbon oxide gases, C2H6 is the typical hydrocarbon gas from 450 ºC to 650 ºC. Under a 750 ºC thermal fault, C2H4 becomes the dominant fault gas. For mineral oil, less CO2 and CO are generated. CH4 is the typical gas of the occurrence of a thermal fault. C2H6 is the typical gas generated by 450 ºC and 550 ºC thermal faults. C2H4 is the typical gas generated by 650 ºC and 750 ºC thermal faults. For both insulating liquids, H2 starts to be generated during a 550 ºC thermal fault and C2H2 starts to appear during 650 ºC and 750 ºC thermal faults. The results of both liquid-Kraft paper insulations reveal that the generated CO2 and CO increase exponentially with the reduction of the DP and increases linearly with the increasing CSN. A linear relationship has been observed between the generation of carbon oxide gases and 2-FAL in oil. Compared with accelerated ageing studies at lower temperatures, more CO is generated at the same level of DP, indicating that different CO generation mechanisms exist under different thermal fault temperatures. Meanwhile, a static CO2/CO ratio in mineral oil immersed insulation is not an optimal indicator of paper degradation, whereas a dynamic ratio change under a thermal fault could be explored in the future as being an indicator of the occurrence of a thermal fault involving paper insulation.

Date of Award	1 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Zhongdong Wang (Supervisor) & Qiang Liu (Supervisor)