Effect of Moisture and Aging of Kraft Paper Immersed in Mineral Oil and Synthetic Ester on Bubbling Inception Temperature in Power Transformers

Ghada Gmati (), Issouf Fofana, Patrick Picher (), Oscar Henry Arroyo-Fernàndez, Djamal Rebaine, Fethi Meghnefi, Youssouf Brahami and Kouba Marie Lucia Yapi
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Ghada Gmati: Department of Applied Sciences, University of Quebec in Chicoutimi, Saguenay, QC G7H 2B1, Canada
Issouf Fofana: Department of Applied Sciences, University of Quebec in Chicoutimi, Saguenay, QC G7H 2B1, Canada
Patrick Picher: Hydro-Québec Research Institute (IREQ), Varennes, QC J3X 1S1, Canada
Oscar Henry Arroyo-Fernàndez: Hydro-Québec Research Institute (IREQ), Varennes, QC J3X 1S1, Canada
Djamal Rebaine: Department of Applied Sciences, University of Quebec in Chicoutimi, Saguenay, QC G7H 2B1, Canada
Fethi Meghnefi: Department of Applied Sciences, University of Quebec in Chicoutimi, Saguenay, QC G7H 2B1, Canada
Youssouf Brahami: Department of Applied Sciences, University of Quebec in Chicoutimi, Saguenay, QC G7H 2B1, Canada
Kouba Marie Lucia Yapi: Department of Applied Sciences, University of Quebec in Chicoutimi, Saguenay, QC G7H 2B1, Canada

Energies, 2025, vol. 18, issue 17, 1-18

Abstract: Bubbling Inception Temperature (BIT) is a critical metric that indicates the temperature at which gas bubbles form due to cellulose decomposition in a paper–oil insulation system. It serves as a key indicator of the thermal stability of transformer insulation, offering valuable insights into its performance under elevated temperatures. Building on findings from a companion study that examined the BIT of Kraft paper (KP), thermally upgraded Kraft paper (TUK), and aramid paper in mineral oil, this research expands the analysis to assess the impact of moisture, aging, and alternative dielectric fluids. Using the same customized experimental setup featuring precise dynamic load control, real-time bubble detection, and continuous monitoring of moisture and temperature, this study evaluates BIT across four distinct oil–paper aging stages: new (0 h) and 2 weeks, 4 weeks, and 6 weeks of accelerated thermal aging. This approach enables a comparative analysis of BIT in various paper–oil systems, focusing on both mineral oil and synthetic esters, as well as the influence of different moisture levels in the paper insulation. The results show that BIT decreases with aging, indicating reduced thermal stability. Furthermore, KP impregnated with synthetic ester exhibits a higher BIT than when impregnated with mineral oil, suggesting that synthetic esters may offer better resistance to bubble formation under thermal stress. Based on these results, empirical BIT models were developed as a function of degree of polymerization (DP) and water content in paper (WCP). This study further demonstrates how these models can be applied to quantify safety margins under emergency overloading conditions, providing a practical tool for operational decision-making in transformer thermal risk management.

Keywords: bubble formation; Bubbling Inception Temperature (BIT); Kraft paper; mineral oil; synthetic esters; moisture content; transformer insulation; water content in paper (WCP) (search for similar items in EconPapers)
JEL-codes: Q Q0 Q4 Q40 Q41 Q42 Q43 Q47 Q48 Q49 (search for similar items in EconPapers)
Date: 2025
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