The degradation of the insulation system in liquid-filled power transformers is a serious concern for electric power utilities. The insulation system’s ageing is accelerated by moisture, acids, oxidation products, and other decay particles (soluble and colloidal). The presence of these ageing by-products is detrimental to the insulation system and may further lead to premature ageing and serious consequences. The ageing mechanisms of oil-paper insulation are complex, highly interrelated, and strongly temperature-dependent. The operating temperature of the transformer insulating system has a direct relationship with the loading profile. The major aspect that is witnessed with the fluctuating temperatures is moisture migration and subsequent bubble evolution. In other words, gas bubbles evolve from the release of water vapor from the cellulosic insulation wrapped around the transformer windings. The models presented in the existing standards, such as the IEC Std. 60076-7:2018 and the IEEE Std. C57.91:2011, are mainly based on the insulation temperature, which acts as a key parameter. Several studies have investigated the moisture dynamics and bubbling phenomenon as a function of the water content in the paper and the state of the insulation system. Some studies have reported different prototypes for the estimation of the bubble inception temperatures under selected conditions. However, there are various attributes of the insulation system that are to be considered, especially when expanding the models for the alternative liquids. This paper reviews various evaluation models reported in the literature that help understand the bubbling phenomenon in transformer insulation. The discussions also keep us in the loop on the estimation of bubbling behavior in alternative dielectric liquids and key attributable factors for use in transformers. In addition, useful tutorial elements focusing on the bubbling issue in transformers as well as some critical analyses are addressed for future research on this topic.

The degradation of transformer oil-paper insulation is one of the major concerns for utilities and transformer owners. Temperature and moisture are amongst the key variables to be considered while determining the load profiles that can be safely applied to transformers. In this paper, important parameters used to monitor the degradation of the oil-paper insulation are introduced in the existing thermal and moisture models reported in the literature. The standard IEC 60076-7 loading guide model is improved by including the viscosity of the liquids with a controlled aging history to estimate the behavior under hot-spot conditions. Similarly, moisture model is improved by including the acidity of oils and degree of polymerization of the paper. This allows a better representation of the relative moisture saturation of the liquids and the bubble inception temperature. In this work, potential models are reported to understand the moisture dynamics of cellulose paper and predict the bubbling inception temperature under specific loading profiles. The simulation analysis is also spanned to different aging conditions of mineral oil and cellulose based oil-paper insulation. The developed models may be helpful to monitor the moisture in paper and predict the bubbling temperature while taking into consideration temperature variations along with the degradation of the insulation system inside the unit.