Improvements to the Construction of Bubble Inception Formulae for Use With Transformer Insulation

Moisture in transformer insulation is a topic of major interest to power system equipment operators. Moisture plays a large role in the degradation of insulation within the transformer, impacting on the longevity and the failure rate of this crucial asset. Therefore, understanding of how moisture behaves in the transformer insulation has been the focus of many studies. One particular matter that can occur during high-temperature events is the formation of bubbles of water vapor which are released from the cellulosic paper wrapped on the transformer windings. During such events, the presence of these bubbles could lead to an electrical failure of the transformer, and so it is desirable to be able to set operational criteria which prevent such occurrences. A formula which can predict the temperature at which bubbles will form from the paper insulation can allow operators to restrict transformer operation in order to avoid such situations. However, it has proven difficult to generate a formula with global applicability that is simplistic enough for generic use. This paper assesses the current formula against existing data in literature and compares a suggested alternative, drawing on fundamental science to assist in development of both formulae. The alternative formula is based on a better representation of bubbling activity and requires fewer input variables. Finally, adjustments are suggested to the formulae which allow them to be used more generally throughout the transformer fleet, including on service-aged transformers and with alternative insulation materials.