Analysing the power transformer temperature limitation for avoidance of bubble formation

Failure of a transformer is a costly incident which can impact power system operation and result in loss of supply to customers.Transformers can fail through a number of mechanisms,one of which is from the formation of bubbles within the insulation.Currently,the loading guidance is set to avoid certain overload temperatures as high temperature is the main cause of bubbles forming.This paper analyses the limit set within the loading guides,comparing the most commonly cited temperature limit of 140℃ with values taken from across the full spectra of available studies in literature.It is concluded that there is sufficient evidence for further investigation into formation of bubbles from transformer insulation as employing a single temperature limit may be inadequate.Use of a single temperature value is unsuitable,given the wide range of potential scenarios that the transformer fleet can present.It has become the go-to rationale that the moisture content of the solid insulation is the key driver for transformer bubbling temperature.This is true,however,it is shown that other factors such as solid insulation aged condition(determined through degree of polymerisation value)and the insulation type are also important considerations.