Is there a function which gives the relationship between the transformer life vs percentage of overloading? What is the cause for degrading of life of the transformer due to overloading?
Any general or "average" formulas would not capture the specific details on a unique design. Rather, the life assessment would require modelling the transformer in detail, looking at all the "bottlenecks" as to thermal limits. The assessment would also include bushings and other contact points, along with all insulated conductors current status. Based on this, and the actual load (as provided) and temperature conditions (as measured) to better assess the insulation loss of life. The overloading of a unit, in where it is used at the top for sustained periods of time and/or above the indicated rating, will cause a temperature rise affecting the properties of the insulation system such as the mineral oil, paper and pressboard (cellulose). As these are organic products, they will degrade over time and lose the capability to withstand the stresses a transformer might see in daily life (e.g. short circuits, energizing, vibrations).
Lifetime is then a combination of temperature, oxygen content, acidity of the oil and the moisture content in the insulation. Temperature is mainly dependent on the transformer design, the loading, the cooling facilities and the ambient temperature. Thus, if overloaded and used at the peak of or above the rating, it will risk shortening the remaining life expectancy of the unit if no preventive maintenance actions are periodically performed. One could think of Oil Regeneration and/or Low Frequency Heating (LFH) drying as to means to prolong the life time of the unit.
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