Assessment of Natural Ester Retrofilling Effects on Transformer Lighting Impulse Behaviour Regarding to Safety Margins of Transformer Insulation: A Case Study on a 180 MVA, 400/36 kV Power Transformer

The use of natural and synthetic esters has significantly increased over the past 20 years, providing advantages in fire safety, environmental impact and transformer life expectancy. Nowadays application of these liquids in distribution transformers is well established and their use in power transformers is growing. Some of the transformers that use esters as liquid insulation were initially designed to use these type of liquids; other transformers were retrofilled during their service life after some years operating with mineral oil. One of the challenges to retrofill large power transformers is the redistribution of the electric field within the oil paper insulation that takes place after changing the liquid. Esters have higher dielectric constants than mineral oil, resulting in a more homogeneous field distribution between the paper and the liquid. Previous studies analyzed this effect under AC stress; this paper extends the analysis to the assessment of the insulation of a 180 MVA transformer subjected to lightning impulses.