Factory and Retro-Filled Natural Ester Distribution Transformers Subjected to Overload Conditions

Southern California Edison (SCE) began utilizing natural esters in all pole mounted distribution transformers in 2018. SCE also began a retro-fill program for all useable pole mounted units which had been removed from service. It immediately became apparent that the measured parameters normally associated with condition assessment of mineral oil filled units did not compare to those from ester filled units. Without a clear understanding of the dielectric changes in the ester fluid and cellulose in response to thermal stress, condition assessment and failure analysis was found to be difficult at best. In addition, the same questions were raised regarding retro-filled units where the winding insulation was initially at least, saturated with mineral oil. It was decided that a simulation should be performed to compare factory filled vs retro-filled units.