Evaluation and on-Site Retrofilling Procedures with Natural Ester for a 110 Kv Power Transformer

Retrofilling of mineral oil immersed transformers with natural esters has emerged as a promising solution for enhancing fire safety and environmental sustainability of transformers. Therefore, retrofilling transformers has become a trend worldwide and there are numerous retrofilling cases for distribution tranformers currently. However, there exists limited reported cases of retrofilling power transformers, especially when retrofilling on site which places higher demands on the retrofilling procedures to control residual mineral oil content. In this paper, a SZ11-50000/110 (110 kV 50 MVA) power transformer was selected to be retrofilled with Cargill FR3<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TM</sup> (a soybean-based natural ester) on site. This paper discusses detailed procedures and technical considerations for retrofilling the 110 kV power transformer on site, and also diplays transformer handover test results before and after retrofilling. The results of insulating oil tests demonstrated that the fire point of oil sample was <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$350.1^{\circ} \mathrm{C}$</tex> which met K-class less flammable insulating liquids requirements after retrofilling. Additionally, the handover tests results were qualified after retrofilling. This case provides reference procedures and field data to further support retrofilling on site of power transformers.