Initial investigation of a thermal performance qualification method for transformer insulating liquids

During the development of IEEE C57.154 - IEEE Standard for the Design, Testing, and Application of Liquid-Immersed Distribution, Power, and Regulating Transformers Using High-Temperature Insulation Systems and Operating at Elevated Temperatures - in 2012, there was a need to define quantitatively an operating temperature limit for insulating liquids. The limits for transformer hottest spot and average winding temperature are based on the thermal class of the solid insulation in the insulation system, while the top liquid temperature depends on the performance of the liquid insulation itself. The suggested limits for mineral oils, from Table 9 in IEEE C57.91-2011, are $105^{\circ}\mathrm{C}$ for continuous loading and $110^{0}\mathrm{C}$ for loading above nameplate rating. However, these ratings, and those for silicone liquids and natural and synthetic esters, lack quantitative validation. Based on the experience and tests presented by supplier members of the working group at that time, temperature limits were included in Tables 4 and 6 of IEEE C57.154-2012. For natural and synthetic ester liquids the values of 13 were defined, respectively, for "normal life loading" and "loading above nameplate rating". For silicone liquids 155 values were used. When a working group was formed in 2019 for the revision of this standard, a task force was created to revisit these limits and propose a testing procedure for defining such temperatures. This was considered especially relevant for defining limits applicable for new brands and types of alternative liquids. This paper presents the first round of an accelerated aging test referred to as Phase 1 which was performed by this task force. Three liquid classes were chosen for this initial test: mineral oil (HyVolt® II), natural ester (FR3® liquid) and synthetic ester (Midel® 7131) liquids. Sealed stainless steel bottles containing only the investigated liquids were aged in ovens at in four different laboratories, namely, Cargill R&D Laboratory in Plymouth, MN, U.S.A., Weidmann in St. Johnsbury, VT, U.S.A., FM Global in Norwood, MA, U.S.A., and Vegoor in Colombo, PR, Brazil. After defined time intervals reaching up to 2,688 hours (112 days), one bottle was removed from the oven and the liquid was analyzed for commonly measured properties, excluding dissolved gases. By tracking the variation of properties over time, we hope to better understand the liquids' key degradation indicators over the associated time periods and improve the accuracy of thermal class ratings for insulating liquids.