Revised sealed tube test procedure for thermal verification of liquid-immersed transformer insulation systems

Abstract

Over the last few years, there have been numerous claims that the thermal performance of cellulose can be improved by the presence of ester liquids. However, solid verification of this phenomenon by a recognized, standardized test procedure is still missing. The article will present a summary of different historical studies that suggest some improvement of cellulose when immersed in ester liquids. Inaccuracies of these studies and a shortage of some important data will also be discussed. The recent industry discussion on the actual thermal performance of cellulose in natural or synthetic ester liquids resulted in a need for performing thorough studies on this subject. It was agreed within IEC Technical Committee 14 “Power Transformers”, that comprehensive studies shall be made. It is likely that the sealed tube test methodology discussed in this article could be used for developing more systematic conclusions on cellulose-ester thermal performance.A number of standardized test procedures were proposed in the past for new insulation system thermal evaluation. They included the Lockie Test, sealed tube testing and other laboratory testing methods. The Lockie Test was one method used to verify the higher thermal capability claimed for this new insulation system. However, the method uses complete transformers in the procedure and as such is specific to the individual manufacturer’s design. More recently, the dual temperature test has been proposed and has been shown to be a useful procedure, although the equipment necessary is very specialized and complicated. However, this procedure only tests the wire insulation, although it is still the best procedure for testing a system with widely differing thermal capability between the liquid and the solid insulations.The latest revision of IEEE Std C57.100 defines the Lockie Test procedure along with the dual temperature test and the sealed tube test. The sealed tube test was originally developed only as a means of determining equivalent material substitution and has been maligned in the past as an inadequate laboratory test. As with all simulated accelerated testing, this procedure certainly has its weaknesses. However, the 2011 revision of IEEE Std C57.100 now uses a relative thermal index technique rather than the original absolute method, mitigating one of the most problematic issues with the procedure. This new version also addresses many of the other weaknesses of the original method. The end of life criteria is based on either degree of polymerization (DP) or retained tensile as the determining factor. However, since DP is specific to cellulose products, tensile retention is suggested as the end of life criteria which works universally for almost all solid materials for both power and distribution transformers.An example of a new insulation system based on aramid enhanced cellulose paper immersed in mineral oil will be presented in detail, in order to clarify the procedure of full thermal evaluation following the IEEE Std C57.100-2011 sealed tube test procedure.