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

摘要

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