ČVRSTA IZOLACIJA ENERGETSKIH TRANSFOMATORA – STRUKTURA, PRIMENSKE OSOBINE I METODE ISPITIVANjA NAKON FABRIČKOG SUŠENjA I TOKOM EKSPLOATACIJE

Transformers are vital components in electricity transmission and distribution systems. The lifetime of transformers is limited by the lifetime of solid insulation, which is mostly made of cellulose-based materials. During exploitation, the insulation system of the transformer degrades, changing its chemical composition and characteristics, especially at high operating temperatures. The transformer insulation system has its maximum operating temperature below which normal aging is expected during exploitation. Thermally improved solid insulation has higher mechanical strength which is more resistant to degradation in conditions of high operating temperatures during exploitation. The paper presents the chemical composition and basic structure of an insulating paper,as well as techniques and principles of thermal improvement of insulation. Thermal improvement of solid insulation is achieved by methods of chemical modification of the pulp or the finished product as well as the application of invasive drying techniques in the final stage of processing in the production of transformers. The most important characteristics of new insulating papers are good mechanical properties (high tensile strength), low moisture content, high degree of paper polymerization, and others. The degree of polymerization of paper is one of the key characteristics of cellulose insulation used to control the quality of new paper samples, assess the future lifetime of solid insulation during transformer operation, and in recent years to assess the state of insulation after drying in transformer production. Results of comparative interlaboratory testing of the degree of polymerization (DP) of new insulating papers (Round Robin Test) within CIGRE D1.01. working groups, presented in the paper, pointed out the problems in determining the DP of new insulation papers after factory drying. It was noticed that the results of individual participating laboratories differ significantly, and the assessment of the condition of the transformer after factory drying within the factory acceptance tests of new equipment (FAT - Factory acceptance test) is questionable. Due to the above, research on how to modify the existing method of determining DP and the application of alternative methods for assessing the mechanical properties of solid insulation has become a priority. In this regard, the task of the new working group CIGRE D1.76. is, inter alia, to propose a modification of the DP determination method according to the IEC 60450 standard and its applicability to new papers after factory drying, as well as to obsolete papers during operation. The paper presents basic guidelines for modifying the DP method to obtain results with greater reliability and reproducibility. In addition to the DP method, other insulation paper testing methods are presented, such as tensile strength measurement, water absorption and retention measurement (WRV,%), and application of infrared spectrophotometry to identify different functional groups, which gives a more detailed insight into cellulose structure. Further work is needed on the development and application of these methods to detect changes in the structure of cellulose after delignification process, chemical modification, and additional factory drying in the transformers production, which leads to the phenomenon of "hardening" of the so-called. hornification.