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Mineral insulating oil manufacturing processes have changed significantly over the last twenty years. Oil base structures have also changed due to that, furthermore, the use of additives have also been improved. As a result, oil degradation kinetics is currently different, oil properties variation during aging must be re-evaluated. This paper describes mineral oil ageing behavior under mild and heavy oxidative environment. Traditional properties as well as unconventional tests are measured along ageing process and results are compared to determine best criteria to set the point of non-return for corrective maintenance action over the oil.

This paper presents the development of a “modified Degree of Polymerization test method” based on IEC 60450 for the DuPont™ Nomex® 910, engineered cellulose composite paper. In order to check how retained tensile strength corresponds to the DP value of 200 for the composite 910 paper, accelerated ageing tests of 910 and TUK were performed and DP vs tensile curves were obtained. Although the thermal insulation system test procedures allow the choice of either DP or percent tensile strength retention as the end-life criteria for cellulose-based papers, our data indicates that the traditional kraft DP of 200 is not a good indication for the composite 910 paper end-of-life criteria.

Dissolved gas analysis (DGA) in insulating oil is performed periodically to access operating conditions of power transformers. DGA results suggest possible incipient faults and maintenance actions for transformer repair. The more information the process gives, the better and cheaper corrective actions can be. To optimize asset management, determine the need for repair in a transformer failure, or choose options for managing operating conditions, an additional tool that allows maintenance engineering to previously identify failure type would be interesting. When an incipient fault occurs, all construction materials involved will be affected and, therefore leave traces in insulating fluid. This paper is a first step in evaluating potential analytical methods to be used in determining the presence of transformers construction materials in insulating fluids due to incipient faults. Insulating oil samples were first contaminated with different transformer construction materials in order to test different analytical techniques to determine the presence of those materials in oil and/or their influence in oil properties. Samples were thermally aged and then tested by different methods to find out contamination effect on physical chemistry properties, such as total acid number, interfacial tension, dielectric loss and others. The presence of added contaminants both soluble as well as particulate material is also investigated. Soluble contamination materials are determined by use chemical analysis techniques such as Fourier Transform Infrared Spectroscopy (FTIR), Gas Chromatography Mass Spectrometry (GC-MS), Inductively Coupled Plasma (ICP), and enhanced DGA. Some nonconventional analysis techniques were also used to evaluate particulate contaminants related added construction materials, such as particles counting, particle quantification index (PQI), and Analytical Ferrography that gives particles nature, shape and size. This work is part of the R&D research project ANEEL PD-222.2017.