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A number of computer models have been recently proposed for simulation of the impulse voltage distribution in the winding of power transformers. Some of these models can predict the winding behavior at high frequencies, some other were developed as a practical tool for designers of lower voltage transformers. The time and expenditure put in development of these models shall be balanced by a similar effort to check the model validity. Such check consists in comparison of the predicted voltages to the actually measured ones.With the advent of high resolution, fast digital recorders and signal processing techniques, this validation can be performed to a much higher accuracy and completeness than the conventional comparison of analog oscillograms to simulated graphs. The paper presents such validation technique applied to a computer model of transformer winding.Problem.An accurate simulation of the transient voltage distribution in HV windings of a large power transformer allows the designer to apply a sufficient insulation in the areas of high stress, and at the same time reduce an excessive insulation of the wires and discs, in order to make the transformer more competitive. A practical importance of a reliable simulation of the winding characteristics has been long recognized by manufacturers of large and expensive transformers. They have found that the savings on an optimized insulation represent a significant amount of money These savings project on the overall cost of a transformer, and often decide on the choice of supplier With the increasingly competitive market, the importance of simulation has also been noticed by makers of medium size and smaller transformers. In general, they do not have a sufficient engineering and research staff to develop and implement the computer model required for simulation of the dielectric stress in the insulating system. However, they would like to use a commercially available program that has been proved in industrial applications.In addition, the very fast transients appearing in the SF6 insulated bus bar add to the complexity of the model that shall take into account the natural winding resonances occurring in the megahertz frequency range This situation has created demand for a computer model of power transformer HV windings applicable at manufacturing and repair facilities of lower voltage transformers, as well as for a highly specialized model that can simulate the winding behaviour up to say 1 MHZ. Several research centers and universities have recently reported progress in development of such scientific tools, or even announced the final product [l to 31. It may be taken for granted that the highly specialized knowledge of the authors of these models warrants the accuracy and practical usehlness of these models.. WM 148-7 PWRDA paper recommended and approved by the IEEE Power System Instrumentation & Measurements Committee of the IEEE Power Engineering Society for presentation at the IEEE/PES 1994 Winter Meeting,However, it should be recognized that an actual HV winding...

Synopsis: Power-factor and insulationresistance measurements are aids in controlling the quality of transformer insulation in the factory. These measurements are affected by a number of factors, which must be considered in their evaluation.Some of these factors are related to the quality of the insulation and some are not. Moisture is a harmful contaminant that may be detected by these measurements. An experimental investigation of the effect of moisture on the power factor, insulation resistance, dielectric absorption characteristics, and the dielectric strength is reported. A correlation has been found between moisture content, as indicated by these measurements, and the 1-minute 60-cycle strength, the 7-hour 60-cycle strength, and the impulse strength of oil impregnated pressboard. The-results of a number of powerfactor measurements on completed oil, Inerteen, and air-insuiated transformers are summarized, Finally, there is a description of how dielectric measurements are applied in helping to control the quality of transformer insulation in the factory. ONE OF the most important components of a transformer is the insulation. It is usually the l:mit-'ng factor in the life of a transformer and its qualitv must be carefully controlled in the factory. Any tests which can be used to help insure high quality insulation in new trAnsformers and to aid in maintaining this insulation in the field are well worth while. Measurements of power factor, insulation resistance, and dielectric absorption characteristics help do this by revealing certain conditions which might not be disclosed until a destructive failure has occurred. They are simple and easy to carry out, and have no harmful effect on the insulation. These tests are being used increasingly as a control of quality in the factory and as a periodic check in service.Users of electric power equipment have become very conscious of this means of preventative maintenance, and manv of them have active programs for making dielectric measurements. Proper interpretation of the results of these measurements often permits reconditioning of the equipment before an expensive failure occurs Programs like these have been found to be very useful.1-4The manufacturer and the user have common interests in insuring long-time J. H. McWHIRTER ASSOCIATE MEMBER AIEE stability of the insulation in service. However, the former has the following additional uses for these measurements: 1. To determine the suitability for use of newly developed materials. 2. To control the quality of insulating materials. 3. To control the process of drying. 4. To make a final check on the assembled transformer.This paper describes how a manufacturer of transformers makes and interprets dielectric measurements and presents supporting data. Test results on insulation samples show the relationship between moisture, as indicated by the various dielectric measurements, and dielectric strength. The effects of other variables besides moisture are discussed, and the results of power-factor measurements on a number o...