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...
