The authors describe a new technique for evaluating the quality of electrical insulation of power system apparatus while the apparatus is energized and in normal service. The use of this method eliminates the need to de-energize equipment for routine maintenance checks of power factor or insulation resistance. It thereby can identify insulation problems so that prompt corrective action can be taken to maintain system reliability. The measurement is relatively simple to perform and has the additional advantage of being insensitive to temperature variations. The validity of the technique has been established by controlled measurements on simulated apparatus and confirmed by actual observations on operating apparatus in outdoor substations as well as in the laboratory.
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