The deterioration of the insulation in high-voltage turbo-generator stators can be followed by means of non-destructive tests. The relative merits of the available methods of testing stator insulation are given, but at present a single test will not reliably indicate the insulation condition. The detection of discharges with an electronic instrument most closely approaches this ideal and has the unique ability of indicating the existence of preferential deterioration near the line terminals of a high voltage stator. A dependable guide to the insulation condition can be obtained only from considering the results of several tests, employing both alternating and direct current. This allows the variation in the insulation of a turbo-generator stator undergoing normal usage to be determined. Departure from the normal variation indicates those machines that have suffered abnormal deterioration. The presence of deleterious discharges in high-voltage bushings at the time of manufacture has been ascertained with the E.R.A. discharge detector. The discharge magnitude is measured directly and has been related to the indirect methods normally used, such as loss angle and audible hiss. For synthetic-resin-bonded-paper bushings of 33????????132kV rating, some discharges are permissible near the working voltage. Discharges in oil-impregnated-paper bushings of 132, 220 and 330kV rating, however, are undesirable at the maximum working voltage. Recommendations are made of the permissible discharge magnitude for the two types of bushings.
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