Abstract :
The need is indicated for informative non-destructive test methods to assess the quality of insulation, both as manufactured and during its service life, and the limitations of high-voltage proof-testing are reviewed. The types of insulation system used for high-voltage stator bars are described briefly, and the nature and cause of defects which can give rise to trouble in service are analysed. Low-voltage test methods available are discussed, with particular reference to low-voltage variable-frequency tests, and some results obtained using this technique are reported. Published work describing high-voltage measurements using d.c. and low-frequency sources are reviewed, and the need is emphasized for a better understanding of fundamental dielectric behaviour under varying frequency and stress conditions. Power-frequency measurements at voltages up to line potential are dealt with at greater length. The aim of such measurements is primarily the detection of voids. They can be of great value in controlling insulation quality and can also be used to monitor the condition of the insulation during maintenance checks in service. Tests dealt with include loss-tangent/voltage and corona discharge measurements, and special bridge tests aimed at separating solid-dielectric and discharge losses. Bridge circuits are described which, combined with oscillograph cyclogram displays, permit the discharge energy to be determined, and also enable solid-dielectric losses to be measured. Test results obtained using a novel bridge circuit of this type are reported.
