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In the detection of pulse-type corona discharges in voids occluded within the insulating systems of electrical apparatus, the corona pulse amplitude and repetition or discharge rate represent two basic quantities upon which much of the present state-of-the-art of corona measurement and interpretation rests. In the past, however, much more attention has been paid to the measurement of the corona discharge pulse amplitude than to that of the pulse recurrence rate. This underlying fact is well borne out by the present specifications, concerning commercially available corona detection sets for use on transformers, capacitors, and cables, which are based in their entirety upon the detectable apparent corona pulse charge expressed in picocoulombs. In so far as the corona testing techniques presently in use invoke the measurement of the corona inception and extinction voltages, they must be regarded ipso facto as go no-go type test methods in the sense that they merely indicate the presence or absence of corona pulse discharges above certain preset sensitivity levels expressed in picocoulombs. In order to characterize the corona discharge pulse behavior in a given insulating system, it is necessary to go further and measure the discharge quantities of both the corona pulse amplitude and repetition rate. Quantitative information concerning the corona pulse pattern density is obtained in terms of the overall corona discharge rate measurements. However, additional meaningful data is readily obtained if the corona discharge rate is measured as a function of the corona pulse amplitude. The resulting corona pulse-height distribution spectrum is proportional to the total discharge energy dissipated and, therefore, represents essentially a fingerprint typical of the corona discharge behavior in the insulating system under the given applied voltage and elapsed time conditions of the test. Evidently, this form of corona pulse-height analysis constitutes a very powerful tool for assessing the corona discharge intensity in the insulating systems of electrical apparatus.
Hydro-Quebec Institute of Research, Varennes, Quebec