Characteristics of partial discharge pulses from operating rotating machines

The recent introduction of 2 GHz multi-channel digital oscilloscopes has made it possible to more accurately measure the waveform characteristics of partial discharge and electrical noise pulses. Waveform measurements were made on several operating hydrogenators equipped with 80 pF “PDA” capacity couplers. In many cases the oscilloscope was triggered on pulses which had been categorized by a PDA-type instrument as due to partial discharge or noise. A wide variety of partial discharge waveforms were encountered. Some pulses had risetimes as short as 3 ns, while others had risetimes as long as 10 ns. Even on the same generator some discharge pulses were very oscillatory, while others were not. Often the oscillatory discharge pulses had waveforms where the second “ring” in the pulse had an amplitude which was significantly higher than the first ring. Noise pulses generally had a longer risetime than partial discharge pulses, and were very oscillatory. These waveform characteristics have important implications for the design of instrumentation which classifies pulses as noise or partial discharge, and for determining the correct magnitude of the pulses