Recognition of phase resolved partial discharge patterns for internal discharges and external corona activity

A laboratory investigation of discharge pattern evolution as a function of electrical aging has been carried out for internal partial discharge (PD) and external corona activity. The setup was composed of three stator bars. The groundwall insulation of only one bar was exposed to high voltage. A second bar was not exposed directly to high voltage but was crossing over the first bar in the end arm region. From the evolution of the phase resolved discharge pattern, monitored over 3000 hours, internal discharges were easily distinguishable from end arm corona. The phase angle at which both types of activity occurred revealed some difference indicating that different discharge mechanisms were involved. A third bar used as an antenna was not subjected to any discharge activity and was monitored to determine the amount of electromagnetic coupling occurring at different frequencies. Some of the information found can be extrapolated to rotating machine monitoring to determine the presence of end arm corona.