DC partial discharge correlated with defect photographs

Partial discharge (PD) data taken by the dc ramp method were correlated with Scanning Electron Microscope (SEM) photos of internal flaws in several groups of high voltage capacitors. These were BaTiO3 capacitors for space use intended for dc service conditions; some were multilayer and some were single layer discs. Most passed the usual screening tests such as IR measurements and DWV, but several capacitors in each set had, compared to others in the same set, unusually high integrated corona charge transfer on ramping up to and also down from rated voltage. Heating at 85°C with leads shorted had to be done before each dc PD measurement to obtain reasonably repeatable data. Subsequent sectioning and SEM photographs revealed that if corona was above a certain level, the capacitors had cracks and delaminations as well as porosity, whereas below this they only had porosity. Data was also obtained on ac corona inception voltage to allow comparison with the above dc ramp test data. Ac and dc test methods are discussed in terms of arriving at approximate acceptance/rejection criteria.