Partial discharge characteristics of artificial defects in mica-epoxy composite materials

Partial discharge (PD) in groundwall insulation of rotating machines is mainly caused by internal defects like delaminations or cracks according to microscopic investigation. Discrimination of the cracks from the delaminations is important for diagnosis of the groundwall insulation because the cracks along external electrical field can initiate electrical tree and reduce residual breakdown voltage. PD characteristics of artificial delaminations and cracks were measured with both ultra wideband detection and conventional detection. As a result PD in crack type voids included distinguishing high magnitude pulses with a few ns rise time and 1-10 ns fall time. Delamination type void had two types of waveforms that are characterized by (1) fall time longer than 40 ns and (2) fast rise time and fall time of a few ns. PD characteristics with different conditions of void surface, void dimensions and ventilation are also discussed