Page 64 - Basic PD Theory

Failure Mechanisms

D. Failure Mechanisms

Failure Mechanism Symptoms Detection Tests Insulation types

Partial discharge, white
Air Gap Discharges powder, insulation PD, visual inspection, Switchgear, motor leads
discoloration
Partial discharge, PD, visual inspection, IR,
Contamination oil/grease/dust, white PI, Hi-pot, power factor, All stator winding types
(High voltage coils)
powder tip-up, tan δ
Partial discharge, white
Electrical connections powder, insulation PD, visual inspection, Any connection
Infrared scan
discoloration
PD, visual inspection,
Electrical slot Partial discharges, white
discharge powder, ozone ozone, coil-to-ground Air-cooled machines
resistance, side clearance
Loose blocking, white Accelerometer, visual High voltage machines with
Endwinding vibration
powder inspection long end arms
PD, power factor, tip-up,
Manufacturing defect Partial discharge Global VPI, resin-rich coils
tan δ, capacitance
Inadequate cable Partial discharge, white
spacing powder, insulation PD, visual inspection Switchgear, motor leads
discoloration
Partial discharge, white
Phase to phase powder, ozone, insulation PD, visual inspection, ozone Air-cooled
discharges monitoring
discoloration
Vacuum, tracer gas, bar
Internal Water Leak Saturated windings capacitance, DC Hi-pot, Direct cooled windings
IR/PI, off-line PD
PD, visual inspection, power All stator winding types (gas
Partial discharge, girth
Thermal cycling factor, tan δ, hi-pot, surge turbines, pump/storage
cracking, shorted turn
testing units, long core machines)
Partial discharge, ozone, PD, visual inspection, wedge Hard systems - epoxy &
Coil/bar movement loose wedges tap, ozone polyester

Semi-con/stress Partial discharge, white PD, visual inspection, power Air-cooled with stress
interface powder, ozone factor, tan δ, ozone control paints
Turn-to-turn shorts, PD?, Surge Test, PD? Multi-turn coils, especially
Surges / Starts
Loose Endwindings Endwinding Inspection motors (VSD’s & IFD’s)
Partial discharge, insulation PD, visual inspection, power All stator winding types
Internal delamination
discoloration factor, tan δ, capacitance (Asphaltic mica)

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