Page 16 - Basic PD Theory

Partial Discharge for Stator Windings

 Winding Contamination
When any kind of conductive contamination from moisture or oil mixed with dust/dirt pollutes a machine, it is possible for
electrical tracking (treeing) to develop across the blocking or along the end arms. Electrical tracking occurs because the pollution
introduces a conductive path between two adjacent coils of different potential, and often from different phases. As a result of
the tracking, PD can arise and attack the groundwall insulation on the surface. Permanent phase-to-phase or phase-to-ground
deterioration may occur and eventual failure of the groundwall is possible. Moisture condensation in the slot section can lead to
problems if a machine has been idle. Oil tends to dissolve and loosen insulation system components and can attract dust that
diminishes heat transfer from the winding surface thus reducing insulation life. In open enclosure machines, oil, in combination
with dust, can clog up cooling air passageways to cause overheating. Conductive particulate contamination could produce a
strong local concentration of partial discharges that could produce small perforations in the insulation. [12]

 Internal Water Leak
Direct water-cooled stators can develop a slow leak of the water from fine cracks in copper strands or brazed connections.
These cracks develop over time as the connections corrode in the presence of minerals within the water. This water migrates
along copper-groundwall interface to the slot area and leads to insulation delamination. PD might occur, but more importantly
there is a loss of mechanical strength. Saturated epoxy usually fails a high-potential test. Failure occurs generally within 10 years
of a confirmed water leak.

1.6 Stator Failure Mechanisms
Stress Failure Mechanism Symptoms Insulation types
Partial discharge, insulation
Internal delamination discoloration All stator winding types (Asphaltic mica)
Thermal
Thermal cycling Partial discharge, girth cracking, All stator winding types (gas turbines,
shorted turn pump/storage units, long core machines)
Partial discharge, white powder,
Electrical connections Any connection
insulation discoloration
Phase to phase Partial discharge, white powder,
discharges ozone Air-cooled
Partial discharges, white powder,
Electrical slot discharge Air-cooled machines
Electrical ozone
Semi-con/stress Partial discharge, white powder,
interface ozone Air-cooled with stress control paints
Turn-to-turn shorts, PD?, Loose Multi-turn coils, especially motors (VSD’s &
Surges / Starts Endwindings IFD’s)
Manufacturing defect Partial discharge Global VPI, resin-rich coils
Partial discharge, All stator winding types
Contamination
oil/grease/dust, white powder (High voltage coils)
Ambient Saturated epoxy, corroded
Internal water leak connections Direct water-cooled windings

Foreign material Insulation discoloration Any winding
Coil/bar movement Partial discharge, ozone, loose Hard systems - epoxy & polyester
Mechanical wedges
Endwinding vibration Loose blocking, white powder High voltage machines with long end arms

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