Page 14 - Basic PD Theory

Partial Discharge for Stator Windings

 Endwinding Vibration
In order to meet the required electrical conditions in high-voltage machines, the manufacturers have to design the stator winding
with long end arms. This is especially true of 2-pole generators. These endwindings are subjected to strong 100/120 Hz electro-
magnetic forces, in both the radial and circumferential directions, that can cause vibration in poorly designed or blocked
machines. In addition, if a motor or pump-storage unit is started direct-on-line or a generator synchronized out-of-phase, high
electromagnetic forces are imposed on its endwindings due to the high stator currents that flow during the starting period, e.g.,
6X full load current at motor starting equates to 36X normal endwinding forces. This vibration may eventually lead to cracks in
the copper conductor strands, cracks in the coil ground insulation at the ends of the core, and local overheating. If end arm
vibration is suspected, an accelerometer should be used to determine the severity. Visual inspection of the blocking, lashing, and
surge support rings may indicate movement in the endwindings.
Endwinding vibration often abrades the insulation at the blocking and tying positions. Because of the materials used, this most
often appears as a white powder or dust. This powder should not be confused with that due to PD. A very thorough visual
inspection will usually confirm the real cause, if PD has not been detected. High-speed, high voltage machines with long end-
arms are subject to this problem.

 Phase to Phase Discharges
In order to reduce the size of the coils and to save copper or reduce losses, manufacturers occasionally fail to leave adequate
clearance between the coils in the endwinding area or the ring bus connections. If two adjacent components from different
phases do not have sufficient spacing between them, it is highly likely that partial discharge activity will occur between the two. In
air-cooled machines, this leaves a white powder residue. The discharges will slowly erode the groundwall and eventually
puncture it. The closer the coils or components are – the faster the failure is. Generally these phase-to-phase faults take several
years to develop, but they produce high quantities of ozone in air-cooled machines. However, if this activity is occurring
between jacketed cable leads in the machine main terminal box, it can cause rapid insulation failure since such cable insulation
has a lower PD withstand capability. The combination of inadequate spacing and a polluted operating environment can provide
a fertile condition for PD activity.

 Poor Electrical Connections
Poorly bolted connections at the terminals or poorly braised leads between coils/bars cause the copper to oxidize giving
rise to sparking, overheating. Electrical connections that are contaminated can also lead to electrical tracking. The
ultimate result is overheating that thermally damages the insulation.

 Foreign Material
A frequent occurrence following a major overhaul is the inadvertent oversight of foreign material left within a winding.
The overall damage to the insulation system is completely dependent on the type of material, position within the
winding, and length of exposure.

1.5 Operational Issues
There are several aging stresses to which stator winding insulation systems are subjected. These include thermal, electrical
(voltage), ambient (environmental, and chemical) and mechanical (vibration and shock). These stresses combine to give rise to
many different deterioration processes since any one factor rarely would occur alone. As the stress level increases, the rate of
aging increases. Some stresses are steady-state, such as, AC voltage, operating temperature, 100/120 Hz vibration, where the
time to failure is proportional to operating hours. Other stresses are transient, such as, thermal expansion/contraction, switching
surges, motor starting, and transient loss of cooling, where time to failure is proportional to the number of starts/events. For
example, with some insulation systems, a 10°C higher operating temperature reduces life by about 50%. An increase of 1% in
stator voltage decreases life by about 10%, while abrasion increases exponentially with coil vibration amplitude.

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