Page 84 - Basic PD Theory

References

No end slot wedges should be loose
No more than 25% of the wedges should be loose
No wedges should be cracked
If any of the above criteria are not met, then a partial or complete re-wedge is recommended. This test is only practical for
windings with individually VPI’d, or resin-rich coils.
Finds: loose coils/bars

9. Ozone Monitoring

An easy method for determining the extent of surface partial discharges in an air-cooled machine is to  Ozone is
hazardous to
periodically test the cooling air for ozone concentration. This can be done using “draeger” tubes or personnel
sophisticated electronic sensors. Ozone is a by-product of electrical discharges in an oxygen-rich health as well
environment and may be sensitive to severe insulation degradation from grading coating deterioration, as corrosive
electrical slot discharges, coil/bar movement, or phase to phase discharges between high voltage
components.
0.1 ppm indicates significant PD (OSHA - personnel exposure are 0.1 ppm for 8 hours)
1 ppm indicates severe PD
Finds: surface discharges

10. Coil-to-ground Resistance
This test is used to validate the effectiveness of the semi-conductive armor and interference fit between the coil/bar surface and
the core. Use a standard digital ohmmeter with a braided wire lead to measure the resistance from the coil surface to grounded
core or frame. Resistances should be < 2000Ω. If resistance is > 5000Ω in line ends, then electrical slot discharges (PD) are
likely to occur.

Finds: problems with the semi-conductive coating

11. Side Clearance Check
This is a simple test to check if there is excessive side clearance between the coil outer surface and core. It only applies
to windings with individually VPI’d, resin-rich, or asphalt bonded mica coils. It involves first removing the stator slot
wedges. Feeler gages are then inserted between the coil and core at several positions along each slot, and the size of the
largest that will fit is recorded. For modern epoxy or polyester bonded coils looseness is indicated if the clearance
measurement is > 0.125mm (0.005in) for a length more than 15-20cm (6-8in). For older asphalt bonded mica coils the
limit is 0.5mm (0.020in).
Finds: problems with coil/bar looseness

12. Insulation Resistance

This test is primarily used to determine whether or not a winding is suitable for return to service or subsequent high potential
testing. It involves application of a relatively moderate direct voltage to the test specimen for a period of 10 minutes while
monitoring the current flow.
Finds: Contamination, moisture, and severe cracks

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