As in the case of asphalt-mica a defect common to these
polyester mica windings is a leakage path to ground
associated with tape separations in the end arm immediately
adjacent to the core. An example will be found in the ramp
test reported in Figure 9. Note that increased leakage
current becomes noticeable at 16 kV and is pronounced at
28 kV. A defect with this characteristic is not serious unless
the winding is exposed to moisture. At Kettle these defects
often are at coils that were involved in lifts at the time of
installation. There have been at least 5 ground faults inservice
or on test since 1976 with no external cause.
Off-line partial discharge measurements at 8 kV line-toground
using the electromagnetic (TVA) probe and peak
pulse meter have been particularly useful at Kettle for
detecting slot discharge before serious damage has
occurred. Corrective action is initiated when readings
exceed 30 mA peak pulse [15]. This usually involves
restoring the semi-conducting coating on the stator coil.
This is accomplished more thoroughly when the front leg of