Page 82 - Basic PD Theory

References

4. Partial Discharge
Similar to the on-line test, the off-line partial discharge test is sensitive to delamination and
semicon/grading coating problems. Tests are usually taken at line-to-ground voltage. Because the  IEEE Std 1434:
Guide to the Measurement
voltage stress is higher than normal on the majority of the coils there is no direct comparison of Partial Discharges in
between off-line and on-line testing. It is often possible with off-line testing to identify the exact Rotating Machinery
problem spot and therefore repair it.
Sensors: capacitive couplers
 RF probe (TVA probe)  TVA - Tennessee Valley Authority
 acoustic probe
 visual enhancement cameras – “Corona Scopes”

Finds: moisture and delaminated insulation

5. Surge Test

Most of the insulation testing described thus far has been for testing the integrity of the groundwall insulation. Because of the
high frequency surges that exist when a generator is started, the turn insulation of the line end coil  IEEE Std 522: Guide
can be subjected to extremely high electric stress. During the manufacturing process a surge test for Testing Turn-to-Turn
is often used to locate defects prior to impregnation. Some companies also use the test as a Insulation on Form-Wound
maintenance tool. The test procedure is to apply a high-frequency surge to two similar specimens Stator Coils for Alternating-
and compare the results or to observe the surge waveform for changes in frequency as the Current Rotating Electric
applied voltage is increased. At high frequency, the majority of the surge will be dropped across Machines
the capacitance of the insulation between the first and second turns. A noticeable difference between the resultant waveforms
would be an indication of a problem. Any misconnection affecting the magnetic field patterns within the winding, such as a coil
reversal, phase-to-phase short, or coil-to-coil short, would cause large irregularities. Not as observable because of the small
change in frequency, < 0.1%, are turn-to-turn shorts. Since it is difficult to detect turn-to-turn shorts, only people truly familiar
with the patterns of the generator being tested should be relied upon for interpretation.
The surge test is essentially a go/no-go test similar to a hipot test of the turn insulation.
For new windings the test is normally performed at the peak voltage value calculated from the above figure and with a rise time
of 0.1 to 0.2 µs. For a 11 kV motor this would be:
√2/√3*11*3.5 = 31 kV
For motors that have been in service, a surge with the same rise time, but with a peak value of 75% of that for a new winding is
normally used, i.e., for a 6.6 kV motor this would be 23.25 kV

Finds: turn insulation problems

6. Direct High Voltage Testing
After a winding has passed the IR/PI test and deemed suitable for high potential testing, you may  IEEE Std 95:
want to perform a high direct voltage test. This involves subjecting the winding to a high direct Recommended Practice for
voltage to see if it will survive over-voltage stress. The hope is that if there is a crack or weak spot Insulation Testing of Large
in the insulation it will “pop” during the test where there is substantial energy stored in the AC Rotating Machinery
winding capacitance but little “follow” current from the test supply and therefore minimal with High Direct-Voltage
damage. It will also “fail” during a scheduled outage when repairs are easier to perform. There are two general procedures for
high potential testing: a proof test used for a new winding or after major repairs and a maintenance test used for routine testing.
Finds: severe cracks

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