Overvoltage tests are used to obtain
assurance concerning the minimum strength of the insulation. Such
tests are made on all or parts of the ground insulation.
Many users of large rotating machines
apply overvoltage tests periodically, generally at the beginning of
the overhaul of related equipment. This allows for the detection and
possible repair of insulation weaknesses during the scheduled outage.
An overvoltage test should be applied
to each phase separately with the remaining phases not under test
being grounded. In this way, the insulation between phases (or lines)
is also tested. This is only practical, however, where both ends of
each phase are brought out to separate terminals, as is usually the
case in generators.
Some motors may have three or four
leads brought out which precludes test between phases. Overvoltage
tests may be performed either with alternating or direct voltage. The
level of overvoltage which should be applied will depend to a large
extent on the type and age of the machine involved, the degree of
exposure to overvoltages, and the level of serviceability required
from the machine in question.
It should, however, be sufficiently
searching to discern any weakness or incipient weakness in the
insulation structure which might lead to service failure. It should
be recognized that if the windings are clean and dry, overvoltage
tests may not detect defects which are in the end turns or in leads
remote from the stator core.
The values of test voltages usually are
selected in the range of 125 to 150% of the rated line-to-line
voltage and are normally held for 1 min:
1) Refer to IEEE Std 4-1968 ,
Techniques for Dielectric Tests (ANSI C68.1-1968 ), for power
frequency testing
2) Refer to IEEE Std 433-1974 ,
Recommended Practice for Insulation Testing of Large AC Rotating
Machinery with High Voltage at Very Low Frequency, for 0.1 Hz testing
and recommended voltage level ratio
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