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