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Moisture, which can enter the insulation of a generator or motor winding from damp air or which can enter the winding of a transformer from wet oil, will make a surprisingly large difference in the insulation resistance. This is clearly shown by the curves of figure 2 for a Grand Coulee generator.

Curve “A” was taken shortly after the generator was placed in service, at a temperature of 36 EC. Curve “C” was taken after a dry-out run of 168 hours on the generator.

Figure 2. - Dielectric absorption curves before and after initial dryout for Grand Coulee unit L-6 108,000-kVA, 120-r/min, 13.8-kV, 60-Hz generator.
Figure 2. - Dielectric absorption curves before and after initial dryout for Grand Coulee unit L-6 108,000-kVA, 120-r/min, 13.8-kV, 60-Hz generator.

The generator winding was, therefore, more thoroughly dried out in curve “C” than in curve “A,” although evaporation of the volatile content of the insulation or other curing or aging effect may have had an appreciable effect. Low insulation resistance resulting from exposure to moisture does not mean that the insulation is unsuitable for operation, particularly if the insulation resistance value is comparable to that obtained from recent periodic tests.

Dry out of thermosetting insulation is not as big a factor, and is sometimes not done, except to cure field applied insulation.

About Author //

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Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

3 Comments

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  3. Megger Tests | EEP
    Oct 13, 2011

    […] detect these punctures.The insulation resistance meter test will show (a) the relative amount of moisture in the insulation, (b) the leakage current over dirty or moist surfaces of the insulation, and (c) winding […]

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