Condition assessment of stator insulation during drying, wetting and electrical ageing

Stator insulation failure due to moisture ingress is one of the common reasons for generator failures. Non-destructive tests such as DC ramp, insulation resistance (IR) and frequency domain spectroscopy (FDS) are usually conducted to assess the conditions of such insulations. This paper presents a l...

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Bibliographic Details
Published in:IEEE transactions on dielectrics and electrical insulation 2013-12, Vol.20 (6), p.2081-2090
Main Authors: Fernando, M. A. R. M., Naranpanawa, W. M. L. B., Rathnayake, R. M. H. M., Jayantha, G. A.
Format: Article
Language:English
Subjects:
Aging
Condition assessment
Current measurement
electrical ageing
frequency domain spectroscopy
Frequency-domain analysis
Generators
Insulation
Moisture
Moisture content
Permittivity
stator insulation
Windings
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Summary:Stator insulation failure due to moisture ingress is one of the common reasons for generator failures. Non-destructive tests such as DC ramp, insulation resistance (IR) and frequency domain spectroscopy (FDS) are usually conducted to assess the conditions of such insulations. This paper presents a laboratory investigation of generator spare windings under drying, wetting and cyclic electrical ageing conditions. First Dc ramp, IR and FDS tests were conducted on asphalt-mica and epoxy-mica insulated spare windings to obtain initial finger print values. Then the drying effects was investigated by DC ramp and FDS measurements conducted on asphalt-mica insulated winding samples by drying them up to three days at 80°C. The wetting effect was studied by wetting the polyester-mica insulated winding samples in a water bath up to one week at 25°C. Further a correlation has been established between the moisture content and FDS results by drying and wetting of two asphalt-mica insulated samples at 70°C and the room temperature respectively. The ageing effects was obtained by electrically stressing two asphaltmica and two epoxy-mica insulated winding samples at 300% of the rated voltage up to 9 weeks in a cyclic manner. For all cases, conductivity (σ) and parameters representing the response function were estimated by the Davidson-Cole and inverse power dependence functions. The estimated parameters under DC ramp, IR and FDS tests were in similar range for new samples. During ageing and drying, a clear variation was noted only on the σ values and FDS results. It was found that FDS results and estimated parameters provide useful information about the condition of generator insulation.
ISSN:1070-9878
1558-4135
DOI:10.1109/TDEI.2013.6678856