Consequences of Moisture Ingress in Oil Impregnated Paper (OIP) Insulated Transformer Bushings

The condenser core of OIP transformer bushing is formed by use of thin aluminium foils and electrical grade insulating kraft papers. This condenser grading uniformly distributes the electric field (radially and axially) over the bushing insulation systems. The condenser core is dried and impregnated with mineral oil. During drying process, the moisture content in paper is reduced from around 7% to less than 0.5%. After achieving the moisture content lower than 0.5%, the paper is impregnated with dried and degassed insulating oil. After drying and processing of insulating oil, the moisture content in oil is reduced to lower than 10 ppm.

Sources of Moisture Ingress

Moisture ingress in OIP bushings may take place through sealing rings/gaskets and could be caused by ageing of sealing rings/gaskets. Also, the moisture ingress may take place through accidental opening of bushing assembly e.g. oil plugs.

Adverse Effects of Moisture Ingress

• Reduced BDV (Breakdown Voltage) of insulating oil

• Increase in dielectric loss (i.e. Tan Delta)

• PD, discharges/tracking on insulating surface

• Thermal instability leading to run away

• Accelerated ageing of OIP insulation

Diagnostic Methods for Evaluation of Moisture Ingress

• Dielectric Frequency Response (DFR): DFR testing is a reliable diagnostic tool for transformer bushings by measuring Capacitance and Tan Delta over a wide range of frequency and thereby assessing the moisture content in the insulating system.

• Moisture content analysis of oil sample drawn from bushings using Karl Fischer method.

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