Moisture effect on conductivity of kraft paper immersed in power transformer vegetable-based insulation oils

Abstract

Vegetable insulation oils in transformers have become a reality. However, they are inherently hydrophilic and can absorb much moisture. Existence of moisture in transformer insulation will generally lower the breakdown voltage of oil. Moisture will also accelerate cellulose deterioration by increasing its dc conductivity. Both oil and cellulose are components of the transformer insulation system. It is therefore important to understand how this class of oils retains their dielectric strength and their relationship with kraft paper (KP) after absorbing so much moisture. This study presents effect of moisture increase in three samples of vegetable oil and its effect on dc conductivity of KP samples immersed in the oils. The dc conductivity of the samples is calculated based on dielectric response polarisation and depolarisation current measured. The result shows that KP in vegetable insulation oil performs better when the oil samples are wetter and drying the vegetable insulation oil accelerates KP degradation. The finding will help to understand of the deterioration of cellulose when immersed in the oil.