Morphological Study on Corrosive Sulphur Development in Transformer Windings

This study presents morphological study on transformer windings with the presents of corrosive sulphur. The thermal ageing experiment was conducted at $150{ }^{\circ} \mathrm{C}$ for 5 days. The mineral oil and Kraft paper's moisture content post drying were maintained less than 30 ppm and $0.5 \%$, respectively. Several analyses were carried out such as Scanning Electron Microscopy (SEM), Energy dispersive X-ray (EDX) and $A C$ breakdown voltage to understand the influence and the formation mechanism of copper sulphide $\left(\mathrm{Cu}_{2} \mathrm{~S}\right)$ deposits on copper strip, the deterioration mechanism of $\mathrm{Cu}_{2} \mathrm{~S}$ on Kraft paper and oil. The presence of $\mathrm{Cu}_{2} \mathrm{~S}$ changes the color appearance of oil to yellowish. The copper strip and Kraft paper have noticeable grey-black deposits on the surface upon unwrapping the paper from copper strip. The SEM results reveals that the $\mathrm{Cu}_{2} \mathrm{~S}$ deposits alter the surface structure of copper strip with numerous granular sediments present on the copper surface. The fibre texture of Kraft paper on the mid $\left(5^{\text {th }}\right)$ layer becomes fuzzy and worst on the $1^{\text {st }}$ layer. The actual dielectric breakdown strength of corrosive oil reduces close to half as compared to non-corrosive oil.