Condition Monitoring of Transformer Oil and Paper

Abstract

Like other machines, transformer too has limited life. However, unlike other machines, it does not have any moving parts, except tap changers or cooling fan or pump motors. The outages, therefore, is not due to wear out. The transformers die because of deterioration of insulation over time. Generally, cellulosic paper and mineral oil form bulk of insulation in the transformer. The insulation is subjected to a variety of stresses, such as thermal, mechanical, electromagnetic, etc. Under the influence of the said stresses and in the presence of oxygen and moisture, the insulation deteriorates continuously over a period of time, eventually leading to failure. If these failures can be predicted with some degree of confidence, sudden failures can be minimized. In view of the above, efforts are being made to understand and analyze factors responsible for determining the life of a transformer. Broadly, ageing of insulating material can be looked upon as a chemical reaction, which takes place at a rate that is influenced by thermal, electrical and mechanical stresses. All chemical reactions lead to product(s), which have physical and chemical characteristics different from the parent material. Therefore, chemical and physical characterization of the reaction product(s) may assist in monitoring the health of the insulation and hence transformer. For instance, cellulose undergoes degradation under thermal stress: the Degree of Polymerization (DP) of cellulose is reduced due to thermal ageing. Besides, it leads to the formation of reaction products, such as carbon monoxide, carbon dioxide, water, furan, etc. Furans are a family of organic compounds like 2-furfuraldehyde, 2 acetylfuran, 5-methyl-2- furfuraldehyde, furfurylalcohol, 5-hydroxymethyl -2- furfuraldehyde, etc. It is therefore, possible to estimate the extent of insulation damage by monitoring reaction product(s). Moreover, these changes in cellulose, at molecular level manifest in change of physical properties.