Transformer diagnostics by means of gas-in-oil analysis

A relatively large amount of heat can be generated at places in the transformer where a (latent) defect exists. This heat will be absorbed by the insulation medium and subsequently dissipated into the outside air. In oil-cooled transformers, such 'extra' heating affects the insulation oil, causing the formation of gases capable of dissolving in the oil. The composition of gases dissolved in the oil provides a clue as to possible latent defects in the transformer. By means of an analysis of the dissolved gases, any latent transformer defects can be detected in time and steps can be taken at a time favourable for the operator. Relatively severe defects in the transformer will result in the formation of gases that no longer dissolve in the oil but, instead, accumulate in the Buchholz relay. This will cause the transformer to trip if a certain gas volume is exceeded. Gas-in-oil analysis has developed into a valuable technique in transformer diagnostics. The composition of the gas dissolved in the oil is determined by gas chromatography. This technique can therefore lead to timely detection of defects and thus prevent expensive repairs. Several examples of transformer failures detected on time by means of gas-in-oil analysis are discussed.