Analysis of transformer oil for transformer condition monitoring

International experience shows that large transmission transformers can have service lives in excess of 50 years. For the operator the concern is to be able to identify the onset of unreliability as the end of life approaches. There are a variety of chemical, electrical and physical condition monitoring techniques which can be applied. For many utilities, the basic method is a regular analysis of an oil sample. Mechanical and electrical faults may arise following short circuits, local overheating at hot spots or leakage flux and eddy currents in the core, and partial discharge or arcing at areas of high stress. Decomposition products from breakdown of the oil, paper or insulating boards, glues etc. are transported through the transformer by the coolant oil. Some are low molecular weight gases dissolved in the oil and can be identified by gas chromatography. Others indicating solid degradation include furans, cresols and phenols and detected by liquid chromatography. Alternative technologies require outages and access to bushing terminals to measure mechanical electric characteristics of the winding and the magnetic circuit. For many these are used as secondary methods to be used for condition audits, or to investigate problems identified by oil analyses.