Algorithm of the microprocessor thermal protection of oil power transformers

This paper surveys the work on the improving of the thermal modelling of oil power transformers. The original algorithm for the hot-spot temperature calculation is exposed. The algorithm describes the physical phenomena better and provides higher calculation accuracy then the algorithms in the valid IEC standards and in the new draft. The algorithm is convenient to be practically realised on the microprocessor thermal relay. The research of the important practical problems of unknown starting hot-spot temperature and of thermal characteristics change during a long-term operation of a transformer is also overviewed. The prospect for future work is a further research of other configuration of the thermal observer. Also, the natural step after the research and extensive experimental verification of the algorithms is the application in commercial microprocessor thermal protection and monitoring systems.