The transformer is an important electrical equipment in power plants, converting the voltage from 20 - 26kV at the generator output to 220 - 230kV. After a period of use, it is necessary to re-evaluate losses, efficiency, and operation mode to have suitable measures to increase working efficiency and limit incidents, as well as prolong the transformer’s life. The authors conducted surveys at 5 power plants (Ca Mau 1 and 2, Nhon Trach 1 and 2, and Vung Ang). The transformers here have a capacity between 231 - 300MVA, and have been operating for 6 - 13 years. Based on measurement data, the authors have built the CLET software (Computing Losses and Efficiency of
Transformer) to analyse and evaluate the status of operation and performance results of the above machines. From there, the authors made suggestions on operation and maintenance to ensure safety and prolong the life of the machine.

Key words: Transformer, thermal power, losses, efficiency, CLET.