Nowadays the economics of power system operation is becoming more and more important. Because of the development of technology, the time based asset management methods are no longer suitable. Ways of asset management which are based on the age of the transformers do not meet today’s economic expectations, so we have to consider other factors to get comprehensive and reliable picture of the condition of our equipment. A measurement-based condition assessment can easily decide whether any action is needed, for example factory or onsite repair, refurbishment or replacement. Nowadays the condition based maintenance has an increasing trend, therefore it becomes possible to create and develop the Instrument Transformer Health Index expert system. This expert system describes the current condition to help avoiding the malfunctions of the power system. The economic approach also cannot be ignored, like design an optimal rate between investment, operating, maintenance, renovation and repair costs.
In this document I summarize the basis of asset management, review the role and importance of instrument transformers in the power system, and then I discuss the diagnostic methods of instrument transformers widespread in national and international practice. All of this contributes to the easier understand of the theoretical foundations and the calculation method of the Instrument Transformer Health Index expert system. The Instrument Transformer Health Index uses not only the results of the measurement, but also other properties, such as voltage level, network location and climate. The health index can be calculated from all available data of the transformer. The document gives specification about the health check measurements and other considered parameters, the health index, and about the opportunities of use this elements. This expert system should simplify the asset management for the users and gives a chance to change their systems to a condition based maintenance. For easier understanding I give example about measurement results, calculation of the health index and about the evaluation.