Health Indexing of Norwegian Power Transformers

Abstract

In this thesis, a health index model for condition evaluation of power transformers is proposed. A health index is a tool that processes service and condition data into a score which describes the overall health of an asset. The motivation behind this is to objectively and confidently assess the condition of power transformers so that reinvestment and maintenance decisions might be justified. This way, the technical lifetime of healthy assets might be safely increased, while risky assets can be identified and taken care of before they fail. Health indexing is particularly useful for evaluation of large transformer fleets, since it makes it easy to identify the assets most in need of additional attention. An important prerequisite for a health index to be useful is, however, that the availability of data is considered in the model design. A health index intended for use in Norway will thus have to be customized to the data availability faced by most Norwegian utilities and transformer users.

In order to identify which assessment methods that are suited for use in a Norwegian health index, four existing health index models have been reviewed. Based on these reviews and the general data collection practices of Norwegian utilities, a health index model has been proposed. Its main inputs for assessment are: Dissolved gas analysis, oil sample analysis, temperature or load history, maintenance history and particular design data. This information is processed through assessment modules that each evaluate different subsystems of the transformer. For these assessment modules to reflect the degradation of transformers in a best possible way, emphasis has been put on identifying the most important failure modes and aging mechanisms.

In order to test the performance of the proposed model, it has been applied to seven different transformers. The results from this analysis show that the model is capable of differentiating between assets in different conditions and that the health index score appears to be a rough, but reliable, indication of the actual condition of a transformer.