Health Indexing of Norwegian Power Transformers
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- Institutt for elkraftteknikk 
In this thesis, a health index model for condition evaluation of power transformersis proposed. A health index is a tool that processes service and condition data intoa score which describes the overall health of an asset. The motivation behind thisis to objectively and confidently assess the condition of power transformers so thatreinvestment and maintenance decisions might be justified. This way, the technicallifetime of healthy assets might be safely increased, while risky assets can be identifiedand taken care of before they fail. Health indexing is particularly useful forevaluation of large transformer fleets, since it makes it easy to identify the assetsmost in need of additional attention. An important prerequisite for a health indexto be useful is, however, that the availability of data is considered in the modeldesign. A health index intended for use in Norway will thus have to be customizedto 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 Norwegianhealth index, four existing health index models have been reviewed. Basedon these reviews and the general data collection practices of Norwegian utilities, ahealth index model has been proposed. Its main inputs for assessment are: Dissolvedgas analysis, oil sample analysis, temperature or load history, maintenancehistory and particular design data. This information is processed through assessmentmodules that each evaluate different subsystems of the transformer. For theseassessment modules to reflect the degradation of transformers in a best possibleway, emphasis has been put on identifying the most important failure modes andaging mechanisms. In order to test the performance of the proposed model, it has been applied toseven different transformers. The results from this analysis show that the model iscapable of differentiating between assets in different conditions and that the healthindex score appears to be a rough, but reliable, indication of the actual conditionof a transformer.