Condition based maintenance for power transformers in service: Application of conventional and advanced diagnostic methods

The reliability of all HV-apparatus is strongly dependant on the ability of the insulating system to withstand the permanent electrical stress without any damage during the expected lifetime, typically more than 40 years in service. If Dissolved Gas in Oil Analysis (DGA) shows an increasing amount of fault gases the results are analyzed and the risk of the defect is judged by specialists. Due to the fact that DGA is a generic method, the location of the fault inside of the transformer is hopeless. Therefore additional diagnostic methods must be used (conventional and advanced) to identify and localize the source of the fault gases dissolved in the insulating oil. In this contribution it will be shown on practical examples, that it is possible to identify and localize the reason for the alarm or spontaneous outage of the transformer by consequent application of all existing methods (conventional and advanced) and by final "in depth" analysis of all available results.