Power transformer diagnosis based on mechanical oscillations due to AC and DC currents

The reliable operation of power transformers is an essential factor for the high availability of electric energy. Reliability demands can be enforced using condition based asset management which relies on different diagnosis measurements and continuous monitoring. This contribution determines the abilities of asset monitoring by regarding transformers' mechanical oscillations during operation. The physical origins of oscillation sources are introduced as well as the common measurement technique using accelerometers. The contribution evaluates the method's principle diagnostic abilities by correlating the actual mechanical status of an active part and resulting mechanical vibrations in a laboratory setup. The practical usability is determined by long-term comparison of measurements on different power transformers in service. Several influencing factors are evaluated: the sensor position, the changing transformer's load and operating temperature during service as well as the position of the on-load tap changer. Additionally, mechanical oscillations are used to determine the influence of superimposed, undesirable DC components on 3-limb power transformers originated by coupling between nearby AC and HVDC lines.