Analysis of the arcing process in on-load tap changers by measuring the acoustic signature

Power transformers are fundamental parts of the power system. In order to keep the availability of power systems high, the reliability of a transformer, in all its parts, must be assured. This requires also paying attention to the reliability of the tap changer, as tap changer failures are major causes of power transformer breakdown. This paper focuses on the development of acoustic diagnostics for on-load tap changers (OLTC). A selector switch-type tap changer was used to detect and characterize different defects that may ultimately lead to a tap changer failure. Among other techniques, acoustic diagnostic was considered first because it is a method which can work online without affecting in any way the normal operation of a power transformer. Special emphasis was dedicated to recreate the arcing conditions in a tap changer. Arcing inside the tap changer chamber is unavoidable, but excessive arcing leads to a multitude of problems, so that efforts have been made to detect acoustically the signature of the arcing phenomenon. In order to have both flexibility and control over the experimental parameters, the tests were conducted on two sets of test objects, i.e. a model and a small-size tap changer. Focus was made to the type of defects that pose the most imminent threat to the tap changer operation, and the most appropriate methods of signal acquisition and processing were singled out. It is shown that these defects can be recognized based on their specific acoustic signature.