Standard for frequency characteristics measurement at power transformers

Frequency characteristics measurement at the power transformers became a requirement imposed to the manufacturers in the specifications of the new products. There is a great interest dealing with the frequency characteristics from the viewpoint of the resonances that may appear between the transformer and the network where it is placed, as well as to detect the mechanical failures that may appear in service. There are a lot of factors that may influence frequency characteristics reproducibility at power transformers not related to their mechanical condition as: differences in measuring circuit layout, measuring equipment parameters variation a.s.o. Many manufacturers provide measuring systems without providing a way to check whether the differences relate to the transformer or to the measuring circuit. A standard is absolutely necessary to compare the results obtained at long time periods. The paper presents a standard that can be used to check the measuring circuit. It is based on some standard circuits easy to manufacture and having small dimensions that can be used both in laboratory and on-site. The influence of the used measuring method and the influence of cables laying mode on result reproducibility are studied on the basis of the above standard.