High-Frequency AC Resistance and its Application in Material Identification for Transformer Windings

Due to the skin effect, the ac resistance of a metallic conductor increases with increasing frequency [1]. The skin effect is strongly related to the metal material of the conductor; therefore, it can be used to identify the material of the conductor by measuring its high-frequency ac resistance [2]–[5]. Transformers are key equipment used in the power grid. In particular, distribution transformers are used widely in the distribution power network, and their performance will have an important impact on the stable operation of the power grid. A transformer is composed of an iron core, windings, insulation oil, etc. The cost of the winding material accounts for approximately 30–40% of the total cost of manufacturing the transformer [6]. Because aluminum is so much less expensive than copper, some unscrupulous manufacturers make aluminum transformers but sell them as copper transformers. These transformers are called ‘aluminum posing as copper’ transformers and they have currently been found. in high voltage (HV) windings, low voltage (LV) windings, or both windings of the transformers.