Network Functions for Online Frequency-Response Analysis to Detect Winding Deformations in Transformers

Frequency-response analysis (FRA) is widely used as a method for the offline diagnosis of winding deformations in power transformers. To apply this method to a transformer in operation, new network functions must be established. These should be suitable for the excitation and response signals of online measurements, and they should eliminate the influence of external equipment that is directly connected to the high-voltage outgoing lines of the transformers. In this paper, based on circuit network analysis, series of transfer functions for online FRA are proposed. These include comprehensive consideration of the transformers and the external electric power grid, including the connection types of the three-phase windings, whether or not the neutral point is grounded, and the mutual coupling between the high- and low-voltage windings. The suitable conditions, appropriate configurations of application, feasibility, and sensitivity of each network function were analyzed. Among them, four functions involve the parameters of the outside network but are easy to measure. Two of the functions are not affected by the outside network. These network functions will help promote online applications of FRA.