This article is intended to familiarise readers with frequency response of stray losses (FRSL) testing for transformers - an invaluable test technique that is rapidly gaining recognition in the industry because of its ability to reveal problems that would be missed by other electrical test methods.
FRSL as a test tool for transformers was first discussed and investigated by Hydro Quebec in the 1970’s because of its ability to detect winding deformation. Hydro Quebec concurrently examined the use of the sweep frequency response analysis (SFRA) method and leakage reactance testing for the same purpose, and ultimately SFRA, together with leakage reactance testing, became the accepted tools for confirming winding deformation.
For some time, FRSL went largely unused. That is, until its seemingly latent diagnostic strengths were exposed. Of particular note was the discovery that FRSL was very useful for revealing short-circuits between individual strands within a conductor bundle. This is a failure mode that, until the advent of FRSL, had been undetectable with electrical test methods.
A conductor bundle may be comprised of any number of individually insulated conductor strands. When two or more of these strands are shorted together, this is not a turn-to-turn fault or even a partial turn-to-turn short-circuit . The latter is a situation when one or more strands within a conductor bundle become short-circuited to one or more strands within an adjacent turn of the conductor bundle. The exciting current test, for example, can reveal a partial turn-to-turn short circuit but is not sensitive to a strand-to-strand short circuit.