Residual Flux in the Closed Magnetic Core of a Power Transformer

When a transformer is de-energized, the magnetic core material will contain a certain amount of residual magnetic flux. This flux may intensify saturation and lead to high transient in-rush current upon remagnetizing. This paper describes a novel method for analyzing and detecting the residual flux, based on the nonlinear magnetizing characteristics (also Phi-I characteristics) of the core. A model of toroidal transformer core was built to simulate transients, which takes the process of initial energization of the nonloaded transformer core into consideration. The relationship between residual flux density and the magnetizing current was then approximated. Measurements of residual flux density for a simple laboratory setup verified that the simulation gave reasonable values.