Use of a Sampling Scheme to Detect Transient Saturation in Protective Current Transformers

Current-transformer (CT) saturation due to primary dc transient offsets presents a problem with regard to fast and predictable operation of protective relays. In this paper, a method is devised and tested for detecting the presence of saturation by sampling the output of the current transformer every millisecond. Successive sampled values are compared and a large difference between these values is used to indicate that saturation has occurred. Results are given that show the circuit operation when actually connected to a CT that is subjected to a transient primary current. Finally, some possible applications of the saturation detector in protection and other areas are suggested.