Development and hardware implementation of a compensating algorithm for the secondary current of current transformers

The conventional method of dealing with current-transformer (CT) saturation is overdimensioning of the core so that CTs can reproduce up to 20 times the rated current without exceeding 10% ratio correction. However, this not only reduces the sensitivity of relays, as some errors may still be present in the secondary current when a severe fault occurs, but also increases the CT size. An algorithm is described for estimating the secondary current corresponding to the CT ratio under CT saturation using the fluxkurrent (hli) curve, and the results of hardware implementation of the algorithm using a digital signal processor are also presented. The main advantage of the algorithm is that it can improve the sensitivity of relays to low-level internal faults, maximise the stability of relays for external faults, and reduce the required CT-core cross-section significantly.