Current related relaying algorithms immune to saturation of current transformers

Current transformers (CTs) supplying protective relays may suffer permanent or transient saturation under severe faults. The majority of protective relays are more or less sensitive to CT saturation. On the other hand, contemporary microprocessor-based relays open the window of opportunity for performing sophisticated checking in order to either reconstruct the secondary current waveform or compensate the measuring algorithms for saturation of CTs. This paper describes algorithms for amplitude and impedance components, resistive to transient saturation of CTs. The presented algorithms combine two procedures: (1) recognition of periods of saturated/linear operation of an observed CT; and (2) variable-length data window measuring algorithms that use unsaturated samples of the secondary current.