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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.

The paper describes a multi-criteria self-organized fuzzy logic based protective relay for three-phase power transformers. The employed protection criteria and the overall relay organization are presented as well as the novel algorithms for off-line pre-installation self-setting of the relay. These algorithms are based on statistical information obtained by mass-simulation using ATP-EMTP package and enable a kind of learning phase similar to that known from artificial neural network applications to power system protection. The example and results of testing demonstrate high stability and sensitivity of the presented relay which operates with average tripping time below half a cycle. Robustness of the relay has been also confirmed.