The relay includes means for obtaining currentand voltage samples on the power transmission line fora particular possible fault type. The current sample ismultiplied by the replica impedance of the powertransmission line, while the voltage sample ismultiplied by a polarizing voltage to provide a firstcomplex expression having a real portion and animaginary portion. The product of the current sampleand the replica impedance is multiplied by thepolarizing voltage to produce a second complexexpression having a real portion and an imaginaryportion. The real portion of the first complexexpression is then divided by the real portion of thesecond complex expression to produce a scale value mwhich is compared against known reference scale valuesfor each zone, so as to determine underimpedanceconditions in any one of the zones.
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