A directional element

摘要

A directional element (40) which produces a quantity zeta o related to the zero sequence source impedance of a power system relative to the location of the directional element (40), from values of zero sequence current (IR) and an adjusted value of zero sequence voltage (3Vo). The impedance quantity ( zeta o) is then compared against first and second threshold quantities (ZOF and ZOR) to identify the direction of a fault. Further, the zero sequence voltage-polarized directional element can be combined with other individual directional elements, induding a zero sequence current-polarized directional element and a negative sequence voltage-polarized directional element to form a universal directional element for unbalanced faults, in which the individual directional elements operate in a particular sequence, with the other directional elements being blocked from operating if a prior directional element in the sequence provides a direction indication. <IMAGE>