Zero sequence voltage-polarized directional element for protective relays

A directional element for use in a relay for protection of power systems, comprising:

• means for obtaining zero sequence voltage and current values for a power signal on a power system having known values of zero sequence local source impedance, zero sequence line impedance and zero sequence remote source impedance;
• means for calculating a value representative of a zero sequence impedance for the power system, in accordance with the following formula:
• ##EQU3## wherein I.sub.R =I.sub.A +I.sub.B +I.sub.C
• V.sub.O =V.sub.A +V.sub.B +B.sub.C
• and
• .theta..sub.LO =zero sequence line impedance angle
• means for establishing first and second threshold quantities which are within an impedance plane region bounded by (1) the zero sequence line impedance plus the zero sequence remote source impedance and (2) the negative zero sequence local source impedance, and wherein the first threshold quantity is more positive than the negative zero sequence local source impedance and less positive than the second threshold minus 0.1 ohms and wherein the second threshold is less positive than the zero sequence line impedance plus the zero sequence remote source impedance and more positive than the first threshold quantity plus 0.1 ohms;
• and
• means for comparing said calculated value against the first and second threshold quantities to identify the direction of a fault relative to the relay, the first threshold quantity being for a forward fault and the second threshold quantity being for a reverse fault.