I.L. 40-386.42-12. 1 INTRODUCTIONBoth the REL301/302 relay systems detect faults in three zones of phase and ground distance.Zones 1 and 2 are forward set, Zone-3 can be set forward or reverse. REL302 has a separatepilot zone (see Section 2-5).The R-X Diagram, Figure 2-1, shows a composite of characteristics available with REL301/302.Zone-1 phase and ground settings are chosen to provide substantial coverage of the protectedline without overreaching the next bus. A setting of 80% of the line impedance is recommended.Faults occurring within the reach of the Zone-1 measurement cause direct tripping without re-gard to any action occurring at the remote terminal.Zone-2 settings are chosen to assure that faults occurring on the next bus are detected. Settingsare chosen (independent of the Zone-1 settings), generally to be 120 to 150% of the line imped-ance. Any fault occurring on the protected line will be detected by this Zone-2 measurement(within the fault resistance and current limitations of the relaying system settings). Zone-2 trip-ping occurs after a time delay of T2 Definite Time or T2 Torque Control Overcurrent Time, de-pendent on setting choice.The Zone-3 measurement has a directional setting choice, and may be chosen to respond toforward or reverse faults. The reverse sensing option is used in conjunction with the T3 time de-lay, chosen to coordinate with adjacent terminal(s) Zone-2 timing. The forward sensing optionproduces time delayed backup to other devices sensing forward faults.Blinder measurements (B1/B4, B2/B3) are available for out-of-step blocking. The inner blindersalso restrict the trip reach of all of the 3-phase fault measuring units (load restriction).2. 2 LINE MEASUREMENT TECHNIQUESLine measurement techniques applied to each zone include:• Single-Phase-To-Ground fault detection• 3-Phase fault detection• Phase-to-Phase fault detection• Phase-to-Phase-to-Ground fault detection2.2.1 Single-Phase-to-Ground FaultSingle-phase-to-ground (ØG) fault detection (Figure 2-2) is accomplished by 3 quadrature po-larized phase units (ph-A, ph-B, ph-C). Equations 1 and 2 are for operating and reference quan-tity, respectively. The unit will produce output when the operating quantity leads the referencequantity.(1)(2)V XG I XZ 0L Z 1L–Z 1L------------------------ I 0+ Z CG–V QSection 2. FUNCTIONAL DESCRIPTION
