Toshiba GRD140 Instruction Manual

⎯ 15 ⎯6 F 2 S 0 7 5 82.1.1.3 Instantaneous Overcurrent ProtectionIn conjunction with inverse time overcurrent protection, additional overcurrent elements provideinstantaneous or definite time overcurrent protection.OC1 to OC4 and EF1 to EF4 are phase fault and earth fault protection elements, respectively. Eachelement is programmable for instantaneous or definite time delayed operation. (In case ofinstantaneous operation, the delayed pick-up timer is set to 0.00.) The phase fault elements operateon a phase segregated basis, although tripping is for three phase only.Selective Instantaneous Overcurrent ProtectionWhen they are applied to radial networks with several feeder sections where ZL (impedance of theprotected line) is large enough compared with ZS (the impedance between the relay and the powersource), and the magnitude of the fault current for a local end fault is much greater (3 times ormore, or (ZL+ZS)/ZS≧3, for example) than that for a remote end fault under the condition that ZSis maximum, the pick-up current can be set sufficiently high so that the operating zone of theelements do not reach the remote end of the feeder, and thus instantaneous and selective protectioncan be applied.This high-set overcurrent protection is applicable and effective particularly for feeders near thepower source where the setting is feasible, but the longest tripping times would otherwise have tobe accepted.As long as the associated inverse time overcurrent protection is correctly coordinated, theinstantaneous protection does not require setting coordination with the downstream section.Figure 2.1.6 shows operating times for instantaneous overcurrent protection in conjunction withinverse time overcurrent protection. The shaded area shows the reduction in operating time byapplying the instantaneous overcurrent protection. The instantaneous protection zone decreases asZS increases.TCTCA B COperate timeFigure 2.1.6 Conjunction of Inverse and Instantaneous Overcurrent ProtectionThe current setting is set 1.3 to 1.5 times higher than the probable maximum fault current in theevent of a fault at the remote end. The maximum fault current for elements OC1 to OC4 isobtained in case of three-phase faults, while the maximum fault current for elements EF1 to EF4 isobtained in the event of single phase earth faults.2.1.1.4 Staged Definite Time Overcurrent ProtectionWhen applying inverse time overcurrent protection for a feeder system as shown in Figure 2.1.7,well coordinated protection with the fuses in branch circuit faults and high-speed protection forthe feeder faults can be provided by adding staged definite time overcurrent protection withtime-graded OC2 and OC3 or EF2 and EF3 elements.