Toshiba GR-200-5 Instruction Manual

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6F2T0207 (0.01)GRE200 (5,6)- 252 -2.22.8 Application guidelinesThe application of settings in the ARC function requires careful consideration of the followingfactors:Frequency and nature of faultsThe frequency and nature of faults on the distribution line help to decide upon the number ofARC shots desired.If the majority of the faults are transient, (e.g. due to lightning), then applying a single-shot scheme would be appropriate.If a significant percentage of faults on the distribution line are semi-permanent (e.g. dueto foreign objects like a small tree branch or small animal that may have fallen across the line),then adopting a multi-shot scheme becomes advantageous.If transient or semi-permanent faults are rare (e.g. overhead distribution line in urbanareas or a feeder that has a significant portion of cable), then non-use of the ARC function willbe appropriate.CB characteristicsThe capability of a CB for ARC duty must be checked. The CB characteristics such as thenumber of successive open-close (OC) duty cycles and the dead time required between an open-close operation will determine the number of ARC shots employed and the minimum dead timebefore an ARC shot can be given.De-ionization timeThe time taken for de-ionization of the fault arc path is an important consideration fordetermining the dead time before an ARC shot can be given. The de-ionization period dependsupon the system voltage, the fault current and the duration of the fault. Capacitive couplingbetween the unfaulted phase and the faulted phase will increase the de-ionizing time; however,this is not applicable for a relay for distribution systems, because tripping is always three-phase. A de-ionization time of at least 100ms may be considered for distribution voltages (66KVand below).System stabilityThe intrinsic stability of an interconnected power system will determine the maximum deadtime allowable before the ARC shot must be issued. This may not apply to conventionaldistribution systems where the generating source is remote from the load centers. However,with distributed generation located closer to load centers, stability becomes an importantconsideration. Transient stability studies can establish the maximum dead time allowed for