source impedance at line end A should be maximized (minimum short circuit power)for this calculation. We get the phase current Ifault7,8,min = 0.46 kATo assure that step 3 detects all short circuits on the adjacent lines out from the localbusbar a phase-phase short circuit is applied at fault points 9 and 10. The sourceimpedance at line end B should be maximized (minimum short circuit power) at thiscalculation. Resulting phase current is Ifault9,10,min = 0.46 kADelay of I> should be set larger than the delay of distance protection zone 2(normally 0.4 s), this means normally at least 0.8 s.Step 3 should be set so that the maximum load current out on the line does not giveunwanted trip.The maximum load current can be estimated as:max,maxmin180 0.843 3 0.9 138= = =× × ×LoadSI kAV2142-ANSI V1 EN (Equation 21)The current setting of step 2 is, if possible, chosen as:)min(7.02.1 min10,9,8,73max,faultstepLoad III ×££×hGUID-C3C741A6-8E7B-4E1E-803A-8BC5CD82831C V1 EN (Equation 22)32210614607.095.08402.1 33 ££×££× stepstep IorIGUID-871F169F-1B5E-43C5-8628-1E010031999B V1 EN (Equation 23)where ƞ is the reset ratio of the overcurrent function. For the overcurrent function inthe IED, ƞ = 0.95.In this case it is not possible to fulfil the requirement above. Priority is given toselectivity, choosing setting 1100 A and accepting that the phase overcurrentprotection cannot act as remote back-up protection for the remote lines.2. Set t3 to 0.8 s3.1.10 Calculating settings for four step residual overcurrentprotection, zero or negative sequence directionEF4PTOC(51N_67N)The residual overcurrent protection is more difficult to set as the ground-fault current ishighly dependent of the network configuration in the power system. In order to achievesetting that assure selective fault clearance a large number of calculations have to be madewith different fault locations, different switching states in the system and differentground-fault types. Below one example of setting of four step residual overcurrentprotection for a line in a meshed solidly grounded system is given.Section 3 1MRK 506 334-UUS AREL650 setting examples70Application manual
