source impedance at line end A should be maximized (minimum short circuit power)
for this calculation. We get the phase current If
ault7,8,min
= 0.46 kA
To assure that step 3 detects all short circuits on the adjacent lines out from the local
busbar a phase-phase short circuit is applied at fault points 9 and 10. The source
impedance at line end B should be maximized (minimum short circuit power) at this
calculation. Resulting phase current is I
fault9,10,min
= 0.46 kA
Delay 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 give
unwanted trip.
The maximum load current can be estimated as:
max
,max
min
180
0.84
3 3 0.9 138
= = =
× × ×
Load
S
I kA
V
2142-ANSI V1 EN (Equation 21)
The current setting of step 2 is, if possible, chosen as:
)min(7.02.1
min10,9,8,73
max,
faultstep
Load
II
I
×££×
h
GUID-C3C741A6-8E7B-4E1E-803A-8BC5CD82831C V1 EN (Equation 22)
32210614607.0
95.0
840
2.1
33
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stepstep
IorI
GUID-871F169F-1B5E-43C5-8628-1E010031999B V1 EN (Equation 23)
where ƞ is the reset ratio of the overcurrent function. For the overcurrent function in
the IED, ƞ = 0.95.
In this case it is not possible to fulfil the requirement above. Priority is given to
selectivity, choosing setting 1100 A and accepting that the phase overcurrent
protection cannot act as remote back-up protection for the remote lines.
2. Set t3 to 0.8 s
3.1.10 Calculating settings for four step residual overcurrent
protection, zero or negative sequence direction
EF4PTOC(51N_67N)
The residual overcurrent protection is more difficult to set as the ground-fault current is
highly dependent of the network configuration in the power system. In order to achieve
setting that assure selective fault clearance a large number of calculations have to be made
with different fault locations, different switching states in the system and different
ground-fault types. Below one example of setting of four step residual overcurrent
protection for a line in a meshed solidly grounded system is given.
Section 3 1MRK 506 334-UUS A
REL650 setting examples
70
Application manual
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