The maximum load current on the line has to be estimated. There is also a demand that
all faults, within the zone that the protection shall cover, must be detected by the phase
overcurrent protection. The minimum fault current Iscmin, to be detected by the
protection, must be calculated. Taking this value as a base, the highest pick up current
setting can be written according to equation
342.
EQUATION1263 V2 EN (Equation 342)
where:
0.7 is a safety factor
Iscmin is the smallest fault current to be detected by the overcurrent protection.
As a summary the pickup current shall be chosen within the interval stated in
equation 343.
Imax
1.2 Ipu 0.7 Iscmin
k
× £ £ ×
EQUATION1264 V2 EN (Equation 343)
The high current function of the overcurrent protection, which only has a short delay of
the operation, must be given a current setting so that the protection is selective to other
protection in the power system. It is desirable to have a rapid tripping of faults within
as large portion as possible of the part of the power system to be protected by the
protection (primary protected zone). A fault current calculation gives the largest
current of faults, Iscmax, at the most remote part of the primary protected zone.
Considerations have to be made to the risk of transient overreach, due to a possible DC
component of the short circuit current. The lowest current setting of the most rapid
stage, of the phase overcurrent protection, can be written according to
EQUATION1265 V1 EN (Equation 344)
where:
1.2 is a safety factor,
k
t
is a factor that takes care of the transient overreach due to the DC component of the fault current
and can be considered to be less than 1.1
Iscmax is the largest fault current at a fault at the most remote point of the primary protection zone.
Section 3 1MRK504116-UUS C
IED application
410
Application manual
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