fault current calculation gives the largest current of faults, Iscmax, at the most remote part
of the primary protected zone. The risk of transient overreach must be considered, due to
a possible DC component of the short circuit current. The lowest current setting of the
fastest stage can be written according to
EQUATION1265 V1 EN (Equation 453)
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 1.05
Iscmax is the largest fault current at a fault at the most remote point of the primary protection zone.
The operate time of the phase overcurrent protection has to be chosen so that the fault time
is short enough that the protected equipment will not be destroyed due to thermal overload
while, at the same time, selectivity is assured. For overcurrent protection in a radial fed
network, the time setting can be chosen in a graphical way. This is mostly used in the case
of inverse time overcurrent protection. Figure
258 shows how the time-versus-current
curves are plotted in a diagram. The time setting is chosen to get the shortest fault time
with maintained selectivity. Selectivity is assured if the time difference between the
curves is larger than a critical time difference.
1MRK 506 369-UUS - Section 9
Current protection
Line distance protection REL670 2.2 ANSI 501
Application manual
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