max
1.2
t
sc
high
I
k I
³
× ×
EQUATION1265 V1 EN-US
(Equation 34)
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
I
scmax
is the largest fault current at a fault at the most remote point of the primary protection zone.
The operate times of the phase overcurrent protection has to be chosen so that the fault
time is so short so that equipment will not be destroyed due to thermal overload, at the
same time as 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. In the figure below is shown 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.
Section 7
1MRK 505 370-UUS A
Current protection
194
Busbar protection REB670 2.2 ANSI
Application manual
Содержание RELION REB670
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