ABB REB670 Applications Manual page 190

Section 7
Current protection
184
protected zone). A 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
³ × ×
I 1.2 k I
t sc max
high
EQUATION1265 V1 EN-US
where:
1.2 is a safety factor
k is a factor that takes care of the transient overreach due to the DC component of the fault current
t
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
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 505 370-UUS A
(Equation 28)
90 shows how the time-
Busbar protection REB670 2.2 ANSI
Application manual