ABB REB670 Applications Manual page 177

1MRK 505 337-UUS -
Application manual
(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
³ × ×
I 1.2 k I
t sc max
high
EQUATION1265 V1 EN
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 and
t
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 times of the phase overcurrent protection has to be chosen so that the fault
time is so short that protected 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. Figure
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.
80 shows how the time-versus-current curves are
Section 7
Current protection
(Equation 31)
171