ABB Relion 670 Series Applications Manual page 161

1MRK 505 370-UUS Rev. K
£
Ipu 0.7 Isc min
EQUATION1263 V2 EN-US
where:
0.7
Iscmin
As a summary, the pickup current shall be chosen within the interval stated in Equation 30.
Im ax
×
1.2
k
EQUATION1264 V2 EN-US
The high current function of the overcurrent protection, which only has a short-delay trip time, must be
given a current setting so that the protection is selective to other protection functions in the power
system. It is desirable to have rapid tripping of faults within a large 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. 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
high
EQUATION1265 V1 EN-US
where:
1.2
k
t
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
90 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.
Busbar protection REB670
Application manual
×
is a safety factor
is the smallest fault current to be detected by the overcurrent protection.
£ £ ×
Ipu 0.7 Isc min
× ×
k I
t sc max
is a safety factor
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
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Section 7
Current protection
(Equation 29)
(Equation 30)
(Equation 31)
155