ABB Relion 670 Series Applications Manual page 147

1MRK 511 401-UUS F
£
Ipu 0.7 Isc min
EQUATION1263 V2 EN-US
where:
0.7
Iscmin is the smallest fault current to be detected by the overcurrent protection.
As a summary, the pickup current shall be chosen within the interval stated in Equation 33.
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
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.
Bay control REC670
Application manual
×
is a safety factor
£ £ ×
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
63
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shows how the time-versus-current curves are plotted in a
Section 8
Current protection
(Equation 32)
(Equation 33)
(Equation 34)
141