ABB Relion 670 Series Applications Manual page 137

1MRK 511 358-UUS A
£
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 31.
Im ax
×
1.2
k
EQUATION1264 V2 EN-US
The high current function of the overcurrent protection, which only has a short delay of the
operation, must be given a current setting so that the protection is selective to other protection in
the power system. It is desirable to have a rapid tripping of faults within as large portion as
possible of the 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. 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
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 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
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.
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
58 shows how the time-versus-current curves are plotted in a diagram. The time
Section 7
Current protection
(Equation 30)
(Equation 31)
(Equation 32)
131