ABB Relion REQ650, Applications Manual

Page: 106 / 340

Im ax
Ipu 1.2
k
³ ×
EQUATION1262 V2 EN
(Equation 15)
where:
1.2
is a safety factor
k is the resetting ratio of the protection
Imax is the maximum load current
The maximum load current on the line has to be estimated. There is also a demand that
all faults, within the zone that the protection shall cover, must be detected by the phase
overcurrent protection. The minimum fault current Iscmin, to be detected by the
protection, must be calculated. Taking this value as a base, the highest pick up current
setting can be written according to equation
16 .
Ipu 0.7 Isc min
£ ×
EQUATION1263 V2 EN
(Equation 16)
where:
0.7
is a safety factor
Iscmin is the smallest fault current to be detected by the overcurrent protection.
As a summary the operating current shall be chosen within the interval stated in
equation
17 .
Im ax
1.2 Ipu 0.7 Isc min
k
× £ £ ×
EQUATION1264 V2 EN
(Equation 17)
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
Section 6
1MRK 505 291-UEN A
Current protection
100
Application manual