ABB REC650 ANSI, Applications Manual

Page: 104 / 370

Im ax
Ipu 1.2
k
³ ×
EQUATION1262 V2 EN
(Equation 29)
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
30 .
Ipu 0.7 Isc min
£ ×
EQUATION1263 V2 EN
(Equation 30)
where:
0.7
is a safety factor
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 Ipu 0.7 Isc min
k
× £ £ ×
EQUATION1264 V2 EN
(Equation 31)
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 511 286-UUS A
Current protection
98
Application manual