ABB REB670 Applications Manual page 186

Section 7
Current protection
180
disconnectors. The manufacturer of the equipment normally gives the maximum thermal
load current of the equipment.
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 I
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 35.
£ ×
Ipu 0.7 Isc min
EQUATION1263 V2 EN
where:
0.7 is a safety factor and
I is the smallest fault current to be detected by the overcurrent protection.
scmin
As a summary the pick up current shall be chosen within the interval stated in
equation 36.
Im ax
× £ £ ×
1.2 Ipu 0.7 Isc min
k
EQUATION1264 V2 EN
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,
I , at the most remote part of the primary protected zone. Considerations have to be
scmax
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 k I
t sc max
high
EQUATION1265 V1 EN
where:
1.2 is a safety factor,
k is a factor that takes care of the transient overreach due to the DC component of the fault current and
t
can be considered to be less than 1.1
I is the largest fault current at a fault at the most remote point of the primary protection zone.
scmax
1MRK 505 337-UUS -
, to be
scmin
(Equation 35)
(Equation 36)
(Equation 37)
Application manual