ABB REB670 Applications Manual page 176

Section 7
Current protection
170
Im ax
³ ×
Ipu 1.2
k
EQUATION1262 V2 EN
where:
1.2 is a safety factor
k is the resetting ratio of the protection
Imax is the maximum load current
From operation statistics the load current up to the present situation can be found. The
current setting must be valid also for some years ahead. It is, in most cases, realistic that
the setting values are updated not more often than once every five years. In many cases this
time interval is still longer. Investigate the maximum load current that different equipment
on the line can withstand. Study components such as line conductors, current
transformers, circuit breakers, and disconnectors. The manufacturer of the equipment
normally gives the maximum thermal load current of the equipment.
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 29.
£ ×
Ipu 0.7 Isc min
EQUATION1263 V2 EN
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 30.
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
1MRK 505 337-UUS -
(Equation 28)
(Equation 29)
(Equation 30)
Application manual