ABB RELION REB670, Руководство по применению

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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.
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
scmin
, 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 32 .
Ipu 0.7 Isc min
£ ×
EQUATION1263 V2 EN-US
(Equation 32)
where:
0.7
is a safety factor and
I
scmin
is the smallest fault current to be detected by the overcurrent protection.
As a summary the pick up current shall be chosen within the interval stated in
equation
33 .
Im ax
1.2 Ipu 0.7 Isc min
k
× £ £ ×
EQUATION1264 V2 EN-US
(Equation 33)
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
scmax
, 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
1MRK 505 370-UUS A
Section 7
Current protection
Busbar protection REB670 2.2 ANSI 193
Application manual