ABB REB670 Series, Руководство по применению

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Im ax
Ipu 1.2
k
³ ×
EQUATION1262 V2 EN-US
(Equation 25)
where:
1.2 is a safety factor
k is the reset ratio of the protection
Imax is the maximum load current
The load current up to the present situation can be found from operation statistics. The
current setting must remain valid for several years. In most cases, the setting values are
updated once every five years or less often. Investigate the maximum load current that the
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 pickup current setting can be
written according to Equation 26 .
Ipu 0.7 Isc min
£ ×
EQUATION1263 V2 EN-US
(Equation 26)
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 27 .
Im ax
1.2 Ipu 0.7 Isc min
k
× £ £ ×
EQUATION1264 V2 EN-US
(Equation 27)
The high current function of the overcurrent protection, which only has a short-delay trip time,
must be given a current setting so that the protection is selective to other protection
functions in the power system. It is desirable to have rapid tripping of faults within a large 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. The risk of transient overreach must be considered, due to a possible
DC component of the short circuit current. The lowest current setting of the fastest stage can
be written according to
1MRK 505 370-UEN D Section 7
Current protection
Busbar protection REB670 149
Application manual