ABB Relion 670 Series Applications Manual page 143

1MRK 511 401-UEN Rev. K
³
Ipu 1.2
EQUATION1262 V2 EN-US
where:
1.2
k
Imax
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 32.
£
Ipu 0.7 Isc min
EQUATION1263 V2 EN-US
where:
0.7
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 33.
Im ax
×
1.2
k
EQUATION1264 V2 EN-US
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
Bay control REC670
Application manual
Im ax
×
k
is a safety factor
is the reset ratio of the protection
is the maximum load current
×
is a safety factor
£ £ ×
Ipu 0.7 Isc min
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Section 8
Current protection
(Equation 31)
(Equation 32)
(Equation 33)
137