Hitachi Relion 670 Series Applications Manual page 122

Section 8
Current protection
The lowest setting value can be written according to Equation 36.
³
Ipu 1.2
EQUATION1262 V2 EN-US
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 37.
£
Ipu 0.7 Isc min
EQUATION1263 V2 EN-US
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 38.
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
116
Im ax
×
k
×
£ £ ×
Ipu 0.7 Isc min
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1MRK511407-UUS Rev. N
(Equation 36)
(Equation 37)
(Equation 38)
Phasor measurement unit RES670
Application manual