ABB RELION RET670 Applications Manual page 533

1MRK 504 163-UUS A
Transformer protection RET670 2.2 ANSI
Application manual
Im ax
³ ×
Ipu 1.2
k
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 423.
£ ×
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 424.
Im ax
× £ £ ×
1.2 Ipu 0.7 Isc min
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
Section 9
Current protection
(Equation 422)
(Equation 423)
(Equation 424)
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