ABB Relion REC670 Applications Manual page 117

1MRK 511 358-UEN A
Current I
IEC05000203 V3 EN-US
Figure 45: Operate and reset current for an overcurrent protection
The lowest setting value can be written according to equation 29.
³
Ipu 1.2
EQUATION1262 V2 EN-US
where:
1.2
k
Imax
From operation statistics the load current up to the present situation can be found. The
current setting must be valid also for some years ahead. It is, in most cases, realistic that the
setting values are 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.
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 pick up current setting can be
written according to equation 30.
Application manual
Operate current
Reset current
The IED does not reset
Im ax
×
k
is a safety factor
is the resetting ratio of the protection
is the maximum load current
Line phase current
Time t
IEC05000203-en-2.vsd
Section 7
Current protection
(Equation 29)
111