ABB RELION 670 Series Applications Manual page 165

1MRK 505 337-UUS A
cause protection operation. Here consideration also has to be taken to the protection reset
current, so that a short peak of overcurrent does not cause operation of the protection even when
the overcurrent has ceased. This phenomenon is described in figure 82.
Current I
IEC05000203 V3 EN-US
Figure 82: Pick up and reset current for an overcurrent protection
The lowest setting value can be written according to equation 34.
³
Ipu 1.2
EQUATION1262 V2 EN-US
where:
1.2
k
I
max
The maximum load current on the line has to be estimated. 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.
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, and
is the maximum load current.
Line phase current
Time t
IEC05000203-en-2.vsd
Section 7
Current protection
(Equation 34)
159