ABB Relion 670 Series Applications Manual page 136

Section 7
Current protection
Current I
Reset current
ANSI09000146 V1 EN-US
Figure 57: Pickup 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.
130
Pickup 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
ANSI09000146-en-1.vsd
1MRK 511 358-UUS A
(Equation 29)
Application manual