ABB Relion 650 Series Applications Manual page 82

Section 7
Current protection
The pickup current setting of the inverse time protection, or the lowest current step of the
definite time protection, must be defined so that the highest possible load current does not 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 30.
Current I
Reset current
ANSI09000146 V1 EN-US
Figure 30: Pickup and reset current for an overcurrent protection
The lowest setting value can be written according to equation 11.
³
Ipu 1.2
EQUATION1262 V2 EN-US
where:
1.2
k
Imax
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 12.
76
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
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Line phase current
Time t
ANSI09000146-en-1.vsd
1MRK 505 287-UUS B
(Equation 11)
Busbar protection REB650
Application manual