ABB REB670 Series, Applications Manual

Page: 162 / 476

The pick up current setting inverse time protection or the lowest current step constant
inverse time protection must be given a current setting 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 91 .
Operate current
Current (A)
The IED does not reset
Line phase current
Time (s)
Reset current
IEC05000203-en-2.vsd
IEC05000203 V4 EN-US
Figure 91: Pick up and reset current for an overcurrent protection
The lowest setting value can be written according to equation 31 .
Im ax
Ipu 1.2
k
³ ×
EQUATION1262 V2 EN-US
(Equation 31)
where:
1.2 is a safety factor,
k is the resetting ratio of the protection, and
I
max
is the maximum load current.
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.
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 I
scmin
, to be detected
Section 7 1MRK 505 370-UEN D
Current protection
156 Busbar protection REB670
Application manual