ABB REB670 Series - Page 163

by the protection, must be calculated. Taking this value as a base, the highest pick up current
setting can be written according to equation 32.
Ipu 0.7 Isc min£ ×
EQUATION1263 V2 EN-US (Equation 32)
where:
0.7 is a safety factor and
I
scmin
is the smallest fault current to be detected by the overcurrent protection.
As a summary the pick up current shall be chosen within the interval stated in equation 33.
Imax
1.2 Ipu 0.7 Isc min
k
× £ £ ×
EQUATION1264 V2 EN-US (Equation 33)
The
high current function of the overcurrent protection, which only has a short delay of the
operation, must be given a current setting so that the protection is selective to other
protection in the power system. It is desirable to have a rapid tripping of faults within as large
portion as possible of the part of the power system to be protected by the protection (primary
protected zone). A fault current calculation gives the largest current of faults, I
scmax
, at the
most remote part of the primary protected zone. Considerations have to be made to the risk
of transient overreach, due to a possible DC component of the short circuit current. The lowest
current setting of the most rapid stage, of the phase overcurrent protection, can be written
according to
max
1.2
t sc
high
I k I³ × ×
EQUATION1265 V1 EN-US (Equation 34)
where:
1.2 is a safety factor,
k
t
is a factor that takes care of the transient overreach due to the DC component of the fault current
and can be considered to be less than 1.1
I
scmax
is the largest fault current at a fault at the most remote point of the primary protection zone.
The operate times of the phase overcurrent protection has to be chosen so that the fault time
is so short so that equipment will not be destroyed due to thermal overload, at the same time
as selectivity is assured. For overcurrent protection, in a radial fed network, the time setting
can be chosen in a graphical way. This is mostly used in the case of inverse time overcurrent
protection. In the figure below is shown how the time-versus-current curves are plotted in a
diagram. The time setting is chosen to get the shortest fault time with maintained selectivity.
Selectivity is assured if the time difference between the curves is larger than a critical time
difference.
1MRK 505 370-UEN D Section 7
Current protection
Busbar protection REB670 157
Application manual