ABB RET650 Applications Manual page 120

Section 7
Current protection
114
the protection must be calculated. Taking this value as a base, the highest pickup
current setting can be written according to Equation 22.
£ ×
Ipu 0.7 Isc min
EQUATION1263 V2 EN-US
where:
0.7 is a safety factor
Iscmin is the smallest fault current to be detected by the overcurrent protection.
As a summary, the operating current shall be chosen within the interval stated in
Equation 23.
Im ax
× £ £ ×
1.2 Ipu 0.7 Isc min
k
EQUATION1264 V2 EN-US
The high current function of the overcurrent protection, which only has a short-
delay trip time, must be given a current setting so that the protection is selective to
other protection functions in the power system. It is desirable to have rapid tripping
of faults within a large part of the power system to be protected by the protection
(primary protected zone). A fault current calculation gives the largest current of
faults, Iscmax, at the most remote part of the primary protected zone. The risk of
transient overreach must be considered, due to a possible DC component of the
short circuit current. The lowest current setting of the fastest stage can be written
according to
³ × ×
I 1.2 k I
t sc max
high
EQUATION1265 V1 EN-US
where:
1.2 is a safety factor
k is a factor that takes care of the transient overreach due to the DC component of the fault
t
current and can be considered to be less than 1.05
Iscmax is the largest fault current at a fault at the most remote point of the primary protection zone.
The operate time of the phase overcurrent protection has to be chosen so that the
fault time is short enough that the protected equipment will not be destroyed due to
thermal overload while, at the same time, 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. Figure
46 shows 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.
1MRK 504 169-UEN A
(Equation 22)
(Equation 23)
(Equation 24)
Transformer protection RET650 2.2 IEC
Application manual