ABB Relion REQ650, Руководство по применению

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Im ax
1.2 Ipu 0.7 Isc min
k
× £ £ ×
EQUATION1264 V2 EN
(Equation 22)
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, Iscmax, 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 I
³ × ×
EQUATION1265 V1 EN
(Equation 23)
where:
• 1.2 is a safety factor
• kt 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
• Iscmax 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 that protected 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. Figure
54 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. Selectivity is assured if the time
difference between the curves is larger than a critical time difference.
Section 6
1MRK 505 291-UEN A
Current protection
110
Application manual