ABB Relion REQ650, Руководство по применению

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max
1.2
t sc
high
I k I
³ × ×
EQUATION1265 V1 EN
(Equation 18)
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
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
49 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.
en05000204.wmf
IEC05000204 V1 EN
Figure 49: Fault time with maintained selectivity
1MRK 505 291-UEN A
Section 6
Current protection
101
Application manual