Hitachi Relion 670 Series Applications Manual page 123

1MRK511407-UUS Rev. N
³
I 1.2
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 current and can be
t
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
56 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.
tfunc1
tfunc2 n
0.01
IEC05000204 V2 EN-US
Figure 56:
The operation time can be set individually for each overcurrent protection.
To assure selectivity between different protection functions in the radial network, there has to be a
minimum time difference Dt between the time delays of two protections. To determine the shortest
Phasor measurement unit RES670
Application manual
× ×
k I
t sc max
10
n
10
Fault time with maintained selectivity
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Time-current curves
Str
n
Fault Current
Current protection
(Equation 39)
10000
en05000204.ai
Section 8
117