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6 F 2 S 0 8 5 7
(Example)
Primar
y
Secondar
y
I
N
I
01
1REF
CT ratio: 2400/1
kct1=2
kct2=3
CT ratio: 3600/1
I
02
CT ratio:
1200/1
max-kct = 3
Therefore, REF is blocked at
I
BLK
=3×2.0=6A (CT secondary)
If the maximum of phase
currents of both primary and
secondary windings is 7200A
(=6x1200A), the REF is blocked.
Reference current: 1200A
(Rated current of neutral CT)
2.10.4 Inverse Time Overcurrent Element OCI and EFI
The OCI and EFI elements have one long time inverse characteristic and three inverse time
characteristics in conformity with IEC 60255-3 as shown in Figure 2.10.4. One of these
characteristics can be selected.
These characteristics are expressed by the following equations and curves.
1
0.1
0.5
1
5
10
50
T=1.0
100
Standard Inverse
Very Inverse
Extremely Invease
5 10 20 30
200
0.2
2
20
Long-time Inverse
2
Current I (Multiple of setting current)
(s)
Operating
time
Long Time Inverse
t = T
×
120
(I/Is)−1
Standard Inverse
t = T
×
0.14
(I/Is)
0.02
− 1
Very Inverse
t = T
×
13.5
(I/Is)
− 1
Extremely Inverse
t = T
×
80
(I/Is)
2
− 1
where,
t : operating time
I : fault current
Is : current setting
T : time multiplier setting
Figure 2.10.4 Characteristics of Inverse Time Overcurrent Element