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7UM62 Manual
C53000-G1176-C149-3
4.0
2.0
1.0
0.5
0.2
0.1
Figure 4-10
Restraining Influence of Higher-Order Harmonics
I
Xf
12
10
5
3
2
1.0
0.5
Setting value e.g. 0.15
0.3
I
DIFF
0.2
0.15
Blocking
0.05
10
20
Figure 4-11
Influence of Frequency in Transformer Differential Protection
4.8 Differential Protection for Transformers (ANSI 87T)
I
fN
I
N
Tripping
Can be set to
e.g. 5th harmonic = 40 %
Can be set to e.g.
I
>/I N = 0.2
DIFF
0
0.1
0.2
0.3
(settable)
I
>>/I
N
DIFF
Setting value e.g. 5.0
Tripping
>/I
(settable)
N
30
40
50
Legend:
I
Differential current = | I
DIFF
I
Current at system frequency
fN
I
Current at any frequency
Xf
within specified range
Can be set to e.g.
I
max n/I N = 4
DIFF
Blocking
I
5f
0.4
0.5
I
fN
Blocking
(Blocking by
2nd harmonics)
60
70
80
+ I
|
1
2
f/Hz
421
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