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