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Settings for OTH, C0, C1 and C2 for Tapped Load Applications
The relay setting OTH and the sequence filter constants, C1, C2 and C0, should be first selected according to normal
conditions following the recommendations. Then, the relay setting and the selected constants need to be checked to
make sure they satisfy the special requirements for tap load applications:
Following the procedure for the general case, the following settings were determined:
C1 = 0.1
C2 = 0.7
C0 = 0
OTH = 0.56 A
i.e. use criterion (2)
=
·
C
0
if
2
V
⋅
⋅
>
LN
0
3 .
C
OTH
⋅
⋅
1
2
Z
R
C
≠
·
C
0
if
2
ο
⋅
+
−
⋅
60
j
0
3 .
C
C
e
⋅
1
2
2
where
V
is the line-to-neutral voltage, in primary Volts
LN
R
is the CT ratio,
c
Z is the total equivalent system impedance looking back into the system from the transformer bank, in high side ohms
Z
is the transformer impedance, in high side ohms
T
ο
−
⋅
+
⋅
j
60
0
3 .
C
C
e
⋅
1
2
2
−
°
⋅
+
⋅
j
60
0
3 .
0
1 .
0
7 .
e
2
⋅
+
°
⋅
j
120
0
3 .
0
1 .
0
7 .
e
2
Thus the settings established for the general case will be valid also for this tapped load application example.
Settings and Application
V
>
⋅
⋅
LN
0
3 .
C
+
1
Z
Z
T
V
>
>
⋅
LN
OTH
0
3 .
⋅
Z
R
C
V
>
>
⋅
LN
0
3 .
OTH
⋅
Z
R
C
138000
=
⋅
0
3 .
⋅
⋅
⋅
. 4
08
240
3
138000
⋅
+
⋅
⋅
(
. 4
08
66
.
65
)
240
ABB REL 356 Current Differential Protection
⋅
R
C
V
ο
+
⋅
120
j
C
C
e
⋅
+
1
2
2
Z
V
ο
+
⋅
j
120
C
C
e
⋅
+
1
2
2
Z
⋅
=
>
. 0
76
40
.
41
. 9
21
OTH
=
⋅
⋅
=
0
3 .
. 0
66
. 2
34
3
(1)
LN
⋅
(2)
Z
R
T
C
⇒
LN
⋅
Z
R
T
C
<
. 0
46
OTH
3-35
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