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4
Transformer Differential Protection (87T and 87HS)
The settings in Addresses 1106, 1126, and 1146 tell
the relay how to process the zero sequence current of
each winding. These addresses only affect the
transformer differential protection functions
(87T and 87HS).
1106 Io1PROCESS (for Winding 1)
1126 Io2PROCESS (for winding 2)
1146 Io3PROCESS (for winding 3, 7UT513 only)
How to process the zero sequence current for each
transformer winding when calculating the
-
matched
currents for the transformer differential protection
functions (87T and 87HS).
Option
Description
-
Io
ELIMINATION
The winding has a ground connection
within the protected zone that is not
fitted with a ground
all three phase currents, the relay
calculates the zero sequence current,
then eliminates it from each phase
current when calculating the matched
phase currents (default).
-
Io
CORRECTION
The winding has a ground connection
(7UT513 only)
within the protected zone that is fitted
with a ground
to the relay. The measured zero
sequence current is used to correct
each phase current when calculating
the matched phase currents.
WITHOUT
The winding has no ground
connection within the protected zone.
The following mathematical model, Section 4.3.6 on
page 50, illustrates how the relay processes zero
sequence current for Io-Elimination and Io-Correction.
The model clearly shows that Io-Correction offers the
maximum sensitivity and selectivity for internal faults.
I
A
A
I
B
B
I
C
C
I
N
Delta
-
Wye Circuit
Figure 4.6
50
-
current CT. Using
-
current CT connected
I
I
1
a
a
I
b
I
2
b
I
c
I
3
c
4.3.6
Zero
Sequence Current
-
Elimination vs. Correction
Mathematical Illustration
Io-Elimination – Refer to Figure 4.6. And
Section 4.3.5 on page 49
I* = Normalized Current
Wye-Winding: Setting "I0-Elimination"
=
−
*
*
*
I
I
I
A
,
E
lim
A
0
=
−
*
*
*
I
I
I
B
,
E
lim
B
0
=
−
*
*
*
I
I
I
C
,
E
lim
C
0
Delta-Winding: Setting "I0-Elimination"
Phase-Shift Correction and I0-Elimination
−
+
=
*
*
*
I
I
I
0
a
1
3
−
+
=
*
*
*
I
I
I
0
b
2
1
−
+
=
*
*
*
I
I
I
0
c
3
2
−
=
−
−
*
*
*
*
I
I
I
I
a
b
1
3
−
=
−
−
*
*
*
*
I
I
3
I
I
a
b
1
1
1
−
=
−
*
*
*
(
I
I
)
I
I
a
b
1
3
⎡
*
I
Delta-Winding
, 1
⎢
Currents after
*
⎢
I
, 2
Transformation
⎢
*
to primary side
⎢
I
⎣
, 3
I
Differential
Currents
I
calculated
by the relay
I
7UT51 v3
[
]
1
=
+
+
*
*
*
*
I
I
I
I
.
0
A
B
C
3
[
]
1
=
+
+
* *
*
*
*
I
I
I
I
0
1
2
3
3
+
*
*
I
I
2
1
−
=
−
*
*
*
* *
I
I
3
I
3
I
3
2
1
0
=
* *
*
I
0
, 1
E
lim
⎤
⎡
⎤
...
1
....
1
....
0
E
lim
⎥
⎢
⎥
1
=
⎥
...
0
....
1
....
1
⎢
⎥
E
lim
3
⎥
⎢
⎥
−
1
....
0
....
1
⎣
⎦
⎥
⎦
E
lim
=
+
*
*
*
I
I
D
,
A
A
,
E
lim
, 1
E
lim
=
+
*
*
*
I
I
D
,
B
B
,
E
lim
, 2
E
lim
=
+
*
*
*
I
I
D
,
C
C
,
E
lim
, 3
E
lim
PRIM-2330C
⎡
⎤
*
I
a
⎢
⎥
*
⎢
I
⎥
b
⎢
⎥
*
I
⎣
⎦
c
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