4
Transformer Differential Protection (87T and 87HS)
7UT51 v3
50 PRIM-2330C
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).
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.
Figure 4.6
Delta
-
Wye Circuit
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
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
-
current CT. Using
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
(7UT513 only)
The winding has a ground connection
within the protected zone that is fitted
with a ground
-
current CT connected
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.
I
A
I
B
I
C
I
1
I
a
I
2
I
3
I
b
I
c
a
b
c
A
B
C
I
N
*
lim,1
**
0
*
1
**
**
0
*
1
*
2
*
3
*
1
*
1
**
*
1
*
2
*
3
*
1
**
*
2
*
3
*
*
1
*
2
*
*
3
*
1
*
)(
3
1
333
0
0
0
Eba
ba
ba
c
b
a
IIIII
IIIIIIII
IIIIII
III
III
III
=−=−
−=−−−=−
+−−=−
=+−
=+−
=+−
[]
*
3
*
2
*
1
**
0
3
1
IIII
++=
*
0
**
lim,
*
0
**
lim,
*
0
**
lim,
III
III
III
CEC
BEB
AEA
−=
−=
−=
[]
.
3
1
****
0
CBA
IIII
++=
Delta-Winding: Setting “I0-Elimination”
Phase-Shift Correction and I0-Elimination
Wye-Winding: Setting “I0-Elimination”
Delta-Winding
Currents after
Transformation
to primary side
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎣
⎡
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎣
⎡
−
=
⎥
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎢
⎣
⎡
*
*
*
*
lim,3
*
lim,2
*
lim,1
1....0....1
1....1....0...
0....1....1...
3
1
c
b
a
E
E
E
I
I
I
I
I
I
*
lim,3
*
lim,
*
,
*
lim,2
*
lim,
*
,
*
lim,1
*
lim,
*
,
EECCD
EEBBD
EEAAD
III
III
III
+=
+=
+=
Differential
Currents
calculated
by the relay
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