Siemens 7UT51 Instruction Manual page 52

4
Transformer Differential Protection (87T and 87HS)
I
A
A
I
B
B
I
C
C
I
N
External Phase to Ground Fault
Figure 4.7
52
I
I
1 a
a
I
b
I
2
b
I
c
I
3
c
External Single Phase to Ground Fault,
Io-Correction
Refer to Figure 4.7
Wye-Winding
Currents:
= ∠
* 0
I 1 180
A
=
*
I
0
B
=
*
I
0
C
= 1 ∠
* 0
I 180
0 3
= ∠
* 0
I 1 0
N
Wye-Winding Currents after
I0-Correction:
= +
* *
I I
A , Corr A
= +
* *
I I
B , Corr B
= +
* *
I I
C , Corr C
Delta-Winding Currents after I0-Elimination
and Phase-Shift Correction:
⎡ ⎤
*
I
⎡
... 1 ....
, 1 E lim
⎢ ⎥
⎢
1
=
*
⎢ ⎥
I ... 0 ....
⎢
, 2 E lim
3
⎢ ⎥
⎢
−
1 ....
⎣
*
⎢ I ⎥
⎣ ⎦
, 3 E lim
=
* *
I I
D , A A
Differential
=
* *
I I
Current: D , B B
=
* *
I I
D , C C
7UT51 v3
Delta-Winding
Currents:
= ∠
* 0
I 1 0
1
=
*
I 0
2
=
*
I 0
3
= − = ∠
* * *
I I I 1
a 1 3
= − = ∠
* * *
I I I 1 180
2 1
b
= − =
* * *
I I I 0
c 3 2
1
= 2 ∠
* 0
I 180
3
N
3
1
= ∠
1
* 0
I 0
3
N
3
1
= 1 ∠
* 0
I 0
N 3
3
⎡
2 ∠
0
0
⎡ ⎤
*
⎤ I
1 ... 0 ⎢ 3
a
⎢ ⎥
⎥
⎢
= 1 ∠
*
1 ... 1 ⎢ I ⎥ 180
⎥
⎢
3
b
⎢ ⎥
⎥
⎢
0 ... 1 *
⎦
I
1
⎣ ⎦ ∠
180
⎢
c
⎣
3
+ =
*
I
0
, Corr , 1 E lim
+ =
*
No Trip !
I 0
, Corr , 2 E lim
+ =
*
I 0
, Corr , 3 E lim
PRIM-2330C
0
0
0
⎤
⎥
⎥
0
⎥
⎥
0
⎥
⎦