CHAPTER 15: THEORY OF OPERATION
PHASE DISTANCE APPLIED TO POWER TRANSFORMERS
D90
PLUS
LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
645
Equations from the
current transformation
and
voltage transformation
columns are used to
derive inputs to the three (AB, BC, and CA) phase distance elements. For example, if the CTs
are located at the delta side of the delta-wye 11 (Dy11) transformer, and a given zone is set
to look through the transformer into the system connected to the wye winding, the CT
location setting for that zone shall be set to “Dy11” and the relay would use –
√
3
I
B
instead
of a traditional
I
A
–
I
B
for the AB phase distance element.
The current supervision pickup setting applies to the currents specified in the
current
transformation
columns.
A distance zone originates at the location of the VTs (regardless of the location of the CTs).
Example system with power transformers
Consider the system shown below. To respond to the indicated fault, a distance relay is
normally applied at the relay point
X
.
Yd5
AB
BC
CA
Yd7
AB
BC
CA
Yd9
AB
BC
CA
Yd11
AB
BC
CA
Transformer
connection
Loop
Current transformation
Voltage transformation
(
)
C
A
B
AB
I
I
I
I
-
-
=
2
3
1
_21P
B
AB
V
V
3
_21P
=
(
)
B
A
C
BC
I
I
I
I
-
-
=
2
3
1
_21P
C
BC
V
V
3
_21P
=
(
)
C
B
A
CA
I
I
I
I
-
-
=
2
3
1
_21P
A
CA
V
V
3
_21P
=
(
)
A
C
B
AB
I
I
I
I
2
3
1
_21P
-
+
=
A
AB
V
V
3
_21P
-
=
(
)
B
C
A
BC
I
I
I
I
2
3
1
_21P
-
+
=
B
BC
V
V
3
_21P
-
=
(
)
C
B
A
CA
I
I
I
I
2
3
1
_21P
-
+
=
C
CA
V
V
3
_21P
-
=
(
)
B
A
C
AB
I
I
I
I
-
-
=
2
3
1
_21P
C
AB
V
V
3
_21P
=
(
)
C
B
A
BC
I
I
I
I
-
-
=
2
3
1
_21P
A
BC
V
V
3
_21P
=
(
)
C
A
B
CA
I
I
I
I
-
-
=
2
3
1
_21P
B
CA
V
V
3
_21P
=
(
)
B
C
A
AB
I
I
I
I
2
3
1
_21P
-
+
=
B
AB
V
V
3
_21P
-
=
(
)
C
B
A
BC
I
I
I
I
2
3
1
_21P
-
+
=
C
BC
V
V
3
_21P
-
=
(
)
A
C
B
CA
I
I
I
I
2
3
1
_21P
-
+
=
A
CA
V
V
3
_21P
-
=
