When the end user enters all these parameters, transformer differential function
automatically determines the matrix coefficients based on the following rules:
For the phase reference, the highest voltage star (Y) connected winding is used. For
example, if the power transformer is a Yd1 power transformer, the HV winding (Y) is
taken as the phase reference winding. If the power transformer is a Yy0 power
transformer the HV winding (Y) is taken as the phase reference winding. If the power
transformer is a Dy1, then the LV winding (y) is taken for the phase reference. If there
is no star connected winding, such as in Dd0 type of power transformers, then the HV
delta winding (D) is automatically chosen as the phase reference winding.
The fundamental frequency differential currents are in general composed of currents
of all sequences, that is, the positive-, the negative-, and the zero-sequence currents.
If the zero-sequence currents are eliminated (see section
"Elimination of zero
sequence currents"), then the differential currents can consist only of the positive-, and
the negative-sequence currents. When the zero-sequence current is subtracted on one
side of the power transformer , then it is subtracted from each individual phase current.
Table 29 summarizes the values of the matrices for all standard phase shifts between
windings.
Table 29: Matrices for differential current calculation
Matrix with Zero Sequence
Reduction set to On
Matrix with Zero Sequence
Reduction set to Off
Matrix for Reference Winding
2 1 1
1
1 2 1
3
1 1 2
- -
× - -
- -
é ù
ê ú
ê ú
ê ú
ë û
EQUATION1227 V1 EN (Equation 3)
1 0 0
0 1 0
0 0 1
é ù
ê ú
ê ú
ê ú
ë û
EQUATION1228 V1 EN (Equation 4)
Matrix for winding with 30°
lagging
1 -1 0
1
0 1 1
3
1 0 1
× -
-
é ù
ê ú
ê ú
ê ú
ë û
EQUATION1229 V1 EN (Equation 5)
Not applicable. Matrix on the
left used.
Matrix for winding with 60°
lagging
1 2 1
1
1 1 2
3
2 1 1
-
× -
-
é ù
ê ú
ê ú
ê ú
ë û
EQUATION1230 V1 EN (Equation 6)
0 1 0
0 0 1
1 0 0
-
-
-
é ù
ê ú
ê ú
ê ú
ë û
EQUATION1231 V1 EN (Equation 7)
Matrix for winding with 90°
lagging
0 -1 1
1
1 0 1
3
1 1 0
× -
-
é ù
ê ú
ê ú
ê ú
ë û
EQUATION1232 V1 EN (Equation 8)
Not applicable. Matrix on the
left used.
Table continues on next page
1MRK 502 048-UEN A Section 6
Differential protection
89
Technical manual
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