Functions
2.14 Differential Protection and Its Protected Objects
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
120
Figure 2-43 Vector group matching for a Yd5 transformer (isolated starpoint)
Deducting on side 2 the currents I
L3
– I
L1
, results in the current I
A
, which has the same direction as I
A
on side
1. Multiplying with 1/√3
matches the quantitative values. The matrix describes the conversion for all three
phases.
Earthed Transformer Starpoint
The following figure shows an example of a YNd5 vector group with earthed starpoint on the Y-side.
The zero sequence currents are eliminated in this case. In the following figure on the right side, the zero se-
quence currents are automatically eliminated by the current difference formation, just as in the transformer
there can be no zero sequence currents outside the delta winding. On the left-hand side, the elimination of the
zero sequence current results from the matrix equation, e.g.
1
/
3
· (2 I
L1
– 1 I
L2
– 1 I
L3
) =
1
/
3
· (3 I
L1
– I
L1
– I
L2
– I
L3
) = 1/3 · (3 I
L1
– 3 I
0
) = (I
L1
– I
0
).
Because of the zero sequence current elimination, fault currents which flow through the CTs during earth faults
in the network if there is an earthing point in the protected zone (transformer starpoint or starpoint earthing
transformer) are neutralised without any special measures from outside.
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