Differential Protection (ANSI 87G/87M/87T)
81
7UM62 Manual
C53000-G1176-C149-3
Figure 2-34
Vector Group Matching for Y(N) d5 (with Earthed Starpoint)
In Figure 2-35 on the left-hand side, a zero sequence current will occur in case of e.g.
an external fault; on the right-hand side, it will not. If the currents were compared
without first eliminating the zero sequence current, the result would be wrong
(differential current despite external fault). Therefore, the zero sequence current must
be eliminated on side 1. The zero sequence current is deducted from the phase
currents. The rule for calculation is shown in the left-hand matrix in Figure 2-34.
Figure 2-35
Example of an Earth Fault Outside the Transformer with Distribution of Currents
2.12.1.3 Evaluation of Measured Quantities
The 7UM62 performs the above calculations (matching of absolute values, vector
group matching in the case of transformers) at every sampling and derives from them
the instantaneous values of differential and stabilizing current. From the differential
current, the fundamental component is determined by means of a Fourier filter, which
provides for effective damping of any interference and aperiodic DC components.
I
A
I
B
I
C
1
3
---
2
1
–
1
–
1
–
2
1
–
1
–
1
–
2
I
L1
I
L2
I
L3
⋅
⋅
=
I
A
I
B
I
C
1
3
-------
1
–
0
1
1
1
–
0
0
1
1
–
I
L1
I
L2
I
L3
⋅
⋅
=
L
1
L
2
L
3
L
1
L
2
L
3
Side 1
Side 2
I
L1
I
L2
I
L3
I
A
√
3
⋅
I
A
I
L1
I
L2
I
L3
L
1
L
2
L
3
L
1
L
2
L
3
