Micom P634 - Vector Group Matching; Zero-Sequence Current Filtering

3.21.3 Vector Group Matching
Vector group matching means that the low voltage-side currents are rotated with
respect to the high voltage-side currents according to the vector group of the
transformer to be protected. Thereby, phase coincidence with the high voltage-
side currents is restored. With the P634, this is achieved by calculating the
relevant vector difference or where appropriate, by sign inversion for the low
voltage-side phase currents (end b, c or d). Care must be taken to avoid
distortion of the amplitude matching by this operation. For all odd vector groups,
this is achieved by means of the factor 1/√3. Using vector diagrams, it can be
shown that the operations listed in the following table will lead to phase
coincidence of the high and low voltage-side currents while maintaining the
amplitude matching. In Fig. 3-88, (p. 3-128), such a vector diagram is depicted
for a transformer having the vector group Yd5 as an example. By subtraction of
each phase current from the cyclically leading phase current and subsequent
multiplication by the factor 1/√3, the desired matching is achieved.
Q6Z0105A
-Iam,B,b
-Iam,C,b
√3⋅Is,2,b
√3⋅Is,3,b
-Iam,A,b
√3⋅Is,1,b
Iam,A,b
Iam,B,b
Iam,C,b
Is,3,a
Is,2,a
Is,1,a
Fig. 3-88: Vector diagram for vector group matching with a transformer having the vector group Yd5
3.21.4 Zero-sequence Current Filtering
Table 3-2, (p. 3-130) shows that the zero-sequence current is subtracted from
the phase currents of winding a and, for all even vector groups, from the phase
currents of windings b, c and d. According to the theory of symmetric
components, the zero-sequence current is calculated as follows:
I
am,0,z
=
1
3
. I
am,A,z
+ I
am,B,z
+ I
am,C,z
z: end a, b, c or d
I
am
: amplitude-matched current
Zero-sequence filtering may be disabled separately for each end.
In general this disabling of zero-sequence filtering is intended for even-numbered
vector groups. Should the side considered here require the setting of an odd-
numbered vector group while at the same time no operational system star point
grounding is provided within the protected area, then, in view of increased
sensitivity with single-pole internal faults, it is recommended that the respective
zero-sequence current is fed to the individual measuring systems again.
P634
3 Operation
3-128 P634/EN M/R-42-A // P634‑311‑653