Seg HighTECH MRU3-2, Руководство пользователя

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12 TB MRU3-2 12.00 E
4.4.6 Zero sequence system
To decide whether a vector system is symmetrically,
the point, the symmetry has to refer to, is always to be
mentioned. Usually this point is the earth potential.
When an earthfault occurs in an isolated or compen-
sated grid, it does not influence the relative position of
the three voltage vectors to each other, mains opera-
tion can be maintained. The vector peak of the missing
phase is forced on the earth potential. For an observer
who takes the earth potential as reference, the star
point shifts by the amount of the missing phase. For
him the voltage vector system is not anymore symmetri-
cal. The exact measure of the shifting results from re-
placing this system in symmetrical components in the
developed zero sequence system.
Note:
Shall the relay evaluate the zero sequence system it is
absolutely necessary that the voltage transformers and
the
MRU3-2 are wired in star connection. The star
points must be earthed, furthermore the MRU2-2 must
be set to Y-connection. In delta connection no zero
sequence system evaluation is possible and thus no
earthfault detection.
When only the phase-to-phase voltages are measured,
the vector star point is not known, thus also the posi-
tion of the star point in regard to the earth potential
cannot be defined.
Symmetrical vector system Vector system with earth fault
U0~0V U0~230V
Phase-to-phase voltage
400 V Phase-to-phase voltage
400 V
star point star point
Figure 4.5: Zero point shifting after earthfault in the isolated grid