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Zero displacement voltage U
Starting from the generating system with star point E
(which is normally earthed), the star point (N) on load
side is shifted in case of unbalanced load. The zero-
displacement voltage between E und N may be
determined by a vectorial addition of the voltage vectors
of the three phases:
U
= - (U
+ U
NE
1N
A displacement voltage may also occur due to
harmonics of order 3, 9, 15, 21 etc., because the
dedicated currents add in the neutral wire.
Earth fault monitoring in IT systems
Via the determination of the zero-displacement voltage it's possible to detect a first earth fault in an
unearthed IT system. To do so, the device is configured for measurement in a 4-wire system with
unbalanced load and the neutral connector is connected to earth. In case of a single-phase earth fault
there is a resulting zero displacement voltage of ULL/ √3. The alarming may be done e.g. by means of a
relay output.
Transformer, secondary side
Because in case of a fault the voltage triangle formed by the three phases does not change, the voltage
and current measurements as well as the system power values will still be measured and displayed
correctly. Also, the meters carry on working as expected.
The method is suited to detect a fault condition during normal operation. A declination of the isolation
resistance may not be detected this way. This should be measured during a periodical control of the
system using a mobile system.
Another possibility to analyze fault conditions in a grid offers the method of the
as described in A3.
P10 Rev A
NE
+ U
) / 3
2N
3N
Device handbook LINAX PQ3000-PQ5000
Load
symmetrical components
83/102
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