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3 Mounting and Commissioning
Implementation of
Zero Sequence
Current Tests
Measuring Differen-
tial and Restraint
Currents
408
For these commissioning tests, the zero sequence current must be at least 2 % of the
rated relay current for each phase, i.e. the test current at least 6 %.
This test cannot replace visual inspection of the correct current transformer connec-
tions. Therefore, the inspection of these connections is a prerequisite.
• Switch on test current.
• Read out the current magnitudes:
Compare the indicated current magnitudes under measurement → secondary →
operational measured values secondary with the actually flowing values:
– All phase currents of the tested measuring location correspond to approximately
1
/
of the test current (for 1-phase transformers
3
– 3I0 of the tested measuring location corresponds to the test current.
– Phase currents and zero sequence current of the other measuring location are,
on transformers, nearly 0.
– The current at the auxiliary 1-phase current input corresponds to the test current
— provided this current is available and included.
Deviation can practically occur only for the single-phase current (if included)
because the connection of the phase currents had been verified already during the
symmetrical tests. In case of deviations:
– Switch off the test source and the protected object (shut down the generator) and
earth it.
– Re-check the assignment or the tested 1-phase input (subsection 2.1.4 under
margin heading „Assignment of Auxiliary 1-phase Measuring Locations").
– Re-check the settings for the magnitude matching (Subsection 2.1.4 under
margin heading „Current Transformer Data for 1-phase Auxiliary Current In-
puts").
– Check the plant connections to the device and the test arrangement and correct
them.
– Repeat test and re-check the current magnitudes.
The differential and restraint currents are referred to the rated currents of the tested
side of the main protected object. If a zero sequence current test does not concern the
main protected object but a different earthed object (e.g. a shunt reactor outside the
main object), the base of the referred currents is the rated current of that 3-phase mea-
suring location to which the 1-phase current input is assigned, i.e. the measuring lo-
cation under test. This must be considered when they are compared with the test cur-
rents.
• Switch on test current.
• If the starpoint current is available:
Read out the differential and restraint currents I-Diff; I-Rest under Measurement
→ Percent Values → Differential and Restraint Currents.
– The differential current of the restricted earth fault protection I
at least one scale less than the test current.
– The stabilisaing current I
corresponds to twice the test current.
Rest REF
1
/
),
2
must be low,
Diff REF
7UT613/63x Manual
C53000-G1176-C160-2
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