If the W2 sequence is not correct, significant residual current is measured or there is a phase inversion, check
for connection faults.
If all angles deviate from the expected values by the same amount, there could be a cyclic inversion of all 3
phases or an incorrect vector group in transformers. In the last case, check the vector-group adaptation.
If all angles differ by 180°, the polarity of a set of current transformers for the second measuring point could
be incorrect.
These problems can be corrected by changing the corresponding parameters of the device configuration but
there is a risk that the root cause of the problem may not be identified and the error remains but hidden by
incorrect configuration.
Measuring Differential and Restraint Currents
To complete the symmetrical tests, check the differential and restraint current measurands. The differential
currents should be very low compared to the restraint current, no more than 10 % of the restraint. The differ-
ential currents will not be zero for any primary test due to transformer magnetizing current and also unless
the transformer tap changer is positioned on the center tap that lines up with the recommended differential
setting defined by the
ICT Multiplier
settings.
If differential current is of a similar magnitude to restraint current, this suggests that a current transformer set
is reversed or the
Vector Group
setting is incorrect.
i
i
NOTE
Ensure that the intended tests is performed for each current path.
Residual-Current Test on the Protected Object
If the neutral point of a transformer winding is earthed, the following residual-current tests are required. If
several neutral points are earthed, it is necessary to perform the residual current tests for each of the earthed
windings.
If the neutral-point current is available and is conducted to the device via a 1-phase current input, the polarity
of the earth current connection is essential for taking the neutral point current into account. This applies to
the differential and restricted earth fault protection.
If no neutral point current is available, the residual-current tests serve to verify the correct treatment of the
residual currents in the differential protection.
Preparation of the Residual-Current Tests
Perform the residual-current measurements from the 3-phase measuring point for which the neutral point is
earthed. For auto transformers, perform the measurement from the upper-voltage side. For transformers, a
delta winding must be present (d-winding or stabilizing winding). The windings not included in the test
remain open, since the delta winding creates the impedance of the residual-current path by itself.
The test setup varies, depending on the particular application. The following figures show typical examples of
the test setup on a earthed star-delta transformer. The neutral-point current is included in the measurement. If
the neutral point is measured by the relay it is included in the test.
!
DANGER
Dead components hazardous due to capacitive coupling of other system parts!
Failure to comply with the safety notes, can result in death, severe injury or considerable material
damage.
²
Primary tests may be performed only on dead and earthed system parts!
Functional Tests
9.8 Configuration Tests for Transformer Differential Protection
Reyrolle 7SR5, Transformer Protection Device, Device Manual
417
C53000-G7040-C015-1, Edition 11.2019
