Since the DTL setting is generally small the pick-up setting can be tested by gradually increasing current until
element operates. The relay should be disconnected from the current transformers for this test.
Apply 2 times setting current if possible and record operating time.
Phase I
set
(Amps) DTL (sec) P.U. Current
Amps
Tolerance Operate Time
2 ⋅ I
set
Tolerance
It is also desirable to check the operating voltage achieved with the setting resistor and all parallel CTs
connected but de-energized. A higher capacity test set will be required for this test. Adequate current must be
supplied to provide the magnetizing current of all connected CTs. Precautions should be taken to ensure that
no personnel are at risk of contact with any of the energized secondary wiring during the test.
Settings Data Voltage Setting Measured
To complete testing of the REF requires primary injection through the phase and residual REF CT in series to
simulate an out of zone fault and ensure stability of the relay. The test can then be repeated with the REF CT
secondary connections reversed to prove operation.
Element Blocking
The Restricted Earth Fault element can be blocked by Binary Input Inhibit. This functionality
should be checked.
Element BI Inhibits
87GH
Check correct indication, trip output, alarm contacts, waveform record.
Check that any shorting links are removed after testing.
87T Transformer Differential Protection – Biased
87T Transformer Differential Protection – Biased
The differential elements are subjected to CT multipliers, Vector Group Compensation and Zero
Sequence filters when applied to power transformers. The complexity of these features can cause confusion
during testing and lead to incorrect relay settings being applied. It is recommended that the accuracy of the
differential elements are tested by secondary injection with simplified differential settings applied to avoid
ambiguity before reinstating the required site settings which can be tested more thoroughly by primary injec-
tion followed by final checking with the protected transformer on load. If a transformer has more than 2 wind-
ings, the first 2 windings should be tested as a pair with no current in the third winding. The third winding
should then be tested against the first with no current in the second winding.
Secondary Injection Testing
The settings used for Secondary Injection test purposes should be:
W1 ICT Multiplier
1x
W1 ICT Connection Yy0,0°
W2 ICT Multiplier 1x
W2 ICT Connection Yy0,0°
W3 ICT Multiplier 1x
W3 ICT Connection Yy0,0°
Secondary testing of the bias characteristic will be greatly simplified by the use of automated numeric protec-
tion test equipment such as the Omicron CMC256 or equivalent. This equipment can be programmed using
setting which match those of the relay to test for accuracy over the whole operating range and give a clear
easy to use graphical display of relay performance against the specified characteristic.
9.10.23
Functional Tests
9.10 Protection Functional Tests
Reyrolle 7SR5, Transformer Protection Device, Device Manual 447
C53000-G7040-C015-1, Edition 11.2019
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