Configuration Tests for Transformer Differential Protection
Current differential protection configuration settings must be aligned with the transformer
Vector Group
arrangement and earthing arrangement to provide correct operation. It is essential that this is tested to detect
errors in connections or configuration as well as checking of 87T protection element sensitivity.
Secondary injection tests can never replace primary tests because they cannot include connection faults or
errors but testing by application of unbalanced phase conditions is also essential to identify errors.
Advanced primary tests can be difficult to apply due to limitations of injection equipment. Advanced secon-
dary testing is more easily applied but application of unbalanced phase conditions on Y-d or D-y transformers
can be complex to model correctly due to zero sequence filtering and interaction between phases. This can
make it difficult to quantify test results such that errors in connections and setting configuration are not iden-
tified.
Checking of settings values and other secondary tests are greatly simplified and made more reliable by config-
uring the protection device as Y-Y 0° for testing and then changing to the required
Vector Group
before
primary testing and checking on load. Alternatively, differential protection settings can be checked by applica-
tion of balanced 3 phase conditions only.
Secondary tests are used to check the following:
•
Checking the setting values
•
Proper response of the protection function
•
Engineering (for example, routing the respective logic signals, display images with measured values,
operation of user logic, and much more).
If you want to perform secondary tests, observe the following instructions.
i
i
NOTE
For tests with secondary test equipment, ensure that no other protection devices are connected and the
trip commands to the circuit‑breakers are open; otherwise, the circuit‑breaker could be activated.
i
i
NOTE
The measuring accuracy to be achieved depends on the electrical data of the used test sources. In addition,
the protection accuracy tolerance is affected by the
ICT Multiplier
setting. The claimed accuracy
applies for
ICT Multipliers
set to 1.
i
i
NOTE
The thermal rating of the device inputs must not be exceeded during testing. Care should be taken to
consider duration of high current tests. Always take precautions for the case when the test is not stopped
by the expected protection operation. Successive testing will create an accumulation of heating. Interrupt
testing to allow cool-down!
For transformers with more than 2 windings, the differential protection can be checked for each winding
separately. This corresponds to the simulation of a fault supplied from 1 side only. If a side has several meas-
uring points, the measuring inputs not included in the test carry no current. The pickup value is tested by
slowly increasing the test current.
The 87T characteristic applies directly to symmetrical 3-phase current in the case of 3-phase protected objects
and the settings can be tested by injection of balanced 3-phase current.
The differential protection is applied after the
ICT Multiplier
and this must be included in the test applica-
tion and evaluation.
The phase shift due to the
Vector Group
setting can be applied directly to the balanced 3-phase test
currents.
For transformers, the differential protection operating current values for 1-phase or 2-phase faults also
depend on the vector group of the transformer. For 1-phase testing, the current values also depend on the
treatment of the winding neutral point and the processing of the neutral point current. For a Y-Y configuration
9.8
Functional Tests
9.8 Configuration Tests for Transformer Differential Protection
Reyrolle 7SR5, Transformer Protection Device, Device Manual
415
C53000-G7040-C015-1, Edition 11.2019
