Differential Protection

I have the below information and hope to help you

Commissioning transformer differential protection schemes involves several levels of testing

1-Hardware tests verify transformer turns ratio, CT turns ratios, and
CT polarity.
2- Functional tests validate the performance of the relay elements
with the installed settings and test the DC control circuits. Trip
tests verify that the relay operates the correct lockout relays and
breakers.

3- In-service or commissioning tests verify the primary and secondary
AC current circuits. We must take into account the transformer ratio
and connection; the CT ratio, wiring, and connections; and the relay
settings.

The last item is, by far, the most challenging aspect of assuring certainty in
commissioning. modern transformer differential relays have settings that
compensate for the difference in the secondary currents, adjusting for the
transformer connection (e.g., delta–wye) and removing zero-sequence
current.
In order to perform commissioning tests, we must apply balanced threephase
currents to the primary system. Some users energize the transformer
to the system and begin to apply load. Ideally, we prefer to perform this
test without connecting to the power system. For example, use a portable
generator or a station service transformer to supply a reduced voltage
three-phase power supply to one of the windings of the transformer and
apply a short-circuit to the remaining winding.

Through this procedure, we can check the following:
1- The phase rotation and angle of the currents
2- Secondary current magnitudes
3- The relationship of the high-side currents to the low-side currents
4 - The operating or differential current (should be nearly zero)
For the transformer given a 240 V AC source, we can
calculate expected relay currents (magnitude and angle). Using relay metering
data, we then observe the measured currents
measured (actual) currents do not match the calculated (expected) currents
and/or we observe differential current, we must perform troubleshooting to
systematically check CT wiring, connections, and relay settings to correct the
discrepancy.

What you want is to determine the Differential current-Restraint current characteristic, as far as I understand. For this you have to inject current on both sides simultaneously. The theory for this characteristic can be found in any book for transformer protection basics.

The Schneider engineers either don't have this knowledge or don't want to complicate their work- it is not very simple to do this, requires strong knowledge both in theory and in practice with the equipment.

The user manual for the protection should have a chapter where this characteristic is mentioned (as theoretical example). To get the real idea of the Differential-Restraint characteristic you have to do tests on site with the equipment installed. Search for it and ask them to test it.

Nikolay

It seems your concern is 'through fault stability of the differential protection' and asking how to test a differential relay!

Primarily go for testing of the current transformer's in detail for individual conformity and further you have to check the overshoot and undershoot of the relay to be within agreeable limits. If all is well it goes well.