3 phase primary short is usually applied to the HV side so that the LVAC supply is connected to the winding
with lowest impedance which will result in a higher current level. The test current that will be produced can be
predicted based on the impedance of the transformer and the applied test voltage. The primary test current is
injected through all of the biased differential CT’s on the LV side.
Injection of 3 phase current in this way will simulate balanced load conditions, or through fault. During injec-
tion, check that the W1 and W2 relay currents are in anti-phase by examination of the relay Differential
Meters in Instruments mode. Check each phase in turn, ensuring that the phase angle for W1 Relay is in
anti-phase with W2 Relay.
When the transformer is eventually energized and carrying load current, the above examination of the W1 and
W2 relay current phase angle should be re-checked for anti-phase to ensure that the correct ICT Connec-
tion settings are applied to the differential protection.
It should be noted that checking of Vector Grouping by phase alignment between W1 and W2 by 3 phase
primary injection or on-load will highlight phase cross-over or connection polarity but will not show incorrect
application of zero sequence filters.
Functional Tests
9.10 Protection Functional Tests
450 Reyrolle 7SR5, Transformer Protection Device, Device Manual
C53000-G7040-C015-1, Edition 11.2019
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