Transformer differential settings depend on more than transformer MVA. The data sheet should include winding ratings, ratios, vector group, grounding, CT ratios/classes and locations, tap range, expected through-fault current, inrush characteristics and any tertiary. Modern relays can compensate ratio and phase internally, but only if the entered topology and CT mapping are correct.
Map every current input to a physical winding
Record the CT location, ratio, polarity and secondary connection for each relay input, then relate it to the transformer winding and reference direction. Include any tertiary or additional current path within the protected zone. The relay's numerical compensation must match the actual primary and secondary connections. Applying a phase correction in both external CT wiring and relay configuration without accounting for both can produce an incorrect differential quantity even when each individual drawing appears plausible.
Account for tap range and zero-sequence treatment
The transformation ratio changes with tap position, while a fixed CT ratio does not. The protection design must accommodate the resulting normal mismatch over the approved tap range. Grounded-star and delta connections also affect which zero-sequence components should appear in the compensated comparison. Use the relay manufacturer's actual algorithm and configuration rules; a generic vector-group label alone does not establish the correct zero-sequence filtering or compensation for every relay design.
Verify both external-fault security and internal-fault sensitivity
Maximum through-faults challenge CT performance and restraint, while lower-current internal faults can challenge sensitivity. Energization adds a different security requirement. Retain the credible source states and CT burdens in the study, and define the commissioning tests that demonstrate the implemented mapping and logic. A complete data sheet enables a defensible setting study; it does not itself provide approved pickup, slope or harmonic-restraint values for the installation.
Technical reference framework
[1] IEEE C37.91-2021 — Protecting power transformers.
[2] IEC 60076-1:2011 — Power transformers, General.
[3] IEEE C57.105-2019 / Cor 1-2023 — Three-phase transformer connections.