Phase sequence and vector group are different checks. Phase sequence describes the order of the three phase quantities in the system; vector group describes winding connections and the angular displacement between sides. A transformer may have the correct vector group yet still be incorrectly terminated relative to the site phase sequence. Commissioning documentation should therefore verify rotation, terminal marking and phase displacement as separate items.
The three checks answer three different questions
Phase sequence identifies the progression of the three phase quantities. Terminal identity establishes which physical terminal corresponds to the designated system phase. Vector displacement establishes the angular relationship across the transformer. A cyclic relabelling can preserve phase sequence while changing terminal correspondence, which explains why one rotation result cannot replace a phasing record. Keep the drawing convention consistent across the utility, transformer, switchboard and converter documents. Ambiguous labels such as “phase 1” should be resolved before several contractors build independent interfaces around them.
Separate equipment tests from installation verification
Factory ratio and connection tests establish the transformer configuration under the test procedure. They do not verify that a site cable has been terminated at the intended bus phase or that a remote voltage reference uses the same labelling. The commissioning plan should therefore define independent records for equipment identity, conductor/terminal correspondence, phase rotation and required displacement. Verification methods, isolation and energization permissions belong to the approved site procedure; this distinction is a documentation requirement, not an instruction to improvise live measurements.
Treat a bus tie as a separate compatibility boundary
A bus supplied from one transformer may run its local loads correctly while still being incompatible with another source at a proposed tie. Check voltage magnitude, phase relationship and source operating conditions at that boundary, together with the switching and protection philosophy. A transformer vector-group label cannot authorize parallel operation by itself. Retain the approved phase-reference drawing in the operating documentation so a later cable replacement or transformer substitution triggers the relevant verification instead of relying on an old rotation sticker.
Technical reference framework
[1] IEC 60076-8:1997 — Application guide.
[2] IEEE C57.105-2019 / Cor 1-2023 — Three-phase transformer connections.