A delta tertiary may stabilize zero-sequence or triplen-harmonic behavior, supply station auxiliaries, support reactive equipment or exist for system-design reasons. Its presence on a one-line diagram does not define its usable load rating. Buyers should ask whether the tertiary is externally accessible, its continuous and short-circuit rating, how it is protected, and what operating purpose is assumed in the transformer design.
An internal delta is not an unused spare feeder
A buried delta may exist to provide an internal circulating path without externally accessible terminals. Adding external connections changes lead, bushing, insulation, protection and fault-duty requirements. Even an accessible tertiary is usable only within its declared voltage, current and thermal limits. Ask whether the offered tertiary is a stabilizing winding, a load-carrying winding or both. Its drawn triangle on a one-line diagram cannot establish a spare MVA allowance for a capacitor bank or a station-service transformer.
Internal circulating duty can coexist with external load
Triplen-related and zero-sequence-related currents may produce internal heating that is not represented by the external tertiary load alone. Adding an auxiliary load can therefore change the design assessment even when its MVA looks modest compared with the main winding. State the expected normal and fault-related duty, harmonic environment and loading on the other windings. The manufacturer should confirm permitted combinations rather than approving an external kVA figure independently of the transformer's full operating matrix.
A reactive connection needs more than a load check
A reactor or capacitor connected to the tertiary changes reactive power flow, fault contribution and potentially switching or resonance behavior. Obtain the relevant pairwise impedances, grounding arrangement, surge requirements and protection boundaries. Identify how the tertiary is isolated and monitored within the approved operating design, without assuming the main winding protection covers every external connection. Preserve its intended purpose in the equipment record so a later project does not treat an already allocated tertiary function as freely available spare capacity.
Technical reference framework
[1] IEC 60076-8:1997 — Application guide.
[2] IEEE C57.105-2019 / Cor 1-2023 — Three-phase transformer connections.