Adding a third transformer to an existing parallel bank is not a simple capacity addition. The new unit changes load sharing, short-circuit current, tap-control behavior and protection coordination for every bank combination. It can also expose old differences that were tolerable with two units. Evaluate one, two and three-unit states using actual existing test data wherever possible.
Seven nonempty combinations need deliberate selection
Three transformers have seven nonempty in-service combinations: three single-unit states, three two-unit states and one three-unit state. Not all must be permitted, but none should be accidentally assumed acceptable. Add bus sectionalization and source availability to build the actual operating matrix. A successful study of the original pair does not cover the two new pairings or the three-unit bank. Identify which combinations are required for maintenance, contingency and normal operation before specifying the new transformer.
Use the actual existing units as the matching target
Retrieve factory test reports, tap schedules and any subsequent modifications for the installed transformers. An old tender value may differ from the measured impedance or ratio now relevant to sharing. Include connection-path changes and cooling capability. The new unit should be assessed against those data and the required bank operating range, rather than specified to copy only the original nominal kVA and percent impedance. Where existing records are missing, define an approved verification plan.
Capacity growth can expose switchgear and control limits
A third source can raise prospective short-circuit current even if each transformer remains within its own load rating. Check the common bus, feeder breakers and upstream protection against the revised fault study. The OLTC controller and communications architecture also need confirmed support for the proposed bank size and failure modes. Report any required control or switchgear modification separately from the transformer price, so the expansion decision reflects the complete system change.
Technical reference framework
[1] IEC 60076-8:1997 — Application guide.
[2] Schneider Electric — Substation including generators and parallel operation of transformers.