A three-winding transformer needs more than one headline MVA value. Each winding has its own rating and current limit, and the permitted simultaneous loading depends on how power is exchanged among the three circuits. Procurement should state the expected transfer cases explicitly—for example HV-to-MV only, HV-to-LV only, and simultaneous MV/LV loading—so thermal design, pairwise impedances and protection studies use the same operating matrix.
Do not add winding ratings as if they were generators
A notional 40/25/15 MVA HV/MV/LV rating schedule does not describe an 80 MVA source. It describes limits at three electrical interfaces. In an HV-fed case, simultaneous MV and LV exports must be supplied through the HV winding, allowing for losses and reactive demand. Their individual limits still apply. If generation can feed an MV or LV winding, the direction and distribution of internal current change. State import and export signs consistently so a study does not silently assume that one winding is always the supply.
Define a loading matrix with power factor
Include HV-to-MV only, HV-to-LV only, simultaneous downstream loading and any cross-feed case expected in operation. For each row, identify winding MVA, real and reactive power, voltage, tap position, duration and cooling stage. Equal MVA totals can create different winding-loss distributions when the power split changes. A heavily loaded tertiary may also impose lead or terminal limitations not visible in the HV rating. Ask the manufacturer to identify prohibited combinations rather than issuing three independent maximum values with no simultaneity statement.
Use the same cases for design and network studies
Pairwise impedance tests are performed under defined winding conditions and must be supplied on clearly stated bases. Convert them consistently before building the three-winding network equivalent. The protection team also needs CT locations, connections, grounding and the permitted power directions. Keep the approved loading matrix with the transformer data sheet and revise both when a tertiary is repurposed. A factory test on one winding pair does not, by itself, demonstrate the temperature performance of every simultaneous loading combination.
Technical reference framework
[1] IEC 60076-1:2011 — Power transformers, General.
[2] IEC 60076-8:1997 — Application guide.
[3] IEC 60076-7:2018 — Loading guide for mineral-oil-immersed power transformers.