A three-phase transformer carrying unbalanced single-phase loads must be checked phase by phase. Total three-phase kVA can look acceptable while one winding phase, neutral conductor or connection path is overloaded. The study should retain phase currents, neutral current, negative-sequence and zero-sequence components where relevant, because connection and core configuration determine how zero-sequence quantities circulate or are transferred.
Read the busiest phase, not just the total
For illustration, a 400 V, 300 kVA three-phase transformer has a rated line current of about 433 A. With a star-connected secondary, phase-to-neutral loads of 120, 90 and 90 kVA add to 300 kVA, yet the first phase draws about 520 A at 231 V. The total equals nameplate while one phase is approximately 20% above its nominal current. Actual permissible duty depends on the design and operating conditions; the arithmetic is a warning against using the three-phase total as the only loading check.
Fundamental imbalance and triplen current are separate
At the fundamental frequency, neutral current is the vector sum of phase currents, not the arithmetic sum of their magnitudes. Balanced positive-sequence currents cancel in the neutral. Zero-sequence harmonic components, including balanced triplen components in a four-wire load, can instead add. Obtain simultaneous phase and neutral measurements with harmonic content rather than reconstructing neutral duty from three unsynchronized clamp readings. Check the neutral bushing, terminal, conductor and grounding arrangement as distinct current paths; their ratings need not be identical.
Use the connection and core data to explain the result
A delta path can carry internal zero-sequence-related current even where the upstream line current does not show the same component. Core construction also affects the zero-sequence magnetic path, so “three phase” is not a complete model description. Ask for the manufacturer's unbalanced-loading limitations and applicable zero-sequence representation. Reallocating single-phase circuits may reduce a phase overload, but it does not automatically remove triplen harmonic neutral duty. Verify the revised phase schedule against peak operation, not only the convenient commissioning load.
Technical reference framework
[1] IEC 60076-1:2011 — Power transformers, General.
[2] IEC 60076-8:1997 — Application guide.
[3] Schneider Electric — Estimation of actual maximum kVA demand.