Triplen harmonic components can add in the neutral of a four-wire star system instead of canceling. This occurs when the relevant components are zero sequence, as with a balanced set of similar single-phase nonlinear loads. Their phase relationships and the transformer connection must be retained when assessing neutral and winding heating.
Balanced fundamental currents do not cancel the third harmonic
In a balanced set of similar phase-to-neutral nonlinear loads, fundamental currents are separated by 120 electrical degrees. At the third harmonic, that displacement becomes 360 degrees, so the third-harmonic components can add in the neutral. If each phase has an illustrative 20 A third-harmonic component in phase with the others, the neutral carries 60 A at that order even though the fundamental neutral current is zero. Actual loads require the measured harmonic phase relationships.
Combine different frequencies by RMS, not simple addition
First add the three phase-current phasors at each individual harmonic order. Then combine the resulting neutral components across different frequencies by the square root of the sum of squares. For example, 60 A at the third and 30 A at the ninth give about 67.1 A harmonic neutral RMS current, not 90 A. Add any fundamental unbalance through the same RMS process. The resulting current still needs a frequency-aware thermal assessment of the neutral conductor and connections.
A delta path redirects components rather than erasing heating
A delta winding can provide an internal path for zero-sequence triplen current components, affecting what appears in external line currents. It does not remove the heating duty in the involved windings or the neutral on the star-connected side. Review the full connection, neutral terminal rating and harmonic loss calculation. A small upstream triplen measurement is therefore not proof that the transformer carries no triplen-related internal current.
Technical reference framework
[1] IEEE C57.110-2018 — Transformer capability with nonsinusoidal load currents.
[2] IEC 60076-1:2011 — Power transformers, General.
[3] IEEE C57.105-2019 / Cor 1-2023 — Three-phase transformer connections.