Harmonic derating depends on the actual spectrum and transformer loss characteristics; it cannot be assigned from one THD number. Higher-frequency current components can disproportionately increase eddy and stray losses. The engineering file should identify the load spectrum, transformer type, winding loss data or approved calculation method, ambient/cooling condition and resulting allowable load.
Separate the loss components before reducing the rating
Basic resistive loss, winding eddy loss and other stray losses do not all respond identically to frequency. A harmonic capability calculation therefore needs either the relevant loss separation or a manufacturer-approved model suitable for the transformer construction. Total load loss from one sinusoidal test is useful, but does not uniquely identify every frequency-dependent component. Record the method and its assumptions so the resulting allowable current is not presented as a universal property of all transformers with the same kVA.
A reduced current still needs a thermal check
Reducing RMS current can offset some additional loss, but the permissible reduction depends on how the harmonic spectrum changes with load. A converter may have a different spectrum at 50% output than at rated output, so scaling every harmonic in direct proportion may be wrong. Include ambient temperature, enclosure and cooling availability in the thermal assessment. The limiting winding or hotspot should be identified where the method supports it, rather than checking only an aggregate loss total.
Specify the useful output that must remain available
For procurement, state the required continuous kVA or kW together with power factor, spectrum and duty cycle. The supplier can then offer a design meeting that duty or declare a supported derating of an existing unit. Compare the two options on actual usable output, losses and operating constraints. A generic instruction to “derate by 20% for harmonics” does not establish that the required load can be served safely or economically.
Technical reference framework
[1] IEEE C57.110-2018 — Transformer capability with nonsinusoidal load currents.
[2] IEC 60076-1:2011 — Power transformers, General.