With nonlinear loads, displacement power factor and total power factor are not the same. Capacitor correction can improve displacement power factor while leaving distortion current unchanged and may even alter resonance conditions. Transformer loading should be based on RMS current and apparent power, while the capacitor/harmonic study separately checks resonance, current amplification and voltage distortion.
True power factor includes waveform distortion
For a single-phase or balanced per-phase calculation, true power factor is P/(Vrms Irms). With approximately sinusoidal voltage and negligible harmonic active power, it can be approximated by displacement power factor divided by √(1 + THDi²), using THDi as a decimal. An illustrative displacement factor of 0.98 with 40% current THD gives about 0.91 true power factor. A near-unity fundamental phase relationship therefore does not establish near-unity total power factor.
Capacitors do not automatically reduce distortion current
A shunt capacitor can supply fundamental reactive power and reduce the source's corresponding reactive-current component. Harmonic currents remain a separate issue, and the capacitor changes the network impedance at harmonic frequencies. As the fundamental source current falls, a THD percentage can even increase without a corresponding increase in absolute harmonic current. Evaluate amperes and spectra alongside the reported percentage rather than interpreting every rise in THD as a proportional increase in transformer heating.
Confirm the net transformer benefit after compensation
Compare pre- and post-compensation RMS current, apparent power, voltage spectrum and calculated losses over the required load range. Include resonance and capacitor-step states, especially at light load. The desired result is useful capacity or loss reduction without unacceptable harmonic amplification or overvoltage. Where both displacement correction and harmonic mitigation are required, specify those objectives separately and verify the combined network response rather than assuming one capacitor-bank selection accomplishes both.
Technical reference framework
[1] IEEE C57.110-2018 — Transformer capability with nonsinusoidal load currents.
[2] IEC 60076-1:2011 — Power transformers, General.
[3] IEC 60076-8:1997 — Application guide.