Capacitor banks can interact with system inductance to create a resonance near a harmonic frequency. The transformer is part of that network through its leakage impedance and source strength. A resonance study needs capacitor kvar/steps, transformer impedance, upstream short-circuit strength, cables, reactors, harmonic sources and switching states; simply checking transformer kVA is not enough.
Use resonance estimates only as a screening calculation
For a simple predominantly inductive source feeding a shunt capacitor bank, the approximate parallel-resonant harmonic order is h ≈ √(Ssc/Qc), with short-circuit power and capacitor kvar expressed in consistent units at the same bus. An illustrative 25 MVA source strength and 1 Mvar bank give h ≈ 5. This identifies a potential fifth-harmonic concern, not a confirmed resonance magnitude or a reactor-selection rule. Resistance, other branches and frequency-dependent behavior alter the response.
Each capacitor step changes the network being studied
A bank with several switched steps presents different capacitances as demand changes. Parallel transformers and alternate upstream sources also change the inductive side of the network. Check credible combinations rather than only the fully connected bank at maximum source strength. A lightly damped state with fewer loads connected may govern harmonic amplification. Retain reactor data, tolerances and any detuning design, since a capacitor bank with series reactors is not represented correctly by kvar alone.
Evaluate capacitor and transformer duty together
The frequency scan identifies impedance peaks, while harmonic load flow estimates currents and voltages for the specified sources. Check capacitor RMS current and voltage, reactor heating and transformer additional losses against the resulting duty. A satisfactory fundamental power factor does not demonstrate acceptable harmonic stress. The engineering report should retain the governing switching state and the assumed harmonic spectrum so a later capacitor-stage addition or converter change triggers the appropriate review.
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.