Harmonic resonance calculation in power-factor correction
The formula is derived from the basic formula for a simple resonant circuit;

\$f_0 = \frac{1}{2 \pi\sqrt{LC}}\$

L and C are calculated from the short circuit impedance and the capacitor bank impedance. The short circuit impedance is assumed to be purely inductive. That assumption is justified by the assumption that the transformer X/R ratio is 5 for three-phase distribution transformers smaller than 150 kVA and progressively higher for larger transformers. X/R is assumed to be 8 for a typical 1 MVA transformer. Since the capacitor and transformer impedances are based on the relevant power frequency, that frequency is the fundamental for the harmonic order calculation. On other words, the formula is valid for either 50 or 60 Hz as long as the capacitor bank VAR rating and the short circuit VA are stated or calculated based on the frequency of interest.

The formula is given in IEEE Std 519-1992. I found the assumptions and derivation in seminar notes printed for a 1990 seminar.