According to ABB switchgear manual, the direct connection of a capacitor to a transformer, together with which it is switched on and off, is possible and permissible on both the HV and LV sides.
According to VDEW specifications, when connecting capacitors on the low-voltage side, the capacitor ratings must be as stated in Table below.
If the capacitor is fitted on the low-voltage side of the transformer, in the case of networks having a high harmonics content, it is necessary to check whether a voltage resonance at a harmonic present in the network (usually the 5th and 7th harmonic) can occur between the capacitance of the capacitor and the leakage inductance of the transformer. The maximum capacitor rating can be defined approximately as
where SrT is the transformer rated power in kVA, and Qc the capacitor rating in kvar, and ukr the rated impedance voltage (in per cent) of the transformer and the feeding network, and v is the number of the highest critical harmonic.