A tap mismatch creates a secondary no-load voltage difference between paralleled transformers. Because the loop impedance is low, even a modest ratio difference can drive circulating current before useful load is supplied. The study should calculate the no-load voltage mismatch at the actual taps and combine it with transformer plus connection impedance; control logic must prevent incompatible tap positions from persisting.
Use the voltage difference around the parallel loop
For a simplified two-transformer model, circulating current is Ic = (E1 − E2)/(Z1 + Z2), with all quantities referred to the same phase-voltage and current bases. The impedances include both transformer and connection paths. A 1% no-load voltage difference with 6% impedance in each branch gives a circulating-current magnitude of about 0.01/0.12 = 0.083 per unit when the branch angles are equal. This illustrative 8.3% current can exist even with no external load.
Do not add circulating and load currents arithmetically
Each branch current is the vector sum of its load-sharing component and its circulating component. The circulating components have opposite directions around the loop, but their effects on the two current magnitudes depend on the load power factor and impedance angles. Simply adding 8.3% to both load percentages gives the wrong general result. Calculate individual winding currents and reactive power for the relevant taps, including light load where circulation can dominate the branch measurement.
Compare actual ratios, not tap-position numbers
Position 8 on one tap changer need not produce the same ratio as position 8 on another. Tap direction, neutral position, step size and the winding carrying the taps can differ. Build a ratio-to-position mapping from the approved data and verify position feedback in the control system. A permissible mismatch must come from the electrical and control assessment; it should not be assumed that one tap apart is always harmless or that matching position labels guarantees zero circulation.
Technical reference framework
[1] IEC 60076-8:1997 — Application guide.
[2] Schneider Electric — Substation including generators and parallel operation of transformers.