UPS Paralleling Circulating Current

Estimate the circulating current between two paralleled UPS inverters from voltage-magnitude and phase-angle mismatch, plus the load-share unbalance for unequal module ratings.

Inputs

Inverter output voltage V V

Voltage mismatch ΔV V

Phase mismatch Δδ °

Output reactance X Ω

Total load P kVA

Module 1 rated kVA

Module 2 rated kVA

Module rated current Iₙ A

Results

Note — Circulating current uses the two-source model I_circ = ΔV/(2X) (magnitude) and I_circ = V·Δδ/(2X) (phase). Real UPS modules use active load sharing with inner impedance control, but this gives the worst-case open-loop estimate. Keep circulating current well below the module rating and confirm against the UPS paralleling manual and a certified engineer.