A UPS is adapted to receive power from a polyphase AC source. The UPS
includes power conversion circuitry adapted to convert the power supplied
by the polyphase AC source to DC power. The power conversion circuitry
includes phase conductors. The UPS also includes a neutral coupled to an
output of the UPS, a plurality of input capacitors coupling the phase
conductors to the neutral, a DC system including a positive DC bus with a
positive DC bus voltage and a negative DC bus with a negative DC bus
voltage, and a control system. The control system is adapted to control a
difference between a magnitude of the positive DC bus voltage and a
magnitude of the negative DC bus voltage by controlling a DC voltage
across the input capacitors.