Redundant uninterruptible power supply systems

A controller for controlling a power supply system that includes a plurality of uninterruptible power supplies (UPSs), a ring bus, and at least one load electrically coupled to the plurality of UPSs and the ring bus, said controller comprising:

• a processor;
• and
• a memory device communicatively coupled to said processor, said memory device storing executable instructions configured to cause said processor to:
• calculate a phase angle for each UPS of the plurality of UPSs, wherein the phase angle is calculated relative to a common reference angle;
• and
• control operation of each UPS based on the respective calculated phase angles by gradually adjusting a phase angle of each UPS using a slow power transfer algorithm to prevent transient oscillations.