Patent ID: 7492058

Claim:
An uninterruptible power supply (UPS), comprising: a) a plurality of modules connected in parallel, wherein each module includes: 1) an AC-DC-AC converter having a DC bus voltage, 2) a subcontroller for controlling the AC-DC-AC converter; b) a memory readable by the subcontroller, wherein the memory stores a coefficient K that is unique for each module, and wherein the memory stores an efficiency lookup table that is unique for each module; c) a voltage measurement device or devices for measuring in each module: a DC bus voltage; an output voltage, and an input voltage; d) an output current sensor for measuring a total aggregate output current from all the modules; e) instructions stored in the memory for instructing the subcontroller to perform the following steps: 1) for each module, calculating an input current reference from a DC bus voltage target and the measured DC bus voltage; 2) for each module, calculating an input current estimate from the coefficient K and the input current reference; 3) for each module, calculating an output current estimate from the input current estimate, an efficiency value from the lookup table, a measured input voltage and a measured output voltage; 4) for each module, calculating a new DC bus voltage target from the output current estimate and an output current target, 5) for each module, adjusting the AC-DC-AC converter using input current reference so that the DC bus voltage changes towards the new DC bus voltage target.