Three-phase uninterruptible power supply maintaining reserve energy sources in idling condition with unbalanced loads

Claim 1

A control system for a three-phase uninterruptible power supply having a ferroresonant power coupling transformer in each phase, an inverter circuit in each phase for generating reserve power signals, means for coupling a centralized DC voltage source to each of said inverter circuits and operative for coupling energy thereto, means for nulling total current flow from the centralized reserve voltage source to all the inverter circuits as a group, said means for nulling comprising: a master phase-lock loop including phase detection means for synchronizing with a primary three-phase signal and including phase dividing means operative to generate a reference phase signal in each phase, separated from each other by 2π/3 radians a first phase-lock loop in each phase operative to phase lock an output signal phase of the inverter to the reference phase signal in each phase, monitoring means for sensing a current output of the centralized DC voltage source, and controllable phase shift means connected in the master phase-lock loop and responsive to current flow sensed by the monitoring means to alter a signal output of the phase detection means and simultaneously shift all three reference phase signals uniformly and thereby simultaneously altering a phase of operation of all the inverters, whereby individual inverters may supply power to and or absorb power from an output of its ferroresonant power coupling transformer and the net total power flow is operative to null current flow in said means for coupling.