Systems and methods for seamlessly transferring a load between two ac sources of ac electrical power

Abstract

Methods for switching the power source supplying an alternating current (AC) electrical load from a first AC power supply to a second AC power supply comprising: (a) determining, during a time period, the zero voltage crossings of at the first and second AC power supplies; (b) estimating for a future time period, based on information obtained in said determining step (a), the times at which a series of future current zero-crossings of said AC load and at least the current zero-crossing of the second power supply; (c) based on said estimating step (b), determining whether during said future time period the time of said zero-crossing of said load current and the zero-crossing of the second power supply are within about 0.1 microseconds of each other; and (d) for a time period during which said zero crossings are estimated to be within 0.1 microseconds of each other, switching said load to said second power supply at said time at which said zero crossings are estimated to be within 0.1 microseconds of each other, wherein said switching: (i) uses a solid-state switching system and microprocessor-based control system for actuating said solid-state switching circuit; and (ii) accounts for any known actuation delay between the actuation signal from said microprocessor and the occurrence of said switching.