US Patent 7521825 Interface switch for distributed energy resources

A method and apparatus for an interface switch having an external terminal electrically connected to an AC voltage Ē at a frequency ω0 (for example a utility supply), and an internal terminal electrically connected to an AC voltage V at a selectable frequency ω (for example a microsource). In one embodiment, the interface switch is closed when the voltage difference between Ē and V and the relative phase angle δEV between Ē and V are both small, and when the higher frequency voltage (as between Ē and V) leads the lower frequency voltage. In another embodiment, the interface switch is closed when the envelope of the voltage difference between Ē and V reaches a local minimum at a point of inflection.