Patent ID: 7427815

Claim:
A method for preventing islanding in a power system that includes a power grid having a feeder connected in circuit with a distributed generator and at least one load, said method comprising: determining a phase shift of a voltage based solely on sequential frequency measurements at an output of said distributed generator; comparing said phase shift to a threshold phase shift that signifies a phase shift due to a disconnect of said grid from said feeder; and is said phase shift is greater than said threshold phase shift, issuing a command for a disconnect of said distributed generator from said feeder, wherein said phase shift is determined by: θ n = 2 ⁢ ⁢ π · ( 1 - f n - 1 f n ) , where θ n is said phase shift and f n and f n−1 are the frequencies at a current zero-crossing and a previous zero-crossing of said voltage, respectively.