scale grid in that as the frequency is liable to fluctuate due to a load fluctuation there is liable to be a false detection of an isolated operation in contrast an abnormal frequency stage detection can be performed such that the frequency at a point when the rising edge of a reactive power fluctuation is detected and the frequency at a point when the falling edge is detected are detected sequentially and an abnormal frequency stage detection is recognized when the order of the difference between the current detection value and the previous detection value is u201c increase u201d u201c decrease u201d u201c increase u201d application_number US 201113179518 A description a main point of the invention is that in a dispersed power source that outputs power generated in interaction with a utility grid a very small reactive current is output when the grid is normal a larger constant reactive current is applied when the grid falls into a power failure condition and it is detected that the output frequency of the dispersed power source has exceeded an abnormal frequency stage detection level and when it is determined that there is an isolated operation when the frequency reaches an isolated operation detection level an abnormal frequency stage detection is such that the frequency at a point when the rising edge of a reactive power fluctuation is detected and the frequency at a point when the falling edge is detected are sequentially detected and an abnormal frequency stage detection is recognized when the order of the difference between the current detection value and the previous detection value is u201c increase u201d u201c decrease u201d u201c increase u201d fig1 shows a circuit example of a grid interactive inverter to which the invention is applied a filter formed.