A method of controlling connection of an AC power output to a load and
power supply grid. The power output is produced by an AC power generating
system which includes an AC/AC converter and is independently operable to
supply power to the load. The converter is controlled in accordance with
a reference generated within the system. To connect the power output to
the power supply grid and to supply the load, the reference is replaced
by another reference derived from the voltage of the grid, so that the
converter is controlled by that other reference, when the power output is
connected to the grid and supplies power required by the load. When the
power output is disconnected from the grid or loss of the grid voltage
occurs, the reference replaces the other reference, and the power
generating system operates independently continuing to supply power
required by the load without interruption.