Patent ID: 7619905

Claim:
A method of controlling connection of a supply of AC power to a load and to a power supply grid, the supply of AC power being generated by an AC power generating system of the kind that comprises a source of power arranged to provide an electrical output, a converter which generates an AC power output to supply the load from the electrical output, and a control unit operable to control the operation of the converter and thereby to supply the power required to the load both when the AC power output of the AC power generating system is connected to the power supply grid, as well as to the load and during independent operation of the AC power generating system to supply the load including in the event of disconnection of the AC power output from the power supply grid, the control unit being operable in response to signals derived from sensed current and/or voltage of an electrical output which is generated by the converter from the electrical output of the source of power, wherein the current and voltage of the AC power output and the voltage of the power supply grid are monitored, and one reference which is derived from the monitored AC power output voltage and which is used as a reference in the operation of the converter to control the generation of the AC power output during independent operation of the AC power generating system to supply the load is replaced by another reference which is derived from the monitored grid voltage when the AC power output is to be connected to the power supply grid such that generation of the AC power output by the converter of the AC power generating system is controlled in accordance with the other reference that is derived from the monitored grid voltage when the AC power output of the AC power generating system is connected to the power supply grid as well as supplying the power required by the load, so that the voltage of the AC power output is changed to correspond to the grid voltage in phase and frequency, the method including the steps of: comparing one output signal produced by a voltage reference generator with the monitored AC power output voltage in a voltage controller which responds by producing said one reference; deriving a voltage reference signal from the monitored grid voltage, feeding that voltage reference signal to said voltage reference generator, operating said voltage reference generator to modify said one output signal so as to change its phase and amplitude progressively towards those of said voltage reference signal, and delaying connection of said AC power output to the grid until after said one reference and said voltage reference signal are substantially overlapping in phase and amplitude; and feeding said voltage reference signal to said voltage controller instead of said one output signal, and operating said voltage controller to compare said voltage reference signal with said monitored AC output voltage to produce said one reference once said one reference signal and said voltage reference signal are substantially overlapping in phase and amplitude.