A self-synchronised robust droop controller provides proportional power sharing among inverters. A self-synchronised mechanism is embedded into the controller for the automatic synchronisation between the inverter output voltage and the grid voltage reducing the complexity and computational burden of the controller. The controller comprises: a power unit where the real and reactive power of the inverter is measured and where current sent to the power unit can be switched between a virtual current generated using a virtual impedance and a measured inverter current; a voltage generation unit to form a sinusoidal voltage reference signal for the controller where first and second integrators generate the voltage amplitude and phase respectively; a third integrator forming a switched input to the second integrator, subtracted from the system frequency, and formed from a scaled power difference between the measured reactive power and a reactive power set-point, and the measured real power and a real power set-point; and a switch to selectively enable a scaled difference between a rated voltage and the output voltage to be added to the scaled power difference between the real power set-point and the measured real power to form the input to the first integrator.
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