1. A controller for a three-phase uninterrupted power supply (UPS), the three-phase UPS providing power to a three-phase load via a series inductor for each phase and a capacitor connected electrically in parallel to the load for each phase, the three-phase UPS providing a voltage E, an output voltage v C, an inductor current i L, and a load current i 0 for each phase, the controller comprising: an inductor current module operative to to provide the inductor current value in each phase; a voltage module operative to provide the output voltage, V, in each phase; a three-phase to stationary coordinate converter operative to convert the measured inductor current and the measured output voltage in each phase into a inductor current in stationary coordinates and a output voltage in stationary coordinates respectively, and to convert a received reference voltage in each phase into a reference voltage in stationary coordinates; an adaptation processor coupled to the three-phase-to-stationary-coordinate-converter and receiving the output voltage and the reference voltage therefrom, the adaptive portion configured and arranged to calculate an estimate of a plurality of pre-selected harmonic components of the inductor current in stationary coordinates and to calculate an estimate of the value of the series inductor corresponding to each phase, and to convert the estimate of the value of the series inductor corresponding to each phase into stationary coordinates; a control vector processor coupled to the three-phase-to-stationary-coordinate-converter and to the adaptation processor, the control vector processor receiving the inductor current, the output voltage, and the reference voltage from the three-phase to stationary coordinate converter, and receiving the estimate of a plurality of pre-selected harmonic components of the inductor current in stationary coordinates and the estimate of the series inductance from the adaptation processor; the control vector processor including a proportional gain processor, the proportional gain processor configured and arranged to provide a proportional gain term as a function of the inductor current, the output voltage, the reference voltage, and the estimated inductance; the control vector processor further including a harmonic compensator processor, the harmonic compensator processor configured and arranged to determine a harmonic compensated control signal as a function of the plurality of estimated pre-selected harmonic components of the inductor current in stationary coordinates and the estimate of the series inductance, reference voltage, the output voltage; the control vector processor providing a control output vector that is a function of the harmonic compensated control signal and the proportional control signal.