Controller design and implementation of indirect current control based utility-interactive inverter system

The utility-interactive inverter with critical load should be able to provide critical loads with a stable and seamless voltage. This letter proposes an indirect current controller which does not include any nonlinear factors in the control block, so that the classical control theory can be applied to the controller design. Also, a phase-locked loop algorithm is proposed to maintain the constant frequency across the critical load during the unintentional islanding since the magnitude is well regulated, but the frequency may vary with the proposed controller. Further, an islanding detection method is proposed for the controller, which does not cause a change in magnitude and frequency of critical load voltage.