Different control objectives for grid-connected converter under unbalanced grid voltage using forgotten iterative filter as phase lock loop

As the grid-connected converter is operating in the unbalanced three-phase grid in the traditional control strategies, the even harmonics of the voltage appear in DC-link and the odd harmonics of the current in AC-link, which will severely affect the performance of the grid-connected converter. Under this circumstance, the unbalanced three-phase grid will bring various negative effects and difficulty to the control of the converter, such as the fluctuation of active and reactive powers, non-sinusoidal and unsymmetrical three-phase currents. First, this study analyses the form of the fluctuation of power based on the instantaneous power theory and establishes a mathematical model of a grid-connected converter under unbalanced grid voltage. And then, a more complete description of the control problem of the converter is given, after three different control objectives, the fluctuation of active/reactive power and the non-sinusoidal and unsymmetrical grid currents, are considered at the same time. A novel method of separating the instantaneous positive and negative sequence components is proposed and well applied during achieving the different control objectives. Finally, the experimental results are compared, showing that the strategy is effective.