Digital boundary controller for single-phase grid-connected CSI

The theory, design, and implementation of the digital boundary controller for the single-phase grid-connected current source inverter (CSI) is presented in this paper. The concept is based on extending an analogy-based boundary control technique originally proposed for voltage-mode converters to deliver regulated voltage into the one for grid-connected CSI that delivers regulated current. A discrete form of the second-order switching surface that resembles state-plane trajectories passing through the target operating point is derived. The controller first senses the voltage and current of the output filter inductor. It will then estimate the output current trajectory after a hypothesized switching action. Finally, it will generate switching command at an appropriate instant, so that the output current will go within the predefined hysteresis band. The ultimate goal of this control method is to make the inverter exhibit good steady-state spectral characteristics and dynamic responses. The CSI can settle into steady state in two switching actions after a large-signal disturbance. The proposed scheme has been successfully applied to a 550 W, 220 V, 50 Hz prototype and the controller is implemented with a digital signal processor TMS320F2808. The steady-state performances and the time-domain dynamic responses of the inverter under different types of large-signal disturbances will be given.