Discontinuous Current Mode Control for Minimization of Three-phase Grid-Tied Inverters in Photovoltaic System

In this paper, a current control method for discontinuous current mode (DCM) is proposed for a three-phase grid-tied inverter to minimize inductors without worsening current total harmonic distortion (THD). In conventional continuous current mode (CCM) control, current THD increases as an inductor value is reduced because a zero-clamping phenomenon occurs due to dead-time. In the proposed DCM current control, a zero-current interval is intentionally controlled and a dead-time-induced error voltage is simply compensated with conventional dead-time feedforward compensation. The validation of the control method is confirmed by simulations and a 700-W prototype. Compared to conventional CCM current control, the current THD is reduced by 97.6% with the proposed DCM current control, whereas the inductor volume is reduced by 70%. In the experiments, the current THD is maintained below 5% over load range from 0.3 p.u. to 1.0 p.u. even when the inductance impedance is reduced to 0.5% of the inverter total impedance.