Voltage THD Reduction with Discontinuous Current Mode Control for Islanded-mode Operation in Single-phase Grid-tied Inverter

This paper proposes a filter capacitor design for a single-phase grid-tied inverter operating under islanded-mode with discontinuous current mode current control to reduce an inverter-side inductance and a filter capacitance without worsening output voltage distortion. The design considers the worst case of load, which is the non-linear load. A voltage source is applied to the circuit model of the load to obtain the harmonics of the current. By using these harmonics and the disturbance transfer function, the output voltage amplitude of the each harmonic components are obtained to calculate the output voltage total harmonic distortion (THD). Then, the filter capacitor satisfying the output voltage THD is determined by the theoretical output voltage THD calculation formula. The simulated output voltage THD agrees with the calculation value with the error of below 10%. As experimental results with 1.5-kVA prototype, the output voltage THD with the CCM/DCM control method and the current feedforward is reduced by 88.8% compared to that of the continuous current mode control method even with the minimized inductor impedance of 0.19% and minimized capacitor admittance of 6.7%.