Current THD reduction for high-power-density LCL-filter-based grid-tied inverter operated in discontinuous current mode

This paper proposes a discontinuous current mode (DCM) feedback current control for a single-phase grid-tied inverter in order to minimize a LCL filter without worsening total harmonic distortion (THD) of a grid current. In DCM, there are two nonlinearities occurring in the transfer functions; the first non-linearity occurs in the duty-ratio-to-current transfer function which worsens the current command response, whereas the second nonlinearity occurs in the disturbance-to-current transfer function which reduces the disturbance effect. In the proposed DCM current control, the first nonlinearity is compensated by utilizing the duty ratio at the previous calculation period in order to achieve the same control performance of the current command response as in continuous current mode. Meanwhile, the second nonlinearity is utilized in order to reduce the disturbance effect when the LCL filter with a small impedance is applied. Furthermore, a design procedure of the LCL filter is introduced under the condition that the impedance of the LCL filter can be minimized without worsening the grid current THD by applying the proposed DCM control. A 1-kW 100-kHz inverter with several LCL filters of different impedances (3.0%, 0.6% and 0.04%) is constructed in order to confirm the operation of the proposed DCM current control. As a result, the grid current THD is reduced from 8.5% to 3.7% at rated load. Furthermore, the inductor volume is reduced by 77.0%, whereas the converter loss is reduced by 17.1%.