Common-Mode Resonance Suppression Strategy for 3-Level Grid-Connected Inverter with the ILCL Filter

In order to suppress the leakage current and the common current flowing into the grid, a low impedance loop is built for common signals by the improved LCL filter (ILCL) which is largely applied into transformer-less 3-level grid-connected PV string inverters. However, this loop may also bring potential common mode (CM) resonance which is highly related to the modulation strategy. Double Fourier transformation (DFT) is applied to compare the theoretical harmonic distribution between the space vector pulse width modulation (SVPWM) method and the alternative discontinuous pulse width modulation method (DPWMA). It is found out that harmonics in middle frequency region with the DPWMA is larger than with the SVPWM. Since the benefit brought by the DPWMA includes not only non-switching losses but also non-current ripple in the voltage-clamped region, it is more preferable in applications where the inverter side inductor expresses soft-saturation property. By analyzing the DFT equation, we optimize the modulation index. Then, a CM resonance suppression strategy is proposed for 3-level ILCL filtered inverter with the DPWMA. Finally, the proposed strategy is verified by the experiment.