Efficient SEPIC Differential Mode Inverter with New Feedforward Control Technique for Selective Harmonic Compensation

In this paper, isolated single-stage three-phase SEPIC differential mode inverter is presented for grid-connected applications. Grid current control is implemented by a two-loop control technique to inject pure AC current into grid and mitigate Negative Sequence Harmonic Component NSHC which is a common $2^{\mathrm{n}\mathrm{d}}$ order component in all differential inverter topologies. The first loop is simply validated by using a PI controller. Also, a novel feedforward control is used in the second loop instead of conventional feedback control method by detecting third-order $3^{\mathrm{r}\mathrm{d}}$ component in the input current of the inverter rather than direct sensing of $2^{\mathrm{n}\mathrm{d}}$ order NSHC from the output currents. Then, a simple PI controller is implemented to mitigate the 3rd component from input current which directly removes $2^{\mathrm{n}\mathrm{d}}$ NSHC from grid output current. Proposed control has higher bandwidth than traditional control method and provides pure input and output current waveforms. On other hand, simple active-clamp circuit, consisting of one switch and two small capacitors, is designed, and integrated to verify ZVS and ZCS of the main and synchronous switches of the utilized SEPIC converters which enhance the efficiency by reducing the switching losses. Moreover, the active-clamp circuit diminishes the peak voltages of the switches and enhances the reliability of the inverter. Inverter operation along with mathematical analysis of the proposed control method and soft-switching operation is presented. Finally, the theoretical assumptions are supported by simulations and experimental results.