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Single-Stage Three-Phase Step-Up SEPIC Differential Grid-Tied Inverter Features an In-depth Mathematical Analysis for Solving Practical Design Issues

High-frequency transformer (HFT) utilization in a three-phase Grid-tied SEPIC-based differential inverter (SEPIC-BDI) increases its popularity in current inverter applications. Also, it provides high power density, low common-mode voltage (CMV), and low leakage current. However, the negative sequence harmonic component (NSHC), a common low-order harmonic in all types of differential inverters topologies, introduces a practical issue in this good topology. Where the leakage inductance of the utilized HFTs increases the percentage of NSHC. Therefore, this paper presents an in-depth mathematical analysis for NSHC and its practical design issues with the SEPIC converters. This analysis is vital during the design and control implementation of SEPIC-BDI. The analysis combines the differential inverter and SEPIC converters characteristics to properly investigate its primary source and provide helpful insight into the design process. It worth noticing that this mathematical analysis is generic for all types of differential inverter topologies and valid for other boost and buck-boost-based differential inverters. Moreover, a two-loop control strategy based on a Type-II controller in the first loop and low-pass filters (LPFs) in the second loop is performed to regulate the injected grid-current and eliminate the NSHC. Finally, the validity of this analysis is verified by numerical simulation and experimental supporting results.