Research On Suppressing Harmonic And Unbalanced Injected Grid Currents Caused By Grid Voltages For The Three-Phase LCL-Type Grid-Connected Inverter

As the interface between the renewable energy based distributed power generation system (RE-DPGS) and the power grid, the grid-connected inverters play an important role of the power conversion. Since the injected grid currents are inevitably affected by the distortion and imbalance of the grid voltages in practice, the suppression of the harmonics and imbalance of the injected grid currents from the grid-connected inverters is critical to the safe running, stability and high-quality operation of the RE-DPGS. Therefore, this dissertation is dedicated to the suppression of the harmonics and imbalance of the injected grid currents from the widely used three-phase LCL-type grid-connected inverters.The averaged switch models of the stationary α-β frame and synchronous rotating d-q frame controlled three-phase LCL-type grid-connected inverter are derived in this dissertation. With these models, the impacts of grid voltages on grid currents are analyzed, and it is found that the grid currents are not only directly affected by the grid voltages, but also affected by the reference error of injected grid currents caused by grid voltages. This part of work provides the foundation for the suppression of the the harmonics and imbalance of the injected grid currents from grid-connected inverters in the subsequent chapters.To suppress the disturbance component of the injected grid currents directly caused by the grid voltages, the full-feedforward schemes of grid voltages for the stationary a-/3frame, synchronous rotating d-q frame and hybrid frame controlled three-phase LCL-type grid-connected inverter are proposed in this dissertation. The full-feedforward function includes three parts, namely, the proportional, derivative and second derivative parts. The characteristics of the three parts are discussed, and thus the necessity of the full-feedforward schemes is proved. Considering the practical applications, the harmonic and imbalance attenuation affected by the LCL filter parameter mismatches is analyzed. By setting the filter capacitor in the LCL filter to zero, the full-feedforward scheme of grid voltages for the three-phase LCL-type grid-connected inverter is reduced to a proportional part, which is the feedforward scheme of grid voltages for the three-phase L-type grid-connected inverter. The correctness of the theoretical analysis and effectiveness of the proposed full-feedforward schemes of grid voltages are verified by the experimental results.To suppress the reference error of the injected grid currents caused by the harmonics and imbalance of grid voltages, the prefilter based phase-locked loop in the synchronous reference frame (SRF-PLL) is analyzed. Prefilters are derived and analyzed in the complex-vector-filter perspective. According to the complex-vector transfer function, prefilters are categorized into first-order, second-order, and multiblock based high-order complex-vector fitlers. And the second-order and multiblock based high-order complex-vector filters and their scarlar realization are derived based on the first-order one. It is pointed out that the first-order and second-order complex-vector filters are the special cases of the multiblock based high-order complex-vector filter. Based on the analysis, the generalized second-order complex-vector prefilter (CVF) is proposed to achieve better dynamic performance of extracting the fundamental positive and negative component of grid voltages under the sudden unbalance grid faults. Besides, the third-order CVF is proposed to achieve better harmonic attenuation while extracting the grid voltage information. Finally, the correctness of the theoretical analysis and effectiveness of the proposed two improved prefilters are verified by the simulation and experimental results.Taking the three-phase LCL-type grid-connected inverter controlled in the stationary α-β frame as an instance, the output impedance of the grid-connected inverter is analyzed. With the impedance based stability criterion of grid-connected inverters, the stability of three-phase LCL-type grid-connected inverters with and without the grid-voltage full-feedforward scheme is analyzed under weak grid condition. It is pointed out that the grid-connected inverter with full-feedforward scheme might not be stable under weak grid condition due to its increased negative phase-shift. A selective-feedforward scheme of grid voltages is proposed for improving the stability of the grid-connected inverter under weak grid condition, and the harmonics and imbalance of the injected grid currents from grid-connected inverters can be effectively suppressed.