Modeling, Analysis And Suppression On Harmonic Interactions Between Grid-connected Inverters And Utility Grid

Grid-connected inverter is the key interfacing equipment connecting renewable energy with utility grid. Grid current quality and its total harmonic distortion are an important performance index. For rejecting low-order harmonics of grid current caused by nonideal factors of inverter side or grid side, filtering high-frequency harmonics caused by pulse width modulation and analyzing harmonic interactions of grid-connected inverter systems with the utility grid, inherent characteristics of filter and closed-loop response characteristics of inverter system are studied and explored in depth. The main contents of this dissertation are as follows:1. Based on the analysis of harmonic attenuations and topology structure characteristics of L-type, LCL-type and LLCL-type filters, a unified circuit model of grid-interfacing filters is proposed. Through topology deductions and performance comparisons of the five-order and the following filters, two new filters, LLCCL1-type and LLCCL2-type filters, are proposed. Based on the analysis of Rd and Rd-Cd passive damping methods of LCL-type and LLCL-type filters, two methods are also introduced into LLCCL2-type filter. Resonance damping effects and high-frequency harmonic mitigations of different damping methods of three high-order filters are analyzed and compared. Simulation results are presented to demonstrate the correctness of the proposed LLCCL-type filters and the effectiveness of the proposed passive damping methods for LLCCL2-type filter.2. A single-loop control system of LCL-tpye grid-connected inverter with passive damping resistor in parallel with filter capacitor is firstly analyzed, including grid voltage synchronization based on SOGI-PLL and current control strategy based on propotional resonant(PR) controller. In addition, current tracking performance of propotional integral(PI) controller and PR controller is compared. In order to avoid loss power from passive damping, equivalent active damping, namely grid current and capacitor current feedback control system, is deduced in detail. Then equivalent impdance network model of double-loop control system of LCL-type grid-connected inverter is developed on the assumption that inverter is a linear system. Simulation results are given to verify the correctness of the designed SOGI-PLL and PR controller.3. To improve current quality of LCL-type grid-connected inverter, accurate equivalent impdance network model is proposed by incorporating dead-time effects and digital control delay into the model. Based on the developed model, harmonic suppression mechanism and performance of grid voltage feed-forward and multi-resonant control schemes is analyzed and compared from the standpoint of output impedance and current gain characteristics of closed-loop system. Then a combined harmonic suppression scheme is proposed. Simulation results are presented to demonstrate the effectiveness of the proposed low-order harmonic suppression strategies.4. Based on equivalent impedance network model of LCL-type grid-connected inverters, harmonic interactions of multiple grid-connnected inverter systems with utility grid are modeled and analyzed. To suppress harmonic interaction phenomena, full grid voltage feedforward control is incorporated into the control of grid-connected inverter control scheme. Simulation results are presented to demonstrate the correctness of harmonic interaction analysis and effectiveness of the proposed active suppression strategy.5. Experimental platform of a grid-connected inverter system is built, which is controlled with DSP TMS320F28335. Both hardware circuits and software programs are developed and tested. Based on the platform, passive damping methods of LCL-type, LLCL-type and LLCCL2-type filters and low-order harmonic suppression strategies for improving grid current quality are verified.