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Research on Control Strategy of Single-phase LCL-Type Grid-Connected Inverter based on Composite Repetitive Control
ABSTRACT With the rapid development of new energy generation technologies such as photovoltaic and wind power, the distributed power generation system (DPGS) based on renewable energy has attracted more and more attention all over the world. Grid- connected inverters, as an essential component of DPGS, play an important role in converting DC into AC between photovoltaic, wind power equipment, and the power grid. However, a lot of harmonics are generated by the dead time of the grid-connected inverter, the background harmonics from the grid voltages, nonlinear loads, etc., resulting in poor control performance, high total harmonic distortion (THD), additional power loss, and even system instability. Therefore, improving the quality of current and researching high-quality current control technologies for grid-connected inverters are of great significance. Repetitive control (RC) is widely used in grid-connected inverter control systems due to its excellent harmonic suppression performance. To improve the output current quality of the grid-connected inverter and improve the robustness and control accuracy of the system, this dissertation takes a single-phase grid-connected inverter as an application target, adopts composite repetitive control technology to reduce harmonics content in the output current of the grid-connected inverter. The main works of this dissertation are as follows. (1) A single-phase LCL-type grid-connected inverter model is created, and the parameters of the LCL filter are designed. Furthermore, to eliminate the resonant peaks generated by the LCL filter, various damping strategies are compared and analyzed. (2) By analyzing the principles, stability, harmonic suppression ability of the conventional repetitive control (CRC), and advantages of proportional-integral (PI) control, the composite repetitive controller composed of RC and PI in series or in parallel structures is introduced. Furthermore, taking the proportional integral multi- resonant repetitive control (PIMR-RC) composed of RC and PI in parallel as an example, parameters design, steady-state response, and dynamic performance analysis are conducted in detail. (3) The fundamental frequency of the power grid may fluctuate at ±0.5 Hz in DPGSs, and the ratio N is the sampling frequency to the fundamental frequency of the power grid may be a fraction. However, CRC has excellent control performance only N is an integer, or it will result in a significant decrease in signal tracking and harmonic suppression performance. To ensure that the repetitive controller can accurately track reference current even when the grid frequency fluctuates and to reduce computational load and memory consumption, based on a Farrow-structure filter, a fractional-order delay PIMR-RC (FOD-PIMR-RC) scheme is proposed, which greatly improve the quality of the grid current against frequency fluctuations. Then, the stability analysis and the harmonic suppression performance of the proposed scheme are analyzed. Finally, the simulation results demonstrate the effectiveness of the proposed scheme. (4) To reduce the computational load and memory consumption, multirate repetitive control (MRC) is adopted in the PIMR-RC system for grid-connect inverters. Although MRC provides a flexible and efficient design solution, it usually adopts a downsampling rate approach. CRC with integer-order phase lead compensation cannot exactly compensate for the system phase lag, which may result in an unstable system in the case of low sampling frequency. Therefore, a fractional-order phase lead PIMR-MRC (FOPL-PIMR-MRC) scheme, employing an infinite impulse response (IIR) filter, is presented for grid-connected inverters. The proposed scheme includes the design of a fractional-order phase lead compensation filter, along with stability analysis, parameter design, and comprehensive simulation analysis. The steady-state and dynamic simulation results confirm that the proposed control scheme effectively achieves accurate phase compensation, enhances the stability margin of the system, and reduces hardware consumption. Additionally, it ensures excellent performance in harmonic suppression.
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