Impedance shaping method to improve grid-connected inverter under weak grid condition

Due to the popularity of renewable energy, e.g., wind power and solar energy, renewable energy generation systems(REGs) have been preferred as eco-friendly power generation systems. Distributed generation resources, including renewable energy sources, are essential for the distributed generation system. Grid-connected inverters(GCIs), as indispensable parts of the distributed generation system, are endangered by the non-ideal conditions. For instance, the harmonics, high impedance in weak grid condition, and instability issues caused by impedance mismatching. In order to address the non-ideal conditions mentioned before, there are various improving technologies being modified to enhance the performance of grid-connected inverters. Generally, the LCL filters are applied between the inverter and the power grid to suppress the harmonics. However, the resonance circuit also leads to distortion and even instability issues. Therefore, the passive damping and active damping schemes are introduced in the topology of the inverter and control block. Besides, active power filters, unlike passive damping using passive damper, utilize active components to compensate for the harmonics and therefore eliminate the negative effect of harmonics. Nonetheless, the variable grid conditions and digital delay are still serious problems for the grid-connected inverter. Firstly, unlike traditional active power filter, especially series active power filter, the adaptive controller for the APF model is required to improve the robustness in the variable grid conditions. According to model simplification, impedance-based stability criterion and bode plot analysis, it is indicated that the proposed control method certainly increases the phase margin and stabilizes the system in the ideal case. Eventually, the proposed series adaptive stabilizer is applied in the tested single-phase grid-connected inverter with a weak grid. Moreover, the experimental results are demonstrated, which prove the correctness of it. Despite the active power filter, the active damping methods are widely applied in the inverter side to achieve the impedance shaping. Unlike the modified active power filter, the active damping method provides virtual impedance to shape the output impedance of inverter with no power loss. However, the negative effect of digital delay in the digital controller and the potential influence of weak grid condition are challenging the performance of control design. Therefore, the hybrid controller is applied and verified to be effective in eliminating the negative effect of digital delay and weak grid. In this thesis, impedance shaping methods, including active damping and adaptive active power filter, are proposed and the performance of inverter will be improved. As the limitation of single-phase inverter, the feasibility of the series adaptive active power filter should be investigated with new challenges. For future works, the application of series adaptive stabilizer will be discussed in three-phase inverter and parallel-inverter system, respectively.Master of Engineerin