Power Quality and Stability—Control Strategy and Grid Impedance Detection of Three-Phase Grid-Connected Inverters Under Weak Power Grids

In recent years, with the rapid development of power electronics technologies, a significant number of power electronic devices are connected to low voltage distribution power networks. As a result, a large number of nonlinear loads are introduced into the power grid, which brings harmonic pollution to the power grid and makes the power grid environment worse. With the rapid application of distributed generation systems in the power grid, the research of grid-connected inverters has attracted increasing attention, and researchers worldwide have conducted in-depth research on the modeling, control strategy, and other aspects of grid-connected inverters. This chapter analyzes the control structure and mathematical model of LCL three-phase grid-connected inverter with inverter current feedback control. Then, the capacitor voltage Lead compensation feedforward method and the point of common coupling (PCC) voltage control reactive power compensation method are used to solve the LCL resonance problem and the PCC voltage drop problem when the three-phase grid-connected inverter operates under a weak power grid. The effectiveness of the proposed scheme is verified through comparative simulation experiments in MATLAB/Simulink, achieving the goal of stable operation of PV grid-connected inverters under weak power grid conditions of short circuit ratio (SCR). Furthermore, this chapter proposes a new grid impedance detection method incorporating the complex coefficient filter (CCF) with full-order capacitor current observer for a T-type three-level grid-connected inverter controlled by the inverter output current feedback. Compared with conventional CCF impedance detection algorithms, the proposed method reduces the number of current sensors and detects the grid impedance accurately. First, based on the sampled inverter output current and grid-connected voltage signals, the grid-connected current is estimated. Then, the CCF method is used to extract harmonics from the grid-connected current and voltage signals to calculate the grid impedance. Finally, the correctness of the full-order capacitor current observer is verified by simulation and the feasibility and effectiveness of the proposed algorithm are verified experimentally based on a laboratory prototype.