In distributed generation systems, the inverter is the main power interface and its stability directly determines the reliable operation of the grid-connected system. As a typical topology for a three-phase four-wire inverter, the LCL-type three-phase four-wire split capacitor inverter (LCL-TFSCI) is taken as the research subject of this paper. Compared with the three-phase three-wire inverter, there is an additional zero-sequence path in the LCL-TFSCI. Therefore, it is not only necessary to consider the stability of the positive and negative sequence system, but there is also the need to consider the stability of the zero-sequence system when performing stability analysis for the LCL-TFSCI. In this paper, a small-signal impedance model considering the zero-sequence loop of LCL-TFSCI is firstly established. Subsequently, the instability risk is revealed when LCL-TFSCI is connected to the grid with parallel compensation capacitors. Through instability analysis, an impedance-reshaping method based on the complex filter and combined differential elements is proposed, which can reshape the impedance characteristic of LCL-TFSCI within the wide frequency range and expand the stability domain of the grid-connected system. Finally, the proposed method is verified by simulation and experiment.

In a distributed generation system, the all-pass-filter phase-locked loop (APF-PLL) is a commonly used method for grid synchronization. However, the coupling effect between APF-PLL and current control loop increases the risk of oscillation instability for the inverter in the weak grid. At present, there are few effective methods to solve the adverse effect of APF-PLL on the inverter-grid interconnection system in the weak grid. Therefore, a small-signal impedance model of the inverter considering the dual d-q frame brought by APF-PLL is first established. Then the reason for the inverter instability caused by APF-PLL in the weak grid is analyzed. Subsequently, an impedance reshaping method based on a modified first-order filter PLL with a complex coefficient filter (CCF-MFOF-PLL) and its parameter optimization design method are proposed. Finally, the experimental results verify that the proposed method widens the stable range of the inverter and ensures the stable operation of the inverter even with the large grid impedance.

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Given the difficulty of accurately setting multiple control parameters in wind turbines, a design method for a pitch controller considering tower load reduction is proposed, which enhances the control performance and reduces both the tower vibration and load. Firstly, the pitch-speed system and the tower fore-aft active damping control are built. In addition, the explicit equation of the tower fore-aft active damping gain is deduced to calculate its initial value. Secondly, the pitch-speed system is identified as an inertial time-delay system using the least squares method. Subsequently, the pitch PI control parameters are set using the Chien–Hrones–Reswick method. Thirdly, the pitch PI control parameters and the tower fore-aft active damping gains are optimized based on the kindred-protected genetic algorithm, which improves the accuracy of the control parameters. Meanwhile, the Pareto method is used to coordinate the control objectives by allocating the weight. Furthermore, the adaptive control is built by fitting the parameters with the wind speed points using the least squares method to enhance the control performance. Finally, the effectiveness of the proposed design method is verified by comparing the control performance with the tower vibration and load.