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As the grid impedance increases, the grid strength gradually becomes weak. In order to have good output characteristics in weak grid, grid-forming (GFM) inverters with LCL filter have gradually become popular. This paper studies grid-current-sensorless control which does not require grid current sensors. Firstly, the grid-current-sensorless control loops are given. Then, in order to make up for the inability to sample the actual grid current, a grid current estimator, which is derivative operator free and low-complexity, is proposed. And the robustness and accuracy of the estimator is verified by simulation. Afterwards, to demonstrate that this control operates stably when the grid strength changes, a joint small-signal stability analysis combining the control strategy and circuit topology is presented. The theoretical feasibility of the control is proved. Finally, to verify the robustness of the control under different grid strengths, the experiments are carried out on a 20kW GFM inverter. The results show that the proposed control has satisfactory steady-state and dynamic performance whether under strong grid or weak grid. • The proposed method is suitable for both strong and weak grids. • The proposed current estimator has low complexity. • The proposed method reduces the use of sensors and reduces the hardware cost. [ABSTRACT FROM AUTHOR]

• The proposed method suppresses odd-, even-order harmonics and DC offsets. • The proposed method is based on multi-consecutive samples. • The proposed method achieves frequency estimation accurately. This paper proposes a multi-consecutive-samples (MCSs) based method to estimate the grid frequency with strong robustness against odd-, even-order harmonics and DC offsets. Analyzing the relationship among the distorted grid voltage and its multiple consecutive sample signals, a MCS-model is established. Accordingly, the robust frequency estimation is proposed based on the MCS-model. Then, the stability analysis and parameter tuning are provided. Compared with the traditional 3CS-based frequency estimation, the proposed method enhances the disturbance suppression capability, especially in DC offsets and even-order harmonics cases. The experimental results further confirm that the proposed method has strong robustness against odd-, even-order harmonics and DC offsets. [ABSTRACT FROM AUTHOR]

• The proposed method is suitable for more electric aircraft grids. • The proposed method has fast dynamics. • The structure of the proposed method is simple. Different from utility power systems, the grids of more electric aircraft (MEA) are frequency-variable, and the frequency change range is from 360 Hz to 800 Hz. Most grid synchronization methods are designed for constant-frequency grids whose frequency varies in a small range around 50 Hz or 60 Hz. Therefore, the dynamic performance of these methods decreases considerably in MEA grids. In addition, the frequency in MEA is higher than that in utility power systems. Grid voltage parameters are thus required to exhibit a fast dynamic response. To resolve this problem, this paper proposes an algebraic method to estimate the positive- and negative-sequence components (PNSC) of grid voltage accurately. By analyzing the relationship between grid voltage and its time delay signals, the linear regression model is established. Then, grid frequency, PNSC are obtained with an algebraic method. The proposed method relies on algebraic operations only and thus exhibits a fast dynamic response. Experiment results verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]