This article reveals that the saturation block (SB) used with the power-angle control loop can enhance the transient stability of grid-forming (GFM) inverters. Three methods of using the SB in the power-angle loop are examined based on power-angle curves. It is found that all can enhance the transient stability of GFM inverters by either decreasing the acceleration area or increasing the deceleration area. At the same time, the threshold value selection of SB in the three schemes is also explained, considering the tradeoff between the transient stability and the response speed. A holistic comparison of the three schemes recommends the scheme with the SB used for power error adopted by the GFM inverters. Finally, experimental tests confirm the effectiveness and correctness of the analysis.

This article analyzes the parametric effect of the dc-link voltage controller (DVC) on the transient stability of grid-forming (GFM) inverters. It is found that the decrease of the proportional gain of DVC may lead to the loss of synchronism (LOS) even if the system is locally stable at the new equilibrium point (EP) after disturbance. Its physical insight is attributed to the unstable limit cycle (ULC) in the phase space according to the bifurcation theory. The GFM inverters whose operating point locates outside of ULC will lose synchronism with the grid. The decrease of the proportional gain of DVC will diminish the ULC, resulting in fewer operating points converging to the new EP and a higher risk of LOS. This bifurcation-based analysis provides a novel perspective for investigating the mechanism of parametric effect for the high-order nonlinear system, which is the article's main contribution. The parametric effect of the integral gain of DVC is also analyzed. A holistic parametric space partition for designing DVC is then drawn from the analysis, and a parametric configuration guideline is provided. The theoretical findings are validated by experimental tests.

This paper presents a modified modular multilevel converter(MMC) with the three-terminal middle-cells inserted between arms and ac-side for medium voltage applications. It is intended to reduce the sub-module(SM) capacitor voltage ripples by controlling the high frequency powers flowing among arm branches for low-frequency output operations without common-mode voltage injected in ac-side. Due to the physical connection among the upper arm, the lower arm and ac-side, the middle-cell can be regarded as the excess degree of freedom to modify the dc-side and ac-side characteristics. Therefore, the common-mode voltage in ac-side can be eliminated by the inverse common-mode voltage produced by the middle-cell in ac-side. Moreover, owing to the unique structure of the middle-cell, the middle-cell capacitor voltage also results in a small ripple due to the high frequency current flowing into it by proper control. The operation principle and control strategy of the middle-cell has been proposed in detail. Simulation and experimental results have verified the effectiveness of the proposed topology.

Realizing continuous sweeping of perfect anomalous reflection in a wide angular range has become a technical challenge. This challenge cannot be overcome by the conventional aperiodic reflectarrays and periodic metasurfaces or metagratings. In this paper, we investigate means to create scanning reflectarrays for the reflection of plane waves coming from any direction into any other direction without any parasitic scattering. The reflection angle can be continuously adjusted by proper tuning of reactive loads of each array element, while the geometrical period is kept constant. We conceptually study simple canonical two-dimensional arrays formed by impedance strips above a perfectly reflecting plane. This setup allows fully analytical solutions, which we exploit for understanding the physical nature of parasitic scattering and finding means to overcome fundamental limitations of conventional reflectarray antennas. We propose to use subwavelength-spaced arrays and optimize current distribution in λ/2-sized supercells. As a result, we demonstrate perfect tunable reflection to any angle. Our work provides an effective approach to designing reconfigurable intelligent surfaces with electrically tunable reflection angles.

Space-time modulation-based metamaterials have recently spurred considerable interest, owing to the fundamental addition of the time dimension to the medium parameters and resulting novel properties and potential applications. However, the implementation of most related structures—e.g., involving step, slab, or gradient discontinuities—has been hindered by the impossible requirement of infinitely or prohibitively large device sizes. Here, we provide a solution to this issue, consisting in a space-time transposition of the conventional Fresnel prism, whereby a copy of the target modulation is periodically reinjected at the input of a Fresnel-reduced finite structure, so as to provide the same anharmonic and nonreciprocal frequency conversion as the target space-time interface in the case of a modulation step. This concept, which may readily extend to slab or gradient modulations, as well as accelerated profiles for space-time chirping operations, may pave the way for the practical development of a wide range of novel microwave and optical space-time systems.