Explore the vast range of titles available.

Power reversal control strategies for different types of hybrid line-commutated-converter (LCC)/modular multi-level converter (MMC) based high-voltage direct-current (HVDC) systems have been proposed with the consideration of system configurations and MMC's topologies. The studies show that the full-bridge (FB) MMC gives better performance than halfbridge (HB) MMCs in terms of power reversal in hybrid LCC/MMC systems. The modulation method employed in this paper can achieve a smooth online polarity reversal for hybrid LCC/FB-MMC HVDC systems. Additional DC switches and/or discharging resistors may be needed to reverse the DC polarity of LCC/HB-MMC HVDC systems. Based on the proposed strategies, the power reversal processes of the studied systems can be accomplished within several seconds. The speed can be changed according to system operation requirements. The effectiveness of the proposed control strategies has been verified through simulations conducted in PSCAD/EMTDC.

Open circuit failure mode in insulated-gate bipolar transistors (IGBT) is one of the most common faults in modular multilevel converters (MMCs). Several techniques for MMC fault diagnosis based on threshold parameters have been proposed, but very few studies have considered artificial intelligence (AI) techniques. Using thresholds has the difficulty of selecting suitable threshold values for different operating conditions. In addition, very little attention has been paid to the importance of developing fast and accurate techniques for the real-life application of open-circuit failures of IGBT fault diagnosis. To achieve high classification accuracy and reduced computation time, a fault diagnosis framework with a combination of the AC-side three-phase current, and the upper and lower bridges’ currents of the MMCs to automatically classify health conditions of MMCs is proposed. In this framework, the principal component analysis (PCA) is used for feature extraction. Then, two classification algorithms—multiclass support vector machine (SVM) based on error-correcting output codes (ECOC) and multinomial logistic regression (MLR)—are used for classification. The effectiveness of the proposed framework is validated by a two-terminal simulation model of the MMC-high-voltage direct current (HVDC) transmission power system using PSCAD/EMTDC software. The simulation results demonstrate that the proposed framework is highly effective in diagnosing the health conditions of MMCs compared to recently published results.

The paper investigates the cavitation erosion (CE) and sliding wear (SW) resistance of cold-sprayed Al/Al2O3 and Cu/Al2O3 composites and studies them in relation to a set of metallic materials such as aluminium alloy (AlCu4Mg1), pure copper (Cu110), brass (CuZn40Pb2) and stainless steel (AISI 304). The coatings were deposited on stainless steel by low-pressure cold spray (LPCS) using Al (40 wt.%) and Cu (50 wt.%) blended with Al2O3 (60 and 50 wt.%, respectively) feedstocks. CE resistance was estimated by the stationary sample method according to the ASTM G32 standard. The SW test was conducted using a ball-on-disc tester with compliance to the ASTM G99 standard. Results obtained for the LPCS coatings show that the Cu/Al2O3 coating exhibits a denser structure but lower adhesion and microhardness than Al/Al2O3. The Al/Al2O3 and Cu/Al2O3 resistance to cavitation is lower than for bulk alloys; however, composites present higher sliding wear resistance to that of AlCu4Mg1, CuZn40Pb2 and stainless steel. The CE wear mechanisms of LPCS composites start at the structural discontinuities and non-uniformities. The cavitation erosion degradation mechanism of Al/Al2O3 relies on chunk material detachment while that of Cu/Al2O3 initiates by alumina removal and continues as layer-like Cu-metallic material removal. CE damage of metal alloys relies on the fatigue-induced removal of deformed material. The SW mechanism of bulk alloys has a dominant adhesive mode. The addition of Al2O3 successfully reduces the material loss of LPCS composites but increases the friction coefficient. Coatings’ wear mechanism has an adhesive-abrasive mode. In both CE and SW environment, the behaviour of the cold-sprayed Cu/Al2O3 composite is much more promising than that of the Al/Al2O3.

Modular multilevel converters (MMCs) are expected to play an important role in future high voltage direct current (HVDC) grids. Moreover, advanced MMC topologies may include various submodule (SM) types. In this sense, the modeling of MMCs is paramount for HVDC grid studies. Detailed models of MMCs are cumbersome for electromagnetic transient (EMT) programs due to the high number of components and large simulation times. For this reason, simplified models that reduce the computation times while reproducing the dynamics of the MMCs are needed. However, up to now, the models already developed do not consider hybrid MMCs, which consist of different types of SMs. In this paper, a procedure to simulate MMCs having different SM topologies is proposed. First, the structure of hybrid MMCs and the modeling method is presented. Next, an enhanced procedure to compute the number of SMs to be inserted that takes into account the different behavior of full-bridge SMs (FB-SMs) and half-bridge submodules (HB-SMs) is proposed in order to improve the steady-state and dynamic response of hybrid MMCs. Finally, the MMC model and its control are validated by means of detailed PSCAD simulations for both steady-state and transients conditions (AC and DC faults).

Explicit topology optimization methods have received ever-increasing interest in recent years. In particular, a 188-line Matlab code of the two-dimensional (2D) Moving Morphable Component (MMC)-based topology optimization method was released by Zhang et al. (Struct Multidiscip Optim 53(6):1243-1260, 2016). The present work aims to propose an efficient and easy-to-extend 256-line Matlab code of the MMC method for three-dimensional (3D) topology optimization implementing some new numerical techniques. To be specific, by virtue of the function aggregation technique, accurate sensitivity analysis, which is also easy-to-extend to other problems, is achieved. Besides, based on an efficient identification algorithm for load transmission path, the degrees of freedoms (DOFs) not belonging to the load transmission path are removed in finite element analysis (FEA), which significantly accelerates the optimization process. As a result, compared to the corresponding 188-line 2D code, the performance of the optimization results, the computational efficiency of FEA, and the convergence rate and the robustness of optimization process are greatly improved. For the sake of completeness, a refined 218-line Matlab code implementing the 2D-MMC method is also provided.

A modular multilevel converter (MMC) is an advanced voltage source converter applicable to a wide range of medium and high-voltage applications. It has competitive advantages such as quality output performance, high modularity, simple scalability, and low voltage and current rating demand for the power switches. Remarkable studies have been carried out regarding its topology, control, and operation. The main purpose of this review is to present the current state of the art of the MMC technology and to offer a better understanding of its operation and control for stationary applications. In this study, the MMC configuration is presented regarding its conventional and advanced submodule (SM) and overall topologies. The mathematical modeling, output voltage, and current control under different grid conditions, submodule balancing control, circulating current control, and modulation methods are discussed to provide the state of the MMC technology. The challenges linked to the MMC are associated with submodule balancing control, circulating current control, control complexity, and transient performance. Advanced nonlinear and predictable control strategies are expected to improve the MMC control and performance in comparison with conventional control methods. Finally, the power losses associated with the advanced wide bandgap (WBG) power devices (such as SiC, GaN) are explored by using different modulation schemes and switching frequencies. The results indicate that although the phase-shifted carrier-based pulse width modulation (PSC-PWM) has higher power losses, it outputs a better quality voltage with lower total harmonic distortion (THD) in comparison with phase-disposition pulse width modulation (PD-PWM) and sampled average modulation pulse width modulation (SAM-PWM). In addition, WBG switches such as silicon carbide (SiC) and gallium nitride (GaN) devices have lower power losses and higher efficiency, especially at high switching frequency in the MMC applications.

Alpha‐momorcharin (α‐MMC), a member of the plant ribosomal inactivating proteins (RIPs) family, has been proven to exhibit important biological properties in animals, including antiviral, antimicrobial, and antitumour activities. However, the mechanism by which α‐MMC increases plant resistance to viral infections remains unclear. To study the effect of α‐MMC on plant viral defence and how α‐MMC increases plant resistance to viruses, recombinant DNA and transgenic technologies were employed to investigate the role of α‐MMC in Nicotiana benthamiana resistance to tobacco mosaic virus (TMV) infection. Treatment with α‐MMC produced through DNA recombinant technology or overexpression of α‐MMC mediated by transgenic technology alleviated TMV‐induced oxidative damage and reduced the accumulation of reactive oxygen species (ROS) during TMV‐green fluorescent protein infection of N. benthamiana. There was a significant decrease in TMV replication in the upper leaves following local α‐MMC treatment and in α‐MMC‐overexpressing plants relative to control plants. These results suggest that application or overexpression of α‐MMC in N. benthamiana increases resistance to TMV infection. Finally, our results showed that overexpression of α‐MMC up‐regulated the expression of ROS scavenging‐related genes. α‐MMC confers resistance to TMV infection by means of modulating ROS homeostasis through controlling the expression of antioxidant enzyme‐encoding genes. Overall, our study revealed a new crosstalk mechanism between α‐MMC and ROS during resistance to viral infection and provides a framework to understand the molecular mechanisms of α‐MMC in plant defence against viral pathogens. Application or overexpression of α‐MMC in Nicotiana benthamiana increased resistance to TMV infection by means of modulating ROS homeostasis through controlling the expression of antioxidant enzyme‐encoding genes.

Electronic power converters play an essential role in power grids, aiming to improve the electrical energy quality and also enabling bidirectional energy transfer between DC lines and circuits. They facilitate the achievement of a sinusoidal current waveform and effective power transfer control with a high power factor. This paper introduces a stationary reference frame based control strategy for grid-connected three phase modular multilevel converters (MMC). This strategy employs conventional PI controllers to track the instantaneous power components that include intentional oscillations at double grid frequency. By employing this method, the MMC converter can maintain an output sinusoidal waveform even under unbalanced grid voltage conditions. Also, there is no need for a transformation from a stationary frame to a synchronous frame, eliminating the requirement for a PLL to estimate the grid voltage phase angle. Furthermore, the use of MMC converter over common two-level and three-level VSC converters is proposed since MMC converters offer merits such as low harmonic components, no need for filters at the DC terminals, no need for filters at the AC side, and low losses, despite some drawbacks such as a large number of IGBT switches or a higher amount of stored energy in the sub-module capacitors. Therefore, the voltage THD and consequently active and reactive powers of the converter have been impressively mitigated by using MMC. To confirm the capability and effectiveness of the proposed method, various simulations are performed in MATLAB/Simulink software. Finally, the results are compared with common methods.

The modular multilevel converter (MMC) is a keystone of modern energy transmission systems. Consequently, there is an ongoing pursue for mathematical models to represent it under different configurations and control approaches. In short, this paper introduces an analytical Thévenin-equivalent model for representing the MMC when it is controlled with inner current- and an outer voltage-loop altogether. The model is based on a linearized representation of the converter and conveys the dynamics of passive components, such as submodule capacitors and arm reactors, as well as both control loops. Besides that, the proposed model is divided into a close-loop transfer matrix and the equivalent impedance matrix, both of which represent the relationships between the ac-side dq voltages and currents. We also propose a framework for implementing electromagnetic–transient simulations using the impedance model of this power electronic converter. The framework reduces a multi-bus power grid to a multi-input multi-output (MIMO) feedback system where impedance/admittance matrices of the MMC and other grid elements compose its loops. For validation purposes, it is considered a three-bus power grid comprising one MMC and another two grid-connected VSC. The proposed model was validated by comparing its results with a switching-level PSCAD model of the system.