A Dual Arm Complementary Hybrid Modulation Strategy Combining NL-SPWM for MMC Applications

In the medium voltage direct current(MVDC)transmission system,a small number of MMC sub modules will reduce the power quality.In this paper,based on the research background of Photovoltaic Medium VoltageDirect Current(PV-MVDC)system,aHybrid Modulation Strategy based on theDecoupledDouble SynchronousReference Frame(DDSRF)control strategy is proposed.The dual armcomplementary hybrid modulation combining nearest-level-SPWM(NL-SPWM)can keep the number of SMs in the ON state constant.Then,the corresponding voltage sharing control algorithm of sub module(SM)is introduced.Through theoretical calculation,the modulation strategy can be found to stabilize the DC voltage and reduce the harmonic content.A32-level MMC systemhas been developed to verity that the proposed hybrid modulation strategy and its SM voltage sharing algorithm have the advantages of restraining circulating current and maintaining capacitor voltage balance.

Coordinated Capacitor Voltage Balancing Method for Cascaded H-bridge Inverter with Supercapacitor and DC-DC Stage

Cascaded H-bridge inverter(CHBI) with supercapacitors(SCs) and dc-dc stage shows significant promise for medium to high voltage energy storage applications. This paper investigates the voltage balance of capacitors within the CHBI, including both the dc-link capacitors and SCs. Balance control over the dc-link capacitor voltages is realized by the dcdc stage in each submodule(SM), while a hybrid modulation strategy(HMS) is implemented in the H-bridge to balance the SC voltages among the SMs. Meanwhile, the dc-link voltage fluctuations are analyzed under the HMS. A virtual voltage variable is introduced to coordinate the balancing of dc-link capacitor voltages and SC voltages. Compared to the balancing method that solely considers the SC voltages, the presented method reduces the dc-link voltage fluctuations without affecting the voltage balance of SCs. Finally, both simulation and experimental results verify the effectiveness of the presented method.

Design and modulation principles of molybdenum carbide-based materials for green hydrogen evolution

The green production of hydrogen from electrocatalytic water splitting is an important base and promising direction for the future of the large-scale application of hydrogen energy.The key of green hydrogen evolution depends on the development of low-cost and highly active electrocatalysts.Molybdenum carbides(MoxC),as a typical of earth-abundant transition-metal material,have accumulated great attention due to their low cost,earth abundance,electrical conductivity,similar d-band state to Pt,and regulated morphology/electronic structures.In this paper,recent researches focusing on MoxC for efficient HER in a wide pH range are summarized from respects of modulation of unique morphology,electronic structure,and electrode interface step by step.Briefly,modulation of morphology influence the apparent activity of catalyst,modulation of electronic structure of active sites by heteroatom doping and designing heterointerface boost intrinsic HER kinetics,and modulation of electrode interface via hybridization of MoxC structures with carbon materials can ensure the fast electron transfer and boost the activity.Besides the above methods discussed,perspective and challenges of designing MoxC as the substitute of Pt-based electrocatalyst for practical hydrogen generation in a wide pH range are pointed out.

Research on Modulation Strategies Based on Multilevel Inverter Universal Hybrid Topology

Based on multi-module-cascaded inverter topology, this study presented a universal multilevel inverter hybrid topology and unified the researches on multilevel inverter topology. According to the freedom of this universal topology, several new hybrid topologies were constructed. Also, based on conventional modulation strategies- multi-carrier SPWM (Sinusoidal Pulse Width Modulation), hybrid modulation strategies were introduced corresponding to hybrid topologies, and a multilevel SVPWM (Space Vector Pulse Width Modulation) technique based on phase-shifted theory was naturally produced. Simulation and experiment results prove that hybrid topologies and corresponding modulation strategies are valid, which lay a foundation for practical application of hybrid multilevel inverter topologies.

Electrocatalytic oxygen evolution activities of metal chalcogenides and phosphides:Fundamentals,origins,and future strategies

The development of inexpensive and efficient electrocatalysts is key to commercializing energy-related electrocatalytic techniques such as water electrolyzers and metal-air batteries.In particular,novel oxygen evolution reaction(OER)pre-catalysts,such as transition metal chalcogenides(TMCs)and phosphides(TMPs),have evolved in recent years from traditional stable OER electrocatalysts,which show superior OER electrocatalytic performance compared with transition metal oxides(TMOs)or(oxy)hydroxides(TMOHs).In this feature article,we summarize recent advances in the development of TMCand TMP-based OER electrocatalysts,as well as approaches to improve the OER performance in terms of morphology,structure,composition,surface engineering,lattice-strained and in-situ transformation in the electrolysis process.In particular,the electrochemical stability of TMCs and TMPs in alkaline electrolytes and the evolution of morphology,structure and composition under OER conditions are discussed.In the last section,we discuss the challenges that need to be addressed in this specific area of research and the implications for further research.

Investigation and Evaluation of Multilevel H-NPC Converter for Electrically Driven Trains

In this paper the operation of a three level H-bridge converter as well as its parallel operations is analyzed and simulated on the computer. Based on the simulation results the operating behavior between （a） a three level H-bridge neutral point clamped convener, （b） a three level back-to-back H-bridge neutral point clamped convener, （c） two three level H-bridge neutral point clamped converters parallel connected is being compared. From the simulation results it is obvious that in the first two cases the ripples, the distortion in primary and secondary winding currents, and the power factor are quite satisfactory and almost identical to each other. In the third case as compared with the first two, it is observed that current harmonics with higher amplitude appear in the primary winding of the transformer.

Overview on Submodule Topologies,Modeling,Modulation,Control Schemes,Fault Diagnosis,and Tolerant Control Strategies of Modular Multilevel Converters

In the present scenario,modular multilevel converters(MMCs)are considered to be one of the most promising and effective topologies in the family of high-power converters because of their modular design and good scalability;MMCs are extensively used in high-voltage and high-power applications.Based on their unique advantages,MMCs have attracted increasing attention from academic circles over the past years.Several studies have focused on different aspects of MMCs,including submodule topologies,modeling schemes,modulation strategies,control schemes for voltage balancing and circulating currents,fault diagnoses,and fault-tolerant control strategies.To summarize the current research status of MMCs,all the aforementioned research issues with representative research approaches,results and characteristics are systematically overviewed.In the final section,the current research status of MMCs and their future trends are emphasized.

Precise Modulation Strategies for Transcranial Magnetic Stimulation:Advances and Future Directions

Transcranial magnetic stimulation(TMS)is a popular modulatory technique for the noninvasive diagnosis and therapy of neurological and psychiatric diseases.Unfortunately,current modulation strategies are only modestly effective.The literature provides strong evidence that the modulatory effects of TMS vary depending on device components and stimulation protocols.These differential effects are important when designing precise modulatory strategies for clinical or research applications.Developments in TMS have been accompanied by advances in combining TMS with neuroimaging techniques,including electroencephalography,functional nearinfrared spectroscopy,functional magnetic resonance imaging,and positron emission tomography.Such studies appear particularly promising as they may not only allow us to probe affected brain areas during TMS but also seem to predict underlying research directions that may enable us to precisely target and remodel impaired cortices or circuits.However,few precise modulation strategies are available,and the long-term safety and efficacy of these strategies need to be confirmed.Here,we review the literature on possible technologies for precise modulation to highlight progress along with limitations with the goal of suggesting future directions for this field.

Rational design of nanoarray structures for lithium-sulfur batteries:recent advances and future prospects

Lithium-sulfur(Li-S)batteries are considered as promising candidates for future-generation energy storage systems due to their prominent theoretical energy density.However,their application is still hindered by several critical issues,e.g.,the low conductivity of sulfur species,the shuttling effects of soluble lithium polysulfides,volumetric expansion,sluggish redox kinetics,and uncontrollable Li dendritic formation.Considerable research efforts have been devoted to breaking through the obstacles that are preventing Li-S batteries from realizing practical application.Recently,benefiting from the no additives/binders,buffer of volume change,high sulfur loading and suppression of lithium dendrites,nanoarray(NA)structures have have emerged as efficient and durable electrodes in Li-S batteries.In this work,recent advances in the design,synthesis and application of NA structures in Li-S batteries are reviewed.First,the multifunctional merits and typical synthetic strategies of employing NA structure electrodes for Li-S batteries are outlined.Second,the applications of NA structures in Li-S batteries are discussed comprehensively.Finally,the challenge and rational design of NA structure for Li-S batteries are analyzed in depth,with the aim of providing promising orientations for the commercialization of high-energy-density Li-S batteries.

Generalized Switched-Capacitor Step-up Multilevel Inverter Employing Single DC Source

In this paper,a new generalized step-up multilevel DC-AC converter is proposed,which is suitable for applications with low-voltage input sources,such as photovoltaic power generation and electric vehicles.This inverter can achieve a high voltage gain by controlling the series-parallel conversion of the DC power supply and capacitors.Only one DC voltage source and a few power devices are employed.The maximum output voltage and the number of output levels can be further increased through the switched-capacitor unit’s extension and the submodule cascaded extension.Moreover,the capacitor voltages are self-balanced without complicated voltage control circuits.The complementary operating mechanism between each pair of switches simplifies the modulation algorithm.The inductiveload ability is fully taken into account in the proposed inverter.Additionally,a remarkable characteristic of the inverter is that the charging and discharging states among different capacitors are synchronous,which reduces the voltage ripple of the frontend capacitors.The circuit structure,the working principle,the modulation strategy,the capacitors and losses analysis are presented in detail.Afterwards,the advantages of the proposed inverter are analyzed by comparing with other recently proposed inverters.Finally,the steady-state and dynamic performance of the proposed inverter is verified and validated by simulation and experiment.

Fault Tolerant Control of Multiphase Multilevel Motor Drives-Technical Review

The multiphase multilevel motor drives are a promising solution for some high-power and high-reliability applications,since they have multiple power conversion routes,a large amount of redundant voltage vectors,higher equivalent switching frequencies and superior harmonic performance.It has great significance to exploit their remedial control strategies in depth to fully utilize their high fault tolerant capabilities.This paper will present an updated technical review of fault tolerant control schemes for multiphase multilevel motor drives.Based on exemplification of the diode neutral-point-clamping three-level(DNPC-3L)inverters and T-type NPC three-level(TNPC-3L)inverters fed asymmetric six-phase PMSM drives,the study is taken for both phase-leg faults and switch faults.Furthermore,the discussion on multiple-switch faults,short-circuit switch faults and switch and leg hybrid faulty conditions are discussed in this paper.

Review of Low Carrier Ratio Converter System

With the rapid development of new energy power generation and high-power traction technology,the voltage and power levels of converter devices have been continuously improved,and the application of high-power converters is becoming increasingly widespread.However,high-power converters are affected by switching losses and heat dissipation,meaning they are not suitable for high carrier conditions.Therefore,research of low carrier ratio converter systems has received increased attention.Based on existing research,the problems of large current harmonics,low observation accuracy,and poor stability that may occur at low carrier ratios are explained.In addition,the topologies,modulation strategies,and control methods of the low carrier ratio converter system are analyzed and classified.Finally,future research directions of low carrier ratio converter systems are proposed.