Characteristics of the electromagnetic wave propagation in magnetized plasma sheath and practical method for blackout mitigation

“Magnetic window”is considered as an effective method to solve the communication blackout issue.COMSOL software package based on the finite element method is utilized to simulate the propagation of right-handed circularly polarized wave in the magnetized plasma sheath.We assume a double Gaussian model of electron density and an exponential attenuation model of magnetic field.The propagation characteristics of right-handed circularly polarized wave are analyzed by the observation of the reflected,transmitted and loss coefficient.The numerical results show that the propagation of right-handed circularly polarized wave in the magnetized plasma sheath varies for different incident angles,collision frequencies,non-uniform magnetic fields and non-uniform plasma densities.We notice that reducing the wave frequency can meet the propagation conditions of whistle mode in the weak magnetized plasma sheath.And the transmittance of whistle mode is less affected by the variation of the electron density and the collision frequency.It can be used as a communication window.

Flexible Large-Area Graphene Films of 50-600 nm Thickness with High Carrier Mobility

Bulk graphene nanofilms feature fast electronic and phonon transport in combination with strong light-matter interaction and thus have great potential for versatile applications,spanning from photonic,electronic,and optoelectronic devices to charge-stripping and electromagnetic shielding,etc.However,large-area flexible close-stacked graphene nanofilms with a wide thickness range have yet to be reported.Here,we report a polyacrylonitrile-assisted’substrate replacement’strategy to fabricate large-area free-standing graphene oxide/polyacrylonitrile nanofilms(lateral size~20 cm).Linear polyacrylonitrile chains-derived nanochannels promote the escape of gases and enable macro-assembled graphene nanofilms(nMAGs)of 50-600 nm thickness following heat treatment at 3,000℃.The uniform nMAGs exhibit 802-1,540 cm^(2)V-1s-1carrier mobility,4.3-4.7 ps carrier lifetime,and>1,581 W m^(-1)K^(-1)thermal conductivity(n MAG-assembled 10μm-thick films,mMAGs).nMAGs are highly flexible and show no structure damage even after 1.0×10^(5)cycles of folding-unfolding.Furthermore,n MAGs broaden the detection region of graphene/silicon heterojunction from near-infrared to mid-infrared and demonstrate higher absolute electromagnetic interference(EMI)shielding effectiveness than state-of-the-art EMI materials of the same thickness.These results are expected to lead to the broad applications of such bulk nanofilms,especially as micro/nanoelectronic and optoelectronic platforms.

基于正交电磁场的等离子体电子密度控制

正交电磁场是一种极具应用前景的降低等离子体电子密度、消除再入通信黑障的控制方法。根据离子质量与动量的守恒原理,建立了正交电磁场控制的物理模型;通过与流体力学方程的比拟,应用隐式格式实现了物理模型的稳定求解。通过数值模拟研究了正交电磁场控制装置中外加电场强度、磁场强度等参数对等离子体电子密度控制效果的影响规律,发现:在电流及磁场作用下,洛伦兹力使得电子发生漂移,导致局部电子密度下降,出现电磁窗口;电压越高、磁场越强,控制效果越好;对于相同磁场,电压较低时,电子密度减小量与电压基本成线性关系,电压超过100 V后,两者关系的非线性逐渐增强。由此,本文制造了电子密度控制原理样机,并在等离子体实验平台中进行了实验,采用发射光谱法和微波干涉法测量诊断了等离子体的电子密度变化情况,在控制区内得到了局部低电子密度区,证实了洛伦兹力作用下等离子体的漂移以及对等离子体电子密度的控制作用。

航天返回舱雷达探测技术研究

雷达在探测航天返回舱黑障区时,因受到等离子体鞘套干扰,长期存在跟踪不稳定甚至丢失等问题。本文从等离子体鞘套与电磁波相互作用机理出发,提出了航天返回舱黑障区电磁特性仿真方法和再入场景回波仿真方法;结合仿真与实测数据研究获取黑障区目标回波特征,提出了基于鞘套快时变特性的目标检测方法;设计了返回舱黑障区认知探测流程,将其应用于多频段雷达装备成功完成了多次返回舱探测任务,保障了黑障区稳定跟踪,基本解决了航天返回舱黑障区雷达探测问题。

考虑次生故障差异化影响下韧性主动提升的输电系统网架重构策略

近年来,我国能源电力加速转型,极端自然灾害频发,国际形势不稳定,使得常规安全风险与非常规安全风险交织,大停电风险有增加的趋势。稳健可靠的网架重构方案对防止恢复中再次发生系统崩溃具有重要意义,针对当前研究中未有效计及恢复中次生故障的不足,该文提出一种考虑次生故障差异化影响下系统韧性主动提升的网架重构策略。首先,讨论在网架重构恢复方案中考虑次生故障影响的必要性;然后,基于分割多目标风险分析方法提出一种系统韧性指标,旨在差异化处理次生故障并突出高风险故障对恢复过程的影响;进一步建立考虑韧性主动提升的网架重构全局优化模型,以增强系统对高风险故障的抵御能力。为提高模型的求解效率,提出一种基于窗口滚动机制的求解策略,可给出兼顾快速性与稳健性的网架重构方案。最后,采用新英格兰10机39节点系统和IEEE 118节点系统验证所提出方法的有效性。

基于KL散度距离处理风电不确定性的负荷恢复分布鲁棒优化

在构建以新能源为主体的新型电力系统的背景下,在恢复控制过程中积极利用新能源并充分应对其对运行安全带来的潜在风险对于减小停电损失具有重要意义。首先,为了表征风电出力的不确定性,采用KL散度(Kullback-Leibler)距离作为筛选极端风电出力场景的控制条件,并据此构建模糊集作为风电出力典型场景。在满足相关运行安全约束的前提下,建立了以最大化加权负荷恢复量为优化目标的分布鲁棒优化模型以制定计及风电的负荷恢复方案。经松弛处理和对偶转换所得到的混合整数二阶锥模型可调用商业求解器求解。以接入规模风电场的IEEE 10机39母线系统为例进行仿真,结果表明:相比于传统的鲁棒优化方法,该方法降低了优化结果的保守性,有助于加快负荷恢复,减小停电损失。

船用堆全船断电事故的故障树分析研究

船用堆供电系统的可靠性对核动力装置的安全性有重要影响。参考某典型船用堆供电系统设计,提出了船用堆供电系统故障树建模基本原则,以全船断电事故为顶事件建立故障树,确定了系统的邻接矩阵,并基于此设计了一种故障树辅助生成及最小割集求解程序。通过定性和定量分析得出全船断电事故的发生概率及各电源设备的重要度,并提出相应的改善措施,为船用堆供电系统的设计、使用和维护保养提供技术支撑。

双极型LCC-HVDC直流系统作为黑启动电源的启动方法及策略

科学合理的黑启动电源及方案是应对电网灾变停电的最有效策略,可加快其系统恢复进程,大幅减少停电损失。双极型换相换流器型高压直流系统(LCC-HVDC)具有调节灵活输电容量大的优势,但其作为黑启动电源存在一定的困难。提出了以双极型LCC-HVDC为黑启动电源的黑启动方法及完整过程,实现向受端无源网络恢复供电,以加速其恢复进程。首先给出了双极型LCC-HVDC作为黑启动电源时所面临的问题及相应的解决策略;进而给出了双极型LCC-HVDC作为黑启动电源恢复向受端无源网络恢复供电的实现过程。该过程包括2个阶段,第1阶段实现双极型LCC-HVDC的仿融冰模式启动;第2阶段为从仿融冰模式过渡到向受端无源网络恢复供电过程。算例证实了该启动方法的有效性和正确性。该方法使双极型LCC-HVDC具备了黑启动能力,并利用直流系统为受端电网的系统恢复过程提供了电压和频率支撑。

基于加权潮流熵和耦合支路介数的大停电故障集构建方法研究

随着电网互联程度的加深,电网因连锁故障引发大停电的概率增大,构建连锁故障集是预防大停电的有效措施之一。首先,研究对电网稳定运行影响巨大的故障支路集,定量评估电网运行状态的加权潮流熵计算方法。然后,结合全过程动态仿真软件,提出基于加权潮流熵和耦合支路介数的大停电故障集构建方法。最后,以某地区电网为例验证所提方法的正确性和有效性。该方法既考虑电网结构变化又考虑电网状态变化,能够准确反映电网故障动态过程。

基于巴西“8·15”大停电事故的仿真分析与思考

2023年8月15日,巴西发生大面积停电事故,对经济社会正常运转造成严重影响。事故暴露出巴西电网结构薄弱、电源支撑不足、技术防线不完善、安全管控有短板等问题,为我国电力安全和新型电力系统建设工作带来诸多启示。为模拟巴西大停电发展全过程,文章基于大电网在线安全分析应用平台,调整某地区电网运行方式,削弱网架结构,并使北部新能源大发,设置包含直流闭锁在内的复杂连锁故障,使电网解列为南北两片区,并采取低频减载、失步解列、高频切机等安控措施,使电网恢复安全稳定运行,对日常调控运行工作具有一定指导意义。

Two-Dimensional Materials in Large-Areas: Synthesis, Properties and Applications

Large-area and high-quality two-dimensional crystals are the basis for the development of the next-generation electronic and optical devices.The synthesis of two-dimensional materials in wafer scales is the first critical step for future technology uptake by the industries;however,currently presented as a significant challenge.Substantial efforts have been devoted to producing atomically thin two-dimensional materials with large lateral dimensions,controllable and uniform thicknesses,large crystal domains and minimum defects.In this review,recent advances in synthetic routes to obtain high-quality two-dimensional crystals with lateral sizes exceeding a hundred micrometres are outlined.Applications of the achieved large-area two-dimensional crystals in electronics and optoelectronics are summarised,and advantages and disadvantages of each approach considering ease of the synthesis,defects,grain sizes and uniformity are discussed.

Evaluating the reliability of distributed photovoltaic energy system and storage against household blackout

Distributed energy resources have been proven to be an effective and promising solution to enhance power system resilience and improve household-level reliability.In this paper,we propose a method to evaluate the reliability value of a photovoltaic(PV)energy system with a battery storage system(BSS)by considering the probability of grid outages causing household blackouts.Considering this reliability value,which is the economic profit and capital cost of PV+BSS,a simple formula is derived to calculate the optimal planning strategy.This strategy can provide household-level customers with a simple and straightforward expression for invested PV+BSS capacity.Case studies on 600 households located in eight zones of the US for the period of 2006 to 2015 demonstrate that adding the reliability value to economic profit allows households to invest in a larger PV+BSS and avoid loss of load caused by blackouts.Owing to the differences in blackout hours,households from the 8 zones express distinct willingness to install PV+BSS.The greater the probability of blackout,the greater revenue that household can get from the PV+BSS.The simulation example shows that the planning strategy obtained by proposed model has good economy in the actual operation and able to reduce the economic risk of power failure of the household users.This model can provide household with an easy and straightforward investment strategy of PV+BSS capacity.

Mitigation of blackout problem for reentry vehicle in traveling magnetic field with induced current

In this paper, a novel solution mitigating the radio blackout problem is proposed, which improves existing traveling magnetic field(TMF)-based methods. The most significant advance lies in replacing the external injection with self-induced current, which does not require electrodes. The improved analytical model is derived to evaluate the electron density reduction taking into consideration the self-induced current for various TMF velocities. The plasma reduction performance is analyzed for several conditions including the total absence of injected current. The results show that the velocity may be used to trade off the injected current and, when sufficiently large, eliminates the need for an injected current while mitigating radio blackout. The effectiveness of this solution to the blackout problem is demonstrated in commonly used aerospace communication bands. With a field strength of less than 0.15 T, increasing the velocity from40 m s^-1 to 3100 m s^-1 is all that is required to obviate the need for an injected current. Moreover,typical reduction ratios for electronic density tolerance(2, 1.9, 1.75 and 3 times for the L-, S-, Cand X-bands, respectively, at an altitude of 40 km) remain unchanged. Increasing the velocity of the TMF is much easier than injecting current via a metal electrode into a high-temperature flow field. The TMF method appears practical in regard to possible future applications.

Controlled Growth of Large-Area Aligned Single-Crystalline Organic Nanoribbon Arrays for Transistors and Light-Emitting Diodes Driving

Organic field-effect transistors(OFETs) based on organic micro-/nanocrystals have been widely reported with charge carrier mobility exceeding 1.0 cm^2V^(-1)s^(-1), demonstrating great potential for high-performance, low-cost organic electronic applications. However, fabrication of large-area organic micro-/nanocrystal arrays with consistent crystal growth direction has posed a significant technical challenge. Here, we describe a solution-processed dip-coating technique to grow large-area, aligned 9,10-bis(phenylethynyl) anthracene(BPEA) and 6,13-bis(triisopropylsilylethynyl) pentacene(TIPSPEN) single-crystalline nanoribbon arrays. The method is scalable to a 5 9 10 cm^2 wafer substrate, with around 60% of the wafer surface covered by aligned crystals. The quality of crystals can be easily controlled by tuning the dip-coating speed. Furthermore, OFETs based on well-aligned BPEA and TIPS-PEN single-crystalline nanoribbons were constructed.By optimizing channel lengths and using appropriate metallic electrodes, the BPEA and TIPS-PEN-based OFETs showed hole mobility exceeding 2.0 cm^2V^(-1)s^(-1)(average mobility 1.2 cm^2V^(-1)s^(-1)) and 3.0 cm^2V^(-1)s^(-1)(average mobility2.0 cm^2V^(-1)s^(-1)), respectively. They both have a high on/off ratio(I_(on)/I_(off))>10~9. The performance can well satisfy the requirements for light-emitting diodes driving.

Chemical Vapor Deposition Growth of Large-Area Monolayer MoS_2 and Fabrication of Relevant Back-Gated Transistor

A closed two-temperature-zone chemical vapor deposition(CVD) furnace was used to grow monolayer molybdenum disulfide(MoS_2) by optimizing the temperature and thus the evaporation volume of the Mo precursor. The experimental results show that the Mo precursor temperature has a large effect on the size and shape transformation of the monolayer MoS_2, and at a lower temperature of <760°C, the size of the triangular MoS_2 increases with the elevating temperature, while at a higher temperature of >760°C, the shape starts to change from a triangle to a truncated triangle. A large-area triangular monolayer MoS_2 with a side length of 145 °m is achieved at 760°C.Further, the as-grown monolayer MoS_2 is used to fabricate back-gated transistors by means of electron beam lithography to evaluate the electrical properties of MoS_2 thin films. The MoS_2 transistors with monolayer MoS_2 grown at 760°C exhibit a high on/off current ratio of 10~6, a mobility of 1.92 cm^2/Vs and a subthreshold swing of 194.6 mV/dec, demonstrating the feasible approach of CVD deposition of monolayer MoS_2 and the fabrication of transistors on it.

HVDC Grid Segmentation Analysis for Blackouts Reduction

Segmentation of large AC systems through DC links introduces a new concept that utilizes the advantages of direct current transmission to improve network reliability and increase power transfer capacity. Technical literature argues that the segmentation of the AC network and the introduction of DC links at these systems connection points bring benefits to system operation, once contingencies generated on one side of the DC connection point would not be reflected on the other side of DC connection, thereby reducing the likelihood of cascading shutdowns and blackouts due to load restraint on transmission lines and transformers. Amidst this scenario, this paper presents a study of the main topics regarding the use of this new network segmentation philosophy, bringing a practical point of view for the use of this concept at the electrical power system planning. The effect of DC segmentation before a contingency that would initiate major outages in an adapted electrical system model IEEE-14 bus is studied and simulations have been performed with test HVAC systems and segmented by HVDC link. The results have been compared, principally in relation to the voltage bus, reactive power generation, system losses and power flow at the lines, and demonstrated that this new concept improved the grid reliability.

Large-area fabrication:The next target of perovskite light-emitting diodes

Perovskite materials show exciting potential for light-emitting diodes(LEDs)owing to their intrinsically high photoluminescence efficiency and color purity.The research focusing on perovskite light-emitting diodes(PeLEDs)has experienced an exponential growth in the past six years.The maximum external quantum efficiency of red,green,and blue PeLEDs has surpassed 20%,20%,and 10%,respectively.Nevertheless,the current PeLEDs are still in the laboratory stage,and the key for further development of PeLEDs is large-area fabrication.In this paper,we briefly discuss the similarities and differences between manufacturing high-quality and large-area PeLEDs and perovskite solar cells.Especially,the general technologies for fabricating large-area perovskite films are also introduced.The effect of charge transport layers and electrodes on large-area devices are discussed as well.Most importantly,we summarize the advances of large-area(active area≥30 mm^(2))PeLEDs reported since 2017,and describe the methods for optimizing large-area PeLEDs reported in the literature.Finally,the development perspective of PeLEDs is presented for the goal of highly efficient and large-area PeLED fabrication.It is of great significance for the application of PeLEDs in future display and lighting.

A novel flexible plasma array for large-area uniform treatment of an irregular surface

In this work,we demonstrate a flexible multi-pin plasma generator with movable electrodes,which can change the shape of the electrode array freely,and then provide a large-area uniform plasma for the treatment of surfaces of different shapes.Discharge characteristics including U-I waveforms and discharge images and sterilization performance under three different electrode configurations(flat-flat,flat-curve,curve-curve)are investigated.Very similar results are acquired between the flat-flat configuration and the curve-curve configuration,which is much better than that under flat-curve configuration.This flexible multi-pin plasma generator offers a simple method to treat different irregularly shaped surfaces uniformly with a single device.Moreover,this device provides a foundation for developing a self-adaption large-scale uniform plasma generator by further introducing automatic adjustment of the position of every electrode driven by motors with discharge current feedback in the following study.Thus it will promote the applications of atmospheric-pressure cold plasmas significantly.

基于时变隐马尔科夫模型的连锁故障预测

近几年,电力系统大面积停电事故在国内外多有发生,而这些事故都是由连锁故障所引起。因此,分析其发展机理、预测其发展路径成为相关学者重点关注的问题。针对目前预测方法比较单一、不能全面考虑连锁故障的影响因素等问题,文章提出时变隐马尔科夫预测模型,该模型改进了马尔科夫模型中的状态转移矩阵;充分考虑了支路保护/断路器拒动、误动概率;系统支路硬件故障概率;支路使用年限等方面因素。文中基于系统拓扑结构计算支路间的故障状态转移概率矩阵,对该矩阵进行不同的改进,通过对比不同模型的预测结果,优化改进,得到连锁故障传播路径,最后,用IEEE 36模型验证所提方法的可行性和准确性。