Hybrid Grid DSMC Method for Chemical Nonequilibrium with Rarefied Flow Heating

The influence of chemical nonequilibrium on the thermal characteristics is explored by using the 2Dhybrid grid direct simulation Monte Carlo(DSMC)parallel method.An improved molecule search algorithm is proposed,which can preserve the high efficiency of area search algorithm.This method can overcome the defects of area search algorithm,and give all information about molecules hitting surface.The heat flux calculation method for a rarefied hypersonic flow is established.In addition,the testing methods of chemical reaction probability for five species of mixed gas with limited speed chemical reactions are also selected.To validate the effectiveness of the present method,hypersonic flow around a cylinder is firstly simulated,and subsequently numerical simulations of the heat flux and flow field characteristics around the blunt body at different heights are carried out in two different cases:the thermal nonequilibrium condition and the thermochemical nonequilibrium condition.Numerical results demonstrate the validity and reliability of the proposed methods.

A Paradoxical Consistency Between Dynamic and Conventional Derivatives on Hybrid Grids

It has been evident that the theory and methods of dynamic derivatives are playing an increasingly important role in hybrid modeling and computations. Being constructed on various kinds of hybrid grids, that is, time scales, dynamic derivatives offer superior accuracy and flexibility in approximating mathematically important natural processes with hard-to-predict singularities, such as the epidemic growth with unpredictable jump sizes and option market changes with high uncertainties, as compared with conventional derivatives. In this article, we shall review the novel new concepts, explore delicate relations between the most frequently used second-order dynamic derivatives and conventional derivatives. We shall investigate necessary conditions for guaranteeing the consistency between the two derivatives. We will show that such a consistency may never exist in general. This implies that the dynamic derivatives provide entirely different new tools for sensitive modeling and approximations on hybrid grids. Rigorous error analysis will be given via asymptotic expansions for further modeling and computational applications. Numerical experiments will also be given.

COMPUTATION OF TURBULENCE FLOW ON HYBRID GRIDS USING k-ω TURBULENCE MODEL AND OSHER SCHEME

An unstructured Reynolds-averaged Navier-Stokes flow solver using the finite volume method is studied. The spatial discretisation is based on the Osher approximate Riemann solvers. A two-equation turbulence model (k-ω model) is also developed for hybrid grids to compute the turbulence flow. The turbulence flow past NACA0012 airfoil and the double ellipsolids are computed, and the numerical results show that the above methods are very efficient.

SOLUTION OF 3-D TURBULENCE NAVIER-STOKES EQUATIONS USING HYBRID GRIDS

Hybrid grids are used for the solution of 3D turbulence Navier Stokes equations. The prismatic grids are generated near the wall, and the tetrahedron grids are generated in the other field. A Navier Stokes solver using Jamson′s finite volume method is developed. The algebraic Baldwin Lomax turbulence model is adopted. The numerical tests show that the above method is very efficient.

Navier-Stokes Computation of Grid Fin Missile Using Hybrid Structured-Unstructured Grids

Viscous CFD computations are performed to predict the aerodynamic coefficients and flowfield for a missile with grid fins by using hybrid grids. The computations arc made at Math numbvrs of 0.7 and 2.0. Full N-S equations arc discretized into finite volume form and solved by an algorithm in LU-SGS. The comparisons between computation and experimental data are made, and the detailed flow structure near grid fin is shown and examined.

Voltage Stabilization of Hybrid Micro-Grid Using Super Capacitors

Scarcity of fossil fuel resources has motivated the researchers to develop renewable energy based power projects. Instead of using a single or independent renewable energy source, it is preferable to use the combination of such energy sources in a distributed way to compensate the power fluctuations of the system and this leads to the concept of hybrid micro-grid energy. Voltage stability is an important parameter for the secure operation of the hybrid-micro grid, and IEEE 1547 Standard defines the limit of the voltage for the successful operation of the micro-grid. Although Vanadium Redox Batteries (VRBs) can help the system to stabilize the voltage when voltage sag occurs when a heavy load is suddenly connected to the system, this stabilization process takes some time. This paper discusses the application of super capacitors to the hybrid micro-grid system, as a higher energy density element, to help the system quickly recover its transient voltage.

ADAPTIVE HYBRID CARTESIAN GRID METHOD FOR VORTEX-DOMINATED FLOWS

An efficient compressible Euler equation solver for vortex-dominated flows is presented based on the adaptive hybrid Cartesian mesh and vortex identifying method.For most traditional grid-based Euler solvers,the excessive numerical dissipation is the great obstruction for vortex capturing or tracking problems.A vortex identifying method based on the curl of velocity is used to identify the vortex in flow field.Moreover,a dynamic adaptive mesh refinement(DAMR)process for hybrid Cartesian gird system is employed to track and preserve vortex.To validate the proposed method,a single compressible vortex convection flow is involved to test the accuracy and efficiency of DAMR process.Additionally,the vortex-dominated flow is investigated by the method.The obtained results are shown as a good agreement with the previous published data.

Hybrid Cartesian Grid Method for Moving Boundary Problems

A hybrid Cartesian structured grid method is proposed for solving moving boundary unsteady problems. The near body region is discretized by using the body-fitted structured grids, while the remaining computational domain is tessellated with the generated Cartesian grids. As the body moves, the structured grids move with the body and the outer boundaries of inside grids are used to generate new holes in the outside adaptive Cartesian grid to facilitate data communication. By using the alternating digital tree （ADT） algorithm, the computational time of hole-cutting and identification of donor cells can be reduced significantly. A compressible solver for unsteady flow problems is developed. A cell-centered, second-order accurate finite volume method is employed in spatial discreti- zation and an implicit dual-time stepping low-upper symmetric Gauss-Seidei （LU-SGS） approach is employed in temporal discretization. Geometry-based adaptation is used during unsteady simulation time steps when boundary moves and the flow solution is interpolated from the old Cartesian grids to the new one with inverse distance weigh- ting interpolation formula. Both laminar and turbulent unsteady cases are tested to demonstrate the accuracy and efficiency of the proposed method. Then, a 2-D store separation problem is simulated. The result shows that the hybrid Cartesian grid method can handle the unsteady flow problems involving large-scale moving boundaries.

Practice and Prospect of AC/DC Hybrid Power Grid in Southern China

China Southern Power Grid is a unique EHV AC/DC hybrid transmission network that operates in China. In its service area, the distribution of energy resources and the development of economy are extremely unbalanced, so long-distance and bulk power transmission are needed; besides, the geography and climate conditions are serious, rains, fogs, lightning and typhoon as well as high temperature are common all the year round. Facing these challenges, the power grid enhanced stability control, improved the equipment and strengthen the network structure. In the future, the power grid plans to optimize the disposition of power sources and build digitalized power system.

Homer’s Feasibility Analysis of a Hybrid System with a Grid Connection Option for the Mauritanian Northern Coast

On Mauritania’s northern coast, wind and solar resources are abundant and must be used effectively. These resources have the potential to completely or partially replace the existing or projected diesel generators. The main objective of this case study is to study the possibility of using a hybrid system (HS) of the type (diesel, wind and storage). The most important part of this case study intended for this area will be to add the solar in a first phase and then the incorporation of an interconnection with the nearby network in a second phase. This interconnection will be secured by mean of medium voltage lines of 33 kV, where the nearest point is located 35 km away. Indeed, the study of the optimization model is carried out through Homer, which was developed by National Renewable Energy Laboratory [NREL]. Thus, it should be noted that the HS is analyzed on the basis of costs ($/kW) and price ($/kWh) and greenhouse gas emissions. Therefore, in order to achieve these techno-economic optimization objectives, this paper introduces a sensitivity analysis that has been proposed to determine the effect of costs on each HS configuration. In the end, HSs are needed for maximum use of renewable resources at the studied site for an uninterrupted power supply.

Modeling and Simulation of a Hybrid Energy Storage System for Residential Grid-Tied Solar Microgrid Systems

Present-day power conversion and conditioning systems focus on transferring energy from a single type of power source into a single type of load or energy storage system (ESS). While these systems can be optimized within their specific topology (e.g. MPPT for solar applications and BMS for batteries), the topologies are not easily adapted to accept a wide range of power flow operating conditions. With a hybrid approach to energy storage and power flow, a system can be designed to operate at its most advantageous point, given the operating conditions. Based on the load demand, the system can select the optimal power source and ESS. This paper will investigate the feasibility of combining two types of power sources (main utility grid and photovoltaics (PV)) along with two types of ESS (ultra-capacitors and batteries). The simulation results will show the impact of a hybrid ESS on a grid-tied residential microgrid system performance under various operating scenarios.

Hybrid Power System Options for Off-Grid Rural Electrification in Northern Kenya

For domestic consumers in the rural areas of northern Kenya, as in other developing countries, the typical source of electrical supply is diesel generators. However, diesel generators are associated with both CO2 emissions, which adversely affect the environment and increase diesel fuel prices, which inflate the prices of consumer goods. The Kenya government has taken steps towards addressing this issue by proposing The Hybrid Mini-Grid Project, which involves the installation of 3 MW of wind and solar energy systems in facilities with existing diesel generators. However, this project has not yet been implemented. As a contribution to this effort, this study proposes, simulates and analyzes five different configurations of hybrid energy systems incorporating wind energy, solar energy and battery storage to replace the stand-alone diesel power systems servicing six remote villages in northern Kenya. If implemented, the systems proposed here would reduce Kenya’s dependency on diesel fuel, leading to reductions in its carbon footprint. This analysis confirms the feasibility of these hybrid systems with many configurations being profitable. A Multi-Attribute Trade-Off Analysis is employed to determine the best hybrid system configuration option that would reduce diesel fuel consumption and jointly minimize CO2 emissions and net present cost. This analysis determined that a wind-diesel-battery configuration consisting of two 500 kW turbines, 1200 kW diesel capacity and 95,040 Ah battery capacity is the best option to replace a 3200 kW stand-alone diesel system providing electricity to a village with a peak demand of 839 kW. It has the potential to reduce diesel fuel consumption and CO2 emissions by up to 98.8%.

基于注意力机制的CNN-LSTM-XGBoost台风暴雨电力气象混合预测模型

极端台风暴雨灾害具有非线性、极差大以及多峰值等特点。为使电网及时获取预警信息,提出一种基于注意力机制的CNN-LSTM-XGBoost台风暴雨电力气象混合预测模型。首先,利用基于注意力机制的卷积神经网络(CNN)辨识关键台风暴雨灾害特征;然后,利用长短期记忆网络(LSTM)训练时间序列预测模型以挖掘台风暴雨时序特征,使用极限梯度提升算法替换模型输出层以缓解过拟合问题;最后,以2023年台风泰利为例验证所提方法的有效性。算例分析表明,所提模型具有较高的准确性,对预测精度的提升可达40.84%以上。

移动群智感知中基于网格混淆的位置隐私保护

针对现有移动群智感知(mobile crowd sensing,MCS)面临的隐私泄露问题,引入一种混合式区块链架构来构建系统,实现MCS的去中心化,并通过私有区块链保护工人隐私记录。采用无证书签密实现用户数据传输过程中的机密性和完整性,保证用户信息的安全性。为了降低位置隐私暴露的风险,提出一种基于网格混淆的位置隐私保护方案(location privacy protection scheme based on grid obfuscation,LPPSGO)。该方案通过H3索引系统划分多精度六边形网格,实现工人位置的空间隐匿,工人可以根据个性化隐私需求扰动自身位置,无须担心真实位置的暴露。仿真实验结果表明,LPPSGO能有效提高MCS系统的任务分配成功率,减少时间开销,与其他位置保护方案相比,安全性更强,覆盖性能更好。

具备自适应重合闸能力的多端口混合式直流断路器

为解决柔性直流电网中混合式直流断路器成本高以及盲目重合闸的问题,提出了一种具备自适应重合闸能力的多端口混合式直流断路器。所提断路器能够在能量耗散期间旁路故障线路中的限流电抗器,配合电容充电回路的导通,将本应经避雷器耗散的限流电抗器能量转移到电容中,从而有效减少了故障隔离时间与避雷器能量耗散压力,并为故障性质的判别提供了能量支持。同时,该断路器通过共享主断路器的方式实现故障隔离,结合避雷器泄能需求的下降,大大降低了断路器的制造成本,在经济性方面优势明显。此外,该断路器利用故障识别电流与电容电压在永久性故障与瞬时性故障下的幅值差异来识别故障性质,实现了自适应重合闸,操作简单且易于控制。最后,利用PSCAD/EMTDC在三端MMC柔性直流系统环境下对所提方案的有效性进行了仿真验证,并通过与其他方案的对比分析,证明了所提方案的优越性。

一种双电容换流型混合式直流断路器

混合式直流断路器可以快速、准确隔离线路故障,在直流电网故障清除难题中发挥重要作用。为解决直流断路器开断能力要求高、限流能力弱、制造成本大、故障隔离速度慢等问题,提出一种双电容换流型混合式直流断路器(Double capacitor commutated hybrid DC circuit breaker,D-C-DCCB)拓扑。构建断路器时,巧妙利用自充电通路实现电容预充电及反向充电后电容能量的耗散。通过电容反向充电与阻感限流支路降低故障电流上升速率及峰值,利用限流电阻与二极管组实现限流电感,完成限流工作后被短路及储存能量耗散,加快故障隔离速度的同时,有效减少了避雷器吸收能量。对所提断路器的拓扑结构及工作原理进行分析及推导,在PSCAD/EMTDC中对所提断路器拓扑进行了四端直流电网仿真验证,与现有的直流电网故障清除拓扑在性能及经济性方面进行了对比,证明了所提断路器具有性能好、经济性高的优点。

考虑条件风险价值的交直流系统两阶段分布鲁棒低碳经济优化调度

考虑到海上风电出力的随机性以及日益突出的生态环境问题,以含柔性直流输电技术(voltagesource converter high voltage direct current,VSC-HVDC)的交直流系统为研究对象,提出了考虑条件风险价值(conditional valueatrisk,CVaR)的两阶段分布鲁棒低碳经济优化模型,构建了基于Kullback-Leibler(KL)散度的概率分布模糊集,同时利用条件风险价值量化了极端场景下的尾部风险,使得模型能够同时考虑概率分布不确定性以及处于最坏概率分布中极端场景下的尾部损失;此外,将阶梯型碳交易机制并入所提分布鲁棒模型中,通过合理利用柔性资源和储能装置,增强系统运行的灵活性,在兼顾运行风险的前提下,降低碳排放量的目标。再者,为了提高计算效率,在列和约束生成算法(column-and-constraint generation method,C&CG)和Multi-cut Benders分解算法的基础上提出了双循环分解算法。最后,在基于改进的IEEE RTS 79测试系统中验证了所提模型及算法的有效性。

含多类型直流的交直流混联电网潮流计算方法适用性分析

作为电网发展的新阶段,交直流混联电网呈现多类型直流参与、大规模交直流互联的特点,而关于统一迭代和交替迭代2种潮流计算方法的适用性尚未得到深入分析。为此基于含多类型直流的交直流混联电网对2种潮流计算方法的运算性能进行对比研究。推导了含常规直流、柔性直流、混合直流的交直流混联电网潮流模型,进而提出了相应的统一迭代法和交替迭代法。通过3个交直流混联电网测试系统和南方电网实际系统数据验证了潮流模型的有效性和潮流算法的准确性,结合系统负荷水平、系统强度、直流嵌入规模等因素对2种潮流计算方法的收敛性能和计算速度进行对比分析。研究结果表明,在含多类型直流的交直流混联电网中进行潮流计算时,统一迭代法的计算效率比交替迭代法高。

构网型和跟网型电力电子装备混联系统惯量响应的匹配问题综述

构网型和跟网型电力电子装备具有异质化调频特性,导致2类装备交直流混联系统惯量响应阶段的动态交互作用机理复杂,传统的惯量参数匹配原则难以优化系统频率响应的问题。阐述了电力电子装备系统的惯量响应匹配问题,并对现有电压源型变流器的调频控制方法进行了概述。从功频响应分析方法、调频功率分配原则和频率响应特性3个方面对电力电子装备系统惯量配置问题的研究现状展开梳理。对电力电子装备混联系统的统一建模及惯量响应匹配问题提出了研究思路,旨在通过惯量支撑功率的匹配控制提升低惯量电力系统的频率稳定性,从系统层面为高比例电力电子装备友好并网提供基础理论和关键技术支撑。

自由节点差分法寻点策略研究及验证

混合网格具有良好的贴体性,适用于Navier-Stokes方程的计算,但在网格交界面处需要通过插值进行流场信息交换,而这一过程会引入误差。陈洁等提出了一种适用于无序网格点的自由节点差分计算方法,该方法可对重叠网格交界面流场进行差分计算,无须采用插值方法进行流场信息传递,解决了插值方法引入误差的问题。但该自由节点差分法的计算模板需要从中心点周围的点云中选择网格点构成,不同选点策略对计算结果的影响不同。针对自由节点差分法的选点需求,本文综合考虑角度、正交性、距离等因素,提出了几种不同选点策略,并利用数值实验进行了验证。结果表明,本文提出的选点策略符合自由节点差分法的构造思想,均能获得稳定的收敛解,其中模板点与中心点距离对计算精度影响最大,是选点策略的首要影响因素。