基于潮流追踪的电网同步性能优化及鲁棒性分析

采用二阶类Kuramoto模型对电网进行合理建模,分别应用临界同步耦合强度和平均同步误差来描述电网的同步能力和鲁棒性.研究发现,发电机的功率分配对线路的传输功率影响较大,而电网中高负荷线路越多,网络越难同步.基于这一发现,首先在发电机功率均匀分配（EG）方式下,计算出每条线路的传输功率,然后基于潮流追踪算法提出一种发电机功率非均匀分配（TG）方式,即在发电总量不变的情况下,增大枢纽发电机节点的功率,减小边缘发电机节点的功率.该发电机功率分配策略可以在一定程度上降低网络的临界同步耦合强度,减小平均同步误差,改善电网的同步性能和鲁棒性.     

基于复杂网络理论的互联电网Braess悖论现象分析

为了探究互联电力网络中的Braess悖论现象, 采用二阶类Kuramoto相振子模型对电网进行动力学建模, 将两个子网通过大度节点相连构建互联电网。当两个子网间有功率传输时, 分别在两个子网内部新增传输线路探究互联电网发生Braess悖论现象的概率并分析其原因。研究发现: 当互联电网中两个子网间的功率传输达到某一临界值时, 受电子网的同步能力远优于供电子网的同步能力, 供电子网新增传输线路引起互联电网发生Braess悖论的概率远高于受电子网新增传输线路引发的Braess悖论概率。通过定义子网序参数对上述现象的产生进行深入分析。本研究对互联电网的拓扑优化具有重要意义。     

基于子网划分的电网关键节点识别

结合电网拓扑结构和潮流追踪技术,提出一种基于子网划分的电网关键节点识别方法。首先,根据发电机节点的邻域信息和功率将发电机节点划分为不同的子集,然后根据电网的系数分配矩阵将负荷节点划分到为其提供最大功率的发电机节点子集中,完成子网划分。接着采用多属性决策法对每个子网的节点进行排序,进一步改进并计算每个子网的结构系数,作为衡量子网重要性的指标。根据子网重要性,从每个子网中提取特定比例的候选关键节点,对这些候选节点依据多属性决策法重新排序,得到关键节点的最终排序。以IEEE14、IEEE57和IEEE118三种节点系统为例进行分析,得到各个系统的子网划分结果和各个标准网络的重要节点排序结果。采用本文方法、PageRank法和多属性决策法分别进行关键节点排序,并对排序靠前的关键节点进行级联故障性能实验和网络效能实验。实验表明,本文算法选择的关键节点对整个网络的传播性能影响最大,优于其他两种关键节点识别方法。     

High‐temperature stability of suspended single‐layer graphene

We report in situ Joule heating on suspended single‐layer graphene in a transmission electron microscope (TEM). Thermally‐driven degradation of pre‐deposited nanoparticles on the membrane is monitored and used for local temperature estimation. By extrapolating the Joule heating power and temperature relation, we find that the suspended single‐layer graphene has exceptional thermal stability up to at least 2600 K. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

基于复杂网络的电网结构健壮性及Braess悖论现象研究

应用复杂网络理论,建立电力系统的改进导纳模型,结合电网拓扑特性和电气特性对电网的级联故障进行研究。通过随机移除传输线引发电网级联故障,研究网络的节点数、平均度、发电站数量以及发电站的分布状况对系统健壮性的影响,并对小世界电网级联故障过程中的布雷斯(Braess)悖论现象进行分析。研究表明：网络的健壮性与其拓扑结构密切相关,平均度较大时,最近邻耦合网络和小世界网络健壮性曲线存在多个分叉点;在小世界结构电网中,一般平均度和节点数越大,发电站的数量越多,电网健壮性越好;发电站分散分布比发电站集中分布的电网健壮性更好。对网络容量增加导致健壮性降低的布雷斯现象进行解释。     

Effect of Joule heating on the electroosmotic microvortex and dielectrophoretic particle separation controlled by local electric field

Dielectrophoresis (DEP) technology has become important application of microfluidic technology to manipulate particles. By using a local modulating electric field to control the combination of electroosmotic microvortices and DEP, our group proposed a device using a direct current (DC) electric field to achieve continuous particle separation. In this paper, the influence of the Joule heating effect on the continuous separation of particles is analyzed. Results show that the Joule heating effect is caused by the local electric field, and the Joule heating effect caused by adjusting the modulating voltage is more significant than that by driving voltage. Moreover, a non-uniform temperature distribution exists in the channel due to the Joule heating effect, and the temperature is the highest at the midpoint of the modulating electrodes. The channel flux can be enhanced, and the enhancement of both the channel flux and temperature is more obvious for a stronger Joule heating effect. In addition, the ability of the vortices to trap particles is enhanced since a larger DEP force is exerted on the particles with the Joule heating effect; and the ability of the vortex to capture particles is stronger with a stronger Joule heating effect. The separation efficiency can also be increased because perfect separation is achieved at a higher channel flux. Parameter optimization of the separation device, such as the convective heat transfer coefficient of the channel wall, the length of modulating electrode, and the width of the channel, is performed.     

Node-wise robustness against fluctuations of power consumption in power grids

We propose a new concept of node-wise robustness of power grids under variation of effective power in one load node using a mathematical model that takes into account the change in voltage and reactive power of load nodes. We employ the topology of the power grid in eastern Japan. We define the robustness as the threshold value of the effective power, above which the steady state loses its stability. We show that the robustness is highly heterogeneous among the load nodes. We find that the shortest path length from generators is most highly correlated with the robustness of the load nodes. We numerically demonstrate that the supply of reactive power enhances the robustness.     

CFQS装置准环对称测试用磁体电源系统设计

CFQS装置是中国西南交通大学(SWJTU)和日本国家核融合科学研究所(NIFS)联合设计制造的中国首台准环对称仿星器,为满足装置0.1 T稳态运行的实验要求,需要为其磁体线圈设计相应的电源系统。本设计方案考虑工程实际估算线路阻抗,考虑工程裕度、电源以负载参数的1.2倍进行计算,并建立电源系统的Simulink仿真模型,分析负载电流纹波大小及网侧谐波含量。根据仿真结果优化设计方案,通过在直流侧增加无源滤波器,减小输出电流纹波,分析直流侧电压代数形式,计算滤波器参数,并仿真调节得到更加符合实际需求的滤波器参数,满足装置准环对称位形的分布磁场精度对磁体线圈电流纹波的要求。     

Numerical simulation and experimental validation of multiphysics field coupling mechanisms for a high power ICP wind tunnel

We take the established inductively coupled plasma(ICP) wind tunnel as a research object to investigate the thermal protection system of re-entry vehicles. A 1.2-MW high power ICP wind tunnel is studied through numerical simulation and experimental validation. The distribution characteristics and interaction mechanism of the flow field and electromagnetic field of the ICP wind tunnel are investigated using the multi-field coupling method of flow, electromagnetic, chemical, and thermodynamic field. The accuracy of the numerical simulation is validated by comparing the experimental results with the simulation results. Thereafter, the wind tunnel pressure, air velocity, electron density, Joule heating rate, Lorentz force, and electric field intensity obtained using the simulation are analyzed and discussed. The results indicate that for the 1.2-MW ICP wind tunnel, the maximum values of temperature, pressure, electron number density, and other parameters are observed during coil heating. The influence of the radial Lorentz force on the momentum transfer is stronger than that of the axial Lorentz force. The electron number density at the central axis and the amplitude and position of the Joule heating rate are affected by the radial Lorentz force. Moreover, the plasma in the wind tunnel is constantly in the subsonic flow state, and a strong eddy flow is easily generated at the inlet of the wind tunnel.     

A novel dynamic model on power failure propagation and its application to load shedding optimization

In this letter, we proposed a novel individual-based dynamical model to study failure propagation in power system. In this model, a transmission line fails with a probability related to additional load transferred by other failed lines, where load upon failure of power lines is based on global and equal redistribution. Besides, the line recovers with a certain probability, and the failure levels of lines is depicted as a probability rather than a Boolean value subject to the mean-field theory. Based on this model, the outbreak threshold of the power system was analyzed. We gave the mathematical expression of the threshold for the outbreak of power failure scale and conducted simulation experiments to verify the theoretical value. Additionally, we discussed the optimal load shedding problem to minimize the objective function related to the cost of failure and load shedding. Lastly, the numerical simulations were provided to illustrate our theoretical results.     

非线性传输线高功率实验北大核心CSCD

设计了基于交叉耦合铁氧体非线性传输线高功率射频微波产生系统,系统由脉冲形成线、非线性传输线以及高功率匹配负载(或组合振子辐射天线)组成。由100kV高压电源和高压微波电缆构成单传输线高功率脉冲形成线,形成线输出脉冲幅度35kV,脉冲半宽60ns。高压脉冲经过非线性传输线的脉冲压缩和调制,与高功率匹配负载相连时,实验得到了峰峰值31kV、中心频率308 MHz、3dB带宽为13%的射频振荡脉冲;与组合振子天线相连时,实验得到了中心频率380MHz、3dB带宽为12%的宽谱辐射。实验结果与数值模拟基本吻合。     

Models for the modern power grid

This article reviews different kinds of models for the electric power grid that can be used to understand the modern power system, the smart grid. From the physical network to abstract energy markets, we identify in the literature different aspects that co-determine the spatio-temporal multilayer dynamics of power system. We start our review by showing how the generation, transmission and distribution characteristics of the traditional power grids are already subject to complex behaviour appearing as a result of the the interplay between dynamics of the nodes and topology, namely synchronisation and cascade effects. When dealing with smart grids, the system complexity increases even more: on top of the physical network of power lines and controllable sources of electricity, the modernisation brings information networks, renewable intermittent generation, market liberalisation, prosumers, among other aspects. In this case, we forecast a dynamical co-evolution of the smart grid and other kind of networked systems that cannot be understood isolated. This review compiles recent results that model electric power grids as complex systems, going beyond pure technological aspects. From this perspective, we then indicate possible ways to incorporate the diverse co-evolving systems into the smart grid model using, for example, network theory and multi-agent simulation.     

Network robustness and fragility: percolation on random graphs

Recent work on the Internet, social networks, and the power grid has addressed the resilience of these networks to either random or targeted deletion of network nodes or links. Such deletions include, for example, the failure of Internet routers or power transmission lines. Percolation models on random graphs provide a simple representation of this process but have typically been limited to graphs with Poisson degree distribution at their vertices. Such graphs are quite unlike real-world networks, which often possess power-law or other highly skewed degree distributions. In this paper we study percolation on graphs with completely general degree distribution, giving exact solutions for a variety of cases, including site percolation, bond percolation, and models in which occupation probabilities depend on vertex degree. We discuss the application of our theory to the understanding of network resilience.     

Non-monotonic increase of robustness with capacity tolerance in power grids

The robustness of different scale power grids is analyzed based on complex network theory in terms of electrical betweenness and weighted efficiency. The robustness of a power grid does not always increase monotonically with the capacity. This property is different from the results obtained in previous studies, which have indicated that the robustness increases monotonically with capacity. To understand the non-monotonic phenomenon, the cascading failure is divided into several sub-stages, and we analyze the number of overloaded nodes and the average remaining load in each sub-stage. The results indicate that the increasing capacity is barely able to reduce the number of overloaded nodes at the beginning of malfunction, which may lead to more nodes being removed subsequently, including certain nodes with many connections or large load. More loads remain in the power grid such that certain nodes cannot take the load. This eventually causes overloading of more nodes and a decline in the robustness of the power grid. The conclusion may be useful for power grid planners seeking to design grids with cost-effective capacity.     

Optimal intentional islanding to enhance the robustness of power grid networks

Intentional islanding of a power system can be an emergency response for isolating failures that might propagate and lead to major disturbances. Some of the islanding techniques suggested previously do not consider the power flow model; others are designed to minimize load shedding only within the islands. Often these techniques are computationally expensive. We aim to find approaches to partition power grids into islands to minimize the load shedding not only in the region where the failures start, but also in the topological complement of the region. We propose a new constraint programming formulation for optimal islanding in power grid networks. This technique works efficiently for small networks but becomes expensive as size increases. To address the scalability problem, we propose two grid partitioning methods based on modularity, properly modified to take into account the power flow model. They are modifications of the Fast Greedy algorithm and the Bloom algorithm, and are polynomial in running time. We tested these methods on the available IEEE test systems. The Bloom type method is faster than the Fast Greedy type, and can potentially provide results in networks with thousands of nodes. Our methods provide solutions which retain at least 40–50% of the system load. Overall, our methods efficiently balance load shedding and scalability.     

Complex dynamics of blackouts in power transmission systems

In order to study the complex global dynamics of a series of blackouts in power transmission systems a dynamical model of such a system has been developed. This model includes a simple representation of the dynamical evolution by incorporating the growth of power demand, the engineering response to system failures, and the upgrade of generator capacity. Two types of blackouts have been identified, each having different dynamical properties. One type of blackout involves the loss of load due to transmission lines reaching their load limits but no line outages. The second type of blackout is associated with multiple line outages. The dominance of one type of blackout over the other depends on operational conditions and the proximity of the system to one of its two critical points. The model displays characteristics such as a probability distribution of blackout sizes with power tails similar to that observed in real blackout data from North America.     

Critical points and transitions in an electric power transmission model for cascading failure blackouts

Cascading failures in large-scale electric power transmission systems are an important cause of blackouts. Analysis of North American blackout data has revealed power law (algebraic) tails in the blackout size probability distribution which suggests a dynamical origin. With this observation as motivation, we examine cascading failure in a simplified transmission system model as load power demand is increased. The model represents generators, loads, the transmission line network, and the operating limits on these components. Two types of critical points are identified and are characterized by transmission line flow limits and generator capability limits, respectively. Results are obtained for tree networks of a regular form and a more realistic 118-node network. It is found that operation near critical points can produce power law tails in the blackout size probability distribution similar to those observed. The complex nature of the solution space due to the interaction of the two critical points is examined.(c) 2002 American Institute of Physics.     

Joule heating and viscous dissipation effects on MHD flow past a semi-infinite inclined plate with variable surface temperature

A numerical investigation is performed to study the MHD free convection flow past a semi-infinite inclined plate subjected to a variable surface temperature. The Joule heating and viscous dissipation effects are taken into account in the energy equation. The governing equations of the flow are transformed into a nondimensional form using suitable dimensionless quantities. A fully developed implicit finite-difference scheme of Crank-Nicolson type is engaged to solve the dimensionless governing equations, which is more accurate, fast convergent, and unconditionally stable. The effects of the MHD, inclination angle, power law, Grashof number, Prandtl number, Joule heating, and viscous dissipation effects are studied on the velocity, temperature, shear stress, and heat transfer coefficients during transient periods. It is observed that the MHD has retarding effects on velocity.     

高温超导直流电缆现状

文中对高温超导直流电缆现状进行了介绍。高温超导直流电缆是用于直流输电的高温超导电缆。高温超导直流电缆发展相对落后于高温超导交流电缆,国际上示范项目也较少。但随着轻型直流输电应用的逐渐发展,高温超导直流电缆越来越引起人们的重视。高温超导直流电缆的本体结构和高温超导交流电缆本体结构类似。高温超导直流电缆具有输电损耗小、适合长距离输电、增加电网稳定等功能。比较适用于背靠背直流输电,工业直流输电,互联网数据中心直流供电,远距离、大容量直流输电等场合。此外,还介绍了目前国际上主要的超导直流电缆项目及研究机构情况。     

Power grid enhanced resilience using proportional and derivative control with delayed feedback

This paper investigates the resilience of an elementary electricity system (machine-generator) under proportional and derivative (PD) control when subject to large perturbations. A particular attention is paid to small power grids, representative of power grid structure in some developing countries. The considered elementary electricity system consists of a consumer (machine), a power plant (generator) and a transmission line. Both Runge-Kutta and Newton methods are used to solve the dynamical equations and the characteristic equations for stability. It is found that the controller increases the resilience of the system. We also show that time delays associated to the feedback loop of the controller have a negative impact on the performance. It is also shown that the asymmetry due to energy demand of different consumers to power plant increases the stability of the system.