Nonuniformity correction factors for maximum mesh-voltages ofcombined grid-multiple rods electrodes

Correction factors for evaluating maximum mesh-voltages associated with combined grid-multiple rods ground electrodes in two-layer soils are calculated for a wide set of ground electrodes to expand the application of available uniform soil expressions to nonuniform soil cases. Square and rectangular grids are examined reinforced by rods of fixed length distributed along the grid perimeter only and along the grid perimeter and grid diagonals. Results of the calculations of 2200 cases generated by varying electrodes and soil parameters are displayed as graphs appropriate for practical application. Approximate analytical expressions for correction factors are also derived matching reasonably well with the calculated data. These expressions offer an insight in the most effective ground electrodes parameters which might serve in the design process     

论水平接地网最大接触电位差的解析计算法

中国电力标准DL/T 621—1997《交流电气装置的接地》所推荐的接地网最大接触电位差计算公式的缺点是不宜用于面积S<30 m×30 m的小型接地网。为此,提出了以正方环为基本几何模型的接地网最大接触电位差新计算公式,它既适用于小型接地网,又适用于大型接地网,而且,它还比以平行导体为基本几何模型的美国《交流变电站接地安全导则》的最大接触电位差计算公式更符合接地网的实际情况,且物理概念和几何概念也更清楚。新计算公式与计算机数值算法比较,相对误差较小,且适用范围大,可供有关工程人员参考使用。     

Resistance to ground of combined grid-multiple rods electrodes

The resistance to ground of a wide set of square and rectangular grid electrodes combined with multiple rods, buried in uniform and two-layer soils, has been analyzed using available exact computer software. Based upon the results of this analysis, analytical, expressions and graphs for electrodes resistance to ground are provided, appropriate for practical application in substation protection     

Method for calculating the ground resistance of grounding grids using FEM

Taking as given that ground resistance is independent of earth fault current, this paper presents a new way of calculating ground resistance of grounding grids using the finite-element method (FEM). Ground resistance is calculated by means of three consecutive tests, with an acceptable number of elements, and fast resolution. The results of the proposed method are compared with those measured experimentally or determined by other authors. Once ground resistance and earth fault current are known, grid potential and touch-and-step voltages are calculated. The main advantage of the method proposed here is that it permits us to analyze symmetrical and nonsymmetrical grids of any shape in uniform, two-layer, and multilayer soils. The method developed in this paper constitutes a fast-resolution industrial application, with acceptable results, for calculating the ground resistance of grounding grids of any shape.     

一种基于深度强化学习算法的电网有功安全校正方法

电力系统有功安全校正对于保障电网安全运行具有重要意义。传统有功安全校正方法无法综合考虑系统潮流分布状态和机组的调整性能，求解效率低、涉及调整的机组多，存在调整反复的现象，在实际应用中具有一定困难。因此，采用深度强化学习算法，提出一种基于深度Q网络(Deep Q Network, DQN)的有功安全校正策略。首先，建立系统有功安全校正模型。其次，采用卷积神经网络(Convolutional Neural Networks, CNN)挖掘电网运行状态深层特征。进一步利用DQN算法通过“状态-动作”机制，以“奖励”为媒介，构建电网运行状态与最优调整机组组合的映射模型，确定调整机组。最后，根据过载线路对调整机组的灵敏度，计算得到调整量。IEEE39节点系统的验证结果表明，所提出的有功安全校正策略在处理多线路过载时可综合考虑系统潮流分布的总体状况和机组调节性能，高效地消除线路过载。     

Bias correction for the maximum likelihood estimates in thetwo-parameter Weibull distribution

This paper provides a simple bias correction method for the maximum likelihood estimates in the 2-parameter Weibull distribution. The unbiased estimates of the shape and scale parameters as well as the percentile points can be obtained by using a simple formula     

Application of combined electric- and magnetic-conductor ground planes for antenna performance enhancement

This paper investigates the application of three different ground planes for antenna performance enhancement. They are the conventional perfect electric conductor (PEC), the perfect magnetic conductor (PMC), and their combination. A half-wavelength dipole in free space is considered as the reference case, and its performance over these ground planes is investigated and compared. It is shown that by using a combined PMC-PEC ground plane, one can achieve a better performance. Next, a microstrip patch antenna is considered as the source and is placed over a high-impedance surface (HIS) or an artificial magnetic conductor (AMC). Here too, adding a PEC ground plane on the periphery of the HIS or AMC improves the antenna gain significantly. As well, the superiority of AMC ground planes, consisting of grounded dielectric slabs loaded with periodic metallic patches without shorting pins (vias), is investigated in detail. Simulation results show that use of such artificial ground planes can enhance the microstrip patch antenna gain and bandwidth remarkably. These enhancements are confirmed by measurement, and high gains of 10.4 dBi and input impedance bandwidths of 28% are measured.     

膜电极性能的影响因素研究

文章以有机多孔膜为电极支撑体制备了亲水电极,用于质子交换膜燃料电池(PEMFC)。考察了电极在全氟代磺酸离聚物(Nafion)的玻璃化温度处理、催化层中Nafion的含量、以及不同厚度的有机多孔膜等因素对电极性能的影响。测试结果表明亲水电极中Nafion分子的有序排列可以减小电极的极化,减小作为电极支撑体的有机多孔膜的厚度有利于电极性能的提高。采用70%Pt/C制备电极时,Nafion的最佳含量为25%,在1000mA/cm2的电流密度下,电池电压为0.615V。     

一种故障分量选相元件接地短路判据的修正

在详细分析故障分量选相原理的基础上,对传统故障分量选相元件的接地短路故障判据进行了修正。修正后的判据仅利用负序故障分量电流相位和正序故障分量电流相位之间的相位差,加入对零序电压分量的阈值判断来区分接地故障。克服了原判据由于零序电流分布因子与正、负序电流分布因子不相等而产生误判的缺点,较好地解决了接地故障误判断问题,同时简化了选相流程。仿真实验验证了该判据的正确性和稳定性。     

Optimal distribution systems reconfiguration for radial and meshed grids

In recent years, convex relaxations of the AC power flow equations have gained popularity. Their success is mainly due to the optimality guarantees they can provide. However, their use has mostly been limited to continuous problems. This paper extends the reach of relaxations to reconfiguration problems with binary decision variables, such as minimal power loss, load balancing and power supply restoration. This is achieved by extending the relaxations of AC power flow equations to bear on the on/off nature of constraints featured in reconfiguration problems. This leads to an approach producing AC feasible solutions with provable optimality gaps, on both radial and meshed grids. In terms of run time, the new models are competitive with state-of-the-art approximations which lack formal guarantees.     

城市电网安全预警与保障决策支持系统

介绍了城市电网在线安全预警与保障决策支持系统的主要功能和技术特点,针对城市电网功角稳定问题不明显、热稳定问题相对突出的特点,对电网的静态安全、电压安全、继电保护定值等进行在线校核,从设备状态评估、电网调度、运行方式及继电保护等多个专业角度实现了城市电网的综合安全预警和在线辅助决策。     

Systematic evaluation for multi-rate simulation of DC grids

With wide applications of power electronic devices in modern power systems, simulation using traditional electromechanical and electromagnetic tools suffers low speed and imprecision. Multi-rate methods can enhance efficiency of simulation by decreasing the scale of systems in small time-steps. However, the existing traditional methods for multi-rate simulation suffer the problems of instability and simulation errors. These have hindered the application of multi-rate simulation in power industry. Therefore theoretical evaluation on different multi-rate simulation methods is crucial to understand the feasibility and limitation of the methods, and to contribute to overcome the drawbacks of the traditional methods. In this paper, the multi-rate simulation performance based on two traditional technologies and a Modified Thevenin Interface are evaluated to provide an overall feasibility of multi-rate algorithms in the power simulation. The Modified Thevenin Interface is proposed to overcome the drawbacks in synchronization. Three theorems are proposed and proved for theoretically analyzing the stability of the simulation methods. Error analyses of the multi-rate methods are performed to identify the relationships between errors and simulation conditions. Besides, the accuracy and efficiency performance in a practical project of VSC-MTDC shows the feasibility and necessity by using multi-rate simulation. Through the theoretical analysis, the issues of stability and accuracy of multi-rate simulation for the DC grids have been better understood, based on which an improved simulation algorithm has been proposed to overcome these issues. Long-term system dynamics of large-scale systems containing DC grids and fast transients of HVDC converters can be investigated simultaneously with high speed and sufficient accuracy.     

电网经济调度分析方法研究

衡量电网调度运行是否经济的最直接手段是对其所产生的经济效益进行分析[1].提出电网经济调度的一种分析方法(四分法),立足于实际购售电毛利较计划购售电毛利所得的增量,即超计划效益,视其为调度机构通过优化电网运行取得的经济调度运行效益,将其分解为"多购多售"、"降低损耗"、"优化购电"和"优化售电"等4个影响方面,计算出这4个因素的经济调度措施对全网经济效益提高所作出的贡献,并通过算例验证了分析方法的有效性.     

接地距离保护现场测试的主要影响因素及解决方案

针对接地距离保护在现场测试过程中存在的问题,通过分析继电保护测试仪的测试原理及接地距离保护的实现原理,叙述了零序补偿系数的不同表达方式,总结出了测试接地距离保护功能的方法及影响接地距离保护动作的因素,指导现场测试提高测试水平及测试效率,确保接地距离保护功能检验质量。     

接地距离保护现场测试的主要影响因素及解决方案

针对接地距离保护在现场测试过程中存在的问题,通过分析继电保护测试仪的测试原理及接地距离保护的实现原理,叙述了零序补偿系数的不同表达方式,总结出了测试接地距离保护功能的方法及影响接地距离保护动作的因素,指导现场测试提高测试水平及测试效率,确保接地距离保护功能检验质量.     

大电网继电保护技术应用与发展

基于互感器技术、保护原理以及通信技术,分析了互感器的特性、选型和非传统互感器的发展;各种继电保护原理特点和特高压保护的配置;通道延时、数据同步、时钟同步以及IEC61850和通信技术的发展对保护系统的影响,提出了在大电网继电保护技术应用中要注意的若干问题.     

Combining prospective motion correction and distortion correction for EPI: towards a comprehensive correction of motion and susceptibility-induced artifacts

Object  

State-of-the-art MR techniques that rely on echo planar imaging (EPI), such as real-time fMRI, are limited in their applicability by both subject motion and B₀ field inhomogeneities. The goal of this work is to demonstrate that in principle it is possible to accurately predict the B₀ field inhomogeneities that occur during echo planar imaging in the presence of large scale head motion and apply this knowledge for distortion correction.     

板栅模具设计中的几个问题

通过对板栅模具设计中遇到的几个实际问题的分析提出处理意见 ,从而达到科学合理地设计板栅模具。     

拉网电池正极板长大的原因与对策

本文就拉网电池极板长大的机理进行了分析探讨,提出了在电池极群顶部添加热熔胶等预防措施,并针对加胶的措施进行了试验验证,结果证明这种措施对于抑制拉网电池正板长大有明显的抑制效果。     

柔性互联交流配电网的换流器并离网统一控制策略

提出一种并离网统一控制策略,在孤岛控制模式的基础上设计了主动功率调节功能,并网运行时按照有功-电压平方下垂的关系自动分配有功功率;离网运行时下垂功率控制自动切换成定交流电网频率和电压的孤岛模式。该方法适用于并网和离网2种不同的运行工况,并且省去了孤岛检测环节,大幅简化了控制系统结构。以四端柔性互联交流配电网系统为例,详细分析了并离网统一控制策略的参数设计方法,并建立电磁暂态仿真模型,针对并离网多种不同工况开展仿真。仿真结果表明,所提方法可以满足柔性互联系统的负荷均衡及转供的性能要求,同时也避免了并离网切换过程的过电流。