EMP作用下的电缆耦合及屏蔽效能试验分析

介绍了三同轴法、线注入法、功率吸收钳法、混波室法等电缆屏蔽效能测试方法.进行了电磁脉冲作用下的同轴电缆和裸线耦合试验,探讨了该环境下同轴电缆屏蔽效能的测试方法.试验分析表明,电缆耦合后的波形为衰减振荡波,振荡周期正比于电缆的电长度,频谱与照射场的频谱不一致,裸线的耦合输出衰减较快.根据试验数据,采用峰值场强法计算的电缆屏蔽效能与峰值电压法的结果吻合,所以可采用峰值场强法对电缆进行电磁脉冲的屏蔽效能测试.电磁脉冲作用下的同轴电缆屏蔽效能较相当频段连续波作用下的略低.     

陕西省330kV输电线路防雷技术分析

在区别线路反击和绕击的基础上,对典型的线路绕击故障进行了分析。指出绕击是造成陕西省山区内330kV线路雷击跳闸的主要原因,并建议采用综合防雷措施降低线路雷击跳闸率。     

Geometric subspace methods and time-delay embedding for EEG artifact removal and classification.

Generalized singular-value decomposition is used to separate multichannel electroencephalogram (EEG) into components found by optimizing a signal-to-noise quotient. These components are used to filter out artifacts. Short-time principal components analysis of time-delay embedded EEG is used to represent windowed EEG data to classify EEG according to which mental task is being performed. Examples are presented of the filtering of various artifacts and results are shown of classification of EEG from five mental tasks using committees of decision trees.     

含贯通导体腔体电磁耦合的混合计算模型（英文）

For studying the coupling effect of electromagnetic irradiation to rectangular enclosure with wire penetration,a new hybrid model is proposed to determine the shielding effectiveness of the enclosure.This model based on rectangular-enclosure equivalent circuit regards the penetrating wire as a monopole antenna,and the frequency-domain results of calculation using this model match with that of computer simulation technology(CST)Microwave Studio.Meanwhile,this new model requires far less computation than traditional methods:3 s for one sample on an average personal computer.Based on the presented model,the relationships between enclosure volume,external length of wire penetration,internal length of wire penetration,and shielding effectiveness are analyzed.In the frequency range from 100 MHz to1 000 MHz,when keeping the length of penetrating wire unchanged,the shielding effectiveness of an enclosure at its center point increases with the cavity volume.There is no obvious effect from the external length of penetrating wire on the shielding effectiveness.If the external length of a penetrating wire is 8 cm unchanged,the shielding effectiveness of the penetrated enclosure at its center point declines with the increase of internal length of the penetrating wire.     

A computer program for designing the lightning shielding systems ofsubstations

A computer program is presented for designing the shielding systems of substations. The program is based on the most up-to-date version of the electrogeometric model (EGM). The calculation method is made free from the conservatism of some earlier versions of the EGM by introducing rational revisions. Shielding designs for typical layouts are also presented     

三维激光扫描技术在输电线路差异化防雷治理中的应用

输电线路差异化防雷评估技术作为掌握输电线路防雷性能、明确防雷重点的有效方法,已得到各级运行管理部门的认可。地形地貌和杆塔结构作为线路绕击防雷性能的影响因素,是防雷评估时需获取的重要基础参数。为研究获取全档距地形地貌参数和杆塔结构参数的技术,以及将这些参数应用于防雷性能评估的方法,提出基于三维激光扫描数据建立线路走廊三维模型,根据此模型提取差异化防雷评估所需的全档距地形地貌参数和杆塔结构参数,进而进行全档距绕击防雷性能评估。经与山区典型220 kV线路比对,评估结果与实际雷击故障记录吻合良好,充分说明了三维激光扫描技术应用于输电线路差异化防雷治理的有效性。     

Tackling SNR at low-field: a review of hardware approaches for point-of-care systems

Objective

To review the major hardware components of low-field point-of-care MRI systems which affect the overall sensitivity.

Methods

Designs for the following components are reviewed and analyzed: magnet, RF coils, transmit/receive switches, preamplifiers, data acquisition system, and methods for grounding and mitigating electromagnetic interference.

Results

High homogeneity magnets can be produced in a variety of different designs including C- and H-shaped as well as Halbach arrays. Using Litz wire for RF coil designs enables unloaded Q values of ~ 400 to be reached, with body loss representing about 35% of the total system resistance. There are a number of different schemes to tackle issues arising from the low coil bandwidth with respect to the imaging bandwidth. Finally, the effects of good RF shielding, proper electrical grounding, and effective electromagnetic interference reduction can lead to substantial increases in image signal-to-noise ratio.

Discussion

There are many different magnet and RF coil designs in the literature, and to enable meaningful comparisons and optimizations to be performed it would be very helpful to determine a standardized set of sensitivity measures, irrespective of design.

     

特高压直流换流站阀厅屏蔽效能及设计要求

为了研究特高压直流换流站阀厅的屏蔽效能和设计要求,提出了频率在150 kHz～80 MHz范围内、电场强度≤1 V/m射频干扰强度(120 dB,基准1μV/m对应0 dB)作为邻近换流阀厅外侧的控制指标。结合换流站现场实测数据进行相应的分析及计算,提出阀厅屏蔽设计要求:(1)屏蔽效能目标值取≥20 dB;(2)阀厅屏蔽一般采用直径6 mm、网孔面积200 mm×200 mm的钢筋网;(3)如果墙壁直接采用双层0.6 mm厚镀铝锌压型钢板,每隔30 cm用铆钉连接,两板重合部分宽度5 cm,则单层板在≤10 MHz范围内的屏蔽效能40 dB,也可满足要求。     

A data-reduction process for long-term EEGs. Feature extractionthrough digital processing in a multiresolution framework

Describes a contribution to a data-reduction process to be used with long-term EEGs. Since typical long-term EECs recorded from depth electrodes are extended over several days, while epilepsy may be characterized by occasional transients, data reduction is an important consideration for the electroencephalographer. The electroencephalographer detects epileptic activity by visual inspection of the EEG, which is a time-consuming procedure for records that are days long. The result obtained with the authors' proposed algorithm is the selection of segments of EEG where a transient is detected; then these segments are reviewed by an expert. The authors' primary objective is to minimize the visual inspection process, presenting to the clinician only selected segments of the EEG. Throughout this article, no distinction is made among the wide variety of epileptiform transients. The only objective of the algorithm is to separate background activity from epileptiform activity     

输电线路绕击跳闸率的改进电气几何模型

在原有的经典电气几何模型的基础上,提出了相应的改进方法.引入击距系数β来描述雷击输电线路时,避雷线的击距与大地击距的不同.同时对原来计算线路绕击跳闸率的方法进行改进:采用距离差来计算绕击跳闸率.最后采用此种方法,针对典型的单回500 kV酒杯塔,计算和分析了不同保护角和地面倾角时对绕击跳闸率的影响.     

Lightning faults on distribution lines

The authors have developed low-cost instrumentation to differentiate between outages caused by lightning and those caused by other phenomena. The theories used to develop this coincident lightning events detector (CLED), the experimental design used for testing the CLED, and the test results are discussed. The following conclusions are drawn from lightning fault data collected in the Florida test area: shielding from other nearby structures is a major consideration when assessing the lightning fault rate on distribution lines and, for typical distribution lines in the USA. A range of values from 0.6 to 0.8 for the shielding factor would be appropriate     

Estimating complexity from EEG background activity of epilepticpatients

We analyze the EEG background activities of five epileptic patients and compare them with the EEGs of five control subjects by means of methods from nonlinear dynamical system theory. We divided the EEG time-series data into frames to select those that fulfilled the stationarity condition, which is necessary in order to apply our methods. Then, we reconstructed the chaotic dynamic attractors expressed in the phase space and calculated their correlation dimensions, which are a measure of the complexity. Furthermore, we recorded an EEG during a petit mal epileptic seizure, whose attractor is a 3-D plot and the correlation dimension will also be presented     

铁塔横担侧向避雷针的绕击保护效果分析

为评估在杆塔横担上安装侧向避雷针的防雷电绕击效果,提出建立三维的电气几何模型,计算侧向避雷针对导线的绕击保护距离,并总结了其安装和使用的规律。针对110～500kV典型杆塔线路的计算表明：侧向避雷针能较好地保护杆塔附近的重点绕击危险区域,从而可以有效地降低线路的绕击跳闸率。研究还表明,侧向避雷针对导线的保护效果会受到针...     

Single trial estimation of event-related potential components using spatiotemporal filtering and artificial bee colony optimized Gaussian kernel mixture model

Single trial estimation of event-related potential (ERP) components is an open research topic in neuroscience. In this article, we have proposed a method to improve the performance of spatiotemporal filtering by decreasing its dependency to prior estimates of ERP components. For this purpose, we have used a mixture of Gaussian kernels instead of a raw prior signal, and the parameters of the Gaussian kernel are estimated using artificial bee colony algorithm. The algorithm starts with one Gaussian kernel, and after optimizing its parameters, another Gaussian kernel is added. This procedure goes on until the stopping criterion is reached. The efficiency of the algorithm is tested for one single uncorrelated component and two correlated components for synthesized electroencephalogram (EEG) signal. Also, the efficiency of the proposed method is presented on real data for extraction of N170 component in real EEG data.     

A numerical study of the efficiency of a cylindrical electromagnetic shield

Electromagnetic shields are widely used for protecting electrical equipment and cable lines from the influence of natural and artificial electric and magnetic fields. A 3D mathematical model of a solid cylindrical shield for protection from the effect of an alternating electromagnetic field produced inside a cable has been proposed. The model has been constructed using the finite-element method in the ANSYS HFSS software. Two electromagnetic-field sources have been considered: noise in the inner conductor (cable core) and a coaxial circuit, in which the shield is an outer working conductor. The electromagnetic-field distribution over the cable is described by the Maxwell equations, which are transformed into the Helmholtz vector equation with respect to the electric-field component so that the finite-element method may be used. The electromagnetic parameters of the considered media are constant and isotropic. The shielding properties have been studied for a copper cylindrical shield in the frequency range of 1–100 MHz. As a result of the mathematical solution, the distributions of electromagnetic-field intensities in the model have been obtained. To determine the shielding properties of a cable, the transfer impedance and shielding attenuation have been used. The adequacy of the model and technique for determining the efficiency of the screening has been verified by means of comparison between the obtained data and results of the analytical model.     

基于GaN器件的固态射频电源应用研究

随着电力电子技术的发展,射频电源由电子管电源发展成现在的晶体管射频电源。氮化镓GaN(gallium nitride)作为第三代宽禁带半导体材料的典型代表,具有宽禁带、高临界击穿场强、高电子饱和漂移速度以及高导通的AlGaN/GaN异质结二维电子气2DEG(two-dimensional electrons gas)等优点。GaN功率器件与硅(Si)功率器件相比,具有导通阻抗低,输入、输出电容小等特性,这些特性使得GaN功率器件高开关速度、低损耗。在E类功率射频电源的基础上,采用GaN功率器件设计制作了一款开关频率为4 MHz、功率可调的全固态射频电源实验样机。通过电路的设计和优化,样机的输出功率为21.4 W时,效率达到了96.7%;同时,采用专为射频电源生产的Si功率器件替换掉样机上的GaN器件,实验数据验证了GaN器件开关速度快、损耗低,可大幅度提高射频电源的效率。     

Circuit modeling and efficiency analysis for wireless power transfer system with shielding

It is an effective way to use magnetic shielding scheme in magnetic coupled wireless power transfer (WPT) system to reduce magnetic flux leakage. However, the additional shielding material would affect the equivalent circuit model and the power transmission efficiency of WPT system. Although the impact of some common shielding materials on the transmission efficiency has been studied using finite element simulation, there is still a lack of understanding about the impact to the equivalent model in theory. In this paper, the equivalent circuit and transmission efficiency of WPT system are analyzed theoretically under the influence of different shielding materials, and the relationship between material electrical characteristics and transmission efficiency is presented. In our WPT prototype with two 166-mm diameter coils under ferrite shielding and aluminum shielding, the achieved maximum transmission efficiencies are 95.7% for 100-kHz resonant frequency and 97.9% for 1-MHz resonant frequency under the ferrite shielding, and the efficiency errors between the analytical and experimental results are less than 5% for 100-kHz resonant frequency and 10% for 1-MHz resonant frequency.     

A Random Forest Classifier-based Digital Protection Scheme for Busbar

This paper presents a random forest (RF) classifier-based digital protection scheme which provides an effective discrimination between internal and external faults on a busbar. The measured current signals of all the bays (lines) connected to a busbar have been used as feature vectors. The system and fault parameters have been varied to generate a wide variety of simulation cases (33,600) consisting of both internal and external faults. By giving post-fault data of one cycle duration of all the bay currents as an input to the RF classifier, and taking only 30% of the total data (33,600) for training and remaining 70% of the total data for testing, an accuracy higher than 98% has been obtained. The PSCAD/EMTDC software package is used to model a prevailing 400-kV Indian busbar system for the purpose of authentication of the presented technique. The presented technique successfully differentiates between internal faults and external faults and remains unaffected against the change in system and fault parameters. In addition, the proposed scheme maintains stability under the Current Transformer saturation phenomena particularly during a heavy-through fault. A comparative analysis of the proposed scheme with the recently proposed scheme using support vector machine classifier clearly shows its superiority.     

Power frequency magnetic field management using a combination ofactive and passive shielding technology

Power frequency magnetic fields are difficult and expensive to shield, particularly when the fields originate from sources with complicated field patterns. A method is discussed and test data presented which suggests that carefully combining active and passive shielding techniques can produce results which are importantly superior to either technique when used alone. The results are generally consistent with an analysis which considers sources as a superposition of elementary source fields with different order terms     

Low-Power Circuits for Brain–Machine Interfaces

This paper presents work on ultra-low-power circuits for brain–machine interfaces with applications for paralysis prosthetics, stroke, Parkinson's disease, epilepsy, prosthetics for the blind, and experimental neuroscience systems. The circuits include a micropower neural amplifier with adaptive power biasing for use in multi-electrode arrays; an analog linear decoding and learning architecture for data compression; low-power radio-frequency (RF) impedance-modulation circuits for data telemetry that minimize power consumption of implanted systems in the body; a wireless link for efficient power transfer; mixed-signal system integration for efficiency, robustness, and programmability; and circuits for wireless stimulation of neurons with power-conserving sleep modes and awake modes. Experimental results from chips that have stimulated and recorded from neurons in the zebra finch brain and results from RF power-link, RF data-link, electrode-recording and electrode-stimulating systems are presented. Simulations of analog learning circuits that have successfully decoded prerecorded neural signals from a monkey brain are also presented.