HVDC ground electrode heat dissipation in an N-layer soil

The temperature distribution in an N-layer soil, due to high-voltage direct-current (HVDC) injection via ground electrodes, was computed using finite-difference methods. The temperature profiles for various ground electrode shapes buried in a two-layer soil were computed using these same techniques. The results obtained were then compared with results obtained experimentally in the laboratory. A sensitivity analysis was performed on the effect of the height of the top layer of a two-layer earth model. The computed results agree very well with those obtained experimentally. The effect of varying the height of the top layer of a two-layer earth was then investigated. The computations indicate that the temperature can be predicted as a function of this height     

直流接地极极址勘测的研究

直流工程建设初期,直流接地极的选址合理性会影响交流电网的直流偏磁状况。针对典型土壤,从直流电流透深以及直流偏磁计算的角度,研究了直流接地极极址勘测时的测量范围以及测量深度。在运用大地电磁(MT)法进行直流接地极勘测时,推荐测量范围为待选极址周围70 km;测量深度推荐值为70 km,测量深度必须穿过高阻层,直到电阻率下降到临界电阻率以下,否则会给直流接地极评估带来显著误差。同时给出了MT法的测量点布置方法。     

锂离子动力电池的散热及优化

基于锂离子电池对温度的敏感性,采用往复流对电池散热温度场进行仿真分析和优化,探究温度及温差对电池使用性能和安全性能的影响。根据电池组在不同环境温度下的仿真结果,制定散热控制方案并进行实验验证,其结果表明:优化后电池的温度均匀性明显提高,电池温度和温差可控制在电池使用的理想区间。     

Effects of DC ground electrode on converter transformers

The effects of transferred DC voltage to a converter station ground from the DC ground electrode were studied with a time-domain analysis model of a converter substation and interconnected system. The model explicitly represents all grounds of the system (AC and DC) and the nonlinear magnetization characteristics of the converter transformer. The transferred DC voltage from DC ground electrode to station ground has been shown to cause the flow of direct current in the windings of the converter transformer. The effects of these currents have been studied with a parametric analysis. The major parameters considered were the separation distance between station ground and DC ground electrode, and the source voltage. The study suggests that for separation distances larger than 10000 feet, the transferred DC voltage to the station ground is less than 1% of the DC ground potential rise. For smaller separation distances, a substantial percentage of the DC ground potential rise is transferred to the station ground, which causes the flow of direct current in the winding of the converter transformers     

用于HVDC的电子式直流互感器的可靠性预计

直流互感器是高压直流输电工程中的重要设备,电力系统要求其具有较高的可靠性。为此在简要介绍了电子式直流互感器的系统结构后建立了它的可靠性框图和数学模型并推导出其失效率和平均无故障时间的数学表达式;根据美国军标MIL-HDBK-217F修订通告2和我国军标GJB/Z299-1998用计数法和应力法预计得出电子式直流互感器的失效率为15.51×(106h)-1,平均无故障时间为64474h,且指出激光器的失效率是影响整机可靠性的主要因素。     

Heat dissipation and non-equilibrium phonon distributions in molecular devices

Using a density functional approach we compute vibrations of a styrene molecule adsorbed on a Si(100) substrate and the electron-phonon coupling of these modes. A non-equilibrium Green’s function approach is used to compute the partially coherent transmission in molecular junctions due to electron-vibration scattering. The electronic power dissipated into molecular vibrations allows to set a rate equation for the phonon population in the vibrational degrees of freedom of the molecule. The rate of phonon decay is computed using a microscopic approach which includes a first-principle calculation of the coupling of the molecular modes with the vibrations of the contacts. In turn, the calculated phonon lifetime is used to correct the phonon propagator. A self consistent loop allows to compute the steady state non-equilibrium phonon population of the molecular junction under bias condition.     

新型电动式接地极安装装置的研制

定期测量变电站接地网接地阻抗是保证电力系统安全稳定运行的重要技术手段之一,目前常用的方法为电压-电流表三极法,测量过程中需要使用锤头人工锤击安装电流极和电压极辅助进行测量,存在安装时间长、拆除困难、安全风险高、单根电流极接地电阻大等弊端。针对上述问题,研制了一款基于压力感应的新型电动式接地极安装装置,配套设置中空注液腔的接地极,实现人工安装方式的电力替代,单人即可操作,遇到障碍时可自感应破除障碍,安装完毕后可实现降阻剂打压降阻,提高了变电站接地网接地阻抗测试的工作效率和安全性。     

直流输电接地极电流对不同结构变压器影响研究

利用PSCAD/EMTDC软件建立系统模型,仿真了直流接地极电流对组式、三相三柱式和三相五柱式等不同结构变压器的影响.仿真结果表明:组式变压器由于具有独立的磁通回路,磁阻小,受直流偏磁影响最大;三相三柱式变压器铁芯中无直流通道,磁阻大,受直流偏磁影响较小;而三相五柱式变压器虽有磁回路,但铁芯面积比单相变压器小,受直流偏磁影响介于前二者之间.     

Design optimization of high voltage bushing using electric field computations

This paper illustrates the use of electric field computation to optimize the design of gas filled high voltage composite bushings. The United States Navy employs these bushings in high power very low frequency/low frequency transmitting stations. Commercially available 2D and 3D computational packages based on the boundary element method were employed to analyze the electric fields. The optimized design uses both internal and external elements for electric stress grading at critical parts of the bushing. It has been shown that the location and magnitude of the maximum electric field have been optimized which should result in a substantially higher corona free operating voltage     

A new artificial treatment for the reduction of resistance in ground electrode

This study describes a resistance reduction additive (RRA) for reducing and maintaining reduced resistance of ground electrodes over time. The RRA employed was a mixture of inorganic salts, some of which occur as residues from industrial mineral processing in Chile. Chemical characteristics of the mixture are described, as well as results of measurements of electrical resistance of ground electrodes over time with and without RRA treatment. Measures are also given for current intensities and loss of electrode mass in test electrodes buried below ground with RRA and within cell containers having a specific capacity. Small scale tests were made on the effects of impulse current in test cells containing electrodes treated with the RRA.     

Negative corona in air using a point/cup electrode system

Calculations and simulations of potential and charged-particle distributions have been made by various workers for single pulses along the axis of point/plane electrode systems but not for the overall current. The difficult geometry makes this inevitable. In this paper the authors describe measurements of corona currents in air using a high-symmetry `point/cup' electrode system comprising a hemispherical-tipped point which is concentric with a concave hemispherical electrode; and simulations based on a spherically symmetric geometry. The simulations assumed avalanches to originate from the random release of electrons in the inter-electrode gap. The predicted onset voltages and the shape of the experimental curves for corona current vs. Voltage were in good agreement with the experimental results over a range of point-tip radii (0.5 to 3 mm) and pressures (0.02 to 0.10 MPa). Space charge effects were not included in the simulation so it follows that the accumulation of space charge is negligible at the onset voltage in air, although it is likely that it will have an increasing effect as the voltage increases     

Positive corona in air using a point/cup electrode system

The authors have previously published some measurements of negative corona currents in air using a spherically-symmetric point/cup electrode system, and also the related simulations. In this paper the corresponding current/voltage measurements and simulations for positive-point corona, again using the highly symmetric electrode system, are described. Although the corona onset voltages are again predicted with reasonable accuracy for different point radii and air pressures, the currents are very much lower, presumably due to the effect of positive space charge accumulating close to the point electrode (since the simulations assume the absence of space charges). In agreement with this assumption, the streamer pulses are only seen at or just above the onset voltage. Only avalanches occur from about 1 kV above the onset voltage.     

Optimization of HV electrode systems by neural networks using a newlearning method

To avoid a large number of iterations, optimization of electrode shapes has been done by artificial neural networks (NN). Two practical examples have been considered, an axisymmetric single-phase GIS bus termination and an axisymmetric transformer shield ring. The shape of the electrodes has been taken as quarter-ellipse or half-ellipse because an ellipse has more flexibility than a circle. For NN, the so-called resilient propagation algorithm, learning faster than the standard back-propagation algorithm, has been employed. The training sets as well as the test sets of NN have been prepared by charge simulation method     

Selective and independent activation of four motor fascicles using a four contact nerve-cuff electrode

Any one of the four motor nerves in the cat sciatic nerve could be activated selectively and independently, from threshold to saturation, using a self-sizing spiral cuff electrode containing four radially placed monopolar contacts. These studies were carried out in nine adult cats with acute implants. Of the 36 possible fascicles, 23 fascicles could be activated selectively with current stimuli applied to a single contact and ten of the remaining fascicles could be activated selectively with current stimuli applied to two contacts, "field steering." In three experiments, time constraints precluded attempting selective activation through "field steering" techniques. In eight of the ten cases where "field steering" was used, a positive and a negative current source (anodic steering) were required to achieve the desired fascicle and in the remaining two cases, two negative current sources (cathodic steering) were required. The relative distance from the electrode contacts to each fascicle was well correlated to the order in which each fascicle was activated. In seven experiments, carried out in two animals, selective activation was verified by collision block techniques. The results of these experiments support the hypothesis that selective and independent activation of any of four motor fascicles in the cat sciatic nerve is possible using a four contact self-sizing spiral cuff electrode. Furthermore, in a more general case, these results support the concept of a "tunable" electrode that is capable of "steering" the excitation from an undesirable location to a preferred location.     

Human nerve stimulation thresholds and selectivity using a multi-contact nerve cuff electrode.

Testing of the recruitment properties and selective activation capabilities of a multi-contact spiral nerve cuff electrode was performed intraoperatively in 21 human subjects. The study was conducted in two phases. An exploratory phase with ten subjects gave a preliminary overview of the data and data collection process and a systematic phase with eleven subjects provided detailed recruitment properties. The mean stimulation threshold of 25 +/- 17 nC was not significantly different than previous studies in animal models but much lower than muscle electrodes. The selectivity, defined as the percent of total activation of the first muscle recruited before another muscle reached threshold, ranged from 27% to 97% with a mean of 55%. In each case, the muscle that was selectively activated was the first muscle to branch distal to the cuff location. This study serves as a preliminary evaluation of nerve cuff electrodes in humans prior to chronic implant in subjects with high tetraplegia.     

直流输电系统接地极线路故障研究

天广、高肇和兴安等由德国Siemens公司设计的直流输电系统中为例,首先简介了所设置的接地极线路故障相关保护及运行实例,然后从理论上分析了单极大地运行方式下接地极线路发生接地故障和断线故障时各特征量的变化特点,并通过RTDS实时仿真系统进行了验证;接着根据这些特点,提出了可以准确判明故障类型的改进措施,确保准确检测接地极线路接地故障,并通过启动线路故障重启动功能,提高直流输电系统运行的可靠性.     

泡沫镍电沉积Co-Ce合金镍电极的性能

为了提高动力MH/Ni电池的高倍率充放电性能,在泡沫镍骨架表面电沉积具有针状结构、高比表面积的Co-Ce合金,Ce含量为0.19%,用其作镍电极的基板.实验结果表明:600次循环中,镍电极10 C充电接受能力平均提高36%,放电比容量平均提高42%.XRD分析显示:Co-Ce合金基板表面生成的CoOOH是增加基板与活性物质界面电子导电能力的主要因素.     

Extended analysis of ground impedance measurement using the fall-of-potential method

An extended analysis of ground impedance measurement using the fall-of-potential method will be presented. An interesting curve that represents the exact locations of the potential probe when the potential and current probes are in different directions is obtained for the first time. Curves representing measurement error are also presented for the case when the potential probe is placed in locations where the correct ground impedance cannot be measured. A similar analysis for ground impedance measurements in multilayer soils is also discussed. The study presented in this paper has extended the theory of the conventional fall-of-potential method and the results obtained can serve as a practical guide for ground impedance measurements made using this method.     

Transient analysis of heat dissipation due to a HVDC groundelectrode

The temperature of the soil surrounding a high-voltage direct-current (HVDC) ground electrode was computed at various points in time using finite difference methods. The temperature response for various ground electrodes was computed using these same techniques. The results were then compared with results obtained experimentally in a laboratory. It is suggested that peak temperature calculations may be desirable for periodic short-term operation of a HVDC ground electrode. Since the surrounding soil temperatures may not have reached a steady state, a transient analysis may be necessary