Positive Creeping Discharge along Aerial Insulated Wire Generated by Lightning Stroke

There are two causes, inductive lightning surge and direct lightning stroke, as the aerial insulated wire accidents accompanying lightning in a power distribution system. When the overvoltage due to the inductive lightning surge invades to the wire core, the creeping discharge can develop along the wire surface from the free end of the binding wire just after a flashover of the post insulator at the wire supporting point. This creeping discharge may give rise to the accidents such as a melting or snapping of the wire. The creeping discharge along the wire surface has the positive or negative polarity. Positive creeping discharge is restricted to the area where a positive lightning generates. Only a few examples have been reported on the positive creeping discharge, and its characteristic has many unsolved points. In the previous studies, we have observed the positive creeping discharges along the wire surface under the negative inductive lightning surge with the peak values in the range |V_m| ≤80 kV. In this study, the positive creeping discharges are examined newly in the range |V_m| > 80 kV. It is reported that the positive creeping discharges are greatly affected by the negative corona discharges generating from the wire surface in |V_m| ≥ 95 kV.     

Mobile imitator of three-component lightning spark current

A unique mobile imitator of three-component lightning spark current is described. The operational principle of the imitator is based on forming pulse current in a load by imposing individual components in a certain sequence. Crowbar discharges for operational voltage of 1.3 MV are used in the imitator, which allows one to generate during a load pulse components of lightning spark current of aperiodic form with amplitudes of 200 and 100 kA and front durations of 3 and 0.2 μs, respectively, and a quasi-constant component of current with transferred charge of above 200 C.     

架空线路感应雷过电压产生机理与计算方法

架空配电线路裸露在空气中,极易遭受雷击产生雷电过电压,导致线路保护装置跳闸甚至线路电气设备元件的损坏,从而造成供电中断,影响了广大用户的生产和生活。对配电网架空线路感应雷过电压产生机理进行了详细的探讨,提出静电感应分量是配电网线路感应雷过电压的主要构成部分。并研究了目前常见的计算雷击导线附近大地时架空线路感应雷过电压的HC/idalen模型,并通过仿真分析表明大地电导率对架空线路感应雷过电压有一定的影响。     

雷电电流的频谱和能量分析

为了研究风电场中强大电磁干扰源雷电流的基本特性,利用脉冲函数作为2.6/50μs雷电流的解析表达式,将其与双指数函数表示的雷电电流进行比较说明其可用性。之后采用傅里叶变换和仿真计算的方法对雷电流的幅频特性以及所含能量进行了研究,得到相应的频谱图和能量分布图。经过分析得到的结论是:雷电流的频率在低频范围之内,能量主要集中在1～10 kHz之间,且雷电流脉冲上升越陡频率分量的初始振幅值越小,但是其包含的高频分量却越多。     

25～100 MHz频段闪电脉冲辐射能量频谱特征

对闪电不同放电过程中辐射能量频谱特征的观测分析结果表明，经25MHz高通滤波器对闪电辐射能量的低频成分抑制后，所得到闪电不同放电过程期间的宽带辐射信号，其能量谱分布有较大的差异，在25－100Mhz频段内，地闪较低频段的辐射强于云闪、云闪、地闪预击穿过程 和K变化过程峰值辐射能量处于相对较高的频段，表明这些过程可能以较小尺度的放电为主要特征：梯级先导和直窜先导过程初始阶段辐射能量主要集中在较高频段，而接近地面时则集中在较低频段，表明先导在向地面发展过程中放电尺度逐渐增大。     

Quadratic programming approach to reactive power optimization on the primary feeders

Contents In this paper we suggest the use of the quadratic programming technique to determine the optimum size and location of shunt capacitors on radial distribution feeders so as to maximize overall savings, including the cost of capacitors. The saving function which is of quadratic form is maximized for a set of linear inequality constraints by using quadratic programming. — For quadratic programming, efficient alogrithms have been developed which can easily be implemented on digital computers. — The approach is illustrated by an application to a typical distribution feeder of 23 kV.

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Blindleistungsoptimierung an primären Einspeisungen mit Hilfe quadratischer Programmierung

Übersicht Diese Arbeit stellt die Anwendung der quadratischen Programmierung zur Ermittlung optimaler Werte und Orte von Parallelkapazitäten an radialen Verteilungen mit dem Ziel der Kosteneinsparung vor. Die Methode ist an einer 23-kV-Verteilung entwickelt und überprüft worden.

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Nomenclature A M×N array of constraint coefficients - B M vector of constrain limits - C N×M array of quadratic coefficients, from objective function - E Energy loss caused by reactive current in the feeder - E Reduction in energy loss - I _ck Capacitor current injected at nodek - Î _k Maximum reactive load current at nodek - K _c Annual cost per unit of installed capacitor ($/A) - K _E Per unit energy production cost ($/kWh) - K _P Annual cost per unit of power loss ($/kW) - m _k Reactive load factor for loadk - M Number of constraints - N Number of structural variables - P N vector of linear term coefficients from objective function - P Peak power loss caused by reactive current in the feeder - P Reduction in peak power loss - R _k Line resistance for sectionk - S Net saving resulting from peak power and energy loss reduction - T Total period of the reactive load curve - X _k Line reactance for sectionk     

A simple method for mapping the B1 field distribution of linear RF coils

Inhomogeneity of the radio frequency (RF) field B₁ leads to intensity variations in MR images and to spatial dependence of spectral line amplitudes. In this paper, a simple method of measuring the B₁ field components of an unsegmented linear coil is described. The method is designed for the coils operating up to 20 MHz. The B₁ field distribution is replaced by the static magnetic field caused by DC current flowing through the coil. The technique involves rotating the coil 90° so that measured B₁ component is aligned with B₀ and measuring the shift of resonance frequency using a spectroscopic imaging sequence. Experimental results were in good agreement with the theoretical computations.     

Estimation of breakdown voltages of surface flashovers initiated from triple junction in SF6 gas

In this paper, a method is proposed to estimate the minimum breakdown voltages of surface flashovers initiated from the triple junction at which a solid insulator, SF₆ gas, and a metal electrode come in contact. Using four kinds of test samples, the flashover voltages V_f are measured. The voltages V_f become lower than the theoretical voltages V_s of streamer inception in the case of the test sample with the large electrode area at the triple junction. To understand the phenomena quantitatively, we estimated the real partial discharge voltages V_c from the voltages V_f, and investigated the relationship between the voltages V_c and the theoretical values V_s. The values of V_s/V_c, which characterize the properties of the partial discharges, can be shown by the product λS_90%E_av P of the following parameters: (1) coefficient λ representing the surface roughness of electrode at the triple junction; (2) effective area S_90% of the electrode; (3) average field E_av at the area S_90%; and (4) gas pressure p. The minimum surface flashover voltages in the insulators for coaxial electrode systems can be estimated precisely, considering the properties of V_s/V_c.     

Magnetic noise caused by output waveform harmonics of voltage-source inverters

Harmonic components included in the output waveforms of inverter circuits cause magnetic noise, that is, audio noise, from the output transformers or load motors. To estimate the magnetic noise due to the harmonic components, we have already proposed the equivalent noise voltage E_s and current I_s. In this paper, by applying the aforementioned E_s, we compare quantitatively the magnetic noise in the various ordinary voltage-source inverters, both PWM inverters with multipulse waveforms and PAM inverters with multistep waveforms. Then, to reduce the magnetic noise, the output waveforms of the PWM inverters are improved by applying E_s into the idea of harmonic elimination. Moreover, by employing PAM system as voltage regulation in the aforementioned method, the more improved output waveforms can be obtained with lower noise and also lower distortion.     

Semiconductive behavior of passive films formed on Fe-Cr alloy

Semiconductive properties of passive films formed on Fe-18Cr alloy in a borate buffer solution were investigated using photoelectrochemical response and electrochemical impedance spectroscopy. Band gap energy of the passive films, E_g, was estimated from a photoelectrochemical spectrum that could consist of two components. E_gs were recognized as approximately 2.4 eV and 3.4 eV, which were mainly derived from Cr hydroxide layer and Cr oxide layer, respectively. The values were almost constant independent of passivation time, film formation potential and additive element. Positive photocurrents generated from the outer part hydroxide and the inner oxide in the passive film increased as applied potential increased, which indicates that the passive films behaved as an n-type semiconductor. Mott-Schottky plot of capacitance showed positive slope. This means that the passive films behaved as an n-type semiconductor. Donor densities of the passive films estimated by the Mott-Schottky plots depended on passivation time, film formation potential and additives. Corrosion resistance of Fe-18Cr alloy was discussed in terms of the semiconductive properties.     

Models for the frequency dependence of coercive field and the size dependence of remanent polarization in ferroelectric thin films

Abstract

We have confirmed the theory of Ishibashi and Orihara for the frequency dependence of coercive fields in ferroelectric thin films.SrBi₂NbTaO₉, SrBi₂Ta₂O₉, and PbZr_1-x Ti _x O₃ (PZT) data from DeVilbis et al. reveal a power-law dependence of form E_c (f) = Bf^d/α , where d is the dimensionality of domains (ca. unity in uniaxial ferroelectrics such as SBT and SBNT) and α is approximately 6, in agreement with theoretical predictions. Models are also presented for the polarization and fatigue data from NEC of micron-sized thin-film strontium bismuth tantalate capacitors, emphasizing behaviour at unsaturated voltages.     

An efficient algorithm to calculate intrinsic thermoelectric parameters based on Landauer approach

The Landauer approach provides a conceptually simple way to calculate the intrinsic thermoelectric (TE) parameters of materials from the ballistic to the diffusive transport regime. The approach relies on the calculation of the number of propagating modes and the mean free path for each mode. The modes are calculated from the energy dispersion (E(k)) of the materials, which require heavy computation and are often restricted to energy relations on sparse momentum (k) grids. Here an efficient method to calculate the distribution of modes (DOM) from a given E(k) relationship is presented. The two main features of this algorithm are: (i) its ability to work on sparse dispersion data, and (ii) creation of an energy grid for the DOM that is almost independent of the dispersion data therefore allowing for efficient and fast calculation of TE parameters. The effect of K-grid sparsity on the compute time for DOM and on the sensitivity of the calculated TE results are provided. The algorithm calculates the TE parameters within 5% accuracy when the K-grid sparsity is increased up to 60% for all the dimensions (3D, 2D and 1D). The time taken for the DOM calculation is strongly influenced by the transverse K density (K perpendicular to transport direction) but is almost independent of the transport K density (along the transport direction). The DOM and TE results from the algorithm are bench-marked with, (i) analytical calculations for parabolic bands, and (ii) realistic electronic and phonon results for Bi₂Te₃.     

补偿电动势在直流叠加法在线检测XLPE电缆绝缘老化技术中的应用

电缆屏蔽接地化学电动势ES 较大而护套绝缘电阻RS 较小时 ,会在检测回路形成较大干扰电流 ,并造成在线叠加电流被干扰电流“淹没”。通过电路分析和模拟实验发现 ,采用精密可调补偿电势可以有效抵消ES 产生的干扰电流 ,使直流叠加法无须更改测量接线 ,即可适用于较大ES 干扰情况下XLPE电缆绝缘的在线检测 ,提高了抗干扰能力 ,拓宽了检测范围     

Modeling of microwave-excited gas discharges

Content This paper is dealing with the power density versus electric field characteristic of microwave-excited discharges on the example of typical CO₂ laser gas discharges.The fundamentals of the method are a simple electric equivalent circuit for the excitation structure and the assumption of a fixed reduced electric field intensity within the plasma. This operating field is enforced by the plasma under stationary conditions and can be calculated using an electron balance under consideration of the non-Maxwellian electron energy distribution function. The calculations turn out that c.w. excitation with microwaves is involved with considerable plasma power densities. In the case of the CO₂ laser this requires special discharge channel designs for effective gas cooling. Besides a discussion of the plasma characteristic, measurements taken from a microwave applicator are presented and compared to the calculations.

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Modellierung mikrowellenangeregter Gasentladungen

Übersicht Der Artikel behandelt die Berechnung der Feldstärke-Leistungsdichte-Kennlinie mikrowellenangeregter Gasentladungen am Beispiel von Entladungen in typischen CO₂-Lasergasgemischen. Das Berechnungsverfahren beruht auf einem einfachen, auf die meisten Anregungsstrukturen anwendbaren elektrischen Ersatzschaltbild und der Tatsache einer vom Plasma erzwungenen stationären Brennfeldstärke. Letztere wird über eine Elektronenbilanz unter Berücksichtigung der nicht Maxwell'schen Elektronen-Energieverteilung ermittelt. Es zeigt sich, daß bei einer CW-Anregung mit Mikrowellen hohe Mindestleistungsdichten im Plasma auftreten, wodurch beispielsweise im Fall des CO₂-Lasers kein Laserbetrieb ohne sehr effektive Maßnahmen zur Gaskühlung möglich ist.Neben einer Diskussion der Plasmakennlinie werden Vergleichsmessungen an einer konkreten Anregungsstruktur vorgestellt.

     

The influence of wave front duration of impulse voltage on creeping discharges along aerial insulated wire

In high‐voltage aerial distribution systems, creeping discharges progress along the cable surface from the free end of the binding wire when overvoltages caused by a lightning surge have invaded the central line of an insulated cable. Consequently, various accidents such as punch‐through breakdown, melting, or snapping of a cable, often occur at these systems. In our previous studies, it has been clarified that the lengths and aspects of creeping discharges under a 1.2/50 µ s impulse voltage condition can be markedly affected by changes in the electric field strength on the cable surface. However, lightning impulse surges which may invade the central line of a cable have various wave front durations. This will further complicate creeping discharge phenomena due to lightning. In this paper, we report the influence of the wave front duration on both the lengths and the aspects of the creeping discharges which progress on the cable on application of lightning impulse voltages. It has been shown that the behavior of negative creeping discharges reveals pronounced changes in response to the duration of the wave front of the applied voltage. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 147(2): 30–38, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10263     

High‐current lightning discharges in winter

Lightning electric‐field waveforms related to power line faults in winter have been identified. Most of those waveforms appear to be associated with upward lightning discharges with absolute peak currents of over 100 kA. They are quite different from common return‐stroke waveforms, and the lightning discharges which produce these characteristic waveforms are called GC (Ground to Cloud) flashes. These high‐current lightning discharges are distributed around the coastline in different ways depending on their polarities. The spatial distributions of high‐current lightning discharges around Japan are also investigated. It is revealed that the region of Honshu Island along the coastline of the Sea of Japan belongs to the area in which the density of high‐current lightning flashes is the highest in Japan through the year. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 170(1): 8–15, 2010; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20874     

Approximate calculation of triboelectric charge density (σ) of nanoscale contact electrification

We have analytically approximated the triboelectric charge densities generated by triboelectrification reaching equilibrium over time in contact-mode TENG. It is assumed that the surface density of states (SDOS), N_s(E) is constant. The amount of electron transfer at the interface is estimated by calculating the change in vacuum energy level due to the charge density induced in the upper metal electrode and the lower metal electrode within the nanoscale separation range. Numerical results show that N_s(E) and work function difference are dominant compared to dielectric constant (material property) and dielectric thickness (structure parameter).     

Structural Origin of Dimensional Effect in ZnO Varistors

The relations among breakdown field E _1mA/cm ² (electric field at 1 mA/cm²), nonlinear coefficient α (measured via current-voltage, I-V, characteristics), and change in breakdown field (ΔE _1mA/cm ²/E _{1 mA/cm} ²) (obtained via current impulse measurement), and thickness d of ZnO varistors were investigated. The dimensional effect refers to the variation in E _{1 mA/cm} ², α, and ΔE _{1 mA/cm} ²/E _{1 mA/cm} ² with thickness of the samples. The dispersion of the ZnO grain size and the aspect ratio of the ZnO grains are used in characterizing the microstructure to represent the degree of heterogeneity of the grain size distribution and irregularity of the shape of the ZnO grains. The distribution of the ZnO grain size is statistically analyzed and found that the critical thickness d _c increases linearly with the dispersion of the ZnO grain size. The breakdown electric field can be empirically termed as E _{1 mA/cm} ² ∝ exp(bd), where d is the thickness and b represents the transitional behavior for the curve of the E _{1 mA/cm} ² versus d plot. Based on the inflecting response of this curve, b ₁ represents the small thickness domain prior to the inflection and b ₂ represents the large thickness domain in the post-inflection domain. It is observed that b ₂ is directly proportional to aspect ratio of the ZnO grains. Also it is revealed from the analysis that b ₁ increases with the increase in the critical electric field while b ₂ decreases with the increase in the critical electric field. Based on these observations it is suggested that the dimensional effect of ZnO varistors originates from the distribution of the grain size and proven via experiments involving the measurements of E _{1 mA/cm} ², α, and ΔE _{1 mA/cm} ²/E _{1 mA/cm} ². The dimensional effect is the macroscopic expression of population behavior of varistors that is consisting of the ZnO grains as well as the grain boundaries.     

Modal based stability analysis of power systems using energy functions

In this paper a physically based decomposition technique is exploited to perform direct stability analysis of power systems using the energy function method. The slow-coherency analysis decomposes the system into r areas associated with (r- 1) slow modes of the linearized system. The centres of inertia (COIs) of these areas from the slow subsystem and the rest of the fast modes are associated with different areas. The system transient energy function E is decomposed into E_slow associated with the slow variables of the slow subsystem, E_fast associated with the fast variables of the r-areas and E_fast-slow which is a function of both fast and slow variables. Depending on the fault location and strength of connection between areas, the sum of E_slow, E_fast-slow and E_fast of the disturbed arease constitutes a good approximation to the system energy function. Using this partial E-function and the potential energy boundary surface (PEBS) method, both E_cr and t_cr are computed. Numerical results on a 10-machine 39-bus system are presented in support of the technique.     

EnergyDispersion Relations for Holes in Silicon Quantum Wells and Quantum Wires

We calculate the energy dispersion relations in Si quantum wells (QW), E(k _2D), and quantum wires (QWR), E(k _1D), focusing on the regions with negative effective mass (NEM) in the valence band. The existence of such NEM regions is a necessary condition for the current oscillations in ballistic quasineutral plasma in semiconductor structures. The frequency range of such oscillations can be extended to the terahertz region by scaling down the length of structures. Our analysis shows that silicon is a promising material for prospective NEM-based terahertz wave generators. We also found that comparing to Si QWRs, Si QWs are preferable structures for NEM-based generation in the terahertz range.