Frequency dependent and transient characteristics of substationgrounding systems

In spite of the existence of a number of analytical models aimed for transient analysis of large grounding systems, more detailed analysis of the influence of different parameters on the transient performance of large ground grids subjected to lightning current impulse is not available. This paper presents analysis of the influence of soil conductivity, location of feed point, grid size, depth, conductor separation, ground rods, and shape of the lightning current impulse, on the transient performance of ground grids with sizes ranging from 10×10 m² to 120×120 m² and with 4 to 124 meshes. Maximal transient ground potential rise and frequency dependent impedance are analyzed in time and frequency domain, respectively. Computations are made with computer model based on the electromagnetic field theory approach, taking accurately into account frequency dependent characteristics of large ground grids. Instead of usual simple approximations of the lightning current impulse, recorded channel base currents from triggered lightning are used for the time domain analysis     

The soil ionization influence on the lightning performance of transmission lines

Contents The paper presents a mathematical model for analyzing impulse characteristics of complex grounding grids in the cases with soil ionization. The model is based on the grounding grid equivalent circuit with lumped and current dependent parameters, described by a system of differential equations. The proposed model is convenient for practical application. It is incorporated into the present lightning overvoltage computation program. The soil ionization influence on the grounding grid impulse characteristics and lightning performance of 400 kV overhead line for various magnitudes of the soil ionization gradient, soil resistivities, grounding grid configurations as well as impulse current shapes and magnitudes is illustrated.

---

übersicht Diese Arbeit befa?t sich mit einem mathematischen Modell zur Analyse von Impulsformen auf ein komplexes Erdungssystem im Falle der Erdionisation. Dieses Modell beinhaltet verteilte Parameter der Leitungen und stromabh?ngige Parameter, welche mit einem Differentialgleichungssystem beschrieben werden. Dieses vorgestellte Modell ist gültig für praktische Anwendungen. Es wurde schon in ein bestehendes Simulationsprogramm zur Berechnung von Blitzüberspannungen eingebunden. Der Einflu? der Erdionisation auf Impulsformen am Erdungssystem und die überspannungen auf 400 kV Freileitung für verschiedene Amplituden des Erdionisationsgradienten, Leitf?higkeiten der Erde, Anordnung des Erdungssystems, Forme und Amplitude des Strom wird vorgestellt.

---

---

Received: 12 February 1999     

不同结构土壤中接地网冲击特性的测量与分析

变电站接地网的冲击特性研究对于接地网的优化设计具有重要意义,为能系统真实反映接地装置在不同土壤结构中的冲击特性,基于量纲相似性原理,利用冲击电流发生器进行了接地网在均匀及双层土壤中的冲击接地模拟试验。在均匀土壤中测量了5种埋深、3种冲击电流注入点位置时对应的接地网冲击接地电阻值,并测量了典型情况下接地网的地表电位分布情况;在双层土壤中测量了3种土壤厚度、3种冲击电流注入点位置时对应的接地网冲击接地电阻值。通过分析不同试验条件下接地网模型的冲击接地电阻及电位分布得到了冲击电流注入点位置、地网埋深及土壤结构等因素对接地网的冲击接地特性的影响规律。     

Characteristics of the effect of an impulse supply voltage on the structure and characteristics of a surface discharge

Results of an experimental investigation of surface barrier discharge in a three-electrode system are presented. An analysis is done of the flow intensity of charged species extracted from the plasma layer of the surface discharge by the field of high dc potential of the third electrode. A high unipolar periodic impulse voltage of 15-kHz frequency, impulse duration from 0.5 to 45 ms, and 150-ns impulse-front duration is used to form the surface discharge. A It is revealed that the voltage-impulse duration has a significant effect on the intensity of charged species flow (extracted current), the character of the influence being dependent on the voltage-impulse polarity. The influence of voltage-impulse duration on the surface-discharge current is revealed as well. Pictures of the structure of surface discharge at the rise and the fall of the voltage impulse made by means of a high-speed camera are presented. The difference of the discharge structure for different polarities of voltage impulse is revealed. It is suggested that the surface-discharge characteristics are dependent on a combination of a number of factors, such as the parameters of the impulse voltage, volume charge in the gas gap, and the charge on the barrier surface, as well as the material and surface structure of the barrier.     

基于电磁场的杆塔接地体冲击特性计算分析

输电线路杆塔接地体的冲击接地阻抗决定了雷击输电线路或杆塔时的塔顶电位,从而影响线路绝缘子串两端的过电压水平,直接关系到线路的耐雷水平。接地体在雷电流作用下的冲击特性表现为火花效应和电感效应,使接地体的冲击特性明显区别于工频特性。基于电磁场理论建立了输电杆塔典型接地体冲击接地阻抗计算模型,通过模型试验验证了其准确性,得到了接地体在不同土壤电阻率和射线长度下的冲击接地阻抗,并与国际著名的接地计算软件CDEGS工频接地阻抗计算结果对比分析,进一步揭示了接地体的工频和冲击特性的差异,同时验证了接地体冲击电流下存在有效长度。     

DC interruption characteristic of vacuum circuit breaker

A high‐speed vacuum circuit breaker (HSVCB) has been investigated. HSVCB makes high‐frequency current superimposed on a fault current so that the current is forced to be zero and is interrupted. Its interruption performance is considered to be dependent on the rate of change of the current (di/dt). As a fundamental research, we investigated the di/dt– dv/dt characteristics and the insulation recovery characteristic after interrupting the counter‐pulse current for various contact materials of AgWC, CuW, and CuCr. The results revealed that the case where the gap length is larger is better in a current interruption performance. Moreover, it was found that di/dt is not dependent on the insulation recovery characteristics, but the magnitude of interruption current is of great influence. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 161(1): 17– 25, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20362     

考虑土壤非线性特性的接地网冲击特性分析方法

考虑土壤的非线性特性对接地系统冲击特性的影响,基于场路结合思想提出一种新的接地网冲击特性频域分析方法,并进行了接地网的冲击特性模拟试验。推导了考虑土壤非线性特性时接地极的等值半径后经过合理假设建立了更符合实际散流情况的接地系统分析计算模型。该模型待求变量少,计算时间短,兼顾了计算效率和计算精度,适用于分析接地网的冲击暂态特性。计算结果与软件包CDEGS的计算结果及冲击试验结果吻合得较好,进一步验证了该方法的准确性。     

接地装置冲击特性研究分析

为研究接地装置的雷电冲击特性,以火花效应和电感效应的研究为主线探讨了土壤的放电特性、接地装置的高频响应及土壤电气参数的变化规律;比较了电路理论、电磁场理论以及动态建模等3种暂态建模方法;评析了目前接地装置冲击试验的进展。通过归纳与分析,指出了这一研究领域中迫切需要解决的技术问题及初步解决方法:利用X射线胶片或高速成像仪器采集土壤放电图像以研究土壤的放电形式及放电区域的尺寸;确定土壤的临界击穿场强及残余电阻率,并用于暂态建模中;区分火花效应与土壤电气参数频变性以及火花效应与电感效应的影响范围;建议分别采用分布参数电路模型、集中参数电路模型、电磁场模型分析伸长接地体、复杂接地装置及接地装置的频域特征;采用高电压、大容量冲击电流试验系统研究高幅值、波前时间短的冲击电流作用下接地装置的冲击特性,并用于指导冲击接地电阻的现场实测。     

Influence of water absorption on high‐frequency characteristics of insulation layers of printed circuit boards

The influence of water absorption on complex permittivities of various insulating materials used for printed circuit boards was investigated in the frequency range of K‐band (18–26 GHz). Experimental results revealed that the influence of water absorption on complex permittivity, the dielectric loss factor tanδ, in particular, was considerably high. For example, the dielectric loss of polyimide film at 85°C, 85% RH became 10 times larger than that at room temperature and humidity. Since the complex permittivities, however, retained the initial values when the specimens are dried, the water in the specimen should exist freely at the internal interfaces of the materials. The increase of permittivity directly affects the performance of printed circuit boards at high frequencies, so that it is important to clarify the influence of water on dielectric properties of insulations. Copyright © 2007 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Electric fields of grounding electrodes with frequency dependent soil parameters

The profile of the electric field developed at the ground level above a horizontal grounding electrode due to the impression to the electrode of waves resembling lightning currents of first and subsequent strokes was calculated using an electromagnetic model. The simulation considered both assumptions constant values for soil parameters resistivity and permittivity and frequency dependent values. This dependence was given by an expression derived from field measurements. It was found that the frequency dependence of soil parameters is relevant, mainly at low conductivity soils and short front-time current waves, leading to a significant decrease of the electric field profile.     

Lightning Response of a Grounding System Buried in Multiple Layers in Earth Using the PEEC Method Based on the Quasi-static Complex Image Method

Abstract

A mathematical model of the partial-element equivalent circuit method is developed in this paper that can be used to accurately calculate lightning current along a high-voltage alternating current substation grounding system buried in multilayered soil. Combined with the fast Fourier transform (FFT), the conduction effect of the components of the current harmonics is analyzed, and the effect of capacitance and induction between layers of soil are examined. Furthermore, the effects of the frequency-dependent conductivity and permittivity of the soil are considered. To speed-up calculation, the quasi-static complex image method and a closed form of Green’s function are used. Through the inverse FFT, the model can calculate the distribution of lightning current in any grounding system. This can be used to study the performance of the system’s response to transient lightning.     

冻土冲击特性的试验研究

我国西部地区的输变电工程经过多雷的永冻土地区,了解冻土的冲击特性是输变电工程雷电防护的基础。该文通过试验方法对冻土的起始击穿场强进行研究,并对温度、水分、含盐量等因素对冻土的起始击穿场强的影响规律进行探讨,同时结合土壤导电机理对试验结果进行了初步的解释。以水平接地体为例说明温度对冲击接地电阻的影响。试验表明,土壤的起始击穿场强随着温度、含水量以及含盐量的升高而降低;接地体的工频接地电阻和冲击接地电阻随温度的降低而增加,冲击系数却随着温度的降低而降低。     

10 kV系统冲击与工频续流联合试验回路设计

为检验10 kV带间隙防雷装置在冲击闪络后熄灭工频续流电弧的能力,设计了一种冲击试验与工频续流试验相结合的试验回路。为了产生较高电压下的高幅值工频续流,采用LC串并联谐振回路产生工频续流,对该回路中各元件的参数进行计算并给出合理数值,最后利用同步控制回路提取冲击信号来导通工频续流回路,实现冲击试验与工频续流试验的同步。计算结果表明,该联合试验回路能在产生1.2/50 μs冲击波的同时产生频率为50 Hz的正弦电流波且电流的振荡能够持续至少100 ms,满足带间隙防雷装置在冲击闪络后,检验工频续流下熄灭电弧能力的要求。     

Backflashover simulation of HV transmission lines with enhanced counterpoise groundings

This paper deals with the backflashover phenomenon in HV overhead transmission lines (TLs) with towers grounded by means of long counterpoises, typically used in high resistivity soils. Although long counterpoises are effective in reducing the grounding impedance at power frequency, their impulse response may affect negatively the TL backflashover rate. The study is focused on the enhancement of TL lightning performance obtainable by adding a small number of vertical grounding rods to horizontal counterpoises. An extensive ATP-EMTP parametric analysis, based on a detailed 161 kV TL model including the leader progression model (LPM) of line insulations and the non-linear transient behaviour of grounding systems, has been carried out taking into account several different values of soil resistivity as well as varying numbers, locations and lengths of additional vertical rods. Critical lightning current ranges causing backflashover have been then predicted for several lightning waveforms, of different severity. The paper shows that the addition of a few vertical grounding rods significantly improves TL lightning performance by increasing critical lightning current values, even if their influence is negligible at power frequency. The authors have ascertained an increment of average critical backflashover current up to ∼55% for some typical TL grounding system configurations and lightning waveshapes.     

Impacts of DFIG‐based wind farm integration on its tie line distance protection and countermeasures

Modern power grids, especially high‐voltage grids, are increasingly integrated with wind farms. The correct action of distance protection for the wind farm tie line plays an important role in the safe and stable operation of the power system. However, based on the fault characteristics of wind farms based on the doubly fed induction generator, considering crowbar action and weak infeed properties, this paper reveals some serious defects of conventional distance protection. The result shows that, due to the influences of mixed data window and damped current component with near‐rotor‐speed frequency, the fault distance calculation result obtained by the fundamental‐component‐based distance protection is inaccurate. To tackle this problem, a transient‐based distance protection countermeasure along with a faulted phase selection technique is proposed. Specifically, the transient phase voltage characteristic for different fault types is analyzed to select the faulted phase. Considering the effect of high‐frequency components, pre‐fault voltage compensation, and transducer characteristics, a distance calculation procedure including a low‐pass filter, the reconstruction of voltage at the fault point, and an iterative fault distance calculation is proposed. The comprehensive performance of the proposed method is validated by the simulation case studies using the PSCAD /EMTDC software. © 2017 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

An accurate time‐domain model of transmission lines with frequency‐dependent parameters

Linear lossy two‐conductor transmission line can be modelled as dynamic two ports in the time domain, via the describing input and transfer impulse responses. This convolution technique is very effective when dealing with networks composed of transmission lines with frequency‐dependent parameters and non‐linear and/or time‐varying circuits. The paper carries out an accurate analysis of this model, in the most general case of lines with frequency‐dependent parameters. For such lines it is not possible to evaluate analytically the impulse responses, nor is it possible to catch them numerically, due to the presence of irregular terms, such as Dirac pulses, terms that numerically behave as Dirac pulses, and functions of the type 1/t^ρ with 0 < ρ <1. A simple method is proposed to evaluate exactly all the irregular terms of the impulse responses: once these irregular parts have been extracted, the regular remainders are easily evaluated numerically. This method is applied to analyse lines with frequency‐dependent parameters of practical interest, such as superconductor transmission lines, power lines above a finite conductivity ground, lines with frequency‐dependent dielectric losses and lines with normal and anomalous skin‐effect. Numerical simulations are carried out for illustration. Copyright © 2000 John Wiley & Sons, Ltd.     

外部汇流母排对压接型IGBT器件内部多芯片并联均流特性的影响

压接型IGBT器件内部芯片之间的动态均流特性直接影响着IGBT器件的坚固性与可靠性。考虑到并联均流实验的困难,现有的压接型IGBT芯片级并联均流研究通常都是通过提取器件内部封装结构的寄生参数,并结合IGBT芯片的等效电路模型,在电路仿真环境中开展的,不考虑器件外部电磁条件对器件内部电流分布的影响。然而,该文通过9枚压接型IGBT芯片的并联均流实验发现,各个通流支路之间存在显著的动态电流不均衡,而且电流的分布特性不仅与内部并联芯片的相对位置有关,还与连接器件的外部汇流母排存在明显的关联。为了揭示器件内部电流分布特性与外部汇流母排之间的耦合关系,该文对被测器件与外部汇流母排进行三维有限元建模,从频域和时域2个方面,计算IGBT器件内部的电磁场分布特性。频域计算表明,由于外部汇流母排与内部并联芯片存在磁场耦合(即电感耦合),当频率超过一定数值后,外部汇流母排会对各个通流支路的电流产生显著影响。时域计算进一步再现了并联均流实验中外部汇流母排对各个通流支路上动态电流分布的影响规律。结果表明,在压接型IGBT器件的设计和应用中,不仅需要关注器件内部芯片间的相对位置对动态均流特性的影响,同时也要关注外部汇流母排引入的电磁不对称性。最后提出一种对称化的母排设计方案,并通过三维有限元计算,证实对称化母排设计可明显改善器件内部的动态均流特性。     

MOA冲击电流模型的建模仿真及试验研究

为保证金属氧化物避雷器(metal oxide arrester,MOA)的质量,在出厂试验或验收试验等中常采用冲击电流发生器模拟雷电流来检验设备耐受冲击电流稳定的能力。仿真分析中直接采用雷电流函数模型代替冲击电流回路来研究MOA的非线性特性,忽略了电流波形差异对负载特性的影响。文中介绍了雷电流模型的研究现状,分析了冲击电流发生回路与常用雷电流模型的基础理论,选取双指数函数模型、Heidler函数模型、脉冲函数模型,并基于MATLAB/Simulink搭建仿真电路,结合试验数据进行仿真分析,给出了ATP-EMTP软件中常用模型的波形参数。研究结果可为后续MOA在特快速瞬态过电压的特性研究、电路模型仿真分析及防雷研究提供参考。     

网络参数对低压宽带电力线信道的影响

在电力线载波信道的研究中,目前有关电网结构对电力线载波性能的影响研究较少,多径效应和频率选择性衰落现象难以从现有模型得到合理解释,也对电力线节点间的通信性能和路径拓扑中继理论支持不足,制约了宽带电力线载波通信性能的提升。针对此,通过使用基于传输线理论的方法对低压电力线通信信道仿真建模,分析了不同网络拓扑结构对频率为2~30MHz的低压宽带电力线信道特性的影响。仿真得到不同电力线长度、分支长度、单节点分支数、分布式分支数和负载阻抗的低压宽带电力线信道频率响应,并通过反傅里叶变换得到对应的信道时域冲激响应。结果表明,电力线信道频率响应中的峰值点和陷波频点的位置和衰减会受到上述网络参数的影响,时域冲激响应也会因此而衰减和失真。本文的研究对于宽带电力线通信技术在能源互联网中的有效应用具有重要意义。     

冲击接地电阻测量装置的研制

给出了一种采用冲击电流发生器产生冲击电流模拟雷电流(冲击电流)测量接地电阻的装置。该测量原理是以冲击电流发生器产生的电流作为测量电流,由数据采集器采集得到测量回路中接地体内流过的电压、电流波形信号,经A/D转换后,数字信号通过快速傅里叶变换(FFT)变换到频域,并在频域内通过频谱法计算得到冲击阻抗值,最后分离冲击阻抗中的电阻和感性分量。给出了实验室、现场杆塔试验结果及EMTP仿真验证结果。