A Fourier transform technique for calculating cable and pipetemperatures for periodic and transient conditions

An underground pipe-type cable system is represented by a thermal impedance network. A ladder network of resistances/capacitances represents the cable out to the outer surface of the pipe. The earth, adjacent pipe-type cables, and cable images are modeled by a frequency dependent thermal impedance found by solving the heat transfer differential equation. The heat input to the system is conductor I² R loss. The heat input can be a periodic signal or a transient of up to 300 h. A fast Fourier transform (FFT) is used to obtain heat input in the frequency domain. The frequency domain thermal input at the conductor is divided by the thermal admittance seen by the conductor and an inverse FFT is used to obtain conductor temperature as a function of time. A similar procedure obtains shield and pipe temperature. Iteration is used to model conductor electrical resistance change with temperature. The ambient temperature and temperature due to dielectric loss is added in to obtain final values     

阻抗分解法及其在无砟轨道阻抗求解中的应用

轨道电路广泛应用于高速铁路控制系统,轨道阻抗是轨道电路设计中的重要参数之一。采用有限元法求解无砟轨道阻抗,其求解属于三维开域问题;同时考虑到导体趋肤效应,直接采用有限元分析时,存在模型剖分规模巨大、求解收敛困难等。为解决上述问题,提出了阻抗分解的方法,将无砟轨道阻抗分解为内阻抗和外阻抗,其中内阻抗利用无钢筋的二维有限元模型求解,外阻抗利用钢轨电流均匀分布的三维有限元模型求解。由此避免了同时对多个小尺寸单元的剖分,降低了剖分规模。以位于大地上方的单根导体阻抗的求解为例,分别采用阻抗分解法及解析法计算导体阻抗,两者计算结果相对误差小于2.5%,从而验证了阻抗分解法的正确性。应用阻抗分解法计算了山西中南部铁路双块式无砟轨道的阻抗,给出了该段无砟轨道在各频率下的阻抗取值范围。两组实测数据随频率变化趋势与计算结果一致,且符合阻抗变化范围。     

单铝层导线输电网电热协调潮流模型及其解法

受钢芯磁滞和涡流损耗作用,单铝层导线交流电阻随电流呈明显的非线性变化,但当前电热协调潮流模型忽略了线路电阻与电流的关系,对结果精度造成了影响。为此,文中以LGJ120/25单铝层导线为对象,搭建了相关实验平台,对不同温度和电流条件下的导线交流电阻进行了实测,结合参数估计方法建立了计及电流变化影响的交流电阻修正模型。在此基础上,将线路电流作为电力系统潮流状态量,提出了针对单铝层导线输电网的电热协调潮流模型及其解耦计算方法。算例结果表明,对于含单铝层导线的高压输电网而言,所提模型能得到更为准确的电热协调潮流计算结果,有效避免线路运行温度裕量被高估的现象,减少线路过载风险,为单铝层导线输电网的安全、经济运行提供依据。     

直埋金属管道对其内铠装通信电缆的电磁屏蔽计算

建立了一种考虑感性耦合与阻性耦合影响时 ,铠装电缆套加装金属管道后电磁屏蔽计算的通用模型。该模型是在场路结合的基础上 ,对导体采用线性化单元离散 ,建立相应的方程组 ,求得导体中的电流分布 ,从而可对地下通信电缆系统进行电磁屏蔽的近似计算。最后给出了计算实例     

双馈异步发电机序阻抗建模简化方法

新能源发电系统的序阻抗模型广泛应用于系统的稳定性分析和谐振分析。双馈异步发电机进行序阻抗建模时可以使用电机的电压和磁链方程或等效电路。虽然等效电路使建模过程得以简化,但是其建立的模型忽略了励磁支路的影响,且没有对简化的条件进行分析,使此类建模方法的使用产生困难。因此,基于是否计及励磁支路的电机等效电路,利用谐波线性化方法建立了双馈异步发电机转子侧的序阻抗模型,在此基础上分析两种阻抗模型之间的差异并明确简化模型的使用条件。仿真结果验证了所建立序阻抗模型的准确性,分析表明当电机的励磁支路阻抗远大于定转子阻抗时,简化模型可代替准确模型。     

A new method for measurement of earth fault loop impedance

A novel method for accurate and quick measurement of earth fault loop impedance in a low-voltage system is presented. The theoretical principle of the method is based on sampling of the tested circuit voltage at properly chosen moments. The method makes possible the simultaneous measurement of impedance, resistance, and reactance. The digital display is calibrated such that quantities can be read directly     

配电网消弧线圈自动跟踪补偿装置的初步设计与仿真研究

我国3～66kV配电网一般为中性点不接地系统,近几年随着城市电网改造,越来越多的电缆线路代替架空线路,线路对地电容增大。当电网的单相接地电流大于一定值时,接地电弧不容易自行熄灭,使故障扩大,此时采用中性点经消弧线圈接地方式。消弧线圈可以补偿电网的单相接地电容电流以及减缓弧隙恢复电压上升速度,保证电弧可靠熄灭。本文针对10kV配电网设计消弧线圈自动跟踪补偿装置,采用晶闸管投切电容式(TSC)的调谐原理,并在传统设计的基础上改进设计。最后,利用PSCAD/EMTDC软件对中性点经消弧线圈接地系统的各种状态(包括正常运行和单相接地故障)进行仿真,对本文的设计进行了验证,证明了其可行性与可靠性。     

An advanced computer model for grounding system analysis

An advanced methodology and a computer model for analysis of practical grounding systems based on the method of moments are presented. The practical methodology is applicable to both simple and complex grounding systems. The grounding system may consist of cylindrical conductors as well as rectangular earth conductors or metallic surfaces buried in earth or located on the surface of the earth. The method consists of computing an equivalent circuit model of the earth embedded electrodes and conductive soil. Uniform or two-layer soil can he accommodated. The problem of computational efficiency is addressed. Several innovations have been introduced to minimize execution time, including adaptive segmentation of grounding structures and adaptive computation of self and mutual impedances. The procedure enables accurate computation of touch and step voltages, body currents, grounding system impedance, voltage profile, etc. The method and computer program have been validated with actual system measurements     

直埋金属管道对其内铠装通信电缆的电磁屏蔽计算

A universal model of telecommunication cable surrounded by the buried metal pipeline for calculating their current distribution and shielding effects is presented, which takes the inductive coupling and resistive coupling with each other in the presence of a uniform half space conductive medium(earth) into account. The model is based on the combination of the circuit theory and the electromagnetic field theory. By means of dividing the conductor element using a linear interpolation method, a discrete equation group is constructed to solve the current distribution and then calculate the shielding effect of the underground telecommunication cable system. In fact, the model developed can be generalized easily to solve the current distribution of the multi-conductor system. The validation of the model is verified by a practical problem case.     

Multiconductor analysis of underground power transmission systems: EHV AC cables

It is widely ascertained that the multiconductor analysis is a powerful tool which can solve any structurally complex circuit (e.g., high-speed railway supply system, gas insulated lines, etc.). Moreover, this method can be also used as an in-depth analysis of the electric networks after the power-flow studies when they introduce simplifying hypotheses especially in the presence of asymmetry. In this paper, the multiconductor cell analysis has been applied to AC underground cable lines (UGC). This multiconductor procedure based on the use of admittance matrices, which account for the line cells (with earth return currents), different types of sheath bonding, possible multiple circuits, allows predicting the steady-state (and faulty) regime of any cable system. The method calculates the proportion and behavior of the phase currents carried by each parallel conductor, the circulating current in the sheath of each cable and the stray current in the earth. Moreover, some comparisons have been made with traditional programs showing the great accuracy of multiconductor cell model.     

Proposal of earthing resistance measurement method without auxiliary electrodes

This paper proposes an earthing resistance measurement method for a simple earthing electrode without two auxiliary electrodes. The proposed method can measure the earthing resistance by evaluating loop impedance of an earth return circuit composed of an earth electrode of the under test and a lead wire, a return wire, and the earth soil. The return wire is placed on the soil surface without any terminations. The earthing resistance of the electrode of the under test can be given by the earth return circuit impedance at a resonant frequency design between 100 kHz and 1 MHz. The measurement conditions for the proposed method are determined by numerical calculations. Earthing resistance deviations between the proposed method and the existing method are ?13% to +22% for earthing resistance values with 50, 100, or 300Ω. It is found that the proposed method can be used to evaluate an earthing resistance around 100Ω for a simple earthing electrode. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 154(3): 16–26, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20196     

基于波动量法的系统侧谐波阻抗幅值估计精度评价方法

系统谐波阻抗是衡量用户谐波发射水平的关键参数,准确估计其值具有重要的意义。文章基于波动量法,针对系统谐波阻抗的估计幅值构建了系统谐波阻抗幅值估计精度指标集,包括固有偏差指标和量测不确定度指标。固有偏差指标可定量评价由波动量法原理本身局限性造成的估计偏差;量测不确定度指标可综合衡量由量测、干扰等客观原因引起的量测偏差。然后提出了幅值估计精度指标的实用化计算方法。利用PSCAD仿真软件对典型系统进行了蒙特卡罗仿真分析,验证了幅值估计精度指标集可有效地筛选出较理想的工况和较精确的估计值,并在一定程度上量化估计值的精确度,可作为判定估计值是否满足工程误差要求的实用化依据。     

变电站接地网的频域有限元方法

变电站接地网的安全性问题已成为电力系统电磁兼容的重要研究课题之一。文中基于有限元方法，将接地网一维导体单元与三维土壤单元相耦合，并考虑导体阻抗的频变特性，研究变电站接地网的接地性能。与三维有限元的A-V方法相比，该文计算方法的计算量大大降低，同时可以考虑频率对接地网接地性能的影响。通过与矩量法和测量结果进行比较，验证了该计算方法的正确性和有效性。利用计算得到的接地网导体轴向电流和土壤泄漏分布电流，进一步计算了接地网产生的空间磁场强度。结果表明，文中方法与矩量法的计算结果略有差异，原因在于矩量法中未考虑土壤泄漏分布电流的影响，文中计算方法可以方便地应用于复合土壤结构的接地网的接地性能分析。     

三峡左岸龙泉换流站接地极导流系统设计

直流接地极是直流输电系统中的一个重要组成部分,导流系统的安全运行对接地极的安全运行起着至关重要的作用,导流系统的设计核心是为获得比较均匀的电流分布特性。三峡左岸龙泉换流站接地极导流系统设计拟订2 种方案:方案1 是利用终端塔兼作引流塔,可节省1 基塔,引流线也较短,但引流线与馈电环不对称,阻抗差别大;方案2 是终端塔导线接入中心塔后,由中心塔用架空线接入各分支引流塔,然后利用电缆连接馈电棒导流,这种导流方式整个引流线布局比较对称,阻抗比较均衡。龙泉换流站采用的是后者。     

Approximate method for calculating the impedances of multiconductors with cross sections of arbitrary shapes

This paper proposes an approximate impedance formula of a conductor with an arbitrary cross section, and also a method of approximating the conductor by an equivalent circular conductor. Thus, a multiconductor system consisting of arbitrary cross section conductors is represented by circular cross section conductors, the impedance of admittance of which are evaluated by the existing impedance and admittance formulas. The calculated results by the proposed method agree satisfactorily with measured results and numerical results by a finite-element method which is, at present, the only way of calculating such impedance, but requires a large computation time and large memory. The proposed method might be very useful in evaluating the impedance and admittance of a multiconductor system consisting of arbitrary cross section conductors.     

DC/DC变换器精确频响特性分析与测量

采用一种非平均模型法对DC/DC变换器的精确频响特性如输入阻抗、输入输出电压传递函数等进行了分析.由于电力电子变换器同时具有连续和离散时间系统特性,平均模型法和离散模型法均不能对其进行精确的动态特性分析.提出将DC/DC变换器的切换开关等效成一个时变电路,从而将整个系统看成一个线性时变系统,使系统同时具备精确性和易于分析性.在此基础上,对DC/DC变换器的频响特性进行了解析求解分析和实验测量.实验证明,该方法克服了平均模型法和离散模型法的频率范围限制,在开关频率附近及开关频率之上均能精确反映DC/DC变换器的动态特性.     

Broken conductors protection system using carrier communication

This paper presents a detection and signaling system designed to identify and locate high impedance faults caused by broken conductors on distribution primary feeders. Unlike conventional protection systems, which perform current sensing, the working principle of the proposed system consists on monitoring the voltage unbalance along a feeder. This allows the system to detect a fault occurrence even in cases when the conductor touches a high impedance earth surface (for instance asphalt). This system has an additional advantage of giving an indication of the location of a fault since it involves measurements at multiple points on a feeder. In order to detect the voltage unbalance produced by a broken conductor, a new sensor was developed which is sensitive to the electric field generated by primary feeders. A carrier communication channel is associated to each sensor allowing the high impedance fault occurrence information to reach the protection equipment located closer to or at the substation     

汽车线束等效模型及其在电磁辐射敏感度仿真中的应用

线束是导致汽车设备或系统不能满足电磁兼容限值的主要原因之一,建立计算简便且能准确反映电磁特性的线束模型是汽车电磁兼容建模的要点。文中建立了用于电磁辐射敏感度计算的线束等效模型。首先提出了等效波阻抗的定义并导出了计算方法;根据原始导线终端阻抗与等效波阻抗的大小关系将其分成最多四组等效导线,从而使复杂的线束得以简化。给出了多导体传输线单位长度电容矩阵和电感矩阵的计算方法,推导了等效导线的电感矩阵和电容矩阵。验证了基于有限元方法的电磁场仿真软件HFSS计算导线感应电流的有效性。对9根导线组成的线束等效处理及其共模电流的仿真结果表明,文中的等效方法是正确的。该等效模型能有效降低线束建模的复杂度,可用于研究汽车线束高频条件下的电磁辐射敏感度问题。     

基于改进的稳态法获取三相系统谐波阻抗

在三相电力系统,当各相间存在耦合阻抗以及三相系统不对称运行时,应用稳态法和瞬态法不能获取准确的谐波阻抗,本文提出改进的稳态法来获取系统侧谐波阻抗。首先求取相模变换矩阵,将三组可测数据解耦成单变量数据,简化计算过程;其次,在MATLAB/Simulink中建立单相仿真模型,对稳态法和瞬态法进行对比;第三,建立存在耦合阻抗且不对称运行的三相仿真模型,应用稳态法与改进的稳态法计算系统侧谐波阻抗。实验表明稳态法比瞬态法获取阻抗值更加准确,且受干扰因素较少;选择稳态法获取系统侧谐波阻抗,经相模变换改进后,提高了准确率,并简化了计算过程。     

Three-phase modeling of unbalanced distribution systems during open conductors and/or shunt fault conditions using the bus impedance matrix

This paper presents a digital simulation technique for unbalanced three-phase distribution systems subjected to a shunt or a series fault. The method is also applicable to multiple faults which involve a shunt fault at one location and blown fuses or open conductors at another. The method utilizes the phase frame representation of network elements. The imbalance conditioned by large single-phase loads, untransposed feeders, conductor bundles, etc., is reflected in the polyphase impedance matrix. Consequently, line removals, additions, impedance changes, conductor and phase openings can be simulated by modifying the base-case impedance matrix. Thus, the proposed method makes it possible to analyze any type of single or multiple fault occurring anywhere in the system.