40.5kV真空断路器开断并联电抗器的过电压防护

针对国内40.5 kV真空断路器应用于开断并联电抗器时绝缘事故多发的情况,进行了一系列现场试验,并对几种常见的不同原理及型式的过电压保护装置进行了分析和比较。研究结果显示,保护装置除了要考虑相对地保护,更要考虑相间过电压抑制。对于所述工况,可有效限制相对地、相间过电压的保护装置包括无间隙组合式MOA、3间隙组合式MOA以及阻容保护,且对于阻容保护,接在电抗器侧效果要明显好于接在开关侧。分析比较结果对于实际应用具有一定的指导意义。     

Improved calculations for no-load transformer switching surges

This paper deals with a modeling of components and calculation of transient overvoltages that build up on the transformer primary terminals after the transformer has been switched off by a vacuum circuit breaker (VCB). The transformer is connected to the circuit breaker by a cable. The transient overvoltages are calculated for different cable lengths, and the cumulative probability of different arc angles is investigated. In this study two cases are considered: transient overvoltages due to steady-state magnetizing current switching, and transient overvoltages due to inrush current switching. It is shown that the case of inrush current switching is worse, as virtual current chopping is possible. The cable is modeled by pi sections, whereas the transformer model is based on a terminal impedance. The VCB re-ignitions are modeled by means of withstand voltage characteristics and high-frequency quenching capability. Due to shortage of field tests, this work uses only literature references to compare the results with actual measurements     

Switching surge overvoltages on the feeding system of a MAGLEV train

This paper investigates switching surge overvoltages due to a section switchgear between coils and a feeding cable, and surge overvoltages due to a ground fault in the feeding cable. The maximum overvoltage due to switchgear closing is about 25 kV on the coil and 15 kV on the cable sheath. The coil overvoltage becomes greater as a source voltage reaches its peak. The overvoltages due to switchgear breaking is dependent on the breaking current and they reach 115 kV on the coil and 15 kV on the cable sheath when the current is 50 A. The fault surge overvoltage is about 24 kV on the coil which is smaller than the switching overvoltage. On the other hand, the cable sheath overvoltage reaches 32 kV, which is the largest among various overvoltages investigated in this paper, and could result in sheath insulation breakdown. The coil overvoltage due to current breaking by a section switchgear may cause coil insulation breakdown when the breaking current is large. Both the coil and sheath overvoltages can be controlled to less than the insulation level by arresters. © 1998 Scripta Technica, Electr Eng Jpn, (4): 58–66, 1998     

Analytical and numerical modelling of a stationary temperature field in a three dimensional electric heating system

Contents In the paper, a stationary temperature field in a three dimensional system of a direct floor heater was modelled. This installation was described by an elliptic boundary problem. On this basis two independent simulations, analytical and numerical, were carried out. In the former, the cable core was modelled with thin axes and the triple Fourier series was used. In the numerical simulation the heat sources were right octagonal prisms inscribed into the cable core. In this case the finite element method was applied. The results of both simulations are approximate with good accuracy. The results obtained are presented graphically.

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Analytische und numerische Modellierung des stationären Temperaturfeldes einer dreidimensionalen Fußbodenheizung

Übersicht In der vorliegenden Arbeit wurde das stationäre Temperaturfeld im direkten dreidimensionalen System der Fußbodenheizung modelliert. Die Anlage wurde in der Form eines elliptischen Randproblems beschrieben. Hierauf wurden zwei unabhängige Simulationen durchgeführt: analytische und numerische Simulation. In der ersten Simulation wurde die Kabelstränge mit Hilfe von dünnen Achsen modelliert, und es wurde eine dreifache Fourierische Reihe genutzt, in der numerischen Simulation Wärmequellenals achteckige, regelmäßige Prismen, die in die Kabelstränge einbeschrieben wurden. Im letzten Beispiel wurde die Finite-Elemente Methode benutzt. Die Ergebnisse der beiden Simulationen sind sich in hinreichender Genauigkeit ähnlich. Die gewonnenen Ergebnisse wurden in graphischer Form dargestellt.

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List of symbols A dimensionless constant, determined by (9b) - (2a, 2b, l) dimensions of the floor panel Fig. 1a - B_mni coefficients of the series (8) - {G} heat source vector - G(z) gate function determining the position and length of the heating cable along the axis OZ (Fig. 1b) - g _k (x, y, z) volumetric power density of the system with thek-th cable section - K number of cable sections - k index of thek-th cable section (k=1, 2, 3,...,K) - P _k active power of thek-th cable section - q _k linear power density of thek-th section of the heating cable core - R cable radius - r radius of cable core - {T} node temperature vector - T(x, y, z) total temperature field in the floor panel - T _m mean temperature - T _k (x, y, z) temperature field component from thek-th cable section (with the others switched off), - T ₀ air temperature (far from the floor surface), - u dimensionless filling factor of the lengthl by the cable (Fig. 1b, u 0, 1) - (x, y, z) coordinates of a point in the floor panel - (x _k,y _k,z) coordinates of the position of thek-th section of the cable core (forzul, (1–u)l) - averaging coefficient of heat transfer to air (sum of the radiation and convection coefficients) - _n successive positive roots of equation (9a) - (x–x _k), (y–y _k) Dirac's deltas shifted tox _k andy _k respectively - convergence index of series (10b) and (10c)-ratio of the module of the sum of the last ten terms to the module of the total sum (table 1) - heat conductivity matrix - averaging heat conductivity of micro-rein-forced concrete - _c cable core heat conductivity - v _k(x,y,z) k-th component of an increase in the thermal field of over the value T₀ caused by thek-th section of the cable (with the others switched off) - V _k ⁽¹⁾ two dimensional component of an increaseV _k (x, y, z), determined by eq. (10b) - V _k ⁽²⁾ three dimensional component of an increaseV _k (x, y, z), determined by eq (10c) - l(z) unit step function The work (Code No W/WE/3/96) was carried out in Biaystok Technical University under the financial support of State Committee for Scientific Research, Poland.     

Design and testing of heavy pulse current switches based on photothyristors

This article presents the results of testing heavy pulse current switches based on photothyristors. Transient processes are considered that are observed in the semiconductor switch when the capacitor is discharged upon inductance and with a signal-shaping network with an inductor and crowbar diodes. The limit currents are found for the semiconductor structure, at which thermal generation peaks are observed in oscillograph records of direct voltage drop. The energizing of the photothyristor is analyzed. The need for forcing RC circuits to ensure fast and steady transition of the semiconductor structure to the conductive state is shown. The transient process observed at reverse recovery of photothyristors in the discharge circuit with inductance is considered. The need for snubber circuits to suppress pulse overvoltage is established. The particulars of the transient switching of current to the crowbar diodes and the generation of pulse overvoltages at reverse recovery of photothyristors are considered for a discharge circuit with crowbar diodes. The parameters of snubber circuits to suppress these overvoltages are determined. The validity of the accepted engineering solutions is confirmed by the results of tests at switching of a pulse current of up to 100 kA.     

真空开关开断并联电容器组时操作过电压EMTP计算模型的设计与分析

根据真空开关开断并联电容器组时的操作过电压产生机理并结合电力系统的典型电路和试验数据,使用Electro-magnetic Transient Program(EMTP)建立了能模拟这种过电压的计算模型,进行仿真计算,使用方便,可靠性和准确性高。     

真空断路器投切电动机的操作过电压研究

蒲城发电厂１号发电机厂用系统的６ＫＶ开关于１９９７年更换为北京开关厂生产的新一代６ＫＶ真空断路器和真空接触器。真空断路器具有结构简单,维护方便,运行安全和适于频繁操作等优点,已在电力系统得到广泛应用。     

对1000kV电网操作过电压及相位控制高压断路器的讨论

根据500kV及750kV电网操作过电压的数据以及1000kV电网应有的过电压水平,指出了对于1000kV电网断路器关合开断可能引起的多种操作过电压(如开断并联电抗器及开断中小短路电流的的重燃过电压等)必须给予重视.近年来国外用以抑制操作过电压的装置除并联电阻的高压断路器及氧化锌避雷器以外,相位控制高压断路器日益得以广泛应用.文章介绍了国外相控高压断路器的应用情况,总结了相控高压断路器的关键技术,并对将其应用于我国1000kV电网提出了几点建议.     

Earthing effects of uncoated underground cables and transferred potentials

Contents A model for the analysis of earthing effects of uncoated metallic sheathed underground cables is suggested. The model includes the proximity effects between the cable sheath and the earth electrodes of the transformer stations at the cable's terminals. Diagrams for assessing apparent sheath impedances to earth as well as transferred potentials and dangerous voltages at terminal stations are constructed appropriate for practical design.

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Wirkung von Erdkabeln mit nichtisolierten Mänteln und die erzeugte Potentialverteilung

Übersicht Ein Modell für die Analyse der Erderwirkung von Kabeln mit nichtisolierten Mänteln wird vorgestellt. Das Modell berücksichtigt die gegenseitigen Beeinflussungen zwischen Kabeln und Erderanlagen an den Kabelenden. Diagramme für die Beurteilung der Erdausbreitungsimpedanzen von Kabeln, der erzeugten Potentialverteilung und Berührungs- und Schrittspannungen entlang der Kabeltrassen werden für die praktische Anwendung angegeben.

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List of symbols J fault current at the HV/MV station flowing into the earth - u _k potential of earthing system elementk - i _k current emanating into the soil from elementk - E _t,E _s touch- and step-voltages - Y longitudinal admittance of a cable sheath segment - Y _n admittance to earth of the earthing structure bonded with the terminal station earth electrode - r _kj mutual earth resistance for elementsk andj - r _kk self resistance to earth of elementk - r _kM mutual earth resistance of elementk and pointM - Z _eq equivalent system impedance to earth as viewed at HV/MV station - Z _a apparent cable's impedance to earth - l(l ₁) cable (characteristic) length - s sheath's segment length - H, d cable's depth of burial and diameter - D diameter of the equivalent plate modeling HV/MV station electrode - l _r,d _r rod length and diameter - earth resistivity     

220kV电力电缆线路合闸操作过电压分析

在一侧断路器断开状态下,某220 kV电缆线路线遥控合闸后两套纵联保护动作事故跳闸。巡检发现,该电力电缆C相接头击穿。为探究电力电缆在合闸过程中的过电压情况,分析事故原因,对修复后的电缆线路进行与事故情况同一状态下的线路投切试验,监测投切过程中的电压和电流情况。同时,针对线路空载合闸过程中电缆沿线各处可能存在的过电压情况,采用电磁暂态计算程序PSCAD,对该电缆线路合闸过电压进行仿真分析计算。最后,针对实测和仿真结果提出了运维对策。     

Stationary thermal field in a long duct of an electrical heating system

Contents The subject of this work is the analysis of a stationary thermal field in a long duct of an electrical floor heating system. Parallel sectors of the heating cable are modelled by heat sources of the emitted power linear density. The resultant thermal field is described by a partial differential equation of the elliptic type with boundary conditions of the second and third kind. The problem is solved by the method of eigenfunctions. The results are reduced to a dimensionless form and numerically processed. The field distributions obtained are presented in graphical form, with the number of sectors of the heating cable in the duct being changed.

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Das stationäre Temperaturfeld im langen Kanal einer elektrischen Fußbodenheizung

Übersicht Der Artikel analysiert das stationäre Temperaturfeld im langen Kanal einer elektrischen Heizung. Parallele Kabelstrecken wurden durch Wärmequellen ersertzt. Das Temperaturfeld wird mit Hilfe der partiellen Differentialgleichung des elliptischen Typs dargestellt. Dieses Problem hat man durch die Eigenfunktion gelöst. Die Ergebnisse wurden in eine dimensionslose Form umgewandelt und numerisch bearbeitet. Die Feldverteilungen wurden graphisch dargestellt, wobei die Anzahl der Kabelabschnitte verändert wird.

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List of symbols A dimensionless constant, determined by (6b) - 2a, 2b dimensions of the cross-section of the duct (Fig. 1.) - B _mm coefficient of the series (5), determined by (8) - c _k relation of linear density of the power of heaters; thek-th to the first (q _k /q ₁) - K number of heaters - k index of thek-th heater (k=1,2,...,K) - L(x/2b, y/2b) total dimensionless thermal field in the duct - L _k (x/2b, y/2b) dimensionless component of thermal field caused by thek-th heater (with the others turned off) - q _k linear density of the power of thek-th heater [W/m] (k=1,2,...,K) - S cross-section of the duct - T=T(x, y) total temperature field in the duct - T _k (x, y) component of the temperature field in the duct caused by thek-th heater (with the others turned off) - T ₀ temperature of air far from the top surface of the duct - v _k (x, y)=T _k (x, y)–T ₀ k-th component of an increase in the thermal field in the duct over the valueT ₀, caused by thek-th heater (with the others turned off) - (x, y) Cartesian coordinates of a point in the duct - (x _k ,y _k ) coordinates of the position of thek-th sector of the heating cable, averaging coefficient of heat transfer to air (sum of the radiation and convection coefficients) [W/(m ² K)] - averaging coeffiecient of heat transfer to air (sum of the radiation and convection coefficeints) [W/(m ² K)], - _n successive positive roots of (6a), (6b) - (x–x _k ), (y–y _k) Dirac's deltas shifted tox _k andy _k respectively - _m0 Kronecker's symbol - averaging heat conductivity of micro-reinforced concrete [W/(mk)]     

1000kV交流输电线路的故障激发过电压研究

在简述1000kV特高压输电线路故障激发过电压的基础上,研究了国家电网公司规划建设的晋东南-南阳-荆门单回百万伏级输变电工程在线路上单相或多相接地或相间故障引起的过电压和故障切除时的过电压两种情况下由故障激发的操作过电压,并提出了包括沿线装设氧化锌避雷器、使用可控并联电抗器以及装设并联开关电阻等限制故障过电压水平的措施。仿真计算表明,并联电抗器补偿容量越大,故障激发过电压越低;而仅在线路两端装设避雷器可在一定程度上降低单相重合闸故障引起的过电压,但过电压水平仍然比较高,还需要进一步采取措施;如同时使用并联合闸电阻和氧化锌避雷器,则能明显限制故障激发过电压的水平。     

High-voltage,high-frequency electromagnetic oscillations in LC circuits

It is shown experimentally that, in high-frequency circuits, current oscillations are accompanied by extremely high overvoltages. These oscillations arise due to the switching on of a finite part of the inductance coil during the half of the oscillation period t = T/2, and the inductance operation in autotransformer mode. It is shown that the peculiarities of this mode depend on the velocity of the propagation of the electron-conduction current through the coil wire. The effect of preliminary bias on HF oscillations in the LC circuit is considered. An analysis of commonly used electrical circuit overvoltage protection facilities is carried out.     

电缆出线变电站连续两次雷击仿真及防护措施

针对某电缆出线变电站近区连续遭受两次雷电绕击的事故,基于PSCAD/EMTDC搭建仿真模型并进行了过电压的仿真计算.雷击点位于变电站进线第二基杆塔处,仿真分析了发生第一次雷电绕击时,绝缘子片数和出线电缆的长度对绕击过电压的影响,以及发生第二次绕击时不同的避雷器配置方案对热备用状态下断路器过电压的影响.仿真结果表明,绝缘子片数的增加会增大绕击过电压幅值,不同电缆长度对限制过电压的效果具有明显的差异;发生第二次雷击时,通过在站内断路器断口侧及变电站第一基杆塔绝缘子旁安装避雷器,能够有效减小断路器过电压幅值.     

Temporary overvoltage duty of surge arresters in noneffectively grounded systems

Important characteristics of metal oxide surge arresters depend on the specified temporary overvoltage duty requirement due to the volt–ampere characteristics. However, there are few reports on the duty of surge arresters in noneffectively grounded systems. The temporary overvoltages of 1.43 pu proposed in effectively grounded systems is too low compared with nearly pu in a line‐to‐ground fault. The overvoltages due to a combination of a line‐to‐ground and load rejection, where not only the main power circuits but also control systems such as AVRs participate, is rare but probably the severest case. Here the overvoltage duty of surger arresters is clarified for various residual voltage levels and system conditions. The severest condition is that in a system including a cable line because of operation of the UEL of the AVR. © 1999 Scripta Technica, Electr Eng Jpn, 128(4): 38–46, 1999     

真空断路器开断并联电抗器过电压试验研究

针对国内35kV与20kV系统真空断路器开断并联电抗器系统事故多发情况,开展35kV系统工况下的现场试验,分析了开断后的暂态过程及过电压的机理,并在实验室进行了20kV系统真空断路器在额定电流下投切电抗器的截流试验,研究截流值和过电压规律。结果表明:截流过电压幅值远小于重燃过电压,多次重燃过电压是真空断路器开断并联电抗器频繁出现高过电压的主要原因;在系统典型额定工况下,300A负载电流下的截流值比100A负载电流下小,真空断路器的截流值随着开断电流的增大而减小的理论在系统额定电流下同样成立。     

An imitation model of a power-supply system of oxygen-converter manufacturing to analyze switching overvoltage

This paper considers switching overvoltage that occurs in the power-supply system of oxygen-converter manufacturing when a circuit vacuum breaker is triggered. The particular features of such systems are analyzed, and a typical power supply scheme is considered. It is noted that overvoltage manifesting a highfrequency behavior can be produced as a result of the peculiarities of arc interruption in vacuum (current chopping, voltage escalation, and arc reignition). Such overvoltage negatively affects electric motors with insulation that have the lowest margin of dielectric strength. Conventional means of overvoltage protection—such as conventional spark-gapped arresters, metal-oxide surge arresters, and spark gaps—are inefficient at high-frequency overvoltage because of a sharp increase in their voltage-second characteristics at low pulse duration. This paper suggests using RC circuits as protection from such overvoltage. Their application is accompanied by quite exact simulations since an incorrect choice of their parameters can worsen the commutation process. The main particular features of construction of a simulation model to study switching overvoltage and to analyze protection functioning are considered. A depiction of the simulation model and results of simulations are also presented. The validity of the suggested model is determined.     

Additional no-load losses in inverter-fed high-speed cage induction motors

An analysis of additional high-frequency losses in high-speed cage induction motors with closed rotor slots under sinusoidal and inverter supply at no load is presented. Calculated losses are compared with experimental results obtained by measurement on a 270 kW, 16,200/min, prototype motor, at both the delta and star connections of the stator winding. The main additional losses of the prototype motor investigated due to inverter supply are eddy current losses in the stator winding. Both measurement and calculation show that, at the delta connection, the additional losses are higher due to the lower modulation degree of the inverter, causing higher harmonic voltage amplitudes.List of symbols B magnetic flux density, T - b _sheet width of iron sheet, m - b _bar mean width of rotor bar, m - b _sd mean stator tooth width, m - b _Qm mean slot width, m - b _LT square wire side, m - f frequency, Hz - h _bar rotor bar height, m - h _br rotor slot bridge height, m - h _sy stator yoke height, m - I current (rms value), A - k ordinal number of voltage harmonic - k _C Carters factor - k _dHy hysteresis deterioration factor for teeth - k _dFt eddy-current deterioration factor for teeth - k _Fe iron stack fill factor - K _l1k first-order skin effect coefficient of stator inductance for kth voltage harmonic - K _l2k second-order skin effect coefficient of stator inductance for kth voltage harmonic - K _1k first-order skin effect coefficient of stator resistance for kth voltage harmonic - _2k second-order skin effect coefficient of stator resistance for kth voltage harmonic - k _yHy hysteresis deterioration factor for yoke - k _yFt eddy-current deterioration factor for yoke - k _w1 winding factor for fundamental space harmonic - l _b length of winding overhang, m - l _Fe core length, m - L _dc inductance at d.c. current, H - L _r rotor inductance, H - L _s stator inductance, H - M torque, Nm - m _T number of vertically arranged coils in slot - m _TZ number of vertically arranged conductors in slot - n _N rated speed, 1/min - p number of pole pairs - p _Ft eddy current losses per iron mass at 50 Hz, 1 T, W/kg - p _Hy hysteresis losses per iron mass at 50 Hz, 1 T, W/kg - P _Cu copper losses, W - P _Fe iron losses, W - P _Fe,Hy hysteresis losses, W - P _Fe,Ft eddy-current iron losses, W - P _fr friction and windage losses, W - Q _r number of rotor slots - Q _s number of stator slots - R _s stator phase resistance, - R _r rotor phase resistance, - R _dc resistance at d.c. current, - s slip - s _k slip for kth harmonic - U voltage (rms value), V - V volume, m³ - N number of turns per phase - z ₁ average number of conductors in one turn, lying side by side - z ₂ average number of vertically arranged conductors per turn - air-gap length, m - magnetic flux, Wb - electric conductivity, S/m - permeability ( ₀=4×10^–7 H/m: permeability of vacuum), H/m - _r relative permeability - _Fe mass density, kg/m³ - _s leakage coefficient for stator winding - _p pole pitch, m - _sd stator tooth pitch, m G. Joksimovi is on research leave from University of Montenegro, Montenegro, Yugoslavia     

Comparative analysis of techniques for control of switching overvoltages during transmission lines energization

The energization of long transmission lines can cause high overvoltage stresses not only along the transmission line, but also in the rest of the network. The traditional method of limiting switching overvoltages to acceptable levels is the use of circuit breakers equipped with pre-insertion resistors. The present paper describes a study comparing this traditional method with two other alternatives for the limitation of switching overvoltages during line energization in an actual 500 kV transmission system: the use of metal oxide surge arresters at both line closing of circuit breaker poles. Digital simulations were made with PSCAD/EMTDC software and the degree of shunt compensation is considered as an independent parameter.