Steep fronts at transient recovery voltages appearing with the interruption of inrush currents of transformers

High lightning overvoltages do not appear in underground substations connected to transmission cables. Consequently, it is very important to thoroughly investigate switching overvoltages and to achieve rational insulation coordination for apparatus installed in such underground substations. This paper discusses the occurrence of steep fronts at transient recovery voltages (TRV) appearing at circuit breakers when the inrush currents of transformers are interrupted. Caused by a steep front at the TRV, reignitions occur at circuit breakers, resulting in the generation of high overvoltages with high frequencies. The overvoltages are among the highest switching overvoltages appearing at the terminal of a transformer. The authors clarified the mechanism of the generation of steep fronts at TRV by means of EMTP analysis, as well as by carrying out tests in a high‐power laboratory. © 2001 Scripta Technica, Electr Eng Jpn, 135(1): 24–32, 2001     

Overvoltage control during restoration

During the early stages of restoring high-voltage overhead and underground transmission lines, the authors discuss three related overvoltage areas: sustained power frequency overvoltages, switching transients, and harmonic resonances. During restoration certain line-transformer configurations when energized generate a combination of sustained, transient, and harmonic overvoltages which may cause surge arrester failures and thus system faults. or the subsequent flash-over and damage to equipment. The energy stored in long high-voltage lines is large causing significant transient or resonant overvoltages which must be carefully considered during restoration. With the help of analytical tools such as the electromagnetic transients program (EMTP) and those described it should be possible to establish general procedures and restoration guidelines to avoid overvoltages     

Estimation of switching transient peak overvoltages during transmission line energization using artificial neural network

Overvoltages are one of the most frequently encountered problems during line energization. At the time of restoration transmission line switching is also one of the major causes, which creates overvoltage. The magnitude and shape of the switching overvoltages vary with the system parameters and network configuration and the point-on-wave where the switching operation takes place. Though detailed electromagnetic transient studies carried out for the design of transmission systems, such studies are not common in a day-to-day operation of power system. However it is important for the operator to ensure that peak overvoltages resulting from the switching operations are well within safe limits. This paper presents an Artificial Neural Network (ANN)-based approach to estimate the peak overvoltage generated by switching transients during line energization. In proposed methodology Levenberg–Marquardt method is used to train the multilayer perceptron. The developed ANN is trained with the extensive simulated results, and tested for typical cases. The simulated results presented clearly show that the proposed technique can estimate the peak values of switching overvoltages with good accuracy.     

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     

The Canadian Electrical Association: a leader in cable research

The history and cable research efforts of the Canadian Electrical Association (CEA) are reviewed. Since the inception of the first cable research project in 1977, the CEA and its cosponsors have funded 409 research programs. Research topics have included detectors for underground faults, water tree inhibitors, cable identification methods, ageing of cross-linked polyethylene (XLPE) cables, rural underground distribution, the effects of switching surges on cable ageing, ampacity of power cable, maximum temperature operation, and final breakdown mechanism of XLPE cables. Current cable research projects are listed     

基于温度概率预测的地下电力电缆可靠性模型研究

地下电力电缆周围气象温度具有不确定性。通过考虑气象温度对地下电力电缆可靠运行的实时影响,借助广义极值分布(Generalized extreme value distribution,GEV)对气象温度进行概率分布拟合,在此基础上分析了地下电力电缆的温度场和热阻等效电路,提出了基于温度概率预测的地下电力电缆可靠性模型,从而有效预测地下电力电缆的导体温度。数值仿真计算与实际测量结果的比较验证了该预测模型的有效性和准确性,为地下电力电缆可靠运行和温度在线监测提供了重要的技术支撑。     

Switching overvoltages in motor circuits

The switching of HV motor circuits in the megawatt range can cause large overvoltages on motor terminals. The amplitude of the resulting voltage spikes depends on breaker type and circuit characteristics. If the amplitude of switching overvoltages exceeds the basic insulation level of the motor, overvoltage protection circuitry should be installed to prevent a motor insulation failure. The decision whether overvoltage protection circuitry (surge pack) must be installed requires a prediction of the overvoltage level. This paper deals with switching overvoltages in motor circuits with a unit transformer between switchgear and motor. Overvoltages of motor circuits were simulated using specially developed simulation models. The results of three case studies revealed a large impact of cabling and transformer on motor overvoltages. In most of the cases, a unit transformer connected between switchgear and motor will provide an overvoltage protection to the motor. However, the transformer can also be a main cause of large overvoltages when connected to cables of specific length and capacitance. The paper provides some simple rules to recognize such critical configurations     

Frequency characteristics of very fast transient currents in a 245-kV GIS

The conducted as well as the induced voltages on control cables and control circuits due to transient electromagnetic (EM) fields generated during switching operations in a gas-insulated substation (GIS) depend on the waveshape of the very fast transient overvoltages and the associated very-fast transient currents (VFTCs). The aim of this paper is to build a basis for characterizing the VFTC generated in gas-insulated switchgear and the associated equipment during switching operations for the study of transient coupling phenomena. The peak magnitudes of VFTC and their dominant frequency content at various locations have been computed in a 245-kV GIS for different switching operations as well as substation configurations. Finally, the influence of the substation layout on the frequency spectrum, dominant frequencies, and the highest possible frequency component of the VFTC at various distances from the switch have been reported.     

大连城网规划中网架与设备选用原则探讨

为城网改造，结合大连电业局配电网架实际，描述了电缆，架空网2种网架，电缆网几个中心开关站相串接，每个开关站从不同变电所的母线或同一变电所的不同母线取得电源，架空线主网架在不同变电所之间，用2－3个分段形关握手互连。使用环网柜和电缆相结合的方式解决了原有架空网下地问题。根据高可靠性和高互连性的原则，对中低压配电网接线原则做了相关的讨论，并就大连配电网提出了选用设备的一些原则。     

Harmonics and transient overvoltages due to capacitor switching

This work presents a study of the steady-state and transient effects of power factor correction capacitors on the utility and on the customer. In the presence of harmonic-producing loads, capacitors used for power factor correction can cause parallel or series resonance problems which tend to increase the total harmonic distortion (THD) of the voltage and current waveforms. The cases studied in this work considers the addition of a power factor correction capacitor, in the presence of downstream harmonic loads and at the harmonic load site. In both cases the resonance created by the addition of the capacitor caused the harmonic distortion of the voltage and current waveforms to increase. Another problem is transient overvoltages created by switching the capacitor. A case study is reported where the operation of a semiconductor controlled motor drive is effected by transient overvoltages     

高温超导电缆在城市地下输电系统应用的可行性研究

大城市有可能最先采用商业化运行高温超导电缆 ,用于城市地下交流输电系统。其主要应用目标是用于地下电缆工程改造 ,利用现有排管以高温超导电缆取代现有的常导电缆 ,增加地下电缆传输容量以及采用高温超导电缆将巨大电能 (1GVA以上 )输入到城市负荷中心。采用常导电力电缆传输 1GVA以上的电能进入中心城区 ,输电电压一般要求为 5 0 0 k V。在城市中心区不可能建设 5 0 0 k V变电站。 5 0 0 k V电缆线路所需的 5 0 0 k V大长度电缆和相应附件 ,目前尚未研制开发。采用高温超导电缆将有可能降低输电电压等级 ,可以采用 2 2 0 k V高温超导电缆将 1GVA以上的电能输入到城市负荷中心 ,满足特大型城市负荷中心供电需求。采用 110 k V高温超导电缆 ,亦有可能传输 1GVA左右电能。本文通过对交流高温超导电缆系列设计计算对额定电压 35 k V、110 k V、2 2 0 k V的高温超导电缆 ,按不同传输电流 (或传输容量 ) ,以高温超导电缆的传输效率 (损耗与传输容量比 )、高温超导电缆外径限值和超导导体绕制结构限制条件 ,确定高温超导电缆适用性界定条件 ,提出城市地下输电、配电系统用高温超导电缆可行方案。     

强化集肤效应抑制快速暂态过电压

GIS隔离开关投切空载母线时能够产生特快速暂态过电压VFTO;真空开关投切电感性负载时能够产生重燃过电压。这些快速暂态过电压具有很高的变化陡度,能够在电力设备绝缘上不均匀分布,产生严重危害。提出了强化集肤效应抑制快速暂态过电压的方法,它是在导电杆上套上高频磁环串,再在磁环串外套上电阻筒,由此构成同轴结构。此结构能够产生强化的集肤效应,工频电流频率较低,主要通过导电杆,受磁环和电阻的影响很小;快速暂态过电压频率很高,受高频磁环的影响显著,被迫通过外层电阻,从而产生幅值和陡度的衰减。通过模拟试验表明,这一方法对快速暂态的抑制效果显著。     

Resonances and overvoltages in a medium-voltage fan motor drive with long cables in an underground mine

Underground mining is characterized by space constraints forcing the use of long cables to feed large machines, where the application of variable speed drives can generate overvoltages at the machine terminals. This paper presents the analysis and mitigation of harmful overvoltages detected in inverter-fed 1400-hp 13.8-kV motors used for air ventilation in an underground mine. The possible causes of the overvoltages, namely wave reflection and resonance, are described theoretically and simulated. On-site direct voltage measurements at the 13.8-kV motor terminals confirmed that resonance was the dominant phenomenon. Based on this study, a suitable low-pass filter was designed, constructed, and installed. Finally, the adequate operation of such filter is demonstrated by further on-site measurements.     

Stabilizing control for overvoltages in a separated power system with heavy load and large shunt capacitance

This paper deals with overvoltages caused by self-excitation in a separated power system, which has some generators, heavy load and large shunt capacitance of underground cables and/or shunt capacitors. The condition of voltage stability in a separated power system is analysed in the G-B_c (admittance) plane. Even when the system is stable, there is a possibility that the voltage is too high or too low, or the frequency is not in the desired range. These phenomena are also analysed in the G-B_c plane. In stabilizing the separated system, it is necessary to consider not only the active power balance, but also the reactive power balance. Stabilizing control strategy for the separated system is proposed based on the analysis in the G-B_c plane, and the simulation results of the stabilizing control strategy are presented.     

Shunt capacitor placement in radial distribution networks considering switching transients decision making approach

This paper provides a new approach in decision making process for shunt capacitor placement in distribution networks. The main core of the evaluation process is a multi-objective framework to allocate the capacitor banks. The power loss and the total harmonic distortion (THD) are the objective functions of the system under study in a long-term planning horizon. In order to select the executive plan introduced by using a multi-objective model, transient switching overvoltages have been considered. As the size and location of shunt capacitors may result in unacceptable overvoltages, the proposed technical decision making framework can be applied to avoid corresponding damages. In this paper, an iterative conventional power flow technique is introduced. This technique can be applied to evaluate THD for distribution networks as well as other power flow based objectives, such as power losses calculation and voltage stability assessment. The presented framework is a two stage one where at the first stage, a non-dominated sorting genetic algorithm (NSGA-II) augmented with a local search technique is used in order to solve the addressed multi-objective optimization problem. Then, at the second stage, a decision making support technique is applied to determine the best solution from the obtained Pareto front. In order to evaluate the effectiveness of the proposed method, two benchmarks are addressed in this paper. The first test system is a 9-bus distribution network and the second one is an 85-bus large scale distribution network. The simulation results show that the presented method is satisfactory and consistent with the expectation.     

Steady-state operating conditions of very long EHVAC cable lines

In the present paper the authors investigate steady-state capabilities of very long EHVAC underground cable lines, without intermediate compensating stations, installed in meshed networks. Analytical formulas of cable length-loading relationship are presented to this purpose. The study shows that, in optimal operating conditions, most of the ampacity of such long underground cable lines can be exploited for active power transmission. Non-optimal operating envelopes are studied, showing the effect of terminal voltages and line losses on cable utilization and evidencing limited derating under realistic hypotheses. A parametric analysis of loading limits and possible voltage violations along the cable is shown; a simple criterion for optimal utilization of lossy cable lines is also proposed. Line-end shunt compensation requirements for integration of long EHVAC cable lines in transmission networks are then specified. Studies performed for both ideal and real cables and shunt reactors show that active power transmission of 100 km long, 400 kV-50 Hz underground cable lines can attain 90% of their thermal limit, without intermediate compensating stations. Excess reactive power and temporary overvoltages are effectively controlled by line-end compensation around 90%.     

考虑铁磁元件励磁特性的电网暂态计算

介绍含铁磁元件电力网络的合闸暂态过电压的模拟计算方法,其中铁磁元件的非线性特性励磁曲线采用逐点线性化方式加以处理。首先给出了变压器的等值电路模型,再以该模型为基础,运用国际上流行的电磁暂态计算软件ATP分别对计及与不计变压器励磁支路的系统网络进行了合闸暂态模拟,对比模拟结果表明:非线性励磁曲线的考虑能在一定程度上对合闸过电压产生影响。     

采用MATLAB仿真的变电站高压进线温度场和载流量数值计算

随着电力电缆在输配电线路中的广泛应用,准确确定电力电缆及其周围环境温度场的分布和电缆的载流量对于提高电力电缆的使用率、动态调整负荷具有重要的意义。为此,以地下排管敷设的交联聚乙烯电力电缆为研究对象,其实际模型为1个容量为250MVA、额定电压为230kV的变电站的高压进线。根据传热学和有限元法(finite element method,FEM)基本原理,建立了1种基于有限元法的水泥排管敷设电缆温度场计算模型,并对电缆及其周围环境的求解区域进行复合有限三角形单元剖分,即对电缆区域进行较密集的网格划分,而对电缆周围的土壤区域则进行较为稀疏的网格划分,以提高程序的运算精度和运行速度。结果表明:用MATLAB软件仿真,从而得到电缆及其周围环境的温度场分布,迭代计算了排管敷设交联聚乙烯电缆的载流量。证明使用有限元的方法分析地下电缆温度场,为电力工程中电缆载流量确定提供了一个比较可靠的计算方法。     

基于变压器绕组中谐振过电压识别的传输函数

Power transformer insulation systems are subjected to many stresses during normal operation due to lightning and switching.If the spectrum of incoming surge voltage matches the winding one,the corresponding resonance will be excited.Therefore external transients occurring in power systems might trigger internal overvoltages with large maximum value in transformer windings.Overvoltages having such characteristic have been the root cause of many power transformer failures.The paper presents an approach to the identification of sensitive zones in the transformer windings based on the measurements of overvoltages inside windings and frequency dependences of admittance of the power transformer.The frequency characteristic of the transformer winding may determine those regions in the frequency spectrum.The presented approach might be used both for design optimization and diagnostics of distribution and power transformers.     

窄通道下排管敷设电缆群负荷优化

电力管道施工过程中,有时不可避免地要经过道路狭窄或地下管网密集的区域,为了保证安全水平净距,通常将此段电缆群变更为通道较窄的布置方式,但这会导致电缆群载流能力减小。将传热学原理和矩阵论相结合,基于IEC 60287提出了一种窄通道下电缆群不等负荷的矩阵优化方案,针对几种典型排管敷设的窄线路建立了有限元模型并进行温度场仿真,验证了优化方案的准确性;最后通过对实际工程通道变窄前后的负荷进行对比设计了合适的窄线路,结果表明该通流方案可提高电缆的利用率,减小了通道变窄对载流能力的影响,窄线路采用不等负荷通流方案可提高电缆的输电能力,随着回路数的增加最大可提升6.53%。