Development of advanced built-in surge arresters for distributionsystem with new zinc-oxide elements

New zinc-oxide elements with a varistor voltage about twice that of conventional ones have been developed by reducing the ZnO grain size with a new additive, which enables halving of the series number of elements in a metal oxide surge arrester. Using these elements, we have developed advanced built-in surge arresters enclosed in 6.6 kV pole transformers and switchgear. The advanced arresters have been reduced in volume by 25% compared with conventional ones. This paper describes the specification and construction of the advanced arresters and the results of various evaluation tests     

Relation of insulation resistance and deteriorated ratio ofreference voltage of oil-immersed surge arrester elements enclosed inpole transformers

About ten years have passed since oil immersed surge arrester elements enclosed in 6.6 kV pole transformers were first used. At present, technical developments are being demanded to judge the soundness of these oil immersed surge arrester elements by a simple method, allowing one to judge easily whether these elements can be used or not onsite. The authors empirically investigated the correlation between the insulation resistance and the deteriorated ratio of the reference voltage of these oil-immersed surge arrester elements. It was found that the deterioration of the oil-immersed surge arrester elements can be detected by measuring the insulation resistance     

Coordination of surge arresters with medium-voltagecurrent-limiting fuses

Current-limiting fuses (CLFs) are commonly used for unit substation primary protection. Simultaneously, surge arresters are also provided on the primary side of the transformers, more so for the dry-type transformers. The arc voltages generated by CLFs will store energy in the inductive elements of a distribution system. This stored energy divides between the CLF and surge arrester, and the surge arrester may operate. This paper outlines a procedure to calculate the energy diverted to the surge arresters and shows that, in some cases, surge arresters may fail on operation of the CLFs. Guidelines are developed for selection and application of the surge arresters at 13.8-, 4.16-, and 2.4-kV voltage levels     

Evaluation of Breakdown Characteristics of Oil-immersed Transformers under Non-standard Lightning Impulse Waveforms - Method for Converting Non-standard Lightning Impulse Waveforms into Standard Lightning Impulse Waveforms

To lower the insulation specifications (specifically, the lightning impulse withstand voltage) of oil-immersed transformers and thus reduce equipment cost while maintaining high insulation reliability, it is required to identify the insulation characteristics under non-standard lightning impulse waveforms that are associated with actual surge waveforms in the field and quantitatively compare them with the characteristics under the standard lightning impulse waveform. In the previous research, field overvoltages in the lightning surge time region were analyzed, and four typical non-standard lightning impulse waveforms were defined. These four waveforms were used to measure the breakdown voltages and the partial discharge inception voltages on three models of the winding insulation elements of oil-immersed transformers. The average breakdown voltages were evaluated in terms of the overvoltage duration. This paper describes a method for converting of non-standard lightning impulse waveforms into standard lightning impulse waveforms with equivalent stress for the insulation. The constructed algorithm was applied to four examples representing two types of non-standard lightning waveforms. Due to the conversion into standard lightning impulse waveforms, the crest values were reduced by 14% to 26%. This seems to be a potential for reduction of lightning impulse insulation specifications of oil-immersed transformers.     

Mechanism of pole‐mounted transformer damage by backflow lightning and measures against the damage on low‐voltage distribution line

Pole‐mounted transformers are especially vulnerable to lightning damage. The progress of the information society imposes increasingly stringent requirements for the reliability of electric power supply, and this in turn necessitates a reduction in lightning damage to pole‐mounted transformers. Lightning protective devices (surge arresters) are now being installed around the primary bushing of the transformers, which has decreased the number of disconnections around the primary bushing due to lightning. But surge arresters installed on the primary side of the transformer cannot protect it against backflow lightning entering the secondary side of the transformer. The characteristic of transformer damage by backflow lightning is that the electromagnetic force produced by the current flowing into the secondary side deforms the transformer windings. This paper elucidates the mechanism of transformer damage by lightning flowing into the secondary side by comparing actual lightning damage cases with the results of verification tests using a short‐circuit generator. Effective countermeasures against transformer damage by backflow lightning are examined by EMTP calculations, which indicate that neutral grounding on the low‐voltage distribution line is the most effective way of decreasing the current flowing into the transformer. The lower the grounding resistance, the less current flows into the transformer. In addition, decreasing the voltage on the secondary side is important in order to protect the secondary‐side bushing. The calculation results indicate that surge arresters installed around the secondary side of the transformer are effective in decreasing the voltage on the secondary side. © 2010 Wiley Periodicals, Inc. Electr Eng Jpn, 171(2): 1–11, 2010; Published online in Wiley InterScience ( www.interscience. wiley.com ). DOI 10.1002/eej.20921     

Surge protection of high voltage shunt capacitor banks on AC powersystems survey results and application considerations

A survey of power utilities has been carried out to obtain data on high-voltage shunt capacitor bank surge protection practices which presently exist. The questionnaire sought answers to questions relating to voltage rating, size of bank, number of banks, installation practices, switching arrangements, and the application of surge arresters for overvoltage protection. A summary of the results of the survey is presented along with some general comments and conclusions on the findings. The high rate of return on the survey indicates the widespread interest of utilities in surge arrester protection of high voltage shunt capacitor banks. It does not appear to be common practice among electric utilities to apply surge arresters to shunt capacitor banks for overvoltage protection. Where surge arresters are applied, they are often used to protect other equipment, such as transformers and circuit breakers, against overvoltages during capacitor switching     

Impact of surge arrester number and placement on reliability and lightning overvoltage level in high voltage substations

Surge arresters are the most critical equipment for protecting high voltage substations. They play an important role in substations for limiting switching and lightning surges and diverting these surges to ground. On the other hand, surge arrester number and placement for high voltage substations can be determined based on some evaluations in the designing process of substations. Surge arresters can be placed on the both ends of substations, transformers, circuit breakers, reactors, capacitors and also high long bus-bars and etc. Therefore, failure of arresters during overvoltage can put substations in risk condition. Moreover, surge arresters may be inclined to be short circuit during normal operation condition due to ageing process and/or improper quality. This paper attempts to assess reliability of three common substation configurations namely: (1) one breaker and a half; (2) double-bus double-breaker; and (3) ring bus-bar in different placement of surge arresters. At first, maximum voltages on equipment are calculated in different lightning stroke locations through simulation in EMTP-RV. Studies without surge arrester and the presence of surge arrester in different locations are analyzed and compared. Then surge arrester’s placement impacts on the substations reliability indices are calculated in normal operating condition and overvoltage condition by minimal cut set method and simulation results. Analytical studies reveal that surge arrester can increase substations reliability. But for low annual number of lightning stroke, substation reliability may decrease. Also increasing surge arrester number more than substation need reduces reliability.     

Analytical Method for Coordination of Surge Arresters with Current-Limiting Fuses

Many industrial power systems have lightning exposure, requiring surge (lightning) arresters; dry-type transformers, requiring low protective levels; and high available fault currents, making the use of current-limiting fuses desirable. On occasion, current-limiting fuse arc voltages have resulted in destruction of low characteristic arresters. A traditional guideline has been to select arrester types and ratings that will not spark over on current limiting fuse maximum arc voltage?an approach that may not be entirely viable for industrial systems. A step-by-step analytical approach to the selection of surge arresters for use with current-limiting fuses is presented. The method presumes arrester sparkover and is based on determination of system energy, fuse arc voltage and arrester back voltage characteristics, and arrester energy capability.     

Evaluation of breakdown characteristics of oil-immersed transformers under non-standard lightning impulse waveforms - definition of non-standard lightning impulse waveforms and insulation characteristics for waveforms including pulses

To lower the insulation specifications (specifically, the lightning impulse withstand voltage) of oil-immersed transformers and thus cut the equipment cost while maintaining the high reliability in its insulation performance, it is necessary to grasp in an organized way the insulation characteristics under non-standard lightning impulse voltage waveforms that represent actual surge waveforms encountered in the field and compare them with the characteristics under the standard lightning impulse waveform quantitatively. As described in this paper, the first step in a series of study for the purpose above was taken by analyzing lightning surge waveforms and restriking surge waveforms such as disconnector switching surge waveforms at UHV, 500 kV, and 275 kV substations and identifying four typical non-standard lightning impulse waveforms with basic frequencies of 0.24 to 1.0 MHz. Then, two of these non-standard lightning impulse waveforms, the single-pulse waveform which is the most basic type and the waveform with a pulse in the crest and a subsequent flat section, were used to measure the breakdown voltage and the partial discharge inception voltage while changing the parameters, on three models that represent the insulation elements of windings of oil-immersed transformers. Then, the resultant average breakdown voltages were evaluated in terms of the overvoltage durations, leading to a result of formulating them in a unified way. In the tested range, the dielectric breakdown values under non-standard lightning impulse waveforms were higher, marking 52% at the maximum, than those under standard lightning impulse waveforms in all the cases, suggesting a possibility of lowering the insulation specifications of an oil-immersed transformer     

Transmission line arrester energy, cost, and risk of failureanalysis for partially shielded transmission lines

Application of metal oxide surge arresters in power systems has been traditionally linked to electrical equipment protection. The industry has noted a very significant increase in the application of metal oxide arresters on transmission lines in an effort to reduce lightning initiated flashovers. This paper describes a cost-effective installation of surge arresters on a partially shielded transmission line. It compares several options with respect to cost and gains in terms of lightning performance improvement. This paper also presents a new approach to calculate risk of failure of transmission line surge arresters (TLSAs) due to lightning strokes to towers, shield/phase conductors, and illustrates the method with an example for a partially shielded line. Results show that the risk of failure is very small. The installation of TLSAs completed in 1997 demonstrated that it is possible to afford adequate lightning protection levels by selectively applying surge arresters only to the towers most sensitive to backflashover and shielding failures     

Protection of Underground Systems Using Metal-Oxide Surge Arresters

The application of metal-oxide surge arresters on underground systems, and their advantages at the riser pole and system-open points, is shown for arresters now available as well as those anticipated based on current technology. Improvements in protective margins are shown for each type of arrester discussed, with both livefront and deadfront applications addressed.     

Improved zinc oxide surge arresters using high voltage gradient 300V/mm, 400 V/mm ZnO elements

Zinc oxide surge arresters using zinc oxide (ZnO) elements have been widely used for insulation coordination in the world's power systems. These ZnO elements have basically reference voltage of about 200 V/mm. Recently, new ZnO elements having about 1.5, 2 times high voltage gradient zinc oxide element have been developed. This paper describes applications of high voltage gradient 300 V/mm ZnO elements to high performance porcelain type surge arresters and oil immersed surge arresters, and 300 V/mm or 400 V/mm ZnO elements to gas insulated tank type surge arresters for power systems. Adequate selection of voltage gradient of ZnO elements makes more compact design of the surge arresters     

变电站在线监测技术的应用研究

通过对变电站的在线监测方法进行分析,简要介绍了针对变压器、断路器、避雷器、电缆、绝缘子等设备常用的在线监测方法,指出各种方法的优缺点以及发展趋势,从而揭示了目前变电站在线监测方法的研究重点。     

110、220kV城网GIS中ZnO避雷器配置的研究

研究了110、220kV城网GIS中ZnO避雷器的配置和接线方式.结果表明:1)线路入口避雷器应布置在侵入波前沿,并采用V形连接.为此建议采用合成套ZnO避雷器悬挂在线路终端塔上.2)侵入波过电压水平随进出线电缆长度而变化的关系,因电压等级、网络结线、线路结构和参数的不同而差异较大,电缆末端及GIS是否需加装避雷器,宜通过数值计算和具体分析后确定.     

特高压换流站输电设备概述

研究表明,拟建中的800kV特高压换流站采用12脉动换流技术,换流单元有单极单换流器与单极双换流器串联两种选择,其关键影响因素是换流变压器的容量。由于结构复杂,换流变压器故障率是电力变压器的两倍,阀绕组与地以及交流绕组之间的主绝缘结构是特高压换流变压器设计的难点。连接于极线的输电设备主要有平波电抗器、滤波电容器、直流电压分压器、光学电流传感器、直流避雷器等。对于特高压而言,其最主要的问题是由于污秽引发外绝缘闪络以及由于电压分布变化导致的内绝缘故障。     

Insulation coordination for delta connected transformers

It is common practice to install only phase to ground surge arresters for transformer insulation protection against electrical stresses. However, this may not adequately protect the delta connected transformer phase to phase winding insulation. High phase to phase overvoltages at the transformer terminals, due to capacitor switching, have been reported. This paper describes the excessive phase to phase overvoltages that can also occur due to wave propagation and surge transfer through transformers. This paper points out the need for surge arrester arrangements that protect both phase to phase and phase to ground transformer insulations. Surge arrester arrangements, selection criteria, installation practices, and application considerations are discussed in this paper     

Light rail transit DC traction power system surge overvoltageprotection

At present, there are no industry standards or recommended guidelines for the application of DC surge arresters. It appears that such devices are applied based upon a rule of thumb without the benefit of analyzing characteristics, wave shape, and energy content of the expected overvoltage surges against the capabilities of the surge arrester. AC surge arresters are rerated for DC application. A difficulty arises due to a lack of clear published application literature on DC surge arresters, as well as lack of measured data on DC power system transient surges. This leads to the application of surge protective devices, which may be inappropriate. This paper describes the application of DC surge arresters to a typical light rail transit traction power system, and it makes a recommendation for the need of a standard on DC surge arresters test requirements and application guidelines     

一种氧化锌避雷器阻性电流的提取方法

氧化锌避雷器作为电力输变电系统的一个核心保护设备,必须进行在线状态监测。文章在研究氧化锌避雷器常用的在线监测方法的基础上,提出了一种基于谐波分析法的新的氧化锌避雷器阻性电流提取方法。该方法采用准同步DFT谐波分析线性修正算法来实现氧化锌避雷器的阻性电流的测量。仿真实验和现场实验表明,该方法可行、有效,有极高的应用价值。     

Suppression of lightning-induced overvoltages by distribution line surge arresters

The primary aim of surge arresters in power distribution lines is to protect lines and equipment from the voltage induced by nearby lightning strokes. To further improve power systems, methods to protect distribution lines against direct lightning strokes are still needed. An effective measure against direct lightning strokes is to increase the number of arresters. However, if the surge current is too large, some surge arresters absorb energy in excess of their capability and may break; this leads to a line fault. To evaluate the protective effect of the surge arresters against direct lightning strokes to overhead ground wire, the authors measured both the voltage across the surge arresters and the energy absorbed by them using a full-scale model line and a 12 MV impulse generator. The results were compared with simulation results by EMTP. There have been no previous studies making a comparison of this kind.     

Effect of overhead ground wires on reduction of failure rates of surge arresters on power distribution lines

Surge arresters are sometimes damaged by lightning strokes with high energy, in spite of installation of conventional protection methods. In order to reduce the number of failures of surge arresters, we should consider another protection method, such as an increase in the withstand capability of surge arresters and installation of additional overhead ground wires. However, quantitative comparison of these methods for preventing damage to surge arresters against lightning strokes with high energy has never been performed. This paper describes the effects of additional overhead ground wires for preventing damage to surge arresters. The main results are: (1) The failure rate of surge arresters on a distribution line with two overhead ground wires is about one third of that with an overhead ground wire. (2) Installation of a second overhead ground wire six to eight spans from the end of a line reduces the failure rate of a surge arrester at the end as effectively as increasing the withstand capability by a factor of two. © 1998 Scripta Technica. Electr Eng Jpn, 122(2): 12–20, 1998