Evaluation of breakdown characteristics of gas insulated switchgears for non-standard lightning impulse waveforms - method for converting non-standard lightning impulse waveforms into standard lightning impulse waveforms -

To lower the lightning impulse withstand voltage of gas insulated switchgear (GIS) while maintaining the high reliability of its insulation performance, it is important to define in an organized way the insulation characteristics for non-standard lightning impulse voltage waveforms that represent actual surge waveforms in the field and compare them with the characteristics for the standard lightning impulse waveform quantitatively. In the preceding researches, lightning surge waveforms and disconnector switching surge waveforms at UHV, 500 kV, and 275 kV substations were analyzed and five to six kinds of non-standard lightning impulse waveforms with basic frequencies of 0.6 to 5.0 MHz were identified. Then, the dielectric breakdown voltage ? time characteristics were measured under several different conditions on the quasi-uniform SF6 gas gaps and partly the coneshaped insulating spacers that represent insulation elements of GIS for six kinds of nonstandard lightning impulse waveforms. In this paper, the resultant breakdown voltages were evaluated in terms of the overvoltage duration, which led to their formulation in a unified way. On the basis of these insulation characteristics and their unified formulation, the paper investigated a method for converting non-standard lightning impulse waveforms into standard lightning impulse waveforms with equivalent stress for the insulation. When the constructed algorithm was applied to five examples of representative two type waveforms in the lightning surge time region, they were converted into standard lightning impulse waveforms with crest values reduced by 20% to 34%, suggesting potentiality for reduction of lightning impulse insulation specifications of GIS.     

Evaluation of breakdown characteristics of gas insulated switchgears for non-standard lightning impulse waveforms - analysis and generation circuit of non-standard lightning impulse waveforms in actual field

To lower the insulation specifications (specifically, the lightning impulse withstand voltage) of a gas insulated switchgear (GIS) and thus cut the equipment cost while maintaining the high reliability of its insulation performance, it is necessary to define in an organized way the insulation characteristics for non-standard lightning impulse voltage waveforms that represent actual surge waveforms in the field and compare them with the characteristics for the standard lightning impulse waveform quantitatively. In this paper, first, lightning surge waveforms and disconnector switching surge waveforms at UHV, 500 kV, and 275 kV substations were analyzed and five non-standard lightning surge waveforms with basic frequencies of 0.6 to 5.0 MHz were identified. Next, high-voltage circuits that generate these non-standard lightning surge waveforms were designed and constituted using EMTP (electro magnetic transients program) based on a circuit with a gap, inductors, and resistors connected in series and resistors and capacitors connected in parallel. Further, circuits were actually constructed, to obtain voltage waveforms approximately equal to those designed. Finally, the dielectric breakdown voltage-time characteristics were measured under several different conditions for the quasi-uniform SF₆ gas gap that represents an insulation element of a GIS. As a result, it was found that, in the tested range, the dielectric breakdown values for non-standard lightning impulse waveforms were higher than for the standard lightning impulse waveform by 6% to 32%     

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.     

Evaluation of Breakdown Characteristics of Gas Insulated Switchgears for Non-Standard Lightning Impulse Waveforms - Breakdown Characteristics for Non-Standard Lightning Impulse Waveforms Associated with Lightning Surges

To lower the insulation specifications (specifically, the lightning impulse withstand voltage) of a gas insulated switchgear (GIS) and thus cut the equipment cost while maintaining the high reliability of its insulation performance, it is necessary to define in an organized way the insulation characteristics for non-standard lightning impulse voltage waveforms that represent actual surge waveforms in the field and compare them with the characteristics for the standard lightning impulse waveform quantitatively. In the preceding paper, lightning surge waveforms and disconnector switching surge waveforms at UHV, 500 kV, and 275 kV substations were analyzed and five to six non-standard lightning surge waveforms with basic frequencies of 0.6 to 5.0 MHz were identified. In this paper, the dielectric breakdown voltage - time characteristics were measured under several different conditions mainly for the quasi-uniform SF₆ gas gaps that represent an insulation element of a GIS toward four kinds of non-standard lightning impulse waveforms associated with lightning surges. As a result, in the tested range, the dielectric breakdown values for nonstandard lightning impulse waveforms were higher than for the standard lightning impulse waveform by 3% to 32%.     

Dielectric characteristics of oil‐filled transformer in the presence of nonstandard lightning surge waveforms

To achieve a rational insulation design for transformers, it is important to evaluate dielectric strength against surges actually impinging on equipment on‐site. This paper deals with the breakdown voltage characteristics of an oil gap under nonstandard lightning surge waveforms combined with oscillatory voltages. It is found that the breakdown voltages of the oil gap under nonstandard impulse waveforms are higher than standard lightning impulse voltages. The results can be ascribed to V–t characteristics of the oil gap in short‐time impulse voltage ranges. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 146(3): 39–45, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10229     

Insulation characteristics and its evaluation method of N2 gas for nonstandard lightning impulse waveforms

Until recently, SF₆ gas has widely been used as the best insulating medium in substation equipment. However, SF₆ gas was specified to be a greenhouse gas at COP3 in 1997 because of its high global warming potential (GWP), and alternative insulation gases to SF₆ have been sought for a long time. Alternatives using the natural gases are considered to be suitable, but none of them show better properties for insulation as well as good environmental compatibility. Therefore, it is necessary to rationalize the equipment insulation level and reduce the test voltage of electric power apparatus to as low a level as possible. The actual lightning surge waveform (so‐called nonstandard lightning impulse waveform) occurring in the actual field is different from the standard lightning impulse waveform (1.2/50 μs). There are many cases in which the actual lightning surge waveform has a steep rise and large decay of overvoltage, and the insulation requirements are not as severe as those for the standard lightning impulse waveform. In this paper, we focused our research on N₂ gas as an SF₆ substitute and investigated the insulation characteristics of N₂ gas for a single‐frequency oscillatory waveform with various frequencies from 2.7 to 20.0 MHz and damping ratios. Based on the experimental results, it might be possible to reduce the test voltage of N₂ gas insulation by evaluating the crest value of the actual lightning surge waveform that has been converted into an equivalent standard lightning impulse waveform. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 167(2): 10–20, 2009; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20754     

Breakdown characteristics of gas‐insulated switchgear for nonstandard lightning impulse waveforms under nonuniform electric field

To improve the insulation specification of gas‐insulated switchgear (GIS), it is necessary to recognize the insulation characteristics of SF₆ gas during actual surges (called nonstandard lightning impulse waveforms) occurring at field substations. The authors observed the insulation characteristics of SF₆ gas gap under various types of nonstandard lightning impulse waveforms and compared them quantitatively with those obtained with standard lightning impulse waveforms. The experimental results were used to derive an evaluation method for real surges, which was applied to typical surges for various UHV and 500‐kV systems. In the preceding study, therefore, only the case of a quasi‐uniform electric field (with a typical range of field utilization factors in the bus of a GIS) was investigated. In the present investigation, the insulation characteristics of an SF₆ gas gap for a nonuniform electric field were observed experimentally and an evaluation method for converting nonstandard lightning impulse waveforms equivalently to the standard lightning impulse waveform was investigated. © 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 177(1): 11–18, 2011; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/eej.21144     

特高压变压器雷电冲击伏秒特性研究

随着750 kV、1000 kV输电技术的发展,相应的电力变压器和并联电抗器的容量、尺寸和入口电容随之增大,试验回路尺寸亦相应扩大,这使雷电冲击试验电压的波前时间拉长,无法达到国内外标准的要求。根据500 kV、750 kV和1000 kV变压器和电抗器的实际雷电冲击试验波形,结合油纸复合绝缘结构的雷电伏秒特性,分析了不同波前时间对特高压变压器和电抗器绝缘水平的影响。目前变压器的设计计算和试验电压的选取一般按照标准波头进行,而充油设备的雷电冲击伏秒特性表明,雷电冲击试验电压波前时间的长短与绝缘强度有密切关系,波前时间延长可能会对某些纵绝缘的考核偏松,同时对主绝缘的考核偏严。因此,应在特高压变压器、电抗器的设计研制和试验中,考虑和重视雷电冲击波形波前时间延长所带来的影响。     

Insulation characteristics of SF6 gas for nonstandard impulse voltage polarity reversal pulse waveforms

Evaluation of insulation strength for lightning surge that actually enters into substations is important in estimating insulation reliability of gas‐insulated equipment. The standard lightning impulse voltage (1.2/50 µs) is used for factory tests. However, the actual lightning surge waveforms in substations are complex and are usually superimposed with various oscillations. Insulation characteristics of SF₆ gas as a function of such complex voltages have not been sufficiently clarified. This paper deals with gap breakdown characteristics in SF₆ gas under submicrosecond pulses. Breakdown voltages are lower under a polarity reversal condition than under a monopolarity condition. The cause of this difference is discussed while observing discharge propagation using an image converter camera. The electrode size effect is also discussed. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 146(4): 18–25, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10246     

Insulation characteristics of CO2 gas for nonstandard lightning impulse oscillations: Evaluation method of nonstandard lightning impulse waveform for CO2 gas insulation

SF₆ gas is widely used in electric power apparatus such as gas‐insulated switchgears (GIS), because of its superior dielectric properties; however, it has been identified as a greenhouse gas at COP3 in 1997, and alternative insulation gases to SF₆ have recently been investigated. One of the candidates is CO₂ gas, which has lower global warming potential (GWP). However, CO₂ gas has a lower withstand voltage level than SF₆ gas; therefore, it is necessary to rationalize the equipment insulation level and reexamine the insulating test voltage for electric power apparatus as low as possible. From our previous investigation, in SF₆ gas insulation system, we obtained that the insulation requirements of the real surges (called nonstandard lightning impulse waveform) are not as severe as those of the standard lightning impulse waveform. This paper describes the evaluation method for real surges, based on insulation characteristics of CO₂ gas gaps. Furthermore, the method was applied to typical field overvoltage waveform in the lightning surge time region for 500‐kV systems and it is obtained that the equivalent peak value of the standard lightning impulse waveform is possibly reduced by 10 to 15%. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 163(3): 1– 9, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20560     

The electrical breakdown characteristics of oil-paper insulationunder steep front impulse voltages

Disconnecting switch operations in gas insulated equipment cause transient voltages with risetimes as steep as 5 to 20 nanoseconds and magnitudes as high as 2.5 pu. There is very little information on the effect of these transients on oil-paper insulated equipment. There have been reports, however of transformer and bushing failures caused by these transients. The electrical breakdown characteristics of oil-paper insulation under steep front impulse were studied in this project, which was co-sponsored by the Canadian Electrical Association and BC Hydro. V ₅₀ (50% breakdown probability voltage) breakdown data was obtained with steep front (10 ns/2500 μs), lightning and switching impulse waveforms. Insulation breakdown voltage vs breakdown time (V-t) data and multiple impulse breakdown data were obtained with the steep front impulse waveform. The V₅₀ results showed that the breakdown strengths were lower for steep front impulses than for lightning impulses. The multiple impulse breakdown results showed that oil-paper insulation breakdown strength can be lower than 100 kV/mm. These results are alarming, since they suggest that oil-paper insulated equipment subjected to steep front transients will fail at voltages below the lightning impulse design level (BIL). The volt-time data had a discontinuity. The breakdown process at risetimes below about 50 ns was different from the breakdown process at risetimes above 50 ns     

直流电缆雷电冲击电压安全裕度研究

高压直流电缆是柔性直流输电技术的关键装备,直流电缆的绝缘厚度设计以雷电冲击电压安全裕度为重要依据。为了获得直流电缆的安全裕度,文中提出了直流电缆雷电冲击电压安全裕度试验方法,建立了相应的试验回路,对冲击电压波形参数进行了计算、仿真和验证,获得了波前时间为1~5μs、半峰值时间为40~60μs的冲击电压波形;提出了直流电缆雷电冲击电压安全裕度的分析方法,计算了电缆绝缘的电场分布。研究表明,文中所提方法可以获得直流电缆的雷电冲击电压安全裕度,为高压直流电缆绝缘厚度设计提供了依据。     

冲击电压下500 kV油浸倒置式电流互感器局部放电特性研究

为了研究油浸式电流互感器在运行过程中的局部放电特性,搭建500 kV油浸倒置式电流互感器在冲击电压下的局部放电试验回路,对试品施加标准雷电冲击电压和操作冲击电压,使用安装在试品末屏接地引下线处的非接触式高频局部放电传感器采集局部放电信号,通过高采样率示波器显示放电波形。在冲击试验后进行工频局部放电测量、油中溶解气体成分分析、高电压介质损耗因数测试、频域介电谱测试,以检测试品的绝缘状况。研究结果表明:油浸式电流互感器在冲击电压下发生局部放电,绝缘损坏,放电熄灭电压低于正常运行电压,放电无法自熄灭,长期累积作用造成设备故障。     

AC V-t and impulse V-N characteristics of shell-form transformer insulation model

To evaluate an appropriate level of ac test voltage for 1000-kV transformers, it is necessary to make an accurate estimation of ac V-t characteristics. For the level of impulse test voltage, V-N characteristics are important to evaluate effects of impulse stress repetition. Since both of the characteristics are dependent on insulation structures and manufacturing processes of transformers, it is important to make accurate simulations on transformer insulation for reliable insulation data. To obtain some of the basic data for 1000-kV transformer test levels, long-time ac V-t tests and impulse V-N tests of up to 1000 times voltage application were carried out on an insulation model simulating fundamental structure of shell-form transformers. The model is composed of a parallel electrodes system with a compound structure of pressboard barrier and oil gap. The tests showed the following characteristics: ac V-t characteristics of up to several ten days could be described by two lines, each representing short-time and long-time characteristics. V-N characteristics of lightning and switching impulse were expressed by relatively flat lines, which showed a decrease in breakdown voltage by less than 10 percent after 1000 times impulse application.     

V-t characteristics of SF6 gas during lightning surges

The waveform of a standard lightning impulse differs greatly from those of actual lightning surges acting on GIS. This raises the problem of the equivalence of the standard lightning impulse. This report describes the effect of voltage waveforms on insulating performance in an SF₆ gas gap subjected to fast oscillating impulse voltages simulating actual lightning surges, and the evaluation of V-t characteristics by applying the equal area criterion. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 119(4): 1–11, 1997     

Review of research on nonstandard lightning voltage waves

Although the impulse insulation level of a power apparatus is specified under the standard 1.2×50-μs voltage wave, the apparatus seldom encounters such a voltage wave in the field. A wealth of knowledge exists on the behavior of air gaps under nonstandard lightning impulse voltages. The objective of the Task Force was to compile a bibliography on the nonstandard lightning impulse voltages and review the present state of research. This paper reviews the present status of research including the behaviour of air gaps under transient voltages, and the generation and measurement of fast-front impulse voltage waveforms     

Resonant behavior of EHV transformer windings under system originated oscillatory transient over voltages

Grid connected EHV transformers experience various terminal disturbances when in service. The present work attempts to investigate the voltage stresses that may develop on the transformer insulations under a variety of terminal disturbances. A number of standard and non-standard wave shapes like lightning impulse, chopped lightning impulse, steep-front long tail switching surge and oscillatory transient over voltages have been simulated and impressed on the terminals of a 400 kV EHV power transformer operating in the Indian power grid to ascertain how the winding insulations are stressed under these disturbances. Relevant section of the Indian power grid and the transformer has been modeled using Alternative Transient Program (ATP). It has been established that oscillatory system transients can trigger natural resonate frequencies of the transformers causing high voltage stresses on the insulations. Short Time Fourier Transform (STFT) analysis of the oscillatory voltage response of the windings confirmed the presence of resonant frequencies indicating forced resonance. Some remedial measures involving winding design modifications have been suggested in the paper to overcome the problem.     

Results of lightning impulse tests for a 275‐kV full GIS substation

This paper describes the outline and results of the lightning impulse tests for a 275‐kV full GIS substation. The behavior of lightning surges is a very important factor for the rational design of substations and low‐voltage and control circuits inside the substations. For the above reason, we carried out lightning impulse tests for a new 275‐kV full GIS substation. In these tests, we measured voltage induced in low‐voltage and control circuits, transient characteristics of grounding grid, injected voltage and current waveforms, and so on. By investigating these data, we confirmed the behavior of lightning surges inside the substation. We also compared simulated waveforms by EMTP analysis with the measured waveforms. The simulated results agreed well with the measured results. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 146(1): 46–58, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10230     

Characteristics of lightning surges observed at 77 kV substations

In order to improve power supply reliability, it is necessary to prevent lightning faults in transmission lines and substation apparatus. However, faults are caused occasionally in lower-voltage power systems, particularly at the 77 kV level. The governing factor for insulation strength of substation apparatus is the lightning impulse voltage, and it is necessary to know the voltage level and distribution in a substation caused by lightning surges in order to investigate rational insulation coordination. For this purpose, the authors measured lightning surges at two 77 kV conventional substations from 1990 to 1993. In this paper, the characteristics of induced lightning surges and back flashover lightning surges are described. Comparisons of related surge voltages at two substations, the power line phases in grounding faults, and the equivalent capacitance of the substations are also discussed.     

冲击电压下CVT传递特性及其缺陷故障检测研究

电容式电压互感器（CVT）内部存在绝缘缺陷时,其整体运行状态不会出现明显变化,但CVT频率响应特性会发生改变。为准确检测出CVT内部的绝缘缺陷,通过获取CVT宽频电压传递函数进行CVT绝缘缺陷故障检测。搭建了冲击电压试验平台,通过雷电冲击、操作冲击和振荡操作冲击3种冲击波形,研究了不同类型冲击电压波形下的CVT频率响应特性。研究结果表明,由3种不同类型冲击电压获取的CVT电压传递函数主要参数特征基本一致,在主电容充电电压相同的情况下,振荡操作冲击电压可以提高输出电压,高效地获取CVT宽频电压传递函数。通过对比不同绝缘缺陷条件下由振荡操作冲击电压获取CVT电压传递函数的差异,为利用冲击电压频率响应特性检测CVT内部绝缘缺陷提供了支撑。