Lightning Impulse Waveforms Research Articles

Evaluation of breakdown characteristics of CO2 gas for non-standard lightning impulse waveforms - breakdown characteristics in the presence of bias voltages under non-uniform electric field

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 previous work, the dielectric breakdown voltage - time characteristics were measured under several different conditions on the quasi-uniform SF6 gas gap and partly the cone-shaped insulating spacers that represent an insulation element of GIS for six kinds of non-standard lightning impulse waveforms associated with lightning surges and disconnector switching surges. This paper, describes detailed examination in non-uniform electric field, and the pattern in which a disconnector switching surges are superposed on residual DC elements generated through the operation of disconnectors (bias voltages). This is intended to simulate more severe conditions in an actual system. Consequently, it was found that the presence of a bias voltage in the nonuniform electric field resulted in lower breakdown voltage values for single-frequency oscillation waveforms both in the positive and negative polarities. However, the breakdown voltage values under positive-polarity waveforms posing more severe insulation conditions were higher than the ones under the standard lightning impulse waveforms. Despite the presence of a bias voltage, it was possible to evaluate these breakdown voltage values in the positive polarity using the concept of a duration examined in the past.

Experimental study of hard x-rays emitted from metre-scale positive discharges in air

We investigate structure and evolution of long positive spark breakdown; and we study at which stage pulses of hard x-rays are emitted. Positive high-voltage pulses of standardized lightning impulse wave form of about 1 MV were applied to about 1 m of ambient air. The discharge evolution was imaged with a resolution of tens of nanoseconds with an intensified CCD camera. LaBr3(Ce+) scintillation detectors recorded the x-rays emitted during the process. The voltage and the currents on both electrodes were measured synchronously. All measurements indicate that first a large and dense corona of positive streamers emerges from the high-voltage electrode. When they approach the grounded electrode, negative counter-streamers emerge there, and the emission of hard x-rays coincides with the connection of the positive streamers with the negative counter-streamers. Leaders are seen to form only at later stages.

Evaluation of breakdown characteristics of n2 gas for non-standard lightning impulse waveforms - breakdown characteristics under double-frequency oscillation waveforms and pressure-distance characteristics

SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas, an insulation medium used for gas insulated switchgear (GIS), is known for its high global warming potential, hence the search for an effective alternative from an environmental perspective. As a potential alternative, the authors are focusing on N2 gas. To use this N2 gas for actual GIS, the insulation characteristics for actual overvoltage waveforms generated in the field (non-standard lightning impulse waveforms) must be obtained. To do so, the earlier study experimentally obtained and evaluated the insulation characteristics for single-frequency oscillation waveforms as a representative among nonstandard lightning impulse waveforms under the quasi-uniform electric field, which was considered to be a basis for the insulation design of GIS. The present study obtained the insulation characteristics for double-frequency oscillation waveforms to investigate surge waveforms generated in substations in more detail. Consequently, it emerged that negative polarity waveforms were more severe in terms of insulation, as was the case for single-frequency oscillation waveforms. The insulation performance was consistently higher than that for the standard lightning impulse waveforms, which were 1.08 to 1.09 times. In other words, it was confirmed that the insulation specification could be rationalized by about 10%, even for complex oscillation waveforms, such as those of double-frequency oscillation. Furthermore, a study was conducted on the influence of the gas pressure and gap length for single-frequency oscillation waveforms. Consequently, it emerged that the results under the basic experimental conditions were likely to be applicable to actual GIS under wider-ranging conditions.

Evaluation of breakdown characteristics of CO2 gas for non-standard lightning impulse waveforms - breakdown characteristics under double-frequency oscillation waveforms and single-frequency oscillation waveforms in the presence of bias voltage

SF6 gas, an insulation medium used for gas insulated switchgear (GIS), has a high global warming potential, hence the search for an effective alternative is required from an environmental perspective. The authors focus particularly on CO2 gas, which has a low global warming potential, as one of these potential alternatives. In order to apply this CO2 gas to actual equipment, the insulation characteristics for overvoltage waveforms generated in an actual field (called non-standard lightning impulse waveforms) must be obtained. Accordingly, in previous studies, the insulation characteristics for single-frequency oscillation waveforms were experimentally obtained in the quasi-uniform electric field, which is the basis when performing the insulation design of GIS, and its evaluation method was examined. In this paper, to further elaborate the evaluation method established thus, the insulation characteristics in relation to complex oscillatory waveforms generated in actual substations, e.g. double-frequency oscillation waveforms and single-frequency oscillation waveforms on which a dc component (bias voltage) that remains due to the superimposed operation of a switching device, were obtained. Consequently, the results obtained indicated that negative polarity waveforms were more severe in terms of electrical insulation under the double frequency oscillation waveforms as well as under the single-frequency oscillation waveforms. Therefore, when considering a dielectric strength, it is reasonable to conduct experiments using negative-polarity waveforms for the complex oscillatory waveforms, such as the double frequency oscillation waveforms. Furthermore, a study was performed on a method of evaluating insulation characteristics for non-standard lightning impulse waveforms of the CO2 gas gap via a comprehensive approach using the results including the previous ones. As a result, as in the case of studies conducted previously on SF6 gas, the insulation characteristics, including complex oscillatory waveforms generated in actual power systems, could be expressed by a single characteristic line by using the parameter of duration.

Evaluation of breakdown characteristics of N2 gas for non-standard lightning impulse waveforms under non-uniform electric field - breakdown characteristics for single-frequency oscillation waveforms -

SF6 gas has been widely used in electrical power equipment such as gas insulated switchgear (GIS) due to its superior insulation and interruption characteristics. However, in 1997, SF6 gas has been designated a greenhouse gas subject to emission restrictions at COP3 (The 3rd session of the Conference Of the Parties to the United Nations Framework Convention on Climate Change). Therefore, it is important to study the possibility of using environmentally friendly gases as alternative insulation gases for practical gas-insulated apparatuses, such as GIS. In the selection of an alternative gas, it is thought that high-pressure natural gases, such as Air, N2, and CO2 are promising environmentally friendly candidates. The authors are focusing on N2 gas as one of its potential alternatives. In order to use this N2 gas for actual GIS, the insulation characteristics for actual overvoltage waveforms generated in the field (nonstandard lightning impulse waveforms; non-standard-LIWs) must be obtained. The present study, with single-frequency oscillation waveforms as a representative basis, experimentally obtained the insulation characteristics by changing the polarity, frequency, and damping rate of the applied voltage in the non-uniform electric field represented by the presence of metallic particle in the equipment. As a result, the breakdown voltage was lower for the positive polarity under the non-uniform electric field. In this polarity, the breakdown voltage was 1.06 to 1.74 higher than that by standard lightning impulse waveforms even if the frequency and damping rate were changed. These results support the rationalization of insulation for actual surge waveforms under the quasi-uniform electric field.

Evaluation of breakdown characteristics of N2 gas for non-standard lightning impulse waveforms - breakdown characteristics under single-frequency oscillation waveforms and with bias voltages

SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas, an insulation medium used for gas insulated switchgear (GIS), has a high global warming potential, hence the search for an effective alternative is required from an environmental perspective. As one of its potential alternatives, the authors are focusing on N2 gas, which has relatively good insulation characteristics among natural gases and a low environmental impact. However, to use this N2 gas for actual GIS, the insulation characteristics for actual overvoltage waveforms generated in the field (non-standard lightning impulse waveforms; non-standard-LIWs) must be obtained. The present study, with single-frequency oscillation waveforms as a representative basis among non-standard LIWs, experimentally obtained the insulation characteristics of an N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas gap by changing the frequency and the damping factor. Consequently, the breakdown voltage (BDV) was lower for the negative polarity under a quasi-uniform electric field, hence the conclusion that it was reasonable to conduct experiments using negative polarity waveforms in order to discuss the dielectric strength. In addition, even if the frequency and damping factor changed, the BDV remained relatively constant, and that is consistently higher than that for the standard lightning impulse waveforms (standard-LIWs) at a level of 1.14 to 1.24 times. As a result, it was also confirmed for GIS using N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas that the insulation specification could be rationalized by about 20% by converting lightning impulse waveforms from non-standard to equivalent standard-LIWs of 1.2/50 μs. Furthermore, insulation characteristics were obtained under the pattern in which a disconnector switching surges are superposed on residual dc elements generated through the operation of disconnectors (bias voltages). Consequently, the bias voltage had only a minor influence on the insulation characteristics and it emerged that insulation characteristics were likely to be evaluated using a single polarity waveform.

Evaluation of breakdown characteristics of CO2 gas for non-standard lightning impulse waveforms - Method for converting non-standard lightning impulse waveforms into standard lightning impulse waveforms -

SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas, an insulation medium used for gas insulated switchgear (GIS), has a high global warming potential, hence an effective alternative means is expected from the environmental perspective. The authors are focusing on CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas, which has a lower global warming potential, as one of its potential alternatives. To use this CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas for the actual GIS, the insulation characteristics for overvoltage waveforms generated in the actual fields (non-standard lightning impulse waveforms) must be obtained. For this purpose, the preceding study experimentally obtained and evaluated the insulation characteristics for various non-standard lightning impulse waveforms covering the surge waveforms generated in the actual field. In the present paper, the experimental results accumulated to date were comprehensively handled to conduct a study on the evaluation method of the insulation characteristics of the CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas gap for non-standard lightning impulse waveforms. Consequently, similarly to the previous study using SF6 gas, the insulation characteristics could be expressed by one characteristic line using the duration as a parameter. Furthermore, after establishing an evaluation method of the insulation specification for generated waveforms based on the characteristic line and applying it to actual surge waveforms at substations as an example, it emerged that the insulation specification could potentially be reduced by about 10%.

Lightning performance evaluation of quasi-hemispherical air terminations versus a Franklin rod

This research aims to investigate the effect of adding a quasi-hemispherical object at the top of a Franklin rod. This will significantly reduce corona and hence space charge under the high electric fields experienced during the descent of the downward moving leader. By obtaining the appropriate size and shape of the object at the top of the termination the aim is to send a single streamer at precisely the correct moment for it to undergo streamer to leader transition and intercept the downward moving leader. Research as well as preliminary simulation and experimentation point to a critical radius of approximately 0.3–0.35m. A test setup with a 5m air gap has been designed and manufactured with the aim of producing upward leaders from competing air terminations, thus simulating natural lightning conditions more closely. The experimental setup is an inverted rod plane gap and includes all the necessary d.c. biasing circuitry and measuring equipment. Eleven air terminations were tested against the Franklin rod and the results captured with a high speed gated camera. A 1.2/50μs lightning impulse waveform was used during testing. During testing there was no evidence of upward leader formation. The strikes were to the Franklin rod and electric field enhancement proved to be the dominant factor of breakdown. As such the effect of adding a quasi-hemispherical shape to the tip of a rod could not be evaluated as the mechanism of natural lightning attachment was not accurately replicated.

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

AbstractTo 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

Evaluation of breakdown characteristics of CO2 gas for non-standard lightning impulse waveforms under non-uniform electric field - breakdown characteristics for single-frequency oscillation waveforms -

SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas, an insulation medium used for gas insulated switchgear (GIS), has a high global warming potential, hence an effective alternative means is expected from the environmental perspective. As one of its potential alternatives, the authors are focusing on CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas, which has relatively good insulation characteristics among gases with a low environmental impact (natural gases). To use this CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas for actual GIS, the insulation characteristics for actual overvoltage waveforms generated in the field (known as nonstandard lightning impulse waveforms) must be obtained. The present study, with singlefrequency oscillation waveforms as a representative basis, experimentally obtained the insulation characteristics by changing the polarity, frequency, and damping rate of the applied voltage in the non-uniform electric field represented by the presence of metallic particle in the equipment. As a result, the breakdown voltage was lower for the positive polarity under the non-uniform electric field. In this polarity, the breakdown voltage was 1.05 to 1.68 higher than that by standard lightning impulse waveforms even if the frequency and damping rate were changed. These results support the rationalization of insulation for actual surge waveforms under the quasi-uniform electric field.

Development of high frequency circuit model for oil-immersed power transformers and its application for lightning surge analysis

The lightning impulse withstand voltage for an oil-immersed power transformer is determined by the value of the lightning surge overvoltage generated at the transformer terminal. This overvoltage value has been conventionally obtained through lightning surge analysis using the electromagnetic transients program (EMTP), where the transformer is often simulated by a single lumped capacitance. However, since high frequency surge overvoltages ranging from several kHz to several MHz are generated in an actual system, a transformer circuit model capable of simulating the range up to this high frequency must be developed for further accurate analysis. In this paper, a high frequency circuit model for an oil-immersed transformer was developed and its validity was verified through comparison with the measurement results on the model winding actually produced. Consequently, it emerged that a high frequency model with three serially connected LC parallel circuits could adequately simulate the impedance characteristics of the winding up to a high frequency range of several MHz. Following lightning surge analysis for a 500 kV substation using this high frequency model, the peak value of the waveform was evaluated as lower than that simulated by conventional lumped capacitance even though the front rising was steeper. This phenomenon can be explained by the charging process of the capacitance circuit inside the transformer. Furthermore, the waveform analyzed by each model was converted into an equivalent standard lightning impulse waveform and the respective peak values were compared. As a result, the peak value obtained by the lumped capacitance simulation was evaluated as relatively higher under the present analysis conditions.

Evaluation of breakdown characteristics of CO2 gas for non-standard lightning impulse waveforms - breakdown characteristics under single-frequency oscillation waveforms of 5.3 MHz to 20.0 MHz

SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas, an insulation medium used for gas insulated switchgear (GIS), has a high global warming potential, hence the search for an effective alternative means from the environmental perspective. The authors are focusing on CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas, which has a lower global warming potential (GWP), as one of its potential alternatives. In order to use this CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas for actual GIS, the insulation characteristics for actual overvoltage waveforms generated in the field (called non-standard lightning impulse waveforms) must be obtained. For this purpose, the preceding study experimentally obtained and evaluated the insulation characteristics for relatively low frequency oscillation waveforms in disconnector switching surges generated in the actual field. In this paper, the insulation characteristics of the CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas gap for higher frequency oscillation waveforms were experimentally obtained while changing the frequency and damping rate. Consequently, in the high frequency area, if the frequency was raised or the damping rate increased, the dielectric breakdown voltage tended to increase, consistently remaining higher than that for standard lightning impulse waveforms at a level of 1.08 to 1.36 times. Accordingly, it was found that the insulation specification could be rationalized by approximately 10% by converting non-standard lightning impulse waveforms to equivalent standard lightning impulse waveforms. In addition, the influence of gas pressure and gap length was examined and it emerged that the results obtained under basic experimental conditions were likely to be applicable to the conditions close to actual GIS conditions.

Evaluation of breakdown characteristics of CO2 gas for non-standard lightning impulse waveforms - breakdown characteristics under single-frequency oscillation waveforms of 1.3 MHz to 4.0 MHz

SF6 gas, an insulation medium used for gas insulated switchgear (GIS), has a high global warming potential, hence the search for an effective alternative means from the environmental perspective. One of the alternative candidates is CO2 gas, which has a lower global warming potential. In order to use this CO2 gas for actual GIS, the insulation specification must be rationalized by lowering the lightning impulse withstand voltage because the dielectric strength of CO2 gas is lower than that of SF6 gas. To lower the lightning impulse withstand voltage of 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 (non-standard-LIWs) that represent actual surge waveforms in the field and compare them with the characteristics for the standard lightning impulse waveform (standard-LIW) quantitatively. In this paper, with single-frequency oscillations as typical examples of non-standard-LIWs, the insulation characteristics for the CO2 gas gap were experimentally obtained while changing the frequency and damping rate. Consequently, even if the frequency and damping rate were changed, the dielectric breakdown voltage varied little, consistently remaining higher than that with standard-LIW at a level of 1.08 to 1.17 times. Accordingly, it was found that, for GIS using CO2 gas, the insulation specification could be rationalized by approximately 10% by converting non-standard-LIWs to equivalent standard-LIWs. In addition, the influence of gas pressure and gap length was examined and it emerged that the results obtained under basic experimental conditions were likely to be applicable to the conditions close to actual GIS conditions.

Evaluation of Κ-Factor Based on Insulation Characteristics under Non-Standard Lightning Impulse Waveforms

The IEC, which defines the lightning impulse test methods for electric power equipment, is about to introduce the K-factor method for evaluating overshoot of lightning impulse waveforms. There is an urgent need to examine the K-factor method using such parameters as the types and sizes of insulating media and structures. This paper evaluates the K-factor method in the high-voltage test techniques using the insulation characteristics of overshoot waveforms in non-standard lightning impulse waveforms. The insulation elements of oil-immersed transformers and gas-insulated switchgear (GIS) used in this study are oil gaps, turn-to-turn insulations, and gas gaps (field utilization factor ? = 0.45 and 0.60), all of which showed results close to the proposed K-factor curve.

Evaluation of Breakdown Characteristics of Gas Insulated Switchgears for Non-standard Lightning Impulse Waveforms - Breakdown Characteristics for Double-frequency Oscillations under Non-uniform Electric Field

An effective means of rationalization of insulation design for gas insulated switchgear (GIS) is the evaluation of insulation characteristics under actual surge waveforms in the field called non-standard lightning impulse waveforms (non-standard-LIWs). The preceding researches examined quasi-uniform electric field SF6 gas gaps and partly the cone-shaped insulating spacers that represent an insulation element of GIS in order to obtain the insulation characteristics under various non-standard-LIWs. As a result, the breakdown voltages under non-standard-LIWs are higher than those under standard-LIWs, allowing the authors to identify a road map for rationalizing GIS insulation designs. In applying this rationalization technique, experimental study was conducted on single-frequency oscillations to identify the influence of the non-uniform electric field. As a result, the breakdown characteristics were found to be different from those in a quasi-uniform electric field. This paper describes the insulation characteristics under double-frequency oscillations as typical examples of nonstandard-LIWs while varying the applied voltage polarities, frequencies, and damping factors in order to study the influences of non-uniform electric field in further detail. The experimental results show that positive-polarity waveforms pose more demanding insulation requirements and that it is appropriate to conduct experiments using positive-polarity in order to consider dielectric strengths. Moreover, it was confirmed that changing the waveform parameters under positive-polarity did not cause major changes in the breakdown voltages, which remained constantly higher than those under standard-LIWs, and gave little influence to the application of the rationalization techniques.

Evaluation of breakdown characteristics of gas insulated switchgears for non-standard lightning impulse waveforms - breakdown characteristics in the presence of bias voltages under non-uniform electric field

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 previous work, the dielectric breakdown voltage - time characteristics were measured under several different conditions on the quasi-uniform SF6 gas gap and partly the cone-shaped insulating spacers that represent an insulation element of GIS for six kinds of non-standard lightning impulse waveforms associated with lightning surges and disconnector switching surges. This paper, describes detailed examination in non-uniform electric field, and the pattern in which a disconnector switching surges are superposed on residual DC elements generated through the operation of disconnectors (bias voltages). This is intended to simulate more severe conditions in an actual system. Consequently, it was found that the presence of a bias voltage in the nonuniform electric field resulted in lower breakdown voltage values for single-frequency oscillation waveforms both in the positive and negative polarities. However, the breakdown voltage values under positive-polarity waveforms posing more severe insulation conditions were higher than the ones under the standard lightning impulse waveforms. Despite the presence of a bias voltage, it was possible to evaluate these breakdown voltage values in the positive polarity using the concept of a duration examined in the past.

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 iquest 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.

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

AbstractUntil 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

ガス絶縁開閉装置の非標準雷インパルス波形に対する不平等電界下での絶縁破壊特性

To improve insulation specification of a gas insulated switchgear (GIS), it is necessary to recognize the insulation characteristics of SF6 gas under actual surge (called non-standard lightning impulse waveform) occurring in the field substation. The authors had observed the insulation characteristics of SF6 gas gap under various types of non-standard lightning impulse waveforms and compared them with under standard lightning impulse waveform quantitatively. From the investigation of the experimental results, the evaluation method for real surges is shown and the method is applied to typical surges for various UHV and 500kV systems. In the proceeding study, therefore, only for the quasi-uniform electric field (with a typical range of the field utilization factors in the bus of a GIS) was investigated. In this paper, the insulation characteristics of SF6 gas gap for non-uniform electric field were observed experimentally and investigated about the evaluation method of converting non-standard lightning impulse waveforms equivalently to the standard lightning impulse waveform.

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.