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Now that gas-insulated switchgear (GIS) for ac systems are becoming increasingly compact as specifications are rationalized, more consideration of their insulation characteristics for residual dc voltage is required. Furthermore, with dc power transmission technology drawing more and more global attention, clarifying the insulation characteristics of GIS for dc voltage is increasingly important. For GIS insulating epoxy spacer, the present paper experimentally and analytically studied the influence of spacer surface condition on dc withstand voltage performance. In specific terms, initially, an insulating spacer model with an area of non-uniform resistivity present in the surface layer was created and the dc voltage breakdown characteristics were obtained. As a result, it was clarified that the time delay to breakdown increased with lowering the applied voltage, or in other words, even if a low voltage was applied, breakdown may occur after an extended period. Subsequently, using the same non-uniform resistivity model, the change in the electric field distribution over time under dc voltage was investigated through transient electric field analysis. Consequently, it was found that the electric field distribution varied from a capacitive to a resistive field and the maximum electric field was generated in the boundary between the high- and low-resistivity areas. It was further clarified, based on these breakdown characteristics and electric field analytical results, that the presence of an area of nonuniform resistivity on the insulating spacer creepage surface caused electric field concentration over time and breakdown occurred if the electric field in the creepage surface direction exceeded 40 kV/mm or so. Moreover, the breakdown characteristics were investigated where a lightning impulse voltage was superimposed over a dc voltage. Consequently, the creepage breakdown electric field where a dc voltage was applied alone was almost identical to that where the superimposed voltage was applied. It means that the application of the superimposed voltage may potentially allow the insulating spacer withstand voltage performance to be evaluated by a shorter time test for a dc voltage alone or the dc voltage with a superimposed VFTO. [ABSTRACT FROM AUTHOR]

The soundness of a power transformer under an operating voltage is evaluated in partial discharge (PD) test of long-duration induced ac voltage test. The acceptance criterion for this PD test is 500 pC according to IEC standard; however, few basic data backing this criterion are available. To establish a clear criterion for this PD test, the authors initially conducted a study on the harmful PD level of materials themselves constituting the insulation of an oil-immersed transformer and a gas-filled transformer. This PD level was evaluated based on the rate of decline in the lightning impulse breakdown voltage using the insulating materials themselves and the element models simulating the insulating structure which had been exposed to a PD. Consequently, it emerged that insulations of both types of transformers were degraded if exposed to a PD of 7,000 pC to 10,000 pC. With the safety factor for this PD value taken into account considering the long-term operation and the structural difference of an actual transformer, 5,000 pC was deemed as the harmful PD level at the PD occurrence position. Subsequently, using a winding model of a transformer, PD signal propagation characteristics inside the winding were investigated through actual measurement and analysis. As a result, it was found that the PD having occurred inside the winding is measured as the signal significantly damped depending on the position of occurrence. The transmission rate was 2.2% for an oil-immersed transformer and 2.8% for a gas-filled transformer in the respective lowest cases. What should be controlled in the PD test of an actual transformer is the value of the harmful PD level at the PD occurrence position multiplied by the transmission rate at the PD detection position. Therefore, the conclusion was reached that the acceptance criterion in the test should be set to 5,000 pC ??? 2.2% = 110 pC or less for an oil-immersed transformer and 5,000 pC ??? 2.8% = 140 pC or less for a gas-filled transformer, respectively. [ABSTRACT FROM PUBLISHER]

Gas-insulated switchgear (GIS) has widely been used for AC power distribution because of its high reliability and compactness. Recently, DC GIS has been developed with various investigations for dielectric breakdown characteristics of DC gas insulation. GIS insulation is composed of SF6 gas and solid spacers, and it has been recognized that the dielectric performance of DC GIS is mainly influenced by solid spacers. Under DC stress, the electric field is directed one way, the effect of electrification for charges to be accumulated in the spacer must be taken into account and also the effect exists in AC GIS because the switching operations may leave the remnant DC charge on the AC GIS spacer. This paper first describes the effective resistivity (the bulk or the surface) of the solid spacer under the DC stress from the experimental investigation, and the critical factor on the solid spacer that causes reduced dielectric performance of the GIS insulation is studied. Second, the present paper deals with the electrification on the GIS with various levels of surface roughness of the epoxy insulator and metallic electrode. Finally, the DC insulation characteristics of GIS insulator are investigated based on the experimental results. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 168(4): 6–13, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20788 [ABSTRACT FROM AUTHOR]

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 cone- shaped insulating spacers that represent insulation elements of GIS for six kinds of non- standard 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. [ABSTRACT FROM AUTHOR]

To tower 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. [ABSTRACT FROM AUTHOR]

For the development of a "short-duration ac withstand voltage test", an insulation specification of substation equipment, there is a precise method of reliability evaluation using a Weibull distribution function. Regarding this method, there remains a subject of handling coexistence of multiple voltage levels. This paper first defines the two reliability evaluation methods, "independence method" and "accumulation method", applying to Weibull evaluation for coexistence of multiple voltage levels in relation to their physical meanings. Next, the influence of the Weibull parameter values are examined on the cumulative fault probabilities and test voltages calculated using these methods. When the time shape parameter a>1, the accumulation method gives higher values than the independence method; When a=1, the two methods give the same values; When a<1, the former gives lower values than the latter. Then, appropriate reliability evaluation methods are investigated for various insulation media and insulation structures of substation equipment from the viewpoint of inception and development mechanisms of dielectric breakdown and partial discharge. According to the result of engineering evaluation of the presently available data, the independence method may be appropriate for both gas insulated switchgear and oil-immersed transformers. [ABSTRACT FROM AUTHOR]

There is not sufficient data on the breakdown voltage-time characteristics for gas insulated switchgear (GIS) at power-frequency voltages, especially in the long-term region, for complete insulation specifications to be developed. This paper gives voltage- time breakdown characteristics for a 72-kV GIS at power-frequency voltages, both for clean conditions and with metallic particles added, from times of the order of a second to times of the order of 1000 hours. The Weibull parameters were calculated to be n = 95 and a = 0.32 to 0.47 in the clean case; and in the metallic-particle case n = 31 and a = 0.92 to 0.98 for a short-term region and n = 66 and a = 0.40 to 0.47 for a long-term region, with the boundary between these two regions located at 10 minutes. Based on the statistics for the metallic-particle case, the reliability of the power-frequency withstand voltage tests were then estimated using as examples the GIS in a 500-kV system (an effectively grounded system) and a 66-kV system (a non-effectively grounded system). The paper also studied the effect of the type of grounding and the influence of the presence of metallic particles on the test voltage level. [ABSTRACT FROM AUTHOR]

Gas insulated Switchgears (GIS) are widely used as major substation equipment because of their excellent insulation and interruption performance. However, while a GIS is highly reliable, it is a concealed device, meaning diagnostic technology using various sensors is required to detect any insulation abnormality. Among various potential defects inside a GIS, the most problematic is caused by the existence of fine metallic particles, and thus the ability to detect harmful particles in tests at a factory and on site as well as in operation is becoming an important subject. This paper examines, on an experimental basis, the UHF sensor's characteristics of detecting partial discharge (PD) signals generated from particles inside the GIS with those placed freely on the bottom surface of the tank, having adhered to the insulating spacer's surface, and fixed on the center conductor, respectively. Based on previous reports, breakdown characteristics with the existence of particles were arranged in terms of the particle length, and the harmful particle length was estimated. Both results were compared with each other to evaluate the possibility of detecting harmful particles in each location with the UHF sensor. Consequently, it emerged that free particles were detected comparatively easily. However, while those having adhered to the spacer's surface and fixed on the center conductor could be detected in insulation tests at 2 Pu at a factory, detection was difficult in many cases in the field. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In the 2010 revision of IEC-60060-1 "High-voltage test techniques", the conversion of waveforms using the test voltage function (K-factor) was introduced as one of the major revisions. At present, in IEC TC42 "High-voltage and high-current test techniques," a study is conducted on the standards for the lightning impulse voltage test at WG 19 "Adaptation of TC 42 standards to UHV test requirements." The present paper comprehensively evaluates the experimental results of the K-factor values using the largest possible model assuming UHV-class electric power equipment, as well as the past experimental results of the K-factor values. Initially, these experimental results were evaluated from a view point of insulation properties to clarify the characteristics of the Kfactor values. The adaptation itself of the K-factor evaluation method to also cover UHVclass equipment is considered significant. However, for high-voltage class equipment, since the formative time lag of breakdown is longer due to the long insulation length, resulting in a longer breakdown time, the K-factor values are considered lower than the existing K-factor function. Subsequently, the experimental results of the K-factor values were comprehensively evaluated and compared with the existing K-factor function. The K-factor function may possibly have to be reviewed for UHV-class electric power equipment to take lower values than the existing K-factor function. [ABSTRACT FROM PUBLISHER]

At present, a study is being conducted on the lightning impulse (LI) voltage test standard for UHV-class electric power equipment at WG 19 "Adaptation of TC 42 standards to UHV test requirements" in IEC TC42 "High-voltage and high-current test techniques". In the present paper, the past LI voltage test results for UHV-class equipment were initially investigated to determine test waveforms that could be generated. As a result, for a UHV-class transformer and gas insulated switchgear assembly, the front time T1 exceeded 2.2 μs or more, or even considerably 3.0 μs in some cases in order to generate a waveform with an overshoot rate β' of 10% or less using existing test facilities. Subsequently, experiments were conducted with regard to the extension of the T1, which is a prospective way to revise the LI voltage test waveform. Breakdown characteristics with respect to the T11 were measured using the largest possible models assuming UHV-class equipment for three types of insulating media, namely an SF6 gas gap, an oil gap, and a long air gap. Consequently, it was confirmed that extending the T1 up to about 3.6 μs had a minor influence on the breakdown characteristics. In addition, a study was also conducted on the influence of the deregulation of the standard value for the T1 from a viewpoint of insulation properties of equipment for the wave front steepness. As a result, it was concluded that the equipment insulation performance could be appropriately verified even if waveforms with an extended T1 were used. Based on these data, the idea of the standard for lightning impulse test waveforms for UHV-class equipment was proposed as follows: The standard voltage waveform shall be the same as the conventional one for less-than-800 kV equipment with a T1 of 1.2 μs and a time to half value of 50 μs but the upper allowable range of the T1 may be up to 3.6 μs; provided, however, that a note shall be added besides the exceptional provision that, whenever possible, the test should be conducted using a waveform of which the T1 is adjusted to 1.56 μs or less, the same as that for less-than-800 kV equipment. [ABSTRACT FROM PUBLISHER]

The paper reports a new transversal study of dielectric coating effects on high voltage (HV) electrodes in SF6 and eco-friendly gases like CO2, dry air and N2 for real-size gas insulated switchgear (GIS). From a practical viewpoint, the breakdown electric field (BDE) data of the following test conditions are collected; 1) large effective electrode area over about 5 x 104 mm2, 2) actual GIS cylindrical configuration of utilization factor of around 0.6, 3) high gas pressures up to 1.9 MPa for actual GIS. The BDEs of dielectric coated electrodes relative to those of bare electrodes (normalized BDEs) are systematically evaluated as a function of dielectric coating thickness and gas pressure. As a result, the normalized BDEs in percentage terms are 105-150% (average value of around 110%), which does not depend on the gases. Remarkable performances of the normalized BDEs of 120-150%, which correspond to nearly 90% of the theoretical BDEs of the gases, are observed in SF6 and CO2 for "thick casting epoxy coating", "thin coating of comparable dielectric performance to casting epoxy" and "hard anodic-oxide coating & PTFE-immersion" that have fewer defects inside. The normalized BDEs in N2, however, show poorer dielectric performances than the other gases especially in the high pressure region above 1.0 MPa. The dielectric coating effects are more pronounced as increasing the thickness for all the gases. The pressure dependences of the dielectric coating effects are not clearly recognized for all the gases and all the coating conditions except for the "thick casting epoxy coating". The coating effect in N2 decreases slightly as raising gas pressure. The application examples and application limits of the dielectric coatings to GIS are also presented. These new cross-sectional studies of dielectric coated electrodes are an effective approach to get new findings to realize a future eco-friendly compact GIS. [ABSTRACT FROM PUBLISHER]

The degradation characteristic of solid insulators is considered to be one of the key factors to perform a risk assessment of highly aged gas insulated switchgear (GIS). The present study experimentally obtained the insulation characteristics of epoxy insulator mainly with an internal microscopic delamination-shaped defect. In the experiment, a short-time breakdown electric field (EF) was initially obtained and comparison was made with the breakdown EF for the void and crack defects to evaluate the breakdown risk with respect to the defect shape. These defect models (delamination, void, and crack) were designed and produced so that an apparent partial (PD) might be 1 pC in an actual 550 kV-GIS spacer. Consequently, it emerged that the area where the EF was concentrated influenced the breakdown. Of the three types of defect, the degree of EF concentration was the lowest for delamination defects, meaning the lowest breakdown risk for the same. Subsequently, the V-t characteristics were obtained through an EF acceleration test. The V-t characteristics obtained were extrapolated to estimate the breakdown risk in 30 to 50 years operation, which is considered the design life of GIS. As a result, it was determined that the potential for eventual breakdown was extremely low under the normal operating EF, even if a delamination defect equivalent to an apparent PD of 1 pC was present in an actual 550 kV-GIS spacer. Hence, controlling a defect using 1 pC for actual GIS is considered to ensure reliability under long-term operation. [ABSTRACT FROM AUTHOR]

In IEC-60060-1 as revised in 2010, a conversion method for lightning impulse voltage test waveforms using the test voltage function (K-factor function) was adopted. Presently, studies are being conducted on the test standards for ultra-high voltage (UHV)-class equipment. Since the existing K-factor function was established based on the experimental results for more small models compared with the insulation structure of UHV-class equipment, the issue has arisen as to whether this K-factor function can also be applied for UHV-class equipment. The authors experimentally obtained K-factor values using the largest possible model assuming UHV-class equipment in the previous studies. This paper reports the K-factor values and the insulation characteristics with respect to the front time obtained for air insulation under a negative-polarity lightning impulse voltage. According to the results of this experiment, even though the K-factor values for an air gap were likely to be slightly lower than the existing K-factor function in the negative polarity, the tendency of the change with respect to the superimposed oscillation frequency was similar. Furthermore, it was found that even though the breakdown voltage for waveforms with an extended front time was likely to decrease further than that for the standard lightning impulse waveforms, the decline was about 3.1% even for a front time of 4.8 ~\mus. Consequently, it was indicated that a lightning impulse voltage test waveform with an extended front time may help properly verify the insulation performance of a UHV-class air bushing. [ABSTRACT FROM PUBLISHER]

SF₆ 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. 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 N₂ gas for nonstandard 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. In addition, based on the characteristic line, a method of evaluating insulation specifications for generated waveforms was established and applied as an example to actual surge waveforms in a substation. Furthermore, it emerged that the insulation specifications could potentially be reduced by about 10% for actual surge waveforms used in the present study. [ABSTRACT FROM PUBLISHER]

To improve the insulation specification of gas-insulated switchgear (GIS), it is necessary to recognize the insulation characteristics of SF gas during actual surges (called nonstandard lightning impulse waveforms) occurring at field substations. The authors observed the insulation characteristics of SF 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 SF 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 (). DOI 10.1002/eej.21144 [ABSTRACT FROM AUTHOR]

For the development of test conditions for a short-duration power-frequency withstand voltage test, which is one of the insulation specifications of substation equipment, there is a precise method of evaluating reliability using a Weibull distribution function. For this method, "independence method" and "accumulation method" have been proposed in order to handle the coexistence of multiple voltage levels. In this paper, an insulation test based on the one-minute step-up method as condition for the coexistence of multiple voltage levels is used to calculate and evaluate the test results using these two methods. Consequently, the insulation characteristics obtained from the one-minute step-up test are found to have differences from the true values. If the independence method is adopted, there is a certain deviation regardless of the Weibull time shape parameter "a-value" and, the smaller the Weibull voltage shape parameter "m-value", the larger the deviation. If the accumulation method is adopted, on the other hand, the larger the "a-value" or the smaller the "m-value", the larger the deviation. The 50% breakdown voltage value, the standard deviation "σ-value", and the "m-value" that can be obtained from the insulation test based on the one-minute step-up method were evaluated for their deviations from the true values. The results suggest that, when a test is conducted on an oil-filled transformer, the design may turn out to be on the dangerous side due to overestimation of the dielectric strength by around 8%. [ABSTRACT FROM AUTHOR]

The degradation characteristic of solid insulators is considered to be one of the key factors to perform a risk assessment of highly aged gas insulated switchgear (GIS). The present study experimentally obtained the breakdown voltage-time (V-t) characteristics of epoxy insulator with an internal microscopic crack-shaped defect. This defect model was designed and produced so that an apparent partial discharge (PD) might be 1 pC in an actual 550 kV-GIS spacer. During the experiment, the change in the PD magnitude over time and the PD waveform were measured to clarify the PD characteristics until breakdown occurred. Consequently, in terms of the change in the PD magnitude over time, it was likely that the lower the applied electric field (EF), the larger the ratio of the time until the PD magnitude drastically increased (tree induction time) to the total time until breakdown occurred. When 6 kVrms/mm, or the lowest EF in the present test, was applied, two distinctively different trends were observed; one where the PD stably continued and breakdown occurred within several to several tens of hours, and the other where the PD was suspended in mid-course and it was longer than 100 hours until breakdown occurred. Subsequently, the breakdown EF under long-term operation was estimated by extrapolating the V-t characteristics, whereupon a risk under long-term operation was recognized when a micro-defect of 1 pC was present in actual GIS; even though such defect was extremely unlikely. According to the observation results of the PD magnitude, however, the tree induction time was longer at a low EF of the operating EF level and the declining gradient of the V-t characteristics was considered lower. For a rigorous evaluation of breakdown risk under long-term operation, therefore, it was considered that evaluation via simple extrapolation of the V-t characteristics by straight line was not sufficient and the insulation characteristics under a low EF must be obtained. [ABSTRACT FROM AUTHOR]

The insulation performance of gas insulated switchgear (GIS) against lightning surges is verified by applying a lightning impulse (LI) voltage alone. However, the surge voltage generated in an actual system is superimposed over the ac operating voltage to form a waveform that differs from the LI voltage waveform. For UHV- and 500 kV-class GIS in particular, where the ratio of ac operating voltage to the LI withstand voltage is high, the insulation characteristics for this LI and ac superimposed voltage are important factors to study the insulation design. Accordingly, this paper researched the insulation characteristics for the superimposed voltage of element models simulating the SF6 gas gap, creepage surface of the insulating spacer and inside of the insulating spacer, all of which are the main insulating parts of GIS. The ac component ratio of the superimposed voltage ranged from 30 to 70% of that of breakdown (BD) voltage when ac voltage was applied alone to each model, wherein conditions for superimposed phase or similar served as parameters. Subsequently, these results were evaluated from perspectives of an insulation coordination and GIS insulation test. As a result, the BD voltage showed results almost equivalent to those obtained in the case where the LI voltage was applied alone; even when the ac component of superimposed voltage was varied at three different insulation parts. Accordingly, within the 30 to 70% range of superimposed ac voltages, it emerged that the BD voltage was almost determined by the peak value of the applied voltage and was unaffected by the ac component. When comparing with GIS test voltages in the 66 kV to UHV-class, this range of 30 to 70% covers all the ratios of power-frequency and LI test voltages for operating voltages in an actual system. Consequently, the LI voltage test was considered adequate to verify the insulation performance of GIS against surge overvoltage generated during operation whereas superimposition of ac voltage need not be considered. These results are meaningful findings for rationalizing GIS while maintaining its insulation reliability. [ABSTRACT FROM AUTHOR]

The authors are conducting a study on the lightning impulse voltage test standard for UHV-class equipment, including the application of the evaluation method using the Kfactor (test voltage) function. Previously, the actual measurement results of K-factor values assuming UHV-class power transformers and UHV-class gas insulated switchgear were reported. The present paper reports the evaluation results of the influence of LI voltage test waveforms on the breakdown characteristics in the largest possible bushing model, assuming actual UHV-class air insulation. According to the test of the breakdown voltage characteristics with respect to the overshoot rate, it was indicated that the 50% breakdown voltage tended to rise with increasing overshoot. Subsequently, the evaluation using the K-factor function is considered useful for UHV-class air insulation. The evaluation result of the K-factor value based on the experimental results was lower in value than the present K-factor function and the result of the European Project, which is considered attributable to the significant formative time lag of a discharge in the long air insulation distance. Consequently, the K-factor function might be reviewed downward in these cases. Furthermore, according to the experimental results of the breakdown characteristics, when the front time is extended, its extension up to about 3.6 μ?s might possibly be allowed under a standard assuming UHV-class air insulation. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

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. 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 N2 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 N2 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. [ABSTRACT FROM AUTHOR]

The lightning impulse withstand voltage test waveform for electric power equipment is specified in IEC 60060-1 "High-voltage test techniques" as revised in 2010. At present, test standards for UHV-class equipment are under study. Increasing equipment capacity and the digitization of measuring equipment are mentioned as these backgrounds. Withstand voltage test by the standard waveform specified in the previous standard had been difficult (it eventually becomes an overshoot waveform) with increasing equipment capacitance. In response, an evaluation method using the kfactor function (test voltage function) was introduced, whereby the overshoot waveform was converted to the test voltage waveform. To date, concerning these kfactor values, measurement results based on experiments have been reported, but most were from small-scale insulation models with breakdown voltage levels mainly around 100 kV. The present study reports the experimental results of the insulation breakdown characteristics for the lightning impulse withstand voltage test waveform in the largest SF6 gas insulation model possible assuming actual UHV-class gas insulated switchgear. Breakdown voltage and breakdown time were measured with the superimposed oscillation frequency, overshoot rate, and front time as parameters. Following evaluation of the k-factor value based on these experimental results, the k-factor value with the overshoot rate of 10% was almost identical to that of the existing k-factor function. Consequently, evaluation using the existing k-factor function is considered appropriate. Subsequently, the existing k-factor function is most likely to be effective also for UHV-class equipment. In addition, it emerged that changes in the insulation breakdown characteristics due to the extension of the front time were small. It was considered that extending the front time, rather than allowing an excessive overshoot rate, would enable proper verification of the insulation performance as part of standard assuming UHV-class equipment. [ABSTRACT FROM AUTHOR]

SF6 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 CO2 gas, which has a lower global warming potential, as one of its potential alternatives. To use this CO2 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 CO2 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%. [ABSTRACT FROM AUTHOR]

SF6 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 CO2 gas, which has relatively good insulation characteristics among gases with a low environmental impact (natural gases). To use this CO2 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. [ABSTRACT FROM AUTHOR]

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. The authors are focusing on CO2 gas, which has a lower global warming potential (GWP), as one of its potential alternatives. In order to use this CO2 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 CO2 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In the condition settings of a power-frequency withstand voltage test for substation equipment, an approach based on the insulation reliability evaluation using Weibull distribution is available. However, the issue to be discussed remains in this approach. For example, temporary overvoltage during the operation is evaluated through Weibulldistribution parameters obtained from an experiment without operating voltage. For an oilimmersed transformer, the breakdown voltage-time (V-t) characteristics of the oil gap were obtained as basic characteristics in the previous experiment. It was experimented with 0 cm/s, 8cm/s and 16cm/s oil flow velocity as a parameter affecting the voltage application history. In this paper, an insulation characteristic was obtained in a pattern of voltage application assuming temporary overvoltage during operation. The results were compared with the basic characteristics in the previous experiment, and insulation reliability was calculated in trial. Consequently, the insulation reliability evaluation based on the existing V-t characteristic test parameters was found to be practically on the safe side and appropriate. [ABSTRACT FROM AUTHOR]

In the condition settings of a power-frequency withstand voltage test for substation equipment, an approach based on insulation reliability evaluation using Weibull distribution is available. However, the subjects which should be investigated remains in this method. One of the subjects is that evaluating the insulated reliability for temporary overvoltage caused in an aged equipment by the pre-aging Weibull parameter. In the first place, for an oil-immersed transformer, the breakdown voltage-time (V-t) characteristics of the oil gap were obtained as basic characteristics in the previous experiment. In this experiment the presence or absence of the oil flow was used as a parameter affecting the history of the applied voltage. Also, the validity of the present evaluation approach to temporary overvoltage during operation was examined. In this paper, an insulation characteristic was obtained in consideration of the age-related deterioration in the field. The results were then analyzed in comparison with the insulation characteristics of a new insulating oil. As a result, there was little change affecting the result of insulation reliability evaluation in the Weibull parameters. Consequently, it is reasonable to evaluate the insulation reliability by the V-t characteristic of the new insulating oil. [ABSTRACT FROM AUTHOR]

We have investigated the performance of silicone oil as an insulation medium with the aim of developing environmentally-friendly, flame-retardant electric power equipment. In this paper, the insulation characteristics of silicone oil and ester oil, which is the alternative insulating oil candidate, have been investigated and compared. The insulation characteristics examined in this paper are the streaming electrification characteristics including electrification phenomena generated when an insulating oil streams, dielectric breakdown characteristics with increasing temperature and the surface breakdown characteristic when the insulator is placed between electrodes. The range of temperature investigated was 30 to 130 °C. Both kraft and aramid paper were used for the insulator. In the comparison between silicone and ester oil, it was found that the insulation performance of the ester oil was better than that of the silicone oil. We have considered possible mechanisms for these results. [ABSTRACT FROM AUTHOR]

This paper describes the surface breakdown characteristics of silicone oil which has the possibility of application to innovative switchgears and transformers as an insulating medium. At the first step, we have experimentally studied the impulse breakdown characteristics of the configuration with a triple-junction where a solid insulator is in contact with the electrode. The test configuration consists of solid material (Nomex⊗ or pressboard) and liquid insulation oil (silicone or mineral oil). We have discussed the experimental results based on the maximal electric field at the triple-junction. As the second step, we have studied the configuration which may improve the surface breakdown characteristics by lowering the electric field on the insulator surface. [ABSTRACT FROM AUTHOR]