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This article discusses the breakdown voltage characteristics of gas‐filled transformers and reactors. To determine their insulation characteristics, this study has used models simulating their insulating elements: interturn and section‐to‐section insulation. The study measured 50% breakdown voltages in the section‐to‐section and interturn models under standard lightning impulse voltages that were 1.13 and 1.06 times higher than those under the standard switching impulse voltages, respectively. The mechanism of the above‐mentioned facts is discussed, in terms of the formative time lag of the discharge. © 2020 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [ABSTRACT FROM AUTHOR]

To study the dc insulation design of gas insulated switchgear (GIS), the insulation characteristics under lightning impulse (LI) voltage with a superimposed dc voltage (superimposed voltage) must be clarified. The paper experimentally examined the GIS breakdown characteristics under this superimposed voltage. The test models simulating an insulator creepage surface were used for which consideration of the influence of dc voltage among various other GIS insulating elements is particularly important. To be specific, a cylindrical model made of epoxy resin or fiber-reinforced plastic (FRP) as the material and a conical epoxy spacer model were tested. For the cylindrical model, a cap-shaped electrode was placed on the insulator and a small gap was established between the end of the electrode and the insulator. When the dc breakdown voltages were measured using these samples, they were higher for the applied voltage of positive polarity than that of negative polarity for all samples. The post-test observation of the electrification condition revealed greater electrification on the insulator surface for the applied voltage of positive polarity. The electrification charges are considered to have relaxed the electric field and increased the breakdown voltage. Subsequently, the breakdown test was conducted using a superimposed voltage, whereby a foregoing dc voltage was applied to samples for a certain period, whereupon a LI voltage was applied with the dc voltage continually applied. The breakdown voltage when the LI voltage and dc voltage had equivalent polarity was approximately same to the LI alone breakdown voltage. Conversely, when they were opposite in polarity, the breakdown voltage under the superimposed voltage obviously tended to decrease from the LI alone breakdown voltage. It is considered attributable to the fact that the insulator surface was electrified by the foregoing dc voltage and applying LI voltage opposite in polarity to this electrification intensified the electric field where the breakdown started to occur. Accordingly, it emerged that the GIS breakdown characteristics changed significantly depending on the polarity combinations of LI and dc voltages. The influence of these polarities must be taken into consideration when studying the GIS dc insulation design. [ABSTRACT FROM AUTHOR]

The paper investigated the behavior and partial discharge (PD) characteristics of a metallic particle under residual dc voltage, using the setting conditions and length of the particle, the applied dc voltage, and other factors as parameters and a gas insulated switchgear (GIS) bus bar model equivalent to that of an actual 300 kV GIS. A metallic particle repeated reciprocating movement at a relatively high frequency of about 5 times per second between the high-voltage conductor and the sheath when the electric field at the bottom surface of the tank exceeded its lift-off electric field. It emerged that, in the process of this reciprocating movement, PD occurred at the moment when the particle collided with the high-voltage conductor. In addition, when a particle collided with the electrode, the electrical charges moved and the residual dc voltage was damped. Conversely, where the sheath side was insulation-coated, the particle did not move at the normal operating voltage level, and even though it moved if vibration was applied, its movement stopped relatively soon. If a metallic particle exists in GIS and moves, there is concern that the insulating performance may decline significantly. When a metallic particle moves under dc voltage, a relatively stable PD is generated and consequently, for example, the PD measurement is considered an effective way to detect a particle. It is also considered effective to take physical measures, such as using an insulating sheath at the bottom surface of the tank to restrain the behavior of particles. [ABSTRACT FROM AUTHOR]

Now that gas insulated switchgear (GIS) for ac systems is 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. The insulating portions for which the influence of dc voltage must be taken into consideration are solid insulators, such as insulating spacers. Under dc voltage, since the electric field distribution in an insulator differs from that under ac or impulse voltage and is governed by the resistance characteristics, clarifying its characteristics is crucial to study the GIS dc insulation design. As a solid insulator, focusing on fiber-reinforced plastics (FRP) used for GIS, for example, insulating rods, as well as partially treated epoxy resin; this paper experimentally investigated the bulk and surface electric conductivity under dc voltage, using the electric field, temperature, and other factors as parameters. As a result, the bulk electric conductivity of FRP in an edgewise direction exceeded that in the penetrating direction by one digit. It emerged that the electric conductivity of an insulating material with orientation like FRP varied depending on its direction. It was also found that, despite the fact the bulk and surface conductivity depended on the electric field for both FRP and epoxy resin, the variation width was relatively narrow within the range of the actual GIS operating electric field. The bulk and surface electric conductivity were also temperature-dependent, which meant the variation width was relatively wide. Furthermore, the surface electric conductivity was measured in SF6 gas and in the air to investigate the influence of the ambient atmosphere, whereupon it emerged that the electric conductivity was higher in air due to the adherence of moisture. As mentioned above, the electric conductivity of an insulator varies due to various factors, such as the influence of the material orientation, electric field, temperature, and moisture. Consequently, the electric field distribution inside the insulator also changes, meaning these electric conductivity characteristics must be taken into consideration to study the GIS dc insulation characteristics. [ABSTRACT FROM PUBLISHER]

The accuracy of lightning overvoltage evaluation of electrical equipment has been improved based on circuit analysis such as the Electro-magnetic Transients Program, but analyzing phenomena whose plane wave propagation (TEM) mode cannot be assumed remains unsolved. Since numerical analysis of electromagnetic fields such as the finitedifference time-domain (FDTD) method directly solves Maxwell?s equations, it is an appropriate means for solving this issue. In this paper, the lightning surge waveform due to the back-flashover is analyzed using the FDTD method; bearing detailed modeling of transmission towers and incoming substation lines in mind. An open end of a circuit breaker of a 500 kV air-insulated substation was selected as the subject of analysis, and ac operation voltage was considered as an analysis condition. Following FDTD analysis, a 150 kA lightning stroke to the first tower, which is the standard lightning stroke of the lightning protection design, does not cause back-flashover, but a 200 kA lightning stroke to the first tower and a 150 kA lightning stroke to the second did do so. The flashover occurred in the later timing than the result of conventional circuit analysis, and the rise of the lightning surge was also slower. The voltage waveform of the open end of the circuit breaker reached its peak after repeating reflections, and the overvoltage value became lower than the circuit analysis. According to the FDTD analysis, the required withstand voltage value after the waveform evaluation with a 200 kA lightning stroke is 1,337 kV, which is lower than the present lightning impulse withstand voltage (LIWV) 1,550 kV. The lightning stroke current 200 kA is the value adopted in the lightning protection design of the UHV system. Therefore, the existing air-insulated substation equipment is considered highly capable of withstanding overvoltage at the time of a back-flashover. [ABSTRACT FROM AUTHOR]

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 IEC-60060-1 "High-voltage test technique" was revised in 2010 and an evaluation method using the k-factor function (test voltage function) was introduced for the lightning impulse voltage test, which involved the overshoot waveform being converted into the test voltage waveform. Test standards for UHV-class equipment are also presently being studied. To date, authors have measured k-factor values with the largest possible model, assuming actual UHV-class power equipment and proposed a new k-factor function for large-scale equipment. The present paper reports on the evaluation results of expanded uncertainty when measuring a k-factor value on the basis of the ISO/IEC Guide 98-3. As a result, it was clarified that significant uncertainty in measuring a k-factor value is brought about by the definition of the kfactor itself because small differences in the 50% breakdown voltage between the smooth lightning impulse and overshoot waveforms with respective uncertainties at a certain level were calculated. The expanded uncertainty when measuring the k-factor value in the present study for an overshoot rate of 10%, which is particularly important for practical use, was a large value of ?0.23 to ?0.36, or 0.46 to 0.72 in topto- bottom width compared to k-factor values themselves ranging from 0.2 to 1.0. However, this is actually considered a minimum level of uncertainty, which is inevitable in principle, and the best achievable measurement result rather than something caused by any problem in measurement itself. [ABSTRACT FROM AUTHOR]

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. In this paper, to understand the insulation characteristics of epoxy resin, which is widely used for GIS insulating spacers, factors determining the resistivity of the epoxy insulator surface layer under dc voltage were initially investigated on an experimental basis. Consequently, it emerged that the bulk resistance was more dominant than the surface resistance for the dc resistance of epoxy resin due to the dependency of the test sample resistance value on their radius. Since the electric field might be concentrated if some part of this insulator surface layer showed non-uniform resistivity, the influence of the curing agent, one of the potential causes of this non-uniformity, was subsequently investigated with its content as a parameter. As a result, the volume resistivity in the long-term region was likely to decline or vary for epoxy resin containing less curing agent due to the presence of numerous polarized components unreacted with curing agent. In addition, the presence of micro protrusions or similar, if any, on the insulator surface or electrode is considered to cause electrification due to the concentration of electric field on the surface layer. Accordingly, the relationship between their surface roughness and electrification level was investigated using gaps between insulators or an electrode and an insulator facing each other, respectively. Consequently, where the surface roughness of the insulator or electrode was high, a current component with a large damping time constant, considered attributable to electrical charges moving across the gap, appeared after the charging current components and an electrification condition was observed. [ABSTRACT FROM AUTHOR]

The insulation performance of an oil-immersed power transformer against lightning surges is verified by applying a lightning impulse (LI) voltage alone. However, the surge voltage generated at the transformer terminal in an actual system is superimposed over the ac operating voltage. Particularly for UHV- and 500 kV-class, 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 crucial factors to study the insulation design. Accordingly, this paper reviewed the research results of insulation characteristics of transformers for the superimposed voltage. Subsequently, these results were evaluated from perspectives of an insulation coordination and transformer insulation test. The insulation elements of transformers covered were the turn-to-turn and section-to-section insulation, for which the LI test is key for insulation design. As a result, for the section-to-section insulation for a shell-type transformer and the turn-to-turn insulation, the breakdown (BD) voltage for the superimposed voltage remained unchanged from the result when the LI was applied alone. Conversely, for the section-to-section insulation of a continuous disk winding for a coretype transformer, the BD voltage attributable to the superimposed voltage declined by about 10% to 20% from that when the LI was applied alone if the oil-impregnated paper component ratio compared to the oil-gap length was low. This is because the oil gap was primarily responsible for the BD and flaws were generated there when the ac voltage was applied. On the other hand, when this ratio was increased, the oil-impregnated paper part was primarily responsible for BD and the decline in BD voltage could be suppressed. For the section-to-section insulation for the interleaved disk winding, the BD voltage attributable to the superimposed voltage remained constant from that when the LI was applied alone. This is because the BD was determined by the LI partial discharge inception at the turn-to-turn part of the section-to-section insulation, where the superimposed voltage made little impact on BD. Based on the above, the LI test is considered adequate to verify the insulation performance of the section-to-section insulation for a shell-type transformer and for an interleaved disk winding and the turn-to-turn insulation against surge overvoltage. Conversely, for the section-to-section structure of a continuous disk winding, the influence of the ac voltage must be taken into account. These results are key findings for rationalizing transformers while maintaining their insulation reliability. [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]

Secondary arc extinction time is an important factor, which influences system stability because it directly affects the reclosing time. However, few reports exist on a field data of the secondary arc extinction time on transmission lines. This paper describes the investigation results of secondary arc extinction characteristics by analyzing voltage wave shapes during the lightning faults on 550 kV transmission lines. Recovery voltages and secondary arc currents on each transmission line during faults were calculated to consider correlation with the secondary arc extinction time. The secondary arc extinction time is estimated by the existing formula, the secondary arc current and the recovery voltage required for one second reclosing are evaluated. [ABSTRACT FROM PUBLISHER]

The lightning-impulse voltage test of electrical equipment is specified in IEC 60060-1 "Highvoltage test techniques", whereby the standard waveform has a front time and a time to halfvalue of 1.2/50 μs. These values have been unchanged from IEC Ed. 1 in 1962 through to Ed. 3 in 2010, and further study is also ongoing with 1.2/50 μs as a starting point in the currently active WG "Adaptation of TC 42 standards to UHV test requirements". The present paper initially reviewed how studies had been conducted to establish the lightning-impulse test in the U.S.A. and investigated the flow of how the study results linked to IEC Ed. 1 together with the standards in Europe. Eventually, it seems that, in terms of the flow from the U.S.A., the current numerical values of 1.2/50 μs were most probably derived from the lightning surge observation results at the Wallenpaupack-Siegfried 220-kV system of the Pennsylvania Power & Light Company in 1929 and so forth. Subsequently, major characteristics of recent lightning surge observation results were analyzed from the perspective of current actual electrical facilities and equipment. Consequently, points common to the data of 1929 in terms of the average values and the breadth of the distribution emerged in the crest value - front time and crest value - time to half-value plots. At the same time, differences due to the individual conditions of the individual measurement positions were also recognized. The detailed analysis and study of the standard waveform of the lightning-impulse test based on the analysis results are considered issues to be addressed. [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 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. The present study obtained and evaluated the insulation characteristics where a disconnector switching surge was superimposed over the residual dc component (bias voltage) caused by operating a switchgear under a non-uniform electric field, typically represented by metallic particles. Consequently, it was concluded that the breakdown voltage was lower for positive polarity under a non-uniform electric field in the presence of a bias voltage and that it was appropriate to conduct an experiment using a positive polarity waveform in the study of dielectric strength. Furthermore, it emerged that the breakdown voltage for a positive polarity waveform was 1.05 to 1.64 times higher than that for the standard lightning impulse waveform, even if the frequency and damping rate were changed. [ABSTRACT FROM AUTHOR]

A simplified model of corona discharge for finite-difference time-domain (FDTD) computations has been applied to analyzing lightning surges propagating along overhead wires with corona discharge. The FDTD computations simulate the experiments of Inoue and Wagner . In Inoue's experiment, a 12.65-mm radius, 1.4-km-long overhead wire was employed, and in Wagner 's experiment, a 21- or 25-mm radius, 2.2-km-long overhead horizontal wire was employed. The critical electric field on the surface of the 12.65-mm-radius wire for corona initiation is set to E_0 = 1.4, 2.4, or 2.9 MV/m, and those for 21- and 25-mm-radius wires are set to E_0 = 2.2 and 2.1 MV/m, respectively. The critical background electric field for streamer propagation is set to Ecp = 0.5 MV/m for positive voltage application and Ecn = 1.5 MV/m for negative voltage application. The FDTD-computed waveforms (including wavefront distortion and attenuation at later times) of surge voltages at three different distances from the energized end of the wire agree reasonably well with the corresponding measured waveforms. Also, the FDTD-computed waveforms of surge voltages induced on a nearby parallel bundled conductor agree fairly well with the corresponding measured waveforms. [ABSTRACT FROM AUTHOR]

Lightning-induced voltages on a 738-m long overhead two-wire line have been computed using the 3-D finite-difference time-domain (3-D FDTD) method for solving Maxwell's equations. The 3-D FDTD method employed here uses a subgrid model, in which spatial discretization is fine (cell side length is 0.9 m) in the vicinity of overhead wires and coarse (cell side length is 4.5 m) in the rest of the computational domain. The overhead wires having radii of some millimeters are simulated by placing a wire having an equivalent radius of about 0.2 m (≈0.23 × 0.9 m) in the center of an artificial rectangular prism having a cross-sectional area of (2 × 0.9 m) × (2 × 0.9 m) and the modified (relative to air) constitutive parameters: lower electric permittivity and higher magnetic permeability. Induced-voltage peaks computed at different points along the line for the return-stroke speed of 130 m/μs and ground conductivity of 3.5 mS/m agree reasonably well with the corresponding voltage peaks measured in the rocket-triggered lightning experiment of Baker et al., in 1996. [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]

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. In this method, temporary overvoltage is evaluated using an ?independence model?. This ?independence model? assumes no remaining influence of voltage application (i.e. the model obeys the failure probability distribution starting at time 0 each time the voltage is applied). However, the authors have conducted insulation characteristics tests by applying a voltage multiple times at intervals and clarified that the influence of voltage application remains to a certain extent (i.e. the effect assumed in the ?accumulation model? appears) as well as the degree of influence depending on the conditions for oil-immersed transformer. The present paper proposes an evaluation method with the degree of influence of voltage application history taken into consideration. In the proposed method, the portions following an ?independence model? and an ?accumulation model? are respectively assumed and combined after multiplying the relevant coefficient representing the respective degrees of influence. According to the trial calculation based on the proposed method, the required withstand voltage decreased by several percentage points, while it was confirmed that the existing method is on the safer side as a means of evaluating the insulation reliability of temporary overvoltage. [ABSTRACT FROM PUBLISHER]

Global warming caused by such gases as CO is a subject of great concern. Automobile emissions are an especially great problem in this respect. Therefore, hybrid cars are being widely developed and used. Because hybrid cars use electric power and gasoline, their emissions of CO are reduced. The electric motor of a hybrid car is driven by a battery, which has large capacity. Therefore, relays must interrupt a high DC current on switching between the electric motor and the gasoline engine, and hydrogen gas-filled relays are used for the purpose. In interruption tests in which we investigated the basic characteristics of hydrogen gas, the DC current did not reach a current-zero point. Thus, the current must be coerced to zero by using a high arc voltage. The loss coefficient and arc voltages of hydrogen are high, and we therefore performed interruption tests using a high arc voltage. Interruption tests and dielectric breakdowns test of air, pure hydrogen, and a hydrogen-nitrogen mixture indicated that an 80%-20% H-N mixture is the most effective. © 2010 Wiley Periodicals, Inc. Electr Eng Jpn, 174(2): 9-17, 2011; Published online in Wiley Online Library (). DOI 10.1002/eej.21042 [ABSTRACT FROM AUTHOR]

It is important to analyze the phenomena connected with an electromagnetic wave that propagates spherically. A typical example is the propagation of a current perpendicular to the ground, such as a surge current that propagates along a tall structure like a transmission tower. Some distinguishing characteristics reported for a current propagating on a vertical conductor are the attenuation or distortion of the waveform. However, no measurements have been performed by which any distortion of the current waveform and the electric potential on the conductors can be quantitatively characterized. This paper presents the results of experiments to determine the current distortion on a straight wire conductor and the electric potential perpendicular to the conductor. The current waveform was distorted while propagating, although there was no change in the potential waveform. Finite-difference time-domain method results were in good agreement with the experimental results in terms of both the current and potential waveforms. [ABSTRACT FROM AUTHOR]

The reliability evaluation using a Weibull distribution function is studied at the point of handling the coexistence of multiple voltage levels. In this paper, an insulation test based on the one-minute step-up method is used in order to calculate and evaluate the appropriate test results using the "independence model" or "accumulation model". First, the existing measurement data were reviewed, which included data on insulating elements of oil-filled transformers and data on penetration breakdowns of epoxy spacers of gas insulated switchgear. These sets of digital data were subjected to digital processing on a consistent basis, and various parameters were calculated using the same method to summarize their insulating properties. Next, the results of the one-minute step-up test were calculated under the "independence model" and the "accumulation model" to examine which of the models the insulating properties of targets conform to. It was found that, in the one-minute step-up test, the measurement results, particularly the 50% breakdown voltage values, were between the calculated values under the two models in many cases and did not completely conform to either of the assumptions of these models. This is considered to be because part of the voltage application in a previous step partly leaves a cumulative influence. [ABSTRACT FROM AUTHOR]

In lightning impulse withstand voltage tests for substation equipment, waveforms regulated in the standard are sometimes difficult to generate on large equipment of the UHV class due to the level of capacitance of the test equipment "Ct" and the level of the residual inductance of the test circuit including the power supply "Ls". In the present study, the relationships between the capacitance of the test equipment, residual inductance of the circuit, the front time "Tf" of generated waveforms, and the overshoot rate "β" were obtained using a simple equivalent circuit by assigning parameters systematically. Consequently, it has been found that, while Ct and Ls are small, there is a series resistance "Rs" range in which both the Tf and β conditions are met and that, as Ct and Ls grow larger, there is an Rs range in which only one of the conditions is met but not both the conditions, and in the end even the Tf condition alone is not achieved. An increase either in β or Tf increases the testable Ct upper limit or permissible Ls limit value, and an increase in Tf is found to be more effective. [ABSTRACT FROM AUTHOR]

Lightning strokes to transmission lines have been estimated using the electro-geometric model proposed by Armstrong and Whitehead. The observed results of the lightning to large-scale transmission lines are reported here to validate the calculated results, in which ultra high voltage (UHV) designed transmission lines and 500 kV transmission lines were selected as the subjects of the observation. Lightning observations were carried out for direct strokes to phase conductor caused by shielding failures, as well as strokes to ground wires that have rarely been previously reported. The observed results showed that total number of direct lightning strokes to phase conductor were nearly identical to the calculated results based on the conventional method, while observed strokes to upper phase were larger and strokes to lower phase were smaller than those derived from calculations. It was also revealed that lightning strokes to ground wires of UHV designed transmission lines and 500 kV transmission lines were 5.1 times and 2.7 times larger, respectively, than calculated results based on the conventional method. [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 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 non- uniform 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. [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]

In order to rationalize the lightning protection design of distribution lines, lightning phenomena on TEPCO's distribution lines were continuously observed. The observations were carried out with still cameras and lightning surge monitoring sensors. The results elucidate the lightning performance of distribution lines. For example, direct lightning strokes are the major cause of the present distribution line faults. This fact suggests that it is important to investigate observation data on direct lightning strokes. The observation data were analyzed by means of a direct lightning analysis model. The calculated waveforms were compared with the measured waveforms on distribution lines. The probability distributions of the surge currents and voltages were calculated by the Monte Carlo method. These results provide new data on lightning performance. © 2009 Wiley Periodicals, Inc. Electron Comm Jpn, 91(10): 28– 37, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10164 [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, 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. As a result, in the tested range, the dielectric breakdown values for non-standard lightning impulse waveforms were higher than those for the standard lightning impulse waveform by 3% to 36%. In these experiments, parameters other than waveforms were fixed to standard conditions. In this paper insulation characteristics on quasi-uniform SF6 gas gaps were examined while changing conditions on gas pressures, gap lengths, electrode surface roughness, voltage polarities, and bias voltages. As a result, it was confirmed that the results under standard experimental conditions in the preceding experiments can be applied widely to the GIS actual equipment conditions. [ABSTRACT FROM AUTHOR]

Because of the excellent high insulation performance, vacuum is expected to be one of the alternatives of SF6 gas. In order to reduce the use of SF6 gas in substation equipment, the applicability of vacuum circuit breakers (VCB) and vacuum interrupters (VI) should be extended to higher (typically 145-300 kV) voltage. For the high voltage VI, the role of electrical insulation design becomes more important. For accurate insulation design of the VI, we developed an optimization technique to improve the electrical insulation performance. We evaluated several design variables necessary for optimizing the electrode contour of the main contactor and center shield in VI. The electrode area effect on vacuum breakdown characteristics as well as the electric field distribution was considered in the optimization process. In order to verify the accuracy and the efficiency of the proposed technique, we applied it to a practical model of VI. In addition, we examined the influence of the parameters of the area effect, arrangement of VI in VCB on the optimization result. From the calculation results, we confirmed that the optimization based on the area effect had higher availability and higher accuracy than the electric field optimization from the viewpoint of the improvement of the electrical insulation performance of vacuum insulated equipment. [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, 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 obtaining through experiments the insulation characteristics for single-frequency oscillation waveforms as a representative waveform when there is a non-uniform electric field represented by metallic particles in the equipment while changing the applied voltage polarities, frequencies, and damping factors. As a result, it was found that the dielectric breakdown voltages are lower in the positive polarity and it was confirmed appropriate to conduct experiments mainly in positive-polarity in order to consider dielectric strengths. Furthermore, changing the frequencies and damping factors under positive-polarity waveforms did not cause major changes in the feature of the dielectric breakdown voltages, which remained constantly higher (by 1.41 to 2.58 times) than those under standard lightning impulse waveforms. [ABSTRACT FROM AUTHOR]

Since the majority of vacuum circuit breakers in use are of the single-break variety, there have been few studies of double-break vacuum circuit breakers. This report describes our investigations of the interruption characteristics of the double-break vacuum circuit breaker. In this investigation, the post arc current and recovery voltage distribution ratio were measured in order to clarify the relationship between them. The results show that a bias of the voltage distribution ratio occurs when the post arc current value becomes imbalanced. In addition, the measurements of the post arc current for circuit breakers with spiral contacts are very scattered as compared with those with axial magnetic field contacts. Normally, the double-break vacuum circuit breaker consists of two contacts of the same type. However, when two spiral contacts are used, the scattering of post arc current causes a current imbalance. As a result, a bias of the voltage distribution ratio may occur. It appears to be preferable to use double-break vacuum circuit breakers with axial magnetic field contacts. [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]

This paper has investigated the nonuniform characteristics of a counterpoise based on the frequency-dependence of R, L, C, and G and propagation parameters identified from measured results of transient voltages and currents on the counter- poise. It becomes clear that the characteristic impedance is significantly length-dependent (i.e non-uniform, at a high frequency). As the frequency tends to infinity, the maximum value of the characteristic impedance is almost the same irrespective of its geometry, physical constants and soil conditions. However, the characteristic impedance at a section near to source end is far smaller than that near to the remote end. The apparent series impedance increases and the admittance decreases for a counterpoise section farther to the source end. The propagation constant shows less noticeable nonuniformity than that for the characteristic impedance. [ABSTRACT FROM AUTHOR]

Since transmission towers are taller and interphase distances are longer for ultra-high voltage (UHV) transmission lines than for conventional transmission lines, it is important to know the features of direct lightning strokes to phase conductors. With this in mind, the features of direct lightning strokes to UHV transmission lines were observed and 81 datasets were obtained for seven years between 1998 and 2004. Of the 81 datasets, 79 recorded strokes from negative lightning, and the characteristic features of these direct lightning strokes were quantitatively evaluated by Electromagnetic Transients Program analysis of the current amplitude, front duration, and stroke duration. The frequency distribution of lightning strokes to each phase conductor was different from the estimated values obtained from conventional shielding models. Based on the cumulative frequency distribution of the current amplitude observed in this study, a method is proposed for calculating direct lightning stroke current as part of designing lightning protection. [ABSTRACT FROM AUTHOR]

Because of the lower insulation level than for transmission lines, in this study on lightning protection design of distribution lines, not only direct lightning strokes but also induced voltages caused by nearby strokes must be taken into account. Thus, it is necessary to grasp the frequency of occurrence of lightning phenomena around distribution lines. For this aim, lightning phenomena on TEPCO's distribution lines in use were continuously observed for 6 years (1996 to 2001). Through this observation, new interesting statistical data that can be a useful basis for rationalization of lightning protection design of distribution lines were obtained. Two hundred and four lightning strokes were observed through the six-year period. Probability of occurrence of a direct stroke was 22% (45 direct strokes were obtained). The fact that in 47% of direct strokes the electric outage did not occur is very interesting. Moreover, the value of surge discharge current of the arrester in the case of nearby stroke is the basis for conventional theory of grounding system. Therefore, statistical analysis of ZnO discharging current in the case of nearby strokes was carried out. As a result, in the case of nearby strokes, 5% value of cumulative frequency of discharging current is 0.5 kA. This value is half the conventional data. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 155(2): 8–16, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20180 [ABSTRACT FROM AUTHOR]

In order to attain reduction in insulation test voltages, voltage-time (V-t) characteristics under AC voltage and voltage-number (V-N) characteristics under lightning impulse and switching impulse voltages were experimentally investigated on the turn-to-turn insulation model, section-to-section insulation model and barrier-oil-duct insulation model of core- and shell-type transformers in EHV and UHV Classes. First, V-t characteristics for AC voltage were obtained in a short time range of a few tens of milliseconds to a long time range of three to four months. The n-values (inclination) of V-t characteristics are distributed around 40 for the short time and several hundreds for the long time characteristics. These values are of essence in determining the AC withstand voltage test. Next, V-N characteristics for both lightning and switching impulse voltages were acquired for up to 1,000 times of application. The n-values (inclination) of V-N characteristics are in most cases distributed around 70 for both impulse waveforms and about 40 in only turn-to-turn insulation models. This result will be useful for evaluating the effect of frequent surges on apparatus insulation. Finally, insulation coordination studies were performed based on these experimental results. [ABSTRACT FROM AUTHOR]

Gas insulated switchgear (GIS) has been widely applied in power equipment since the late 1960s due to its reliability and compactness. Because epoxy insulators in GIS are important insulation components that affect the dielectric withstand level and the equipment lifetime, high voltage tests and sensitive partial discharge (PD) tests have been carried out on them in the factory. If a latent defect occurs inside an actual epoxy insulator, its size is predicted to be small, such as a micro-defect, because it will have passed rigorous tests and multiple inspections at the factory. Although many studies have clarified the fundamental phenomena of deterioration caused by PD occurring due to defects in inner epoxy, little is known about the actual size of such defects and their effect on insulation properties. Therefore, to assess the risks associated with aging GIS equipment, it is essential to understand the actual size of latent defects in inner epoxy insulators of GIS and the impact the defects have on the insulation performance. The puncture properties and PD characteristics of artificial micro-defects of cracks, voids, and delaminations produced using an accurate estimation technique and precise production control were investigated and are described. The results suggest that an estimation method using electric field analysis is useful for designing the defect shape and that the breakdown voltage depends on the three-dimensional shape of the defect. Techniques for accurate estimation of small defects in epoxy resin and stable production of micro-defects have thus been achieved. [ABSTRACT FROM PUBLISHER]

The technical soundness of an oil-immersed power transformer under an operating voltage is evaluated in partial discharge (PD) test of long-duration ac withstand voltage test. To establish clear control criteria for this PD test, it is necessary to study PD propagation characteristics inside the transformer windings. In this paper, an analysis model was constructed using constants in the oil for a 500 kV-class transformer to analytically obtain PD signal propagation characteristics. For this analysis model, a circuit model for high frequencies up to several MHz was used in order to obtain the characteristics of highfrequency PD propagating inside the winding in detail. Using this model, PD propagation characteristics were investigated in terms of the PD occurrence position, measurement position, and front time as parameters. As a result, it emerged that the propagation pattern was determined by the winding constants as viewed from the PD injection position and the terminal conditions. When the charge amount was the same, the steeper the PD signal front time, the higher the peak voltage value was likely to be because the waveform became more oscillatory. Conversely, assuming an actual test using the ERA device (PD detector developed by Electrical Research Association), it was found that the front time had little impact on the charge amount measured because the oscillatory high-frequency components were removed. As with the previous studies, it emerged that the PD signal transmission rate was lowest when a PD occurred between sections and the signal was detected as one significantly damped to a level of several percent depending on the conditions. [ABSTRACT FROM AUTHOR]

The lightning impulse withstand voltage test waveform for electrical power equipment is specified in IEC 60060-1 ???High-voltage test techniques???. Now that ultrahigh voltage-class (UHV-class) electrical power equipment is being introduced and increasingly used, the electrical capacitance of equipment to be tested has increased, which has eventually hampered the test using the waveform that conforms to the previous standard. To address this issue, the IEC revised the IEC 60060-1 standard in 2010 and now is studying the standard for UHV-class electrical equipment. In this revision, an evaluation method using the k-factor function (test voltage function) was introduced, whereby the overshoot waveform was converted to a test voltage waveform. To date, concerning these k-factor values, measurement results based on experiments have been reported, but most were for small-scale insulation models with breakdown voltage levels mainly around 100 kV. This paper reports the experimental result of insulation breakdown characteristics for lightning impulse withstand voltage test waveforms in the model with the largest oil gap possible assuming an actual UHV-class oil-immersed transformer. Breakdown voltages and breakdown times were measured with the superimposed oscillation frequency, overshoot rate and front time as parameters. Consequently, it was indicated that, as the overshoot rate increased, the 50% breakdown voltage was inclined to increase. Subsequently, the evaluation using the k-factor function is considered to be useful for UHV-class oil immersed transformer. The result of evaluation of the k-factor value based on the experimental result was almost identical to the result of the European Project, which is the basic data of the existing k-factor function. As a result, it emerged that the existing k-factor function itself was most likely to be applicable to UHV-class oil-immersed transformers. Furthermore, since the extension of the front time up to about 3.6 ??s caused only a small change in insulation breakdown characteristics, the extension of the front time up to about 3.6 μs might possibly be allowed in the standard that assumes a UHV-class transformer. [ABSTRACT FROM PUBLISHER]

The soundness of a gas-filled power transformer under an operating voltage is evaluated in partial discharge (PD) test of long-duration ac withstand voltage test. To establish clear control criteria for this PD test, the authors initially conducted a study on the harmful PD level of materials themselves used in a gas-filled transformer. In the actual PD test, since the PD having occurred inside the transformer is measured from the outside, the measured signal is damped due to various influential factors. Since this damping must be taken into consideration to establish control criteria, a preceding study investigated the propagation characteristics of PD signals through actual measurement using transformer winding models. As a result, it was found that the PD having occurred inside the winding is detected as the signal significantly damped depending on the position of occurrence. This paper studies whether PD signal propagation characteristics can be analytically evaluated using the equivalent circuit of the transformer winding. This analytical evaluation, if feasible, can be applied to various winding structures and will make it possible to evaluate the propagation characteristics in further detail as well as the validity of the actual measurement results. The study was conducted on the interleaved and continuous disk windings to make a comparison between the analysis and measurement results of the voltage waveform and the propagation pattern at various locations inside the winding with the position of occurrence and measurement position of the PD as parameters. Consequently, it emerged that the analysis using the equivalent circuit could adequately simulate the propagation pattern and waveform shape of the PD signal. A detailed study using this equivalent circuit also clarified the fact that the PD signal propagation waveform was determined by the winding constant and the terminal conditions. Furthermore, the analytical and measurement results of the transmission rate were almost identical, whereby it was analytically confirmed that the transmission rate significantly decreased depending on the position of occurrence of the PD. It also emerged that, since the frequency characteristics of the PD measurement device may influence the transmission rate, the PD control criteria must be studied taking such influence into consideration. [ABSTRACT FROM AUTHOR]

To establish clear control criteria for a partial discharge (PD) test of long duration ac withstand voltage test of a gas-filled power transformer, the authors have conducted a study on the harmful PD level of materials themselves using the insulating material- and structuralmodels of an actual gas-filled power transformer. In the PD test on an actual transformer, since the PD having occurred inside the transformer is measured from the outside, the measured signal is damped due to various influential factors, such as the position of occurrence, measurement position, and internal structure of the equipment. In particular, in the event of the PD between sections or turns, which might actually occur, significant damping of the signal is anticipated in certain cases due to the influence of the position of occurrence and the internal structure of the equipment, which is a propagation path. Therefore, this damping must be taken into consideration to establish control criteria for the PD. In this paper, to understand the characteristics of this damping, such as its magnitude, two types of winding, interleaved disk and continuous disk, were produced as transformer models to investigate the propagation characteristics of PD signals with the position of occurrence, measurement position, and wavefront duration of the PD as parameters. Consequently, it emerged that, where the PD occurs between sections or turns, the transmission rate at the measurement position was generally low because the PD signal circulates within the PD occurrence parts and does not come outside. Assuming measurement at the bushing test terminal, the lowest transmission rate was 2.8% and 2.0% between sections and turns, respectively. A study was also conducted on the occurrence of the PD against ground, despite the low anticipated potential of actual occurrence, and the lowest transmission rate was 4.0%. Accordingly, the PD having occurred inside the winding of the transformer can be detected with a magnitude of 2% or more; however, from a reverse perspective, the signal is detected as one significantly damped to a level of about 1/50. [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 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]

In a lightning impulse withstand voltage test for large-sized electric power equipment, voltage waveforms sometimes contain overshoot due to the capacitance of the test equipment and the residual inductance of the test circuit including the power source. The authors have been studying fitting methods for extracting a reasonable base curve from this overshoot waveform. In the previous study, each of the fitting methods was applied to the overshoot waveform simulated using a mathematical equation to evaluate the respective merits and demerits. As the test waveform, the present paper used the overshoot waveform actually generated using a large-scale test circuit. This actual waveform was determined as one over which an oscillation waveform of several hundred kHz, which was actually at issue, was superimposed. The correct base curve of the actual waveform was derived through detailed analysis of the equivalent circuit of the test circuit and the overshoot rate was calculated based on this correct value. This correct overshoot rate was compared with that calculated using each of the fitting methods to evaluate which was an optimum fitting method. Consequently, it was found that an overshoot rate close to the correct value could be derived using the new base curve extraction method for the actual waveform. Meanwhile, it should be noted that the kfactor filtering scheme was implemented after the application of various fitting methods, and then the shape parameters of the test voltage waveforms, such as the crest value, wavefront duration, and wavetail duration, were almost identical, which confirmed that fitting methods had only a minor influence on the test voltage waveform. [ABSTRACT FROM AUTHOR]

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. [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]

The authors investigated the influence of a T-branch on propagation properties of PD-induced electromagnetic (EM) wave measured with ultra high frequency (UHF) sensors in 66 kV and 154 kV gas insulated switchgears (GIS). A low pass filtering (LPF) process for the cutoff frequency of TE11 mode was used to eliminate the higher order mode above TE11 of detected signal. The authors discussed the effects of the T-branch on the positive peak amplitude of the first wave Vp of propagating EM wave after the LPF processing. As a result, TEM mode propagates according to the traveling theory based on the distributed constant circuit with considering the characteristic impedance of the T-branch. In addition, a high pass filtering (HPF) process for the cutoff frequency of TE11 mode was used to eliminate the lower frequency component below TE11 of the detected signal. The authors discussed the effects of the T-branch on the peak to peak amplitude Vpp of propagating EM wave after the HPF processing. The authors compared transmission rate THS of propagating EM wave after HPF processing through T-branch in a straight direction with that THR of propagating EM wave after HPF processing turning in a right angle direction at the T-branch. As a result, it was found that THS was larger than THR with the HPF processing. [ABSTRACT FROM AUTHOR]