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The paper focuses on the factors that lower the dielectric performances of real-size gas insulated systems, and show the examples of the measures by using dielectric coatings. The factors studied in this paper are area effects of electrodes and metallic particle motion which are now dominant to determine the size of gas insulated switchgear (GIS). The gases for the systems are basically SF6 but potential gases like N2/SF6 gas mixtures, CO2, dry air and N2 are also included. Area effects of electrodes are studied by reviewing the minimum breakdown data at lightning impulse (LI) of large effective electrode area over about 5 x 104 mm2 under high gas pressures up to 1.9 MPa. The breakdown electric field strength (BDE) or breakdown voltage (BDV) is normalized by the theoretical value and evaluated in terms of effective electrode area and gas pressure. Under the same gas pressure in the range between 0.2 and 0.7 MPa at negative LI, the normalized values of SF6 are lower than those of the potential gases which show similar values. However, the normalized BDVs of N2 at positive LI are further lower than those of SF6 at negative LI. The advanced dielectric coatings enable high dielectric performances over 80% of the theoretical values in SF6 and CO2. Concerning the second factor, metallic particle motion, the experimental data of metallic particle levitation and crossing to high voltage electrode are gathered for dielectric coated and bare enclosures in SF6, N2/SF6> and dry air. The data are classified into two cases of with and without mechanical impulsive force to the enclosure, which simulates the force by the operation of circuit breakers, disconnecting switches and earthing switches. The particle lift-off/levitation electric field strength on the inner surface of dielectric coated enclosure is about 2 to 11 times higher than that on bare enclosure without a mechanical vibration. With a mechanical vibration, the particle lift-off/levitation and crossing electric field strength on dielectric coated enclosure decrease but still about 2 to 3.5 times and 1.3 to 1.6 times higher than those on bare enclosure, respectively. [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]

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]

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 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 has excellent dielectric strength and interrupting capability and is used in various applications such as gas insulated switchgear (GIS) in substations. However, since SF6 has a high global warming potential (GWP), it is imperative to reduce its use and develop recovery technology for its reuse. This paper examined the potential of alternative insulating gases for GIS, determining experimentally the ac and lightning impulse breakdown characteristics of CO2 and N2 gases, which have little environmental impact, in a quasi-uniform electric field. The effect of electrode surface roughness is greater in N2 than in CO2. The impulse ratio in a quasi-uniform electric field was smaller in N2 than in CO2 at low pressure, but this reversed when the pressure reached 1.2 MPa. The dielectric strength of an electrode with a dielectric coating was approximately equal to that of a specular-finish bare electrode in either CO2 or N2 when an ac voltage was applied. When lightning impulse voltage was applied, a significant improvement of dielectric strength was observed if a dielectric coating was applied to the electrode considered to be the initial electron source. [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 potential alternative, the authors are focusing on N2 gas, which has relatively good insulation characteristics among gases with a low environmental impact. 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 obtained and evaluated the insulation characteristics where a surge occurred in the presence of a bias voltage (dc component), which was considered realistic and severe 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 when studying dielectric strength. Furthermore, it emerged that the breakdown voltage for a positive polarity waveform was 1.06 to 1.67 times higher than that for the standard lightning impulse waveform, 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]

Mineral oil with high insulation performance and cooling capability is currently used for oil-immersed transformers. However, mineral oil is derived from petroleum and thus is a limited resource. In a search for mineral oil substitutes, we investigated the characteristics of silicone and ester oils. The electrical performance was assessed in terms of the breakdown and streaming electrification characteristics and the volume resistivity. The burning characteristics of each oil type were also investigated. The temperature dependence of the breakdown voltage in an oil unit and the surface breakdown characteristics in a composite insulating system comprising an insulating oil and an oil-immersed insulator were determined. We also investigated the temperature dependence of the charge density in oil. The charge density in silicone and mineral oils increased with temperature, with maximum values at ~ 100°C and ~ 60°C, respectively. By contrast, the charge density in ester oil decreased with increasing temperature and was maximum at ⩾30°C. [ABSTRACT FROM AUTHOR]