Genyo Ueta

Evaluation of breakdown characteristics of gas insulated switchgears for non-standard lightning impulse waveforms - breakdown characteristics under non-uniform electric field

To lower the lightning impulse withstand voltage of gas insulated switchgear (GIS) while maintaining the high reliability of its insulation performance, it is important to define in an organized way the insulation characteristics for non-standard lightning impulse voltage waveforms that represent actual surge waveforms in the field and compare them with the characteristics for the standard lightning impulse waveform quantitatively. In the 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.

New aspects of UHF PD diagnostics on gas-insulated systems

Gas insulated switchgears (GIS) have been increasingly introduced as main substation equipment since the late 1960s thanks to their high reliability, safety, and compactness. The UHF method to diagnose partial discharge (PD) is broadly employed in the field as an advanced insulation diagnostic technology. There are three primary factors related to the PD signal detection characteristics using this UHF method; namely the frequency spectrum of the PD signal itself inside GIS, the propagation characteristics of the PD signal inside the GIS tank, and the detection characteristics of the sensor measuring the PD signal. Accordingly, to further improvement of the diagnostic technique based on the UHF method, the PD phenomenon itself and its propagation characteristics should be clarified and the measurement system refined. From these perspectives, the present study reviewed the recent and latest findings concerning the PD phenomenon and its propagation characteristics inside GIS as well as the PD measurement system verification methods and diagnostic technologies, and summarized typical examples. Firstly, with regard to the PD phenomenon, measurement up to the high frequency band exceeding 10 GHz (even 30 GHz) clarified that the rise time of the PD current waveform is several tens of picoseconds, shorter than previously known. In the case of PD in micro-defects inside the epoxy insulator, however, the rise time is relatively long, in the order of nanoseconds, and a crack is the most critical defect. Subsequently, with regard to UHF PD diagnostics/monitoring technology, a novel technique using PD current waveform characteristics has been developed, and a more advanced PD diagnostic algorithm has been established by clarifying the influence of the shape and GIS internal structures on the electromagnetic wave propagation characteristics. Less uncertain and simplified calibration and verification technologies are also proposed both for a single UHF sensor and the entire UHF measurement system. These new technologies and further advanced studies in future are expected to make the UHF method more convenient and sophisticated.

Evaluation of breakdown characteristics of gas insulated switchgears for non-standard lightning impulse waveforms under diverse conditions

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.

Dielectric properties of gas mixtures with C 3 F 8 /C 2 F 6 and N 2 /CO 2

Thanks to its excellent insulation and cutoff performances, SF₆ gas has been applied to power equipment since the 1960s and is now widely used for several kV to 1,000 kV-class GIS, GCB, and GIL. However, since 1997, when SF₆ was designated at COP₃ as a greenhouse gas to be reduced, there has been a wish to use an alternative insulating gas to pure SF₆ gas. In the present study, alternative gases were selected from among mixtures excluding SF₆ with the need to reduce GWP (global warming potential) in mind. Gas mixtures containing such substances and with a boiling point of -20°C or less, chemically stable, non-toxic, and not ozone-depleting were prioritized. Furthermore, the availability and environmental performance were taken into consideration when deciding on component gases. Consequently, to launch this series of studies, four types of gas mixtures were used combining a gas in Group A (C₂F₆, C₃F₈) - electronegative gases with relatively high dielectric strength - and a gas in Group B (N₂, CO₂) - gases existing in the natural world. The GWP of SF₆ is 22,800 whereas that of C₂F₆ is 12,200 and that of C₃F₈ is 8,830, or several times smaller than that of SF₆. In the present paper, insulation characteristics were experimentally obtained while varying the mixture ratio under a quasi-uniform electric field assuming GIS. Consequently, compared to the GWP of pure SF₆, the GWP was about 12% to 38% for gas mixtures with C₃F₈/N₂ or C₃F₈/CO₂ and 18% to 70% for gas mixtures with C₂F₆/N₂ or C₂F₆/CO₂. Consequently, it emerged that, while assuming breakdown voltage proportional to gas pressure, the GWP was likely to be reduced by 30% to 90% while maintaining dielectric strength. In addition, a study was conducted on the synergism of a gas mixture through analysis using the Boltzmann equation. Consequently, the synergism was confirmed while its degree varied depending on the type of each gas mixture, and the mechanism thereof was clarified.

Insulation characteristics of oil-immersed power transformer under lightning impulse and AC superimposed voltage

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.

Resistance characteristics and electrification characteristics of GIS epoxy insulators under DC voltage

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.

Breakdown mechanism in C 3 F 8 /CO 2 gas mixture under non-uniform field on the basis of partial discharge properties

This paper investigates simultaneous measurements of partial discharge (PD) current and the corresponding light emission phenomena of C3F8/CO2 gas mixture under a non-uniform field with AC high voltage application in order to discuss the PD properties and breakdown mechanism of new gaseous dielectrics as SF6 substitutes. Perfluoro carbon (PFC) gas mixture is a possible candidate as SF6 substitute because it almost fulfills the environmental aspects, dielectric properties, boiling point, and other requirements of SF6 substitute. The authors simultaneously measured PD phenomena such as time-sequential PD current pulses and corresponding light emission images before breakdown during the positive half cycle of the applied AC voltage where most breakdowns occurred. Consequently, it was found that there were three kinds of current pulse patterns, which depended on the gas pressure and the applied voltage amplitude. It was also confirmed that breakdown occurred around the voltage peak, i.e. 90 degrees. of the applied voltage, irrespective of the PD pattern. Furthermore, the simultaneous measurements enabled the authors to classify the discharge area in the N-shaped characteristic of the breakdown voltage based on discrimination of discharge types between streamer and leader discharges. Namely, the location where leader discharge is generated by the transition from streamer discharge in the N-shaped characteristic was indicated. These experimental results and the simultaneous measurement technique will help clarify the breakdown mechanism for new gases or gas mixtures as SF6 substitutes. The obtained results will also contribute to insulation diagnosis for next-generation gas-insulated power apparatus with the newly developed gaseous dielectrics.

Sensitivity of UHF coupler and loop electrode with UHF method and their comparison for detecting partial discharge in GIS

We introduce in this paper the sensitivity of two UHF couplers, disc-type measuring electrode and loop-type measuring electrode embedded in an epoxy insulator, for detecting partial discharge (PD) and diagnosing GIS. Each of the couplers has sufficient performance for detecting the electromagnetic waves up to 8 GHz. The attenuation characteristic of electromagnetic waves is changed depending upon the location of defects, such as a particle on an enclosure or on a conductor.

Dielectric properties of gas mixtures with per-fluorocarbon gas and gas with low liquefaction temperature

Thanks to its excellent dielectric strength and arc interruption performance, SF6 gas has been widely used for power equipment since the 1960s. However, since SF6 has a high global warming potential (GWP), it was designated at COP3 in 1997 as a greenhouse gas to be reduced; hence the search for an alternative insulating gas to SF6 to be widely used for power equipment. Aiming to reduce the global warming potential (GWP) of existing power equipment using SF6, the present study investigated breakdown characteristics when various alternative gases to SF6 were mixed. Alternative gases to SF6 handled in this report were based on four kinds of gases, namely c-C4F8, 1-C3F6, C3F8 and C2F6, from among per-fluorocarbon gases with relatively high dielectric strength. However, since these base gases have relatively high liquefaction temperature, despite their advantage in terms of GWP and dielectric strength, a gas with a low liquefaction temperature was mixed as an additive. Three kinds of gases were used as additive gases, including CF4, a fluoride gas with a relatively low liquefaction temperature, in addition to N2 and CO2. Accordingly, in this report, various combinations of the four kinds of base gases and three kinds of additive gases were used to obtain the respective insulation characteristics while varying the mixture ratio of the respective gases. Consequently, it emerged that relatively high and positive synergism was likely to be obtainable when the mixture ratio of the base gas was 20%. Furthermore, a comparison of dielectric strength with respect to the GWP revealed that the GWP was likely to be reduced to 10% or less while maintaining about 80% of dielectric strength, under conditions that the upper limit of the liquefaction temperature assuming indoor equipment was 0°C.

Behavior of metallic particles in GIS under DC voltage

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.