H. Okubo

Suppression of Static Electrification of Insulating Oil for Large Power Transformers

Streaming electrification of large power transformers is discussed here from the standpoint of characteristics of insulating oils used in them.

Estimation of fast transient overvoltage in gas-insulated substation

The authors measured FTO (fast transient overvoltages) at various points of a commercial 550 kV gas-insulated substation (GIS) by operating energized disconnectors. Based on the results of these measurements, the possible levels of FTO at various points of the GIS and the probability of occurrence of large levels are discussed. Laboratory tests were also performed by using a recently developed 1 GHz surge sensor to check on the insignificance of high-frequency components due to short-surge impedance discontinuities such as spacers, disconnectors, and short bus branches. Waveforms calculated by rough simulation that ignored the short surge impedance discontinuities are compared with measured waveforms at the commercial substation, proving that simulated FTO levels agreed well with measured values. >

Area effect on electric breakdown of copper and stainless steel electrodes in vacuum

The breakdown field strength E/sub b/(= alpha Vb/d) of gaps made with pure copper and stainless steel depends on the effective area S/sub eff/. It can be expressed by the single experimental equation E/sub b/=K/sub 1/S/sub eff//sup -n/, where K/sub 1/ is a constant, n is about 0.24, S/sub eff/ is the electrode area subjected to more than 90% of maximum field strength, alpha is the shape factor, d is the gap length, and V/sub b/ is the breakdown voltage. The field intensification factor beta /sub l/ obtained through the measurement of prebreakdown current can be expressed by a single experimental equation beta /sub l/=K/sub 2/S/sub eff/n, where K/sub 2/ is a constant. This means that the area of effect of E/sub b/ is represented by the area effect of beta /sub l/ and that the vacuum breakdown may be initiated by field emission. >

PD and BD Probability Distribution and Equi-Probabilistic V-t Characteristics of Oil-Filled Transformer Insulation

Probability distributions of partial discharge (PD) and dielectric breakdown voltage were investigated for the typical components of oil-filled transformer insulation.

Development of Vacuum Interrupters with High Interrupting Capacity

Vacuum interrupters, in which axial magnetic field type electrodes and CuCr contact materials are ccnbined, show excellent performance from all points of view. By this combination of electrode and contact material, the arcing phenomenon is scmewhat different from that of existing combinations. We have investigated the arcing and dielectric phenanena in detail and have found the optimum conditions. As a result, the new vacuum interrupters were developed in which interrupting capabilities have been improved. The application range is fran 12 to 84 kV in rated voltage and up to 5OkA in interrupting current. This type of vacuum interrupter was utilized widely in general field applications.

Developing separate-cooling/sheet-winding gas-insulated transformer

A fireproof, high-voltage transformer based on a concept of separate cooling, aluminium sheet-winding, and polymeric-film/gas insulation has been developed to complete a single-phase prototype transformer (rated at 50 MVA and 275 KV). The development process is described. >

Metallic Particle Motion in 3-Phase SF 6 Gas Insulated Bus

To investigate the behavior of metallic particles in 3-phase gas insulated buses (3-phase GIB), we conducted a test by using a 3-phase 275kV, ac, 60Hz full-scale model. The test results revealed that in a 3-phase electric field, unlike in a single-phase field, metallic particles were trapped on and around the high-voltage conductors and revolved circumferentially. Computer analysis shows that it was closely related to locational and periodic changes in the electric field provided by the 3-phase GIB, and that there were points on the high-voltage conductors in which metallic particles were readily trapped. It also was proved that phenomena peculiar to the 3-phase GIB could be reproduced very well by computer simulation.To investigate the behavior of metallic particles in 3-phase gas insulated buses (3-phase GIB), we conducted a test by using a 3-phase 275kV, ac, 60Hz full-scale model. The test results revealed that in a 3-phase electric field, unlike in a single-phase field, metallic particles were trapped on and around the high-voltage conductors and revolved circumferentially. Computer analysis shows that it was closely related to locational and periodic changes in the electric field provided by the 3-phase GIB, and that there were points on the high-voltage conductors in which metallic particles were readily trapped. It also was proved that phenomena peculiar to the 3-phase GIB could be reproduced very well by computer simulation.

The effect of mechanical contact on breakdown characteristics in vacuum

Using copper-chromium electrodes, an experiment was conducted to clarify the reduction of insulation strength by mechanical contact in vacuum. The experiment consisted of prebreakdown current measurements and SEM (secondary electron microanalyzer) observation of the electrode surface. Experimental results revealed that the field intensification factor obtained from the prebreakdown current was increased by mechanical contact. SEM observation showed that mechanical contact caused microprotrusions to be generated. This phenomenon suggests that a reduction of insulation strength by mechanical contact is caused by field emission from a roughened surface. If mechanical contact is made, dielectric breakdown occurs when the field strength of microprotrusions on the cathode surface reaches a critical value (1.0 to 1.7*10/sup 8/ V/cm). >

Development of insulation technology for high-voltage gas-insulated transformer

Insulation technology development for 275 kV 300 MVA class SF/sub 6/ gas-insulated transformers is described. The main area of development is turn-to-turn insulation using polymer film in sheeting winding. It was experimentally demonstrated that two sheets of film, several tens of micrometer in thickness, had sufficient insulation strength for impulse and short/long-term AC voltage application. Sufficient insulation performance for coil end insulation was demonstrated by a full-scale impulse model, taking into consideration the results of three-dimensional electric field analysis. A 275 kV 50 MVA prototype with an insulation structure equivalent to that of 275 kV 300 MVA units was developed, and a three-month reliability test was conducted. >

INSULATION CHARACTERISTICS OF SEPARATE COOLING SHEET WINDING-TYPE GAS-INSULATED TRANSFORMER

ABSTRACT This paper describes the insulation characteristics of SF6 gas-insulated, high-voltage, large-capacity transformer. This transformer is based on a new concept of separate cooling, aluminum sheet winding, and polymeric film/gas insulation. Regarding insulation development of this transformer, turn-to-turn insulation characteristics were clarified, and long-term reliability was confirmed. End insulation of sheet winding in SF6 gas was investigated, and its impulse test results are discussed with electric field calculation. These experimental results of the insulating component of the winding are summarized, and a single-phase prototype transformer rated 275kV-50MVA was developed.