| Budget Amount *help |
¥3,300,000 (Direct Cost: ¥3,300,000)
Fiscal Year 2006: ¥900,000 (Direct Cost: ¥900,000)
Fiscal Year 2005: ¥2,400,000 (Direct Cost: ¥2,400,000)
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| Research Abstract |
We have examined charging and flashover characteristics of polymeric and glass insulators with 10-50 mm long under DC or AC voltage excitation in vacuum. This is a basic study for developing high voltage VCBs (Vacuum Circuit Breakers) adapted to electric power systems higher than e.g. 100 kV. The high voltage VCB is considered as an alternative of GCB (Gas Circuit Breaker) in order to reduce SF6 that has a high global warming potential. The insulation design of the vacuum bottle made of glass or ceramics is of importance for developing such VCBs with high reliability and compactness. We conducted flashover tests, charge measurements and calculation of surface charge distributions. The insulator was made of PMMA or borosilicate (Pyrex【○!R】), and was in the shape of a right cylinder or a hollow cylinder. The charging characteristics of insulators were investigated by using an electrostatic probe, which was embedded in one of the plane electrodes that holds the insulator and allowed a tim
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e-resolved measurement of the charging process. It is believed that the charging of insulator surface precedes and leads the bridged vacuum gap to the flashover.
One of the purposes of this study is to find out charging mechanism when specimens are subjected to AC voltages. It has been clarified that the charging is characterized by three sequential states; initiation, quasi-stable and stable states, and that the polarity of the charge is positive for these states irrespective of the voltage phase. The charging characteristics with AC voltage are compared to the results with DC voltage excitation. We found that the charge magnitude at the stable state coincides with that obtained by DC. The electric field strength on the grounded electrode, therefore the charge magnitude, decreases with the surface roughness and decreases as the insulation strength is increased. A computer simulation clarified that the transition in surface charge distribution being synchronous to the voltage phase is responsible for causing the quasi-stable state.
The second purpose is to clarify the effect of surface roughness on charging and to obtain useful data for designing a high voltage vacuum bottle. For comparatively short specimens with less than 10 mm in length, wet have clarified previously that charging is depressed as the roughness increases and that the insulation ability improves. The purpose of the present study is to confirm this effect for longer insulators. In the case of right cylinder, the side surface of the specimen was polished to have an average roughness of sub microns, mirror finish, or roughened up to several microns. In the hollow cylinder, the inner side surface of the specimen was polished, while the outer side had a roughness which is enough to prevent from charging. As a result, we have confirmed that roughening the surface prevents the charging and thus improves the insulation strength for a specimen up to 50 mm in length. This improvement can be attributed to the fact that the electrons in vacuum during the charging process have less than 1 μm in hopping height measured from the surface, the surface irregularities act as barriers against those hopping electrons, which is one of the main pre-discharge phenomena leading the insulator to flashover. Less
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