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Flashover Treeing Phenomena on the Surface of MgO Based Insulator under Electron Beam Bombardment

(電子ビーム照射を用いたマグネシア基絶縁体表面の フラッシュオーバートリーイング現象に関する研究)

氏名 Agus Geter Edy Sutjipto
学位の種類 博士(工学)
学位記番号 博甲第255号
学位授与の日付 平成14年8月31日
学位論文題目 Flashover Treeing Phenomena on the Surface of MgO Based Insulator under Electron Beam Bombardment (電子ビーム照射を用いたマグネシア基絶縁体表面のフラッシュオーバートリーイング現象に関する研究)
論文審査委員
 主査 教授 高田 雅介
 審査委員 教授 赤羽 正志
 審査委員 教授 植松 敬三
 審査委員 教授 小松 高行
 審査委員 助教授 安井 寛治
 審査委員 助教授 川本 昴

平成14(2002)年度博士論文題名一覧] [博士論文題名一覧]に戻る.

1. Introduction p.1
1.1 History of Research p.1
1.2 Material p.7
1.3 Purpose of Study p.9
References p.10

2. Appearance of Flashover Treeing under Electron Beam Bombardment p.11
2.1 Theoretical Background p.11
 2.1.1 Breakdown between Two Electrodes p.12
 2.1.2 Breakdown under Electron Bombardment p.14
2.2 Experimental Procedure p.14
 2.2.1 Sample Preparation p.15
 2.2.2 The Use of a Scanning Electron Microscope p.21
2.3 Results and Discussion p.28
 2.3.1 Flashover Treeing Appearance Process p.28
 2.3.2 Treeing Initiation p.30
 2.3.3 Flashover Treeing Appearance of Different Charged Area p.32
 2.3.4 Typical Flashover Treeing p.37
 2.3.5 Effect of Primary Beam Energy p.39
2.4 Electric Field Analysis p.40
 2.4.1 Model Development p.41
 2.4.2 Results p.46
2.5 Summary p.47
References p.51

3. Time of Flashover Treeing of Pure MgO p.52
3.1 Time of Flashover Treeing for the Insulation Evaluation p.52
3.2 Fresh MgO sample p.54
 3.2.1 X-ray photoelectron Spectroscopy Results p.54
 3.2.2 Probability of Time to Flashover Treeing p.55
 3.2.3 Effect of Grain Boundary p.56
3.3 Samples Aged in Environmental Air p.60
3.4 Samples Aged in Argon and Vacuum p.63
3.5 Summary p.64
References p.64

4. Effect of Materials Addition on Time to Flashover Treeing p.65
4.1 Background p.65
4.2 Experimental Procedure p.66
4.3 Effect of CaO Addition p.68
 4.3.1 Phase Diagram p.68
 4.3.2 X-ray Diffraction Pattern p.69
 4.3.3 Microstructure p.71
 4.3.3.1 Scanning Electron Microscope Images p.71
 4.3.3.2 Compositional Maps p.71
 4.3.4 Time to Flashover Treeing p.74
4.4 Effect of SiO2 addition p.75
 4.4.1 Phase Diagram p.75
 4.4.2 X-ray Diffraction Pattern p.76
 4.4.3 Microstructure p.76
 4.4.3.1 Scanning Electron Microscope Images p.76
 4.4.3.2 Compositional Maps p.79
4.4.4 Time to Flashover Treeing p.79
4.5 Effect of Aging in Environmental Air p.81
4.6 Surface Conductivity of CaO and SiO2 added MgO p.84
4.7 Comparison of Time to Flashover Treeing between Insulators p.89
4.8 Summary p.90
References p.91

5. Conclusion p.92

Publications p.94

Presentations p.95

 The ability of a solid insulator to withstand voltage is limited by electrical breakdown. In harsh environment of a country with high lightning strike density, electrical insulators which support power line in transmitting electricity may experience surface breakdown(flashover). The flashover is a progressive cumulative process, involving the initiation and growth of one or more branches that may form a tree-like structure (treeing). The flashover damages the insulator and causes the insulator useless. In order to have good insulators, this thesis deals with evaluating insulators from withstanding flashover treeing appearance under electron beam bombardment.
Magnesia (MgO) with its wide bang gap insulator, high melting point, relatively low thermal expansion coefficient, and alternating current dielectric strength was chosen as an investigated insulator. The insulator surface was charged by electron beam bombardment in a scanning electron microscope (SEM). At a certain time of bombardment, a flashover treeing appeared on the sample surface. From the electric field distribution model, it was considered that increasing bombardment time increased the electric field that might exceed a critical value and result in a flashover treeing.
 The time of bombardment which was needed to create a flashover treeing (hereinafter time to flashover treeing) was used to evaluate the insulation property of high purity MgO. The evaluation was carried out for the fresh (as sintered) and aged samples. It was found that the time to flashover treeing of the sample which was aged in environmental air tended to decrease with increasing aging time. The decreasing of the time to flashover stands for to the deterioration of insulating property of MgO. Therefore, attempts were made to improve the insulation property of MgO by both increasing the time to flashover treeing for increasing voltage breakdown and reducing the effect of aging on the time to flashover treeing for improving the stability. Since flashover treeing process involves secondary electron emission (electron which is ejected from the sample surface during electron bombardment), the improvement was carried out by adding the low emission materials such as CaO and SiO2 to MgO. The results of the effect of CaO and SiO2 addition on the insulation property of MgO can be explained as follows. The CaO addition showed that there was no significant improvement of the insulation property of MgO. On the other hand the SiO2 addition might improve the insulation property of MgO significantly. It was found that the 6 wt% SiO2 addition could delay the flashover treeing appearance until 5.5 times. In addition, when the 6 wt% SiO2 added MgO was aged in environmental air, the time to flashover treeing did not seem to change with increasing aging time.
 The general conclusion is that flashover treeing appearance under electron beam bombardment could be used to evaluate insulation property of MgO. Among the investigated insulators, the 6 wt% SiO2 added MgO was a better insulator for withstanding flashover treeing appearance under electron beam bombardment. This study might give a useful information on improving the insulation property of the electrical insulator placed in harsh environment such as high lightning strike density.

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