Tadasu Takuma

Principle of surface charge measurement for thick insulating specimens

We discuss the method for measuring surface charge density accumulated on the surface of a solid dielectric (insulating specimen) such as a supporting spacer in gas and vacuum insulated equipment. For such thick specimens, the probe response does not correspond to the charge density directly below the probe, so the measurement necessitates multipoint data together with the aid of numerical field calculations. The probe gives either induced charge or floating potential in response to the surface charge. We compare various previously proposed techniques and give a reasonable procedure for analyzing the data.

Improving withstand voltage by roughening the surface of an insulating spacer used in vacuum

This paper describes a simple and reliable method of improving the surface insulation strength of a spacer used in vacuum. The method is to roughen the spacer surface to an average roughness R/sub a/ higher than 1 or 2 /spl mu/m. The material of the spacer examined is SiO/sub 2/, PMMA, PTFE or Al/sub 2/O/sub 3/ and their shape is a right cylinder with 10 mm in height and 54 mm in diameter. The spacer is subjected to a ramped DC voltage and its surface charging is observed by using an electrostatic probe embedded in the cathode. It has been found that R/sub a/ decisively affects the charging, which decreases as R/sub a/ increases. Increasing R/sub a/ larger than about 2 /spl mu/m suppresses the charging until a higher applied voltage is reached, thus improving the insulation property.

Applying a gas mixture containing c-C/sub 4/F/sub 8/ as an insulation medium

This paper studies the possibility of applying a gas mixture containing c-C/sub 4/F/sub 8/ in the gas insulation of power equipment. Environmental aspects such as global warming potential, ozone depletion potential, recycling loss and toxicity are discussed. Insulation characteristics of gas mixtures such as c-C/sub 4/F/sub 8//N/sub 2/, c-C/sub 4/F/sub 8//air, and c-C/sub 4/F/sub 8//CO/sub 2/ are examined experimentally under a quasi-homogeneous or an inhomogeneous electric field condition. Most of the characteristics are compared with those of SF/sub 6//N/sub 2/, which is now widely studied as the most plausible alternative to pure SF/sub 6/ for gas insulation. The experimental results, together with the discussion, suggest that gas mixtures containing c-C/sub 4/F/sub 8/ are possible substitutes for SF/sub 6/ and SF/sub 6//N/sub 2/.

Effective precondition technique to solve a full linear system for the fast multipole method

The fast multipole method (FMM) is an O(N) solver of a full linear system appearing in integral equation methods. We propose a precondition technique for the FMM using the Bi-CGSTAB2 method, which employs a nested FMM having intentionally deteriorated precision. This enables us to utilize the global information residing in the system matrix.

The role of leader re-illumination in the development of surface discharges in SF6 exposed to a very fast transient overvoltage

This paper describes the mechanism of discharge on the surface of an insulating plate in gas exposed to a very fast transient overvoltage (VFTO). The voltage consists of a sequence of steep short pulses on its front. A streak camera and a charge-measuring system were employed for the investigation. The leader channel, which has been formed during the first voltage pulse of the VFTO, flashes repeatedly upon each rise and fall of the successive voltage pulses. The luminosity at the voltages rise consists of the light due to mere flashing of the previous leader channel and the light of the leader discharge in succession. The former phenomenon is called leader re-illumination in this paper. It plays a notable role in the leaders development. Such re-illumination of a leader channel occurs when the steepness of the applied voltage exceeds a critical value. The critical steepness increases with increasing pressure of gas, the thickness and the reciprocal of the permittivity of the insulating plate. A model for the leader discharge developing under VFTO stress is proposed.

Real-time observation of surface charging on a cylindrical insulator in vacuum

By using an electrostatic probe located close to the triple junction on the cathode surface, we have conducted real-time observations of the change in electric field due to surface charging of an insulator in vacuum. A cylindrical sample made of polymethylmethacrylate (PMMA) or Al/sub 2/O/sub 3/ has been exposed to HVDC. An axisymmetric simulation based on the secondary electron emission electron avalanche (SEEA) theory has been performed. The measured results agreed with the simulation concerning the polarity of the accumulated charge as well as the field strength. The simulation also predicts the inception of charging at a voltage well below the measured flashover voltage. These results clearly support the SEEA theory as a macroscopic mechanism of surface charging of an insulator in vacuum.

Calculation of the electric field for lined-up spherical dielectric particles

This paper calculates the electric field in arrangements of dielectric particles. An analytical method is presented in the paper. The method repetitively inserts monopoles and multipoles until all the boundary conditions are satisfied, so that unlike the existing methods, it does not require setting up a linear equation system in the calculation. It can be applied to various types of energization such as a uniform field, spherical electrodes, or planar electrodes. The method and the BEM, a numerical method, have been utilized to investigate the effects of the particle number and particle permittivity. The authors have compared the results by the analytical method with those by the numerical method, and found that the accuracy of the numerical method greatly varies from less than 10/sup -4/% to more than 2%, depending on the condition of the calculation arrangements.

Surface breakdown characteristics of silicone oil for electric power apparatus

This paper describes the surface breakdown characteristics of silicone oil which has the possibility of application to innovative switchgears and transformers as an insulating medium. At the first step, we have experimentally studied the impulse breakdown characteristics of the configuration with a triple-junction where a solid insulator is in contact with the electrode. The test configuration consists of solid material (Nomexreg or pressboard) and liquid insulation oil (silicone or mineral oil). We have discussed the experimental results based on the maximal electric field at the triple-junction. As the second step, we have studied the configuration which may improve the surface breakdown characteristics by lowering the electric field on the insulator surface

Electric fields in composite dielectrics and their applications

Preface. Acknowledgements. 1 Basic Properties of Electric Fields in Composite Dielectrics. 1.1 Background. 1.2 Fundamentals of Composite Dielectric Fields. 1.3 Effect of Conduction. 1.4 Outline of Field Behavior near a Contact Point. 1.5 Outline of the Chapters. References. 2 Electric Field Behavior for a Finite Contact Angle. Introduction. 2.1 Analytical Treatment. 2.2 Numerical Treatment. 2.3 Effect of Volume and Surface Conduction. References. 3 Electric Field for a Zero Contact Angle (Smooth Contact). Introduction. 3.1 Stressed Conductor in Contact with a Solid Dielectric. 3.2 Uncharged Spherical Conductor Under a Uniform Field. 3.3 Stressed Conductor on a Solid Dielectric of Finite Thickness. 3.4 Other Basic Configurations. 3.5 Effect of Volume and Surface Conduction. References. 4 Electric Field Behavior for the Common Contact of Three Dielectrics Introduction. 4.1 Contact of Straight Dielectric Interfaces. 4.2 Perpendicular Contact of a Solid Dielectric with another Solid. 4.3 Numerical Analysis of Field Behavior. References. 5 Electric Field in High-Voltage Equipment. Introduction. 5.1 Finite Contact Angle: Prevention of Field Singularity near a Contact Point. 5.2 Zero Contact Angle in Gas-Insulated Equipment. 5.3 Common Contact of Three Dielectrics. 5.4 Application to High-Field-Emission Devices. References. 6 Electric Field and Force in Electrorheological Fluid: a System of Multiple Particles. Introduction. 6.1 Equivalent Dipole Expression. 6.2 Particles Lined Up Parallel to an Applied Field. 6.3 Particle Chain Tilted to the Field Direction. 6.4 Two-Particle Chain Between Parallel Plane Electrodes with the Minimum Separation. 6.5 Nonhomogeneous Particles. References. 7 Electric Field and Force on Toners in Electrophotography. Introduction. 7.1 Fundamental Characteristics. 7.2 Charged Dielectric Particle on a Conductor. 7.3 Charged Dielectric Particle on a Dielectric Barrier. References. 8 Analytical Calculation Methods.Introduction. 8.1 Variable-Separation Method for Straight Dielectric Interfaces. 8.2 Iterative Image Charge Method. 8.3 Uncharged Conducting Sphere Under a Uniform Field on a Dielectric Plane. 8.4 Re-expansion Method for a System of Particles. References. 9 Numerical Calculation Methods. Introduction. 9.1 General Remarks. 9.2 Charge Simulation Method (CSM). 9.3 Surface Charge Method (SCM). 9.4 Boundary Element Method (BEM). References. Index.

A study on the accuracy of surface charge measurement

The measurement of accumulated surface charge for thick specimens requires multipoint probe outputs to establish the inverse calculation for the determination of an unknown charge distribution. Until now, studies on the various errors associated with the measurement have been conducted only for simplified arrangements mainly in axisymmetric geometry where the charged surface is parallel to the ground. We have numerically analyzed a model measurement set-up more comparable to practical conditions by a highly efficient surface charge method. We have studied the effect of probe position, the induction from charge existing not directly beneath the (probe) sensor and the difference in matrix components computed by two numerical methods. In particular, we have studied the accuracy of the reconstructed charge distributions by numerical simulations of the inverse calculation. It has been shown that the assumed measurement errors make much larger differences in the reconstructed charge distributions, although the influence depends considerably on the assumed charge distribution. Reducing the condition number of the matrix improves the accuracy of the inverse calculation for uniform and linearly changing charge distributions.