Shoji Hamada

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/.

Intercomparison of induced fields in Japanese male model for ELF magnetic field exposures: effect of different computational methods and codes

The present study provides an intercomparison of the induced quantities in a human model for uniform magnetic field exposures at extremely low frequency. A total of six research groups have cooperated in this joint intercomparison study. The computational conditions and numeric human phantom including the conductivity of tissue were set identically to focus on the uncertainty in computed fields. Differences in the maximal and 99th percentile value of the in situ electric field were less than 30 and 10 % except for the results of one group. Differences in the current density averaged over 1 cm(2) of the central nerve tissue are 10 % or less except for the results of one group. This comparison suggests that the computational uncertainty of the in situ electric field/current density due to different methods and coding is smaller than that caused by different human phantoms and the conductivitys of tissue, which was reported in a previous study.

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.

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.

GPU-accelerated indirect boundary element method for voxel model analyses with fast multipole method

Abstract An indirect boundary element method (BEM) that uses the fast multipole method (FMM) was accelerated using graphics processing units (GPUs) to reduce the time required to calculate a three-dimensional electrostatic field. The BEM is designed to handle cubic voxel models and is specialized to consider square voxel walls as boundary surface elements. The FMM handles the interactions among the surface charge elements and directly outputs surface integrals of the fields over each individual element. The CPU code was originally developed for field analysis in human voxel models derived from anatomical images. FMM processes are programmed using the NVIDIA Compute Unified Device Architecture (CUDA) with double-precision floating-point arithmetic on the basis of a shared pseudocode template. The electric field induced by DC-current application between two electrodes is calculated for two models with 499,629 (model 1) and 1,458,813 (model 2) surface elements. The calculation times were measured with a four-GPU configuration (two NVIDIA GTX295 cards) with four CPU cores (an Intel Core i7-975 processor). The times required by a linear system solver are 31 s and 186 s for models 1 and 2, respectively. The speed-up ratios of the FMM range from 5.9 to 8.2 for model 1 and from 5.0 to 5.6 for model 2. The calculation speed for element-interaction in this BEM analysis was comparable to that of particle-interaction using FMM on a GPU.

Effect of conductivity in triple-junction problems

The paper describes the electric field behavior in triple-junction problems for an arrangement with a contact angle between 0 and π/2. We have fully analyzed the effects of volume and surface conductivity by the results of both applying an analytical solution and numerically calculating electric field distributions. The numerical field calculation is performed by the boundary element method. The analytical solution proposed here agrees well with the numerical calculation results. The presence of volume conductivity usually enhances the field singularity at a contact point, while surface conductivity moderates the field to a uniform distribution near a contact point.

Charging characteristics of a solid insulator in vacuum under AC voltage excitation

We have investigated charging and flashover characteristics of a polymeric or glass insulator exposed to AC voltage in vacuum in order to develop compact and reliable high voltage VCBs (vacuum circuit breakers). This paper focuses on charging characteristics of a cylindrical model insulator. The charging of an insulator is investigated using an electrostatic probe that measures the electric field near the triple junction on the grounded electrode. This method allows a time-resolved measurement of the charging process. The insulator was made of borosilicate, fused quartz or polymetyl methacrylate, and was in the shape of a right cylinder with 10 mm in thickness. 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 our previous results with DC voltage excitation. We find that the charge magnitude at the stable state coincides with that obtained by DC. The electric field on the grounded electrode, and therefore the charge magnitude, decreases with the surface roughness, and decreases as the insulation strength is increased. A computer simulation has been conducted to investigate the quasi-stable state, which clarifies that the transition in surface charge distribution being synchronous to the voltage phase is responsible for causing the quasi-stable state.

Gases as a Dielectric

Gas insulation plays a very important role in electric power systems, as GIS (gas-insulated switchgear), GIL (gas-insulated transmission line), GCB (gas circuit breaker), and other power equipment. In the present or conventional gas insulation, SF6 (sulfur hexafluoride) is exclusively utilized as a principal insulating medium. It is needless to say that the use of solid insulating components is also inevitable there for supporting and fixing stressed parts because a gaseous medium alone cannot fulfill these tasks.

Electric field behavior near a contact point in the presence of volume conductivity

The electric field behavior, in particular the field intensification at a contact point, is very important in complex dielectric systems with gaseous or vacuum insulation. The paper describes the electric field behavior at and near a contact point in various arrangements with a zero contact angle when volume conductivity is present in the solid dielectric. Contact conditions are separated into line, point, and surface contact. The effect of volume conductivity is investigated analytically, and numerically by using the boundary element method. The electric field behavior near a contact point principally depends on the absolute value of complex relative permittivity, and volume conductivity usually promotes the field intensification. In the arrangements of point contact or line contact, the position of peak electric field shifts from a contact point when the volume conductivity is higher than a certain value, while in the arrangement of surface contact, the position is usually more or less remote from the contact point, whether volume conductivity is present or not.

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

This paper describes the charging process of a cylindrical insulator made of PMMA or Al/sub 2/O/sub 3/. By using an electrostatic probe located inside the insulator on the cathode surface, we have conducted real-time observation of the electric field change due to the surface charging. An axial-symmetrical simulation based on a secondary electron emission avalanche 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 charging inception at a voltage measured well below the flashover.