Akiko Kumada

Instability phenomenon of high-frequency vacuum arc discharge associated with anode activity

Instability phenomena of high-frequency vacuum arcs with an ∼100-kHz current waveform and active anodes were observed by using high-speed photography and optical emission spectroscopy and analyzed with a conventional theoretical model. Our experimental and theoretical studies demonstrated that the instability mechanism for the transient vacuum arcs was completely different from that for the well-known quasi-stationary diffused arcs, where a cathode process predominantly contributes to the instability phenomena. In the case of the high-frequency vacuum arcs, the instability phenomena were mainly induced by a collaborative anode process operated by anode evaporation and anode voltage drop.

Atomistic modeling of charge transport in polyethylene

The electrical degradation and breakdown in polymeric insulators are correlated with the conduction of charge carriers in the material. However carrier transfer in polymeric insulators remains poorly understood. In order to investigate the relation between the morphology and the carrier mobility of polymeric insulators, hole mobilities in crystalline and amorphous PE was evaluated with multi-scale computational approaches. It is shown that high hole mobility (∼ 10−2 cm2/Vs) in crystalline PE, despite the small electronic couplings is due to the small reorganization energy, which is realized through the intra-molecular hole delocalization, and due to quantum nuclear tunneling. In contrast, owing to the (1) large reorganization energy due to the short persistence length of PE chains and (2) the large energetic disorder due to the conformational disorder of PE chains, the hole mobility in amorphous PE is several orders of magnitude smaller than that in crystalline PE.

Surface potential measurement of stress grading system of high voltage rotating machine coils using pockels field sensor

Stator coil end of high-voltage rotating machines is equipped with stress grading system (SGS), which consists of two dielectric materials (conductive slot coating and stress grading tape/paint (SGT/P)). SGS grades surface potential curve on coil end and prevents surface partial discharge. Improving SGS and raising voltage limit of rotating machines are key to produce more efficient and durable products. Because SGS consists of non-linear materials, actual measurement of surface potential profile along SGS is essential for discussing advanced insulation design. However, there has been no suitable method to measure transient surface potential of as high as 100 kV maximum voltage. The authors have been developing Pockels potential sensor and have been measuring surface potential of coil bar. However, Pockels potential sensor has many advantages such as wide-band frequency response, small delay time and small disturbance to subject, Pockels potential sensor can measure only as high as 20 kV peak due to the close distance between high potential subject and grounded electrode. In this study, surface potential measuring method, which uses Pockels field sensor and includes calculation of surface potential from measured field data, is presented. Measurement system consists of Pockels crystal, laser, and photodetector. The relationship between surface potential and field is numerically calculated in advance and used to estimate surface potential by deconvolution process. Pockels crystal scans 1 mm above subject surface with 1mm measuring pitch. The potential on a sample bar coil under 10kV and 30 kV 50 Hz voltage is successfully measured with this system.

Fundamental studies on switching arcs — Experimental and numerical approaches

Both experimental and numerical approaches have been done on switching arc plasmas for fundamental studies. In the experimental approaches, power-semiconductor switching has been used to control intentional current injections and voltage applications to switching arcs with a high accuracy in time. The systematic experiments provided the interruption probability property for different gas kinds and gas flow rates, as well as the dielectric recovery properties between the electrodes. Time evolutions in electron density were derived by Laser Thomson Scattering (LTS) and Shack-Hartmann (SH) method for different gases. On the other hand, for this nozzle space, numerical models were developed to simulate arc dynamic behaviors with and without local thermodynamic equilibrium (LTE) assumptions for different gases. It was found that the time evolution in electron density derived by the chemically non-equilibrium model is in good agreement with the experimental results by LTS.

Computational insights into carrier transfer and injection in liquid organic insulators

Carrier conduction and injection properties in organic liquid insulator is investigated from an atomistic point of view. «-hexane is chosen as a simple model. Hole mobilities in n-hexane is computed with molecular dynamics, quantum chemical and kinetic Monte Carlo calculations. Hole injection current is computed via semi-phenomenological models with the aid of quantum chemical calculations. At low electric field, hole mobility is comparable to that of ions, however, in electric fields approaching the dielectric breakdown field, hole mobility becomes larger than that of ions. As pointed out for charge injection in organic semiconductors, the success of F-N model is probably accidental because the thickness of the potential barrier is larger than the distance between hopping sites. The injection current rapidly increases with the electric field above 102 kV/mm.

Power dissipation in stress grading as a function of CAT-SG interface topology

Computations indicate that the power dissipated in the stress grading of a motor stator bar during a square wave with 5 kV, 1 µs transitions is relatively independent of the CAT-SG interface topology over a wide range of conditions.