Takehisa Hara

Monte Carlo simulation of surface charge on angled insulators in vacuum

A two-dimensional computer analysis of the surface charging on angled insulators has been performed by using the Monte Carlo simulation method. Results are compared to the charge in equilibrium model, originally proposed by C.H. de Tourreil and K.D. Srivastava (1973). >

Effects of spark conditioning, insulator angle and length on surface flashover in vacuum

A flashover test was conducted in a vacuum gap, bridged by an insulator in the shape of a frustum of a cone. The test gap was subjected to a lightning impulse voltage of 2/50 mu s. The flashover voltages were observed for various cone angles and insulator lengths. Traces on the cathode due to flashovers were observed. A spark conditioning using voltage pulses of reverse polarity is shown to reduce the number of shots needed to obtain a stable flashover voltage. The flashover voltages depend strongly on the cone angle even in the case of the lightning impulse. The flashover fields depend strongly on the insulator length. The traces are distributed not only at the triple junction but also over a wide area of the cathode surface when the gap distance is shorter than 15 mm. This fact indicates the existence of two different flashover processes. >

Time-periodic finite element method for nonlinear diffusion equations

A new numerical calculation method, the time-periodic finite element method, for the nonlinear diffusion equation is proposed. In the method, the equation is closed by the periodic-time of electrical characteristics and is solved by the numerical procedure which is used not for the initial value problem but for the boundary value one. The method is applied to the calculations of the voltage across the nonlinear corona shield region of a rotating machine. The results demonstrate that the method extensively reduces the computing time without losing the accuracy compared to the conventionally used methods.

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.

Effect of low-temperature electrode baking on breakdown in vacuum

Effects of electrode baking at a comparatively low temperature in vacuum have been examined. The temperature of the electrode is controlled well below the melting point of organic insulators, if installed. Currents accompanied by the microdischarges and the conditioning process during breakdown test are observed. It is shown that low temperature baking effectively suppresses the microdischarge, and that baking of the anode is more effective than baking of the cathode. It is also shown that baking reduces the number of voltage applications which are necessary for the spark conditioning. >