Shigeyasu Matsuoka

Two-dimensional potential and charge distributions of positive surface streamer

The fine structure of a surface streamer is investigated by not only a conventional electrostatic probe but also a Pockels sensing system. The Pockels sensing system consists of two optical subsystems: A subsystem for measuring the transient change in the potential distribution along a linear path of 50 mm in maximum length which consists of a continuous Ar ion laser and a streak camera, and the other subsystem for measuring the instantaneous two-dimensional potential distribution on a 25 mm square area which consists of a pulsed laser and a CCD camera. The minimum resolution of the former subsystem is determined by the speed of streak camera, and the later one is the pulse width of the laser. The transient change of the potential distribution along a propagating a positive surface streamer in atmospheric air is measured with this system. From the measured potential profile, the electric field and the charge distributions are also calculated.

The influence of residual charge on surface discharge propagation

The influence of residual charge on propagation of surface discharge is experimentally investigated using high-speed framing and streak cameras, and an electrostatic probe. When consecutive impulse voltages are applied 25 times with a change in polarity, the propagation length of the surface discharge increases gradually from 79 to 164 mm and hardly converges. Under such a condition, the potential gradient in the surface leader channel decreases with the consecutive number of impulses, while that in the surface streamer remains constant and the value is 0.5–0.6 kV mm−1. With the residual charge of a previous opposite polarity discharge on an insulator, the propagation velocity increases to three to eight times as large as that of the surface discharge on a clear insulator without any residual charge. The peak current of the surface discharge with a residual charge also becomes much higher than that without a residual charge.

Highly sensitive Shack–Hartmann sensor for two-dimensional electron density imaging over extinguishing arc discharges

Highly sensitive Shack–Hartmann-type laser wavefront sensors incorporating meniscus microlens arrays with a long focal length of 238 or 467 mm were developed for imaging two-dimensional electron density distributions over extinguishing atmospheric arc discharges. The use of the novel microlens arrays also had the advantage of realizing the spatial synchronization of the measuring system. The highly sensitive Shack–Hartmann sensors were successfully used for the single-shot imaging of two-dimensional electron density distributions over the extinguishing arc discharges with currents of several amperes and were proven to have improved the measurement sensitivity by two orders of magnitude from 1023 to 1021 m−3. Spatiotemporal evolution of the electron density images showed that just before arc extinction the electron densities at outer radial positions with respect to the interelectrode gap were higher than those on the geometrical axis defined by the electrodes.

Influence of tiny metal particles on charge accumulation phenomena of GIS model spacer in high-pressure SF 6 gas

The charge density distribution of the downsized GIS disc spacer made of epoxy resin was measured in 0.5-MPa (abs.) SF6 gas. A dc voltage was applied to the spacer, and the charge density on the surface was measured using an electrostatic probe. Voltage application and charge measurement were conducted in a sealed chamber without opening it. After the voltage application of 60 kV, radial charge patterns appeared on the spacer. Tiny metal particles whose size is smaller than 0.1 mm were observed on the spacer after the voltage application. Metal particles on the spacer may cause the radial charge accumulation in this experiment: if a metal particle adheres on the spacer, partial discharge may occur near the metal particles and negative charges may progress along the surface of the spacer.

Simultaneous imaging of two-dimensional electron density and air-flow distribution over air-blast decaying arc

Sensitive Shack–Hartmann type laser wavefront sensors were applied to simultaneous imaging of two-dimensional electron density and air-flow distributions over decaying arc channels under air blasting with several pressures. Our experimental results showed that higher blasting pressures facilitated the rapid reduction of arc diameters and an increase in the electron densities around the gap centre due not only to the thermal pinch effect but also to air-flow disturbances, although there were no significant effects of the air blasting on the arc conductance.

Residual Charge Distribution of Surface Leader Discharge Under Positive Impulse Voltage

The performance of high-voltage apparatus is seriously restricted by the surface discharge on dielectric materials. A surface charge density distribution scanning system is developed with a dc feedback-type electrostatic probe based on the field-nullifying technique. The residual charge density distribution after the occurrence of a positive leader discharge is measured. Using the 2-D Fourier transformation in the spatial domain and Tikhonovs regularization method, the surface potential distribution and electric field distribution are arrived at. The leader stem and radial lament streamer zone located at the head of the leader can clearly be distinguished from the distributions. There is an obvious knee point that divides the main channel of the discharge into a leader channel and a streamer channel. The knee point is at about 9 kV, which corresponds to about 320 pC/mm2.

Gap length dependence of two-dimensional electron and copper vapour density distribution over vacuum plasma

Shack-Hartmann type laser wavefront sensors were used for simultaneous visualisation of two-dimensional electron and copper vapour density distributions over vacuum plasmas in a breakdown phase and sustainable arc phase. Our experimental results for a 3-mm gap between Cu electrodes showed that the electrons in the vacuum breakdown stage were mainly supplied from the ionisation process of the copper vapour evaporating from the anode. In addition, the electron and copper vapour densities in an intense arc mode for the 3-mm gap were comparable to those for the 1.2-mm gap in our previous experiments. The validity of the observation results was verified by simultaneously recorded voltage and current waveforms demonstrating an inversely proportional decrease in arc column conductance with increasing gap length.

High voltage measuring apparatus based on kerr effect in gas

In testing and monitoring electric power systems, a high voltage measuring system which covers wide frequency range is required. In this study, high voltage measuring apparatus based on Kerr effect in gas is developed. Parallel plate electrodes are placed in a chamber filled with gaseous material up to 0.5 MPa and the electric field between the electrodes is optically measured by Kerr effect in gas. Since the Kerr constant of gas is very small, a White cell structure is utilized to multiply the optical path length and an optical phase modulation technique is also introduced to improve the sensitivity of the measuring system. With the prototype system whose modulation frequency is 50 kHz, ac power-frequency voltage up to 130 kV is measured by using SF6, N2, CO2, and CF3I as the Kerr material. With the modified system whose modulation frequency is raised above 200 MHz, lightning impulses up to 160 kV are measured by using CF3I whose Kerr constant is relatively high.

Intense-Mode Vacuum Arc Characterization by Using 2-D Electron and Vapor Density Image

Shack-Hartmann type laser wavefront sensors were applied to simultaneous single-shot imaging of 2-D electron and copper-vapor density distributions over intense-mode vacuum arc discharges with a pulsed current waveform of 800 A in peak and 24 μs in damping time constant. A parametric analysis of the ion current based on our experimental results suggested that the intense-mode arcs included highly energetic ions moving from cathodes to anodes with a high velocity of 104 m/s and their proportion was larger than or comparable to slow ions with 10-1 m/s drifted from anodes to cathodes by the electric fields. Furthermore, hydrodynamic calculation demonstrated that the copper vapor in the vacuum plasmas dissipated almost instantaneously in a time scale of ~1 μs and it was not residual metal medium but fresh one continuously supplied from the electrodes.

Partial discharge characteristics in composite insulation systems with PPLP for HTS cable

The electrical insulation system of high-temperature superconducting (HTS) cable consists of liquid nitrogen (N₂(l)) and polypropylene laminated paper (PPLP^®). Partial discharge (PD) may occur in butt gaps of the insulation layers and its characteristics imply the insulation performance of HTS cables. N2(l) cooling system is installed in the power system and N₂(l) will flow through the cables during the system operation. Filling the HTS cable with N₂(l) in order to perform pre-shipment inspection is time-consuming and costly for cable manufacturers. Therefore, the authors are trying to find a cost effective method for pre-shipment inspections. One alternative is to use high pressure gaseous nitrogen (N₂(g)) instead of N₂(l). This article investigates PD characteristics such as PD inception electric field (PDIE) and PD extinction electric field (PDEE) in butt gaps of HTS cables in 0.1-MPa to 0.3-MPa N₂(l) and 0.1-MPa to 1.0-MPa N₂(g) environments. For assessing the surface/volume effects, PD characteristics are measured with changing the size of butt gaps. It turns out that PDIE and PDEE in N₂(g) are linearly correlated with those in N₂(l) at any gas pressure in our testing, and PDIE in 1.0-MPa N₂(g) is almost 30% of that in 0.2-MPa N₂(l). It suggests that PD characteristics in N₂(l) can be extrapolated from those in N₂(g).