Hiroyuki Sakakibara

Bio: Hiroyuki Sakakibara is an academic researcher from Nagoya University. The author has contributed to research in topics: Liquid nitrogen & Breakdown voltage. The author has an hindex of 4, co-authored 12 publications receiving 121 citations.

Breakdown mechanism of liquid nitrogen viewed from area and volume effects

Abstract: We investigated the area and volume effects on the breakdown strength in liquid nitrogen (LN/sub 2/) to discuss the breakdown mechanism in cryogenic liquids for superconducting power apparatus. We measured breakdown voltages in LN/sub 2/ with and without thermal bubbles over a very wide range of the electrode size. Experimental results revealed that the breakdown mechanism changed from an area dominant to volume effective region at larger electrode configurations in LN/sub 2/. Moreover, we discussed the contribution rate of area and volume effects to the breakdown strength in LN/sub 2/. It was suggested that a mutual contribution of area and volume effects appeared in breakdown characteristics in LN/sub 2/ under thermal bubble conditions, as a phenomenon peculiar to cryogenic liquids. Consequently, we pointed out that it is very important to consider both thermal bubbles and electrode surface condition for HV insulation of superconducting power apparatus.

High voltage insulation performance of cryogenic liquids for superconducting power apparatus

Abstract: This paper describes high voltage insulation technology of cryogenic liquids for superconducting power apparatus. The insulation of a superconducting transformer is classified into several insulation components. Moreover, we investigate fundamental insulation characteristics peculiar to the superconducting and cryogenic circumstances: area and volume effects on breakdown strength, V-t characteristics, quench induced dynamic breakdown characteristics and so on. Finally, we systematize the electrical insulation of cryogenic liquids, and propose a flow chart for the practical insulation design of superconducting power apparatus.

Supersonic piston synthetic jets with single/multiple orifice

Abstract: We experimentally study a high-speed synthetic jet actuator consisting of a cylinder with an orifice plate at the top and a piston driven by a motor. The experiments are performed for four orifice plates with a single hole or multiple holes with different diameters. The characteristics of the synthetic jets are investigated with cylinder pressure measurement and flow visualization based on the shadowgraph method, where the Mach disks in the shadowgraph images are used for estimating the Mach number of supersonic synthetic jets. A total orifice area on the plate affects cylinder pressure and jet Mach number. The number of the orifices has a direct influence on the downstream region where the jets from different orifices interact with each other. A smaller total orifice area results in a higher jet Mach number, a larger spatial extent of Mach disks, and an earlier formation of the Mach disks in one cycle. The influences of total orifice area are well explained by larger/smaller values of maximum/minimum values of the cylinder pressure for a smaller total orifice area. It is also found that the maximum jet Mach number is represented as a function of the maximum cylinder pressure even for different orifice configurations. Furthermore, for a given operating frequency, the maximum cylinder pressure increases almost linearly with the stroke length divided by an effective diameter of the total orifice area independently of the number of the orifices.

Life Estimation of Water Treed XLPE Cables by Very Low Frequency Voltage Withstand Test

Abstract: 水 トリー はCVケ ー ブル の代 表 的な劣 化形 態で あ り、 水 トリー の発 生 ・進 展 によ り線路 の健 全な 運用 を妨 げ る場 合 が ある 。そ のた め 、水 トリー 劣化 ケー ブル の診断 技術 が 強 く望 ま れて い る。特 にそ のケー ブ ルが今 後何 年間 使用 で き るか といっ た余寿 命 は 、ケ ー ブル線 路 の安定 的な 運用 に欠 か せな い きわめて 有 用な情報 で ある。 一方 、22-77kVク ラス の特別 高圧CVケ ーブル では、水 ト リー が絶 縁体 を橋 絡 して いな い、 いわ ゆる非 橋絡 水 トリー を検出 す る必 要が あ る[1]。この場 合、水 トリー か ら得 られ る 劣 化信 号 は非 常 に小 さ く、 これ らの劣 化 診断 法は 未だ研 究 段 階で ある。 そ こで 、 我々 は、 非橋 絡水 トリー を検 出す る 事 がで き、か つ線 路 の余 寿命 推定 が 可能 な方 法 と して 、耐 圧試 験法 に着 目した。

50 years in the development of insulating liquids

Abstract: The role of electrical insulation is critical for the proper operation of electrical equipment. Power equipment cannot operate without energy losses, which lead to rises in temperature. It is therefore essential to dissipate the heat generated by the energy losses, especially under high load conditions. Failing to do so results in premature aging, and ultimately to failure of the equipment. Heat dissipation can be achieved by circulating certain liquids, which also ensure electrical insulation of energized conductors. The insulating-fluids market is therefore likely to be dominated by liquids, leaving to gases (such as compressed air and SF6) limited applications in power equipment such as circuit breakers and switchgear [1]-[3]. Several billion liters of insulating liquids are used worldwide in power equipment such as transformers (power, rectifier, distribution, traction, furnace, potential, current) [4], resistors [5], reactors [6], capacitors [7], cables [8], bushings [9], circuit breakers [10], tap changers [11], thyristor cooling in power electronics, etc. [12]. In addition to their main functions of protecting solid insulation, quenching arc discharges, and dissipating heat, insulating liquids can also act as acoustic dampening media in power equipment such as transformers. More importantly, they provide a convenient means of routine evaluation of the condition of electrical equipment over its service life. Indeed, liquids play a vital role in maintaining the equipment in good condition (like blood in the human body). In particular they are responsible for the functional serviceability of the dielectric (insulation) system, the condition of which can be a decisive factor in determining the life span of the equipment [13]. Testing the physicochemical and electrical properties of the liquids can provide information on incipient electrical and mechanical failures. In some equipment, liquid samples can be obtained without service interruption.

Breakdown mechanism of liquid nitrogen viewed from area and volume effects

Abstract: We investigated the area and volume effects on the breakdown strength in liquid nitrogen (LN/sub 2/) to discuss the breakdown mechanism in cryogenic liquids for superconducting power apparatus. We measured breakdown voltages in LN/sub 2/ with and without thermal bubbles over a very wide range of the electrode size. Experimental results revealed that the breakdown mechanism changed from an area dominant to volume effective region at larger electrode configurations in LN/sub 2/. Moreover, we discussed the contribution rate of area and volume effects to the breakdown strength in LN/sub 2/. It was suggested that a mutual contribution of area and volume effects appeared in breakdown characteristics in LN/sub 2/ under thermal bubble conditions, as a phenomenon peculiar to cryogenic liquids. Consequently, we pointed out that it is very important to consider both thermal bubbles and electrode surface condition for HV insulation of superconducting power apparatus.

Enhancement of electrical insulation performance in power equipment based on dielectric material properties

Abstract: Research and development results of enhancement techniques of electrical insulation performance for higher electric field application in power transmission/substation equipment, such as transformers, switchgears and cables, are described, especially based on the view point of dielectric materials. Firstly, the electric field analysis, field optimization and field measurement techniques are introduced to discuss higher electric field stress applications in power equipment. Secondly, material types, including gases, liquids, solids, vacuum and their composite systems are discussed to make power equipment with higher insulation performance, lower losses, lower environmental impact and higher reliability. In the process of development, a highly sophisticated new approach to clarify the physical mechanisms of partial discharges was developed and applied. By the introduction and applications of the above mentioned new electrical insulation techniques based on dielectric materials, concepts of future power equipment with higher electric field stress are proposed. This paper is based on the Whitehead Memorial Lecture given at the IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP) 2011 in Cancun, Mexico.