This paper reviews the last twenty years of work concerning surface flashover of insulators in vacuum. It complements an earlier review paper [H.C. Miller, Flashover of Insulators in Vacuum, IEEE Trans. Electr .Insul., Vol. 28, pp. 512-527, 1993]. The surface flashover voltage of insulators in vacuum depends upon many parameters, such as material, geometry, surface finish, and attachments to electrodes, but also on the applied voltage waveform, duration, single pulse or repetitive, and on the process history of the insulator, operating environment, and previous applications of voltage. Suggestions are made for choosing the material, geometry, and processing when selecting an insulator for a particular application. Some specific techniques for improving the holdoff voltage of insulators are recommended.

Flashover of insulators in vacuum: the last twenty years

This report intends to reveal the role of electron migration and its effects in triggering direct current (DC) surface flashover under temperature gradient conditions when using epoxy-based insulating composites. The surface potential and the surface flashover voltage are both measured using insulators that are bridged between two thermo-regulated electrodes. The space charge injection and migration properties under different temperature are detected. The results show that the surface potential rises significantly because of electron migration near the high voltage (HV) electrode under high temperature conditions, thus creating an "analogous ineffective region". The expansion of this "analogous ineffective region" results in most of the voltage drop occurring near the ground electrode, which serves as an important factor triggering positive streamers across the insulation surface. This work is helpful in understanding of DC surface flashover mechanism from a new perspective and also has important significance in design of a suitable DC insulator to avoid surface flashover problem.

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The potentially neglected culprit of DC surface flashover: electron migration under temperature gradients.

The influence of surface charge accumulation on flashover voltage of GIS/GIL basin insulator under various voltage stresses

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.

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

Environmental problem of SF6 gas has been regarded as an up-to-date issue in the field of high voltage engineering Binary gas mixtures such as SF6/N2 are considered to be one of the most promising candidates[1-3], because N2 gas has no greenhouse effect and toxicity

https://iopscience.iop.org/article/10.1088/0022-3727/35/21/311/pdf

Partial discharge and breakdown mechanisms in ultra-dilute SF6 and PFC gases mixed with N2 gas

For higher electrical insulator performance of vacuum circuit breakers (VCB), the surface flashover in vacuum should be improved. In particular, it is important to clarify how strongly surface charges influence the surface flashover mechanism. In this paper, we investigated the surface flashover characteristics based on the existence of surface charges on an alumina insulator in vacuum. We changed the location and magnitude of surface charges and measured surface flashover voltage. From our experimental results, an attempt was made to clarify the influence of charges on alumina dielectrics on impulse surface flashover characteristics in vacuum. From this, we concluded that (1) when surface charges were located near the cathode electrode, a positive charge made the surface flashover voltage lower and a negative charge made the surface flashover voltage higher; and, (2) when surface charges were located in the way of the discharge path, both positive and negative charges made the surface flashover voltage lower. We were able to explain these results by considering the influence of surface charges on the electric field at flashover and on the secondary electron emission avalanche.

Influence of surface charges on impulse flashover characteristics of alumina dielectrics in vacuum

With the goal of reducing SF/sub 6/ gas usage, we investigated partial discharge (PD) and breakdown (BD) mechanisms in ultra-dilute (0 to 1%) SF/sub 6//N/sub 2/ gas mixtures. The experimental results and discussions are given in this paper. In particular, we focused our research on the transition characteristics of the electrical insulation performance with an extremely small amount of SF/sub 6/ content, the PD behavior on the applied voltage, and the relation among prebreakdown streamer, return stroke and BD. Moreover, we discussed the PD and so mechanisms with reference of space charge behavior. From these results, the transition threshold at which the influence of SF/sub 6/ gas on the discharge characteristics began to appear, was clarified to be SF/sub 6/ content k=10 ppm for a total pressure of 0.1 MPa.

Partial discharge and breakdown mechanisms in ultra-dilute SF/sub 6//N/sub 2/ gas mixtures

For higher electrical insulation performance of vacuum circuit breakers (VCB), the surface insulation characteristics in vacuum should be improved. In this paper, we investigated the development mechanism of impulse surface flashover on alumina ceramic insulator in vacuum. We measured the still images and ultra fast framing images of fast developing surface flashover in synchronous with applied impulse voltage and current waveforms. The light emission transition in initiation of surface flashover was also clarified. We found the dark area was formed around the cathode and conducting channel was formed from the anode in developing process of surface flashover in vacuum.

Development process of impulse surface flashover on alumina dielectrics in vacuum

Since SF6 gas was specified to be a greenhouse gas at COP3 in 1997 because of its high global warming potential (GWP=23900), the development of new insulating gases or gas mixtures alternative to SF6 gas is greatly needed. Especially, gas mixtures without the use of SF6 gas will be expected as the ultimate substitutes to SF6 gas.

Partial Discharge and Breakdown Characteristics in N2/O2 and N2/CO2 Gas Mixtures Under Inhomogeneous Electric Field

Partial discharge (PD) measurement is expected to be an effective method for electrical insulation diagnosis and breakdown prediction of the power apparatus. In order to establish the reliable insulation diagnostic techniques, physical mechanism of PD phenomena are required to be clarified. Especially, electronegative gases and gas mixtures are of interest from the viewpoint of high insulation performance due to corona stabilization effect. Although many investigations1,2 have been so far carried out, PD phenomena have not yet been fully understood.

T. Ishida

Papers

Influence of surface charges on impulse flashover characteristics of alumina dielectrics in vacuum

Partial discharge and breakdown mechanisms in ultra-dilute SF/sub 6//N/sub 2/ gas mixtures

Development process of impulse surface flashover on alumina dielectrics in vacuum

Partial Discharge and Breakdown Characteristics in N2/O2 and N2/CO2 Gas Mixtures Under Inhomogeneous Electric Field

Partial Discharge Mechanism and Current Waveforms in Electronegative Gases and Gas Mixtures