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

Epoxy used as insulating material in electronic and electrical devices plays an important role in system reliability. Addition of nanoparticles into epoxy can improve the insulating properties compared with undoped material. However, due to the change of the material structure, trap characteristics and charge behaviors are altered as a consequence. This possibly leads to distortion of electric field that can result in degradation of the insulation. From the viewpoint of safety, it is necessary to investigate charge behavior of epoxy nanocomposites. This paper presents study aimed at clarifying the effect of nano-filler concentration on surface charge accumulation and decay behaviors of epoxy/TiO2 nanocomposites. Samples were prepared by dispersing nano-scale TiO2 into epoxy by mixing with shear force. Corona charging tests were performed at room temperature with a relative humidity of ~ 40%. The charge distribution was measured by means of an electrostatic voltmeter. Obtained results show the dependence of the charge density as well as the charge decay rate upon the concentration of TiO2, varying as a function of the charge polarity, charging time and charging voltage. It is suggested that the charge dynamics is dependent upon the characteristics of localized surface states that are altered by the nanoparticles.

Effects of TiO 2 particles on surface charge of epoxy nanocomposites

Polyimide film, as a special type of engineering plastic film, is a kind of basic insulating material and is widely applied in the aerial, nuclear, microelectronic industry, turn to turn insulation and turn to ground insulation of inverter-fed motors. However, premature insulation failures occur frequently in motors with the disadvantage of pure PI inherent in all organic materials. The existence of surface charge has a great effect on breakdown characteristic and is the main reason leading to dielectric breakdown. Fluorination as change the chemical component in surface layer of polymers should give rise to the corresponding change in electrical properties of the surface layer thus influence the charge injection from electrodes when they are used as an insulator. This paper presents a study aimed at clarifying the effect of fluorination time on surface charge accumulation and decay behaviors of fluorinated polyimide film. Samples were surface fluorinated in a laboratory vessel at about 328 K (55 °C) using a F2/N2 mixture with 20% F2 by volume and 0.05 MPa (500 mbar) for respectively 15, 30, 45 and 60 minutes. Corona charging tests were performed at room temperature with a relative humidity of ~ 40%. The charge distribution was measured by means of an electrostatic voltmeter. Obtained results show the dependence of the charge density as well as the charge decay rate upon the fluorination time of samples, varying as a function of the charge polarity and charging time. It is suggested that the fluorination can significantly improve the decay rate of the surface charge in polyimide films.

Dynamic behavior of surface charge on direct- fluorinated polyimide films

Epoxy is widely used in electronic and electrical devices because of its excellent insulation properties. The addition of nanoparticles to epoxy could change the material structure and then charge behaviors are altered as a consequence. The accumulation of surface charge may cause electric field distortion, which would lead to surface flashover. In this paper, the samples were made by dispersing nano-scale boron nitride (BN) particles in Epoxy with the weight ratios of 5, 10, 20, 30, 40 wt%. The samples were charged by a dc corona triode system using needle-plate electrodes. The distribution of charges was measured with an electrostatic voltmeter and the effect of nanoparticles on charge accumulation and decay behavior was studied. A flashover test was carried out under a dc stress between two finger-type electrodes. The effect of nanoparticles on flashover characteristics and the relation between surface charging and surface flashover were also investigated. The results showed that the traps between nanoparticles and the polymer, which were introduced by the added nanoparticles, could cause surface charge accumulation. In addition, the flashover occurred at a lower voltage because of the accumulated charge on the sample surface.

Influence of surface charge on DC flashover characteristics of epoxy/BN nanocomposites

DC transmission helps to solve the decentralization of distributed wind power generation and other renewable power sources to enhance system performance. But the existence of surface charges by the corona makes earlier insulation failure and plays an important role during the development of the surface flashover. The influences of the ambient temperature and the polarity reversal voltage on the surface charge and the flashover of the heat-shrinkable polymer are not completely clear. In this paper, the carrier mobility and the volume conductivity were employed to explain the temperature dependent surface charge decay results. The initial surface charge density and decay rates served for analyzing the effects of polarity reversal voltage. Furthermore, the flashover experiments of the samples charged by the polarity reversal voltage at different ambient temperatures were conducted. Obtained results show that the descending speed of the surface charge becomes faster at higher temperature. An obvious temperature dependence of the carrier mobility and the volume conductivity is observed. Moreover, the surface charges of the samples charged by the voltage with shorter reversal time decay more rapidly. The flashover voltage increases with the increasing ambient temperature because of less accumulated surface charges in this condition. Considering the accelerated surface charge decay by the increasing temperature and easier charge accumulation during the steady negative period, attentions should be paid to the design of DC insulation and the security of operation.

Effects of ambient temperature on surface charge and flashover of heat-shrinkable polymer under polarity reversal voltage

We investigated the charging characteristics of alumina in vacuum with varying surface roughness, and the electric field distribution. The charging on the alumina surface in vacuum is, in general, strongly influenced by field electron emission (FEE) and secondary electron emission avalanche (SEEA). We varied the surface roughness and the electric field distribution on the alumina surface in order to control the FEE and the SEEA. Under these conditions, we measured the 2-dimensional distribution of surface charging potential on the alumina surface. From the measurement results, we quantitatively discussed charge polarity and charge density on the surface for various types of electric field distributions. Finally, we successfully propose a concept of a charging control technique on the alumina insulator in vacuum for the improvement of electrical insulation performance on the alumina surface.

T. Hosono

Papers

Surface charges on alumina in vacuum with varying surface roughness and electric field distribution