Bao-Hong Guo

Simulation on the dynamic charge behavior of vacuum flashover developing across insulator involving outgassing

A 2D simulation based on particle-in-cell and Monte Carlo collision algorithm is implemented to investigate the accumulation and dissipation of surface charges on an insulator during flashover with outgassing in vacuum. A layer of positive charges is formed on the insulator after the secondary electrons emission (SEE) reaches saturation. With the build-up of local pressure resulting from gas desorption, the incident energy of electrons is affected by electron-neutral collisions and field distortion, remarkably decreasing the charge density on the insulator. Gas desorption ionization initiates near the anode, culminating, and then abates, followed by a steady and gradual augmentation as the negatively charged surface spreads towards the cathode and halts the SEE nearby. The initiation of flashover development is discussed in detail, and a subdivision of flashover development is proposed, including an anode-initiated desorption ionization avalanche, establishment of a plasma sheath, and plasma expansion. The transform from saturation to explosion of space charges and dissipation of the surface charge are revealed, which can be explained by the competition between multipactor electrons and ionized electrons.A 2D simulation based on particle-in-cell and Monte Carlo collision algorithm is implemented to investigate the accumulation and dissipation of surface charges on an insulator during flashover with outgassing in vacuum. A layer of positive charges is formed on the insulator after the secondary electrons emission (SEE) reaches saturation. With the build-up of local pressure resulting from gas desorption, the incident energy of electrons is affected by electron-neutral collisions and field distortion, remarkably decreasing the charge density on the insulator. Gas desorption ionization initiates near the anode, culminating, and then abates, followed by a steady and gradual augmentation as the negatively charged surface spreads towards the cathode and halts the SEE nearby. The initiation of flashover development is discussed in detail, and a subdivision of flashover development is proposed, including an anode-initiated desorption ionization avalanche, establishment of a plasma sheath, and plasma expansion. Th...

Flashover strength improvement and multipactor suppression in vacuum using surface charge pre-conditioning on insulator

Surface charging commonly appears on dielectrics in vacuum in the presence of electron bombardment, seriously aggravating the superficial withstand strength of assorted devices. Nonetheless, a pre-conditioning technique is introduced in this paper capitalizing on surface charges to play an opposite role, enhancing flashover strength and suppressing the multipactor which is frequently found over vacuum insulator. A theoretical study is first performed, incorporated with particle-in-cell simulation to show the critical condition for a single-surface multipactor to initiate. Therewith, it is proven that a negative charge accretion in cathode adjacency can prevent the multipactor from commencing with efficiency. Subsequently, an analytical model is constructed to expatiate multipactor expansion with pre-set surface charges getting involved, illustrating an upper bound of its propagation velocity, influenced by pre-conditioning. Corresponding experiments are also conducted to corroborate previous conclusions, ...

Influence of embedded electrode structure and sputtering gold coating on surface flashover characteristics of PMMA in vacuum

The surface flashover phenomena of insulating materials in vacuum greatly weakens the overall performance of vacuum-dielectric system. The improvement of surface flashover characteristics of insulators in vacuum becomes a significant challenge for the development of high voltage electrical equipment. In this paper, we propose a convenient avenue to achieve the high surface flashover performance of polymethyl methacrylate (PMMA) in vacuum based on adopting two kinds of electrode dielectric contacts, one is to insert electrodes into samples, and the other is to sputter gold coating on the samples surface. Furthermore, the surface electric field distribution of PMMA samples under different embedded electrode structures were simulated in the COMSOL software. The results show that all the two kinds of electrode dielectric contacts treatment can apparently improve the flashover voltage of PMMA in vacuum. For the embedded electrode structure, when the electrode height is higher or lower than the embedding depth, flashover voltage is lower than the structure that electrode height is as high as the embedding depth. And the bigger difference between the electrode height and the embedding depth is, the lower its flashover voltage is and the smaller its dispersion is. Besides, sputtering gold coating on dielectric surface can effectively improve the contact between electrodes and dielectrics. But comparing with the situation of sputtering both bottom side and flank side, the flashover voltage of only sputtering bottom side is much higher and its dispersion is much smaller. This research provides two methods for improving the dielectric surface flashover characteristics in vacuum.

Influence of surface modification by direct fluorination on dielectric flashover characteristics in vacuum

The surface flashover phenomena of insulating materials in vacuum greatly weakens the overall performance of vacuum-dielectric system, which seriously limits its development. The improvement of surface flashover characteristics of insulators in vacuum becomes a significant challenge for the development of high voltage electrical equipment. In this paper, we report a simple and convenient way to achieve the high surface flashover performance of low density polyethylene (LDPE) and alumina ceramics (Al2O3) in vacuum based on direct fluorination technique. The direct fluorination of test samples was performed in a fluorination apparatus for different time of 30, 60, and 90min by using F2/N2 mixture of 12.5% F2 with the pressure and temperature keeping constant as 0.1MPa and 55°C. Then the surface flashover characteristics of LDPE and Al2O3 at different fluorination time was investigated. The results show that for LDPE samples fluorinated with 30 and 60min, the hold-off voltage increased by 21.9% and 34.1% than the virgin sample. However, the hold-on capability decreased apparently for the case of 90min fluorination. However for Al2O3 samples, direct fluorination had little influence on the surface flashover characteristics. This research could make a contribution to understand the influence of direct fluorination on dielectric flashover characteristics in vacuum.

Research on the defect development of cable accessories under AC voltage

As cable insulation failure usually started with PD activity, a PD detection system based on pulse current was built to study the PD characteristics of typical defects in XLPE cable. To facilitate the test, the oil cup test terminal was used. Considering the installation procedure, three different kinds of insulation defects in cable accessory samples were made, which were knife incision defect, metal floating potential defect and misplaced stress cone defect. Under long time power frequency voltage of U», 1.7U0, 2.5U0 and 3.5U0 (U0=8.7kV), this paper proposed a variety of methods to characterize partial discharge of these samples by PRPD spectrum, δ t spectrum and PSA spectrum. Results showed the δ t spectrum presented a clear distribution of the peak and the sub-peak. The distribution regularity of partition probability in PSA pattern before the breakdown was found that the probability in the area 1.1 as from 50% to 60%, and the probability in the area 3.1 was from 30% to 40%.

Characteristics of residual charge distribution on insulator surface under DC voltage in vacuum

For dielectric-vacuum compound insulation systems, the discharges usually occur across the insulator surface with an onset voltage much lower than the breakdown voltage for vacuum gap of the same length. It is no wonder that the surface charge accumulated on the insulator plays an important role in the flashover process in vacuum, especially under DC voltage. In this paper, the off-line observation of residual surface charge on PTFE under DC voltage in vacuum was carried out by a Kelvin electrostatic probe. It indicated that surface charging behaviors had close correlation with electric field distribution. For the geometry without back electrode, i.e., normal electric field was not so strong, surface charging at the first stage was the results of injection of charge carriers from electrode into sample surface layer under strong electric field. For higher excitation, it supported the secondary electron emission (SEE) as a mechanism of surface charging of an insulator in vacuum. However, under geometry with back electrode, i.e., normal electric field was strong enough, dielectric surface would be charged with the same polarity of applied voltage, indicating the injection of carriers into surface states becomes the domain factor in deciding surface charging. This research might be of great help to make a better understanding of surface charging in flashover process, and probably to put forward new measures for increasing the surface flashover voltage.