Guo-Qiang Su

Aging characterization of high temperature vulcanized silicone rubber housing material used for outdoor insulation

During the field service of composite insulators, high temperature vulcanized (HTV) silicone rubber (SIR) material used for insulators sheath and sheds is gradually aging, and thus how to effectively evaluate its aging state has become an inevitable issue. In this paper, artificially corona-aged and naturally site-aged SIR materials are employed as samples, and their properties such as hydrophobicity, leakage current, trap density/energy level, surface microstructure and chemical composition are investigated for comparative study. With the increasing of corona aging intensity, the contact angle of SIR samples drops gradually and recovers more slowly. The hydrophobicity of site-aged insulators also declines, from HC1 for 2-year service to HC5 for 15-year service. The leakage current of corona-aged and site-aged samples both increase with aging, which indicates that aging induces the increase of surface conductivity of SIR material. The peak trap density of corona-aged and site-aged samples increases with the aging duration or service duration remarkably. The changes of these properties are attributed to the changes of micro-structures and compositions in the surface layer of SIR. The scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) analysis results reflect that a hardened inorganic silica-like (SiO2) layer with many polar chemical groups and distributed micro-pores is formed on the surface of SIR material after corona aging. Because of this silica-like layer, the hydrophobicity decreases while surface conductivity and trap density of SIR material increases. Besides the traditional properties like hydrophobicity and surface conductivity, the trap density is expected to be a novel parameter for effective evaluation of aging state of HTV silicone rubber material.

Luminescence evolution from alumina ceramic surface before flashover under direct and alternating current voltage in vacuum

The luminescence evolution phenomena from alumina ceramic surface in vacuum under high voltage of direct and alternating current are reported, with the voltage covering a large range from far below to close to the flashover voltage. Its time resolved and spatial distributed behaviors are examined by a photon counting system and an electron-multiplying charge-coupled device (EMCCD) together with a digital camera, respectively. The luminescence before flashover exhibits two stages as voltage increasing, i.e., under a relative low voltage (Stage A), the luminescence is ascribed to radiative recombination of hetero-charges injected into the sample surface layer by Schottky effect; under a higher voltage (Stage B), a stable secondary electron emission process, resulting from the Fowler-Nordheim emission at the cathode triple junction (CTJ), is responsible for the luminescence. Spectrum analysis implies that inner secondary electrons within the surface layer of alumina generated during the SSEE process also participate in the luminescence of Stage B. A comprehensive interpretation of the flashover process is formulated, which might promote a better understanding of flashover issue in vacuum.

Evolution From Cathode-Initiated to Anode-Initiated Flashover in Vacuum

This paper presents experimentally flashover evolution initiated from cathode to anode via an intensified charge-coupled device camera under the condition of adjusting electric field distribution. These results suggest that a flashover process across insulator surface in vacuum is greatly affected by the local electric field at both cathode and anode triple junctions. Flashover can be initiated at either cathode or anode. This paper might be helpful to clarify the dispute about how the flashover is actually initiated.

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...

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.

Improved algorithm of surface charge density distribution and electric field distribution on insulating materials

Surface charge accumulation on insulating materials is a major factor that greatly distorts the electrical field and thus reduces the insulation level. In this paper, the relationship between the surface charge density distribution and the output voltage of electrostatic probe is discussed, and an improved algorithm is built to calculate the charge density distribution, which employs two-dimensional Discrete Fourier Transformation (2D-DFT) to reduce the amount of calculation and the Wiener filter to reduce the influence of noise. Furthermore, because of the similar relationship between the electric field strength distribution and the surface charge density distribution, the field strength distribution can also be calculated by adopting this algorithm. In addition, the electrostatic probe is taken into consideration in this algorithm, and the spatial resolution of this measurement system including the inverse calculation process is investigated. It is estimated that the spatial resolution of this measurement system with 1mm gap between the electrostatic probe and the surface of the insulation is 0.3341mm. The charge density distribution as well as the field strength distribution is calculated by this improved algorithm when a lightning impulse voltage is applied to a PMMA plate of 1mm thickness, respectively. The results reveal that this algorithm has great accuracy and stability and can exactly calculate the surface charge density distribution and the electric field strength distribution.

Flashover time delay characteristics of grooved insulators under nanosecond pulse 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, which is the main cause leading to insulation failure. It has always been a hot topic to improve the surface insulation strength in vacuum. In this paper, the relationship between the flashover time delay and electric field is studied under nanosecond impulse voltage. Experiment results suggest that the flashover time delay was proportional to N-th power of electric field for grooved insulators with different parameters. For grooved insulator, the flashover channel prefer to develop along the grooves rather than pass over the groove wall. The blocking effect of grooves to the electron multiplication in cathode-anode direction leads to the increased time delay. A narrow band of electric field with respect to time delay is formed over the flat samples. The breakdown electric field of a grooved insulator was increased by more than 100% than that of a flat one. The flashover electric field tends to be saturated when the groove depth is more than 2 mm for both PTFE and PMMA, which is correlated with the trajectory of electrons travelling across insulator surface. Grooves near electrode is more effective to increase surface insulation strength than grooves in middle area, which can be of great help to guide the structure optimization in engineering application.

Electrons stimulated gas desorption of some dielectrics in vacuum

Surface flashover phenomena in vacuum across dielectrics usually happen in many pulsed power devices, high power microwave (HPM) equipment and particle accelerators. Secondary electron emission avalanche (SEEA) model is generally accepted to analyze this phenomena. During the evolution of flashover development, the process of electrons desorption stimulated (ESD) gases plays a dominant role and then leads to final discharge formation. Accurate measurement of ESD gas components and its quantity is very helpful to investigate flashover mechanism and make theoretical simulation. In this paper, an ESD measurement apparatus is set up with a modified quadrupole mass spectrometer (QMS) surround with double cooling shrouds. The incident electron energy varies from 2keV to 7keV. First, residual gas in the experiment chamber is analyzed. Next, pulsed electron beam impacts the sample surface and release ESD gases, and then mass spectrum analysis of ionized gases is carried out by QMS. The differences before and during electron irradiation are regarded as the amount of desorbed gases. Some typical dielectrics are measured including PTFE, PE and Al2O3. It is found that main desorbed gas components are similar, i.e. H2, H2O, CO, CO2, etc. Additionally, electrons irradiation can break chemical bonds to produce new components. H2 accounts for the majority of total desorption gases. Organic material (PE) has higher desorption gas than that of Al2O3.

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.