Boya Zhang

Bio: Boya Zhang is an academic researcher from Xi'an Jiaotong University. The author has contributed to research in topics: Surface charge & Insulator (electricity). The author has an hindex of 14, co-authored 56 publications receiving 684 citations. Previous affiliations of Boya Zhang include Tsinghua University & University of Connecticut.

Surface charge migration and dc surface flashover of surface-modified epoxy-based insulators

Abstract: Epoxy-based model insulators were manufactured and fluorinated under a F2/N2 mixture (12.5% F2) at 50 °C and 0.1 MPa for 15 min and 60 min. Surface charge accumulation and decay behavior were studied with and without dc voltage application. The effect of direct fluorination on surface charge migration as well as on flashover voltage was verified. The obtained results show that the charge decay of epoxy-based insulators is a slow process, but the decay rate increases when an outer dc electric field is applied. The surface charge distribution is changed when a streamer is triggered on the insulator surface. The existence of heteropolarity surface charges can decrease the dc surface flashover voltage to some extent, while the surface flashover voltage is almost unchanged when charges of the same polarity accumulate on the insulator surface. The short time fluorinated insulator can modify the surface resistivity, and the rate of surface charge dissipation is greatly increased under a dc electric field.

Charge accumulation patterns on spacer surface in HVDC gas-insulated system: Dominant uniform charging and random charge speckles

Abstract: Charge accumulation on a solid insulator surface is one of the critical factors for the development of dc gas-insulated equipment since it will lead to the overstress of polymeric insulation due to local field distortion and enhancement. Therefore, it is important to study the charge accumulation phenomenon on spacer surface under dc field. For decades, researchers have made tremendous progress on this subject by measurement and simulation. However, measurement results are quite different by different researchers due to various electrode configurations and experimental conditions. Further, most researchers use potential to represent charge density, which is not rigorous in that many charge density distribution details are hidden behind the potential. As for pure numerical simulation, reports are rather academic and sometimes cannot accord with the real fact. In this paper, attempts are made to characterize the charge accumulation patterns on spacer surface in HVDC gas-insulated system. Surface charge distributions on a model GIL spacer in 0.1 MPa air under DC voltage are obtained by an advanced measurement method, from which the dominant uniform charging pattern and random charge speckles are separated. Mechanism responsible for the dominant uniform charging pattern is discussed with the aid of a simulation model. Results indicate that, in a well-cleaned system, the electric current through the spacer bulk is the principal factor, but gas conduction is not negligible due to some inevitable ion sources. Highly localized pockets of charge are also observed, which are referred to as speckles. They may originate from micro discharges due to tiny metal particles on the spacer surface or microscopic protrusions on the electrodes.

Interpretation of the surface charge decay kinetics on insulators with different neutralization mechanisms

Abstract: Surface charge decay kinetics on corona-charged polymeric insulators were studied. In the experiment, the sample surface exposed to corona was kept open to ambient air whereas the opposite surface was in contact with a grounded plate. The resulting surface potential distribution was scanned in a raster mode using a Kelvin electrostatic probe, from which the relief map of charge density was obtained by an inverse calculation method. The developed procedure allowed for implementation of two study cases: (i) gas neutralization took place under natural conditions, with enhanced concentration of free ions, and with limited gas volume; (ii) surface charge decay occurred only due to bulk neutralization. Three mechanisms were considered responsible for surface charge decay, i.e., bulk neutralization, gas neutralization, and surface conduction. They were discussed separately with the help of numerical models, which quantified the relative importance of each mechanism on the total process of charge decay. The resul...

Suppression of surface charge accumulation on Al2O3-filled epoxy resin insulator under dc voltage by direct fluorination

Abstract: Surface charge accumulation on insulators under high dc voltage is a major factor that may lead to the reduction of insulation levels in gas insulated devices. In this paper, disc insulators made of Al2O3-filled epoxy resin were surface fluorinated using a F2/N2 mixture (12.5% F2) at 50 °C and 0.1 MPa for different durations of 15 min, 30 min and 60 min. A dc voltage was applied to the insulator for 30 min and the charge density on its surface was measured by an electrostatic probe. The results revealed significant lower surface charge densities on the fluorinated insulators in comparison with the original one. Surface conductivity measurements indicated a higher surface conductivity by over three orders of magnitude after fluorination, which would allow the charges to transfer along the surface and thus may suppress their accumulation. Further, attenuated total reflection infrared analysis and surface morphology observations of the samples revealed that the introduction of fluoride groups altered the sur...

Fluoronitrile/CO 2 mixture as an eco-friendly alternative to SF 6 for medium voltage switchgears

Abstract: Recent studies suggest that fluoronitriles compound Novec 4710 possesses a dielectric strength two times higher than that of SF 6 , while with a low global warming potential of only 2400. Mixed with CO 2 buffer gas, the Novec TM 4710 mixtures offer excellent dielectric properties and the possibility to be used as an eco-friendly alternative to SF 6 for medium voltage switchgears. This paper focuses on the comparative study of the breakdown strength and partial discharge (PD) characteristics between the fluoronitrile/CO 2 mixtures and SF 6 under AC voltages. Breakdown tests are conducted under different electrode configurations, i.e., plane-plane, sphere-plane, rod-plane, and needle-plane, at different pressures and electrode gaps. In the uniform field, equivalent dielectric strength to SF 6 at 100 kPa can be reached by a mixture containing 15% fluoronitrile at identical total pressure; while in non-uniform and highly non-uniform fields, SF 6 exhibits higher dielectric strength than 20% mixture. The PD analysis suggests that SF 6 has much more PDs with lower magnitude and the 20% fluoronitrile/CO 2 mixture has less PDs but with higher magnitude. Due to the different PD characteristics, the corona stabilization effect may be responsible for the higher breakdown strength of SF 6 under highly non-uniform field.

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Abstract: : The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Field-dependent charging phenomenon of HVDC spacers based on dominant charge behaviors

Abstract: Spacers are key components that are used to support high voltage conductors in gas-insulated substations or gas-insulated lines. The analysis of the surface charge patterns on spacers remains a difficult task, which requires a comprehensive understanding of the physical mechanism of the gas-solid interface charging phenomenon. In this letter, we reported a field dependent property of surface charge accumulation patterns on spacers under DC stress. We verified this finding through experiment, and further, we put forward a field-dependent charging model based on dominant charge transport behavior under different electric fields. It was found that the charging characteristics of the spacer are dominated by the Ohmic conduction from the volume below an electric field of 2.5 kV/mm. When the electric field stress is higher than 2.5 kV/mm, the charging property of spacers is dominated by the enhanced gas ionization according to Townsend's law. The correctness of this model was verified by surface charge measurement results in literature studies, and a method for determining the dominant mechanism of charge accumulation under different electric fields was proposed.Spacers are key components that are used to support high voltage conductors in gas-insulated substations or gas-insulated lines. The analysis of the surface charge patterns on spacers remains a difficult task, which requires a comprehensive understanding of the physical mechanism of the gas-solid interface charging phenomenon. In this letter, we reported a field dependent property of surface charge accumulation patterns on spacers under DC stress. We verified this finding through experiment, and further, we put forward a field-dependent charging model based on dominant charge transport behavior under different electric fields. It was found that the charging characteristics of the spacer are dominated by the Ohmic conduction from the volume below an electric field of 2.5 kV/mm. When the electric field stress is higher than 2.5 kV/mm, the charging property of spacers is dominated by the enhanced gas ionization according to Townsend's law. The correctness of this model was verified by surface charge measurem...

Surface charge migration and dc surface flashover of surface-modified epoxy-based insulators

Abstract: Epoxy-based model insulators were manufactured and fluorinated under a F2/N2 mixture (12.5% F2) at 50 °C and 0.1 MPa for 15 min and 60 min. Surface charge accumulation and decay behavior were studied with and without dc voltage application. The effect of direct fluorination on surface charge migration as well as on flashover voltage was verified. The obtained results show that the charge decay of epoxy-based insulators is a slow process, but the decay rate increases when an outer dc electric field is applied. The surface charge distribution is changed when a streamer is triggered on the insulator surface. The existence of heteropolarity surface charges can decrease the dc surface flashover voltage to some extent, while the surface flashover voltage is almost unchanged when charges of the same polarity accumulate on the insulator surface. The short time fluorinated insulator can modify the surface resistivity, and the rate of surface charge dissipation is greatly increased under a dc electric field.

The control mechanism of surface traps on surface charge behavior in alumina-filled epoxy composites

Abstract: To investigate the role surface traps play in the charge injection and transfer behavior of alumina-filled epoxy composites, surface traps with different trap levels are introduced by different surface modification methods which include dielectric barrier discharges plasma, direct fluorination, and Cr2O3 coating. The resulting surface physicochemical characteristics of experimental samples were observed using atomic force microscopy, scanning electron microscopy and fourier transform infrared spectroscopy. The surface potential under dc voltage was detected and the trap level distribution was measured. The results suggest that the surface morphology of the experimental samples differs dramatically after treatment with different surface modification methods. Different surface trap distributions directly determine the charge injection and transfer property along the surface. Shallow traps with trap level of 1.03–1.11 eV and 1.06–1.13 eV introduced by plasma and fluorination modifications are conducive for charge transport along the insulating surface, and the surface potential can be modified, producing a smoother potential curve. The Cr2O3 coating can introduce a large number of deep traps with energy levels ranging from 1.09 to 1.15 eV. These can prevent charge injection through the reversed electric field formed by intensive trapped charges in the Cr2O3 coatings.

Understanding surface charge accumulation and surface flashover on spacers in compressed gas insulation

Abstract: Surface charges have the effect of changing the electric field over spacers in compressed gases, which may affect the stable operation of gas insulated equipment and could possibly trigger a catastrophic surface flashover without warning. In this paper, we review the literature on surface charge accumulation and related surface flashover phenomenon on spacers in compressed gases. A summary of experimental research of surface charge accumulation is presented and, the surface charge accumulation mechanism is also summarized, with some newly identified factors in mind that may affect surface charge transport and measurements thereof. Subsequently, the correlation between surface charge accumulation and the surface flashover phenomenon is thoroughly discussed, and the effect of surface charges on surface breakdown under applied voltage pulses of various waveforms is analyzed. Finally, several important research topics in this area are proposed. In addition, some ideas on potential surface charge control methods are presented to aid the design of a better spacer free from surface charges in future studies.