Search

In this paper, an electric field ranging from 0.44-5.24 kV/mm was applied to a gas-insulated line (GIL) model filled with SF 6 at different values of gas pressure and relative humidity (RH). Dark current was measured using high voltage conductors with different surface roughness (S z ). The results show that the dark current reflects a multi-factor-dependent ionization process, which has a strong correlation with electric field strength, RH, and electrode surface roughness. At 45% RH, an S z value of 14.152 µm, and an electric field less than 2.2 kV/mm, the gas pressure was found to have a significant inhibitory effect on dark current. However, at an electric field greater than 2.5 kV/mm, the increase of RH has a significant positive correlation on dark current. From the results, it is verified that the values of conductor surface roughness requirements for AC GIL are too high to be used for DC GIL. We emphasize that the ionization leading to high dark current is due to local protrusions at different environmental conditions. For the DC GIL, the surface roughness of conductors should be minimized to achieve satisfactory performance.

A novel design of HVDC spacer was introduced based on the concept of adaptively controlling surface charges by using nonlinear materials. Unlike the commonly used traditional approaches, in this method, a novel idea for controlling the location of accumulated charges and properly decaying of these charges has been proposed initiatively, which fundamentally solves the problem of surface charge accumulation. The mechanical and electrical test results show that the charge adaptively controlling spacer has high operating capability under the DC voltage and has great industrial application potential.

A spacer is a challenging part of high voltage direct current (HVDC) gas-insulated substations and gas-insulated transmission lines. This paper proposes a novel design of HVDC spacers by self-adaptively controlling the surface charge. The paper briefly reviews dielectric modification methods that aim to limit surface charge accumulation. A novel design of HVDC spacer is introduced that is based on the concept of adaptively controlling surface charges using nonlinear materials, and its surface charge control and regulation abilities are investigated. The surface charge accumulation and decay properties of the novel spacer are compared to a downsized spacer. The results show that the proposed model spacer can restrict surface charge accumulation in the charge adaptive control region of the spacer. Doping the nonlinear material can facilitate surface charge decay in the charge adaptive control region when the electric field exceeds a threshold. These positive results represent a new idea of designing a suitable spacer to avoid surface flashover in HVDC SF 6 apparatus.

Charge accumulation on the surface of high voltage direct current (HVDC) spacers decreases their surface flashover voltage and affects the reliability of the insulation systems. In part I of this study, a novel HVDC spacer in which the surface charges can be adaptively controlled is proposed, and the aim of this paper is to experimentally verify the design. The experimental results of three kinds of model spacers, including a regular spacer, a spacer with a novel shape, and a spacer with charge adaptively controlled are provided. AC and DC surface flashover experiments and a polarity reversal test were performed. The effect of surface charges on triggering the surface flashover phenomenon are discussed. The results show that the surface flashover voltage increases both for the novel shaped spacer and the charge-adaptivelycontrolled spacer under application of AC bias, owing to a modification of the spacer shape that increases the creep distance. However, the surface breakdown voltage is the lowest for the novel shaped spacer under DC application and polarity reversal test. The charge adaptive control spacer displayed more advantages than the other samples under application of DC bias and in polarity reversal tests owing to its superior surface charge dissipation properties. It is hoped that this work could offer a new perspective for the design of a suitable spacer to avoid surface flashover when implementing HVDC energy transmission systems.

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.

The design of spacers for operation under DC stress is a challenging part for gas-insulated substations (GIS) and gas-insulated transmission lines (GIL). This series of papers has proposed a novel way to develop a spacer for operation under high-voltage direct-current (HVDC) conditions. In this part, a model spacer based on adaptively controlling surface charges is briefly introduced and is verified using routine tests, including electric field strength simulations, X-ray imaging, mechanical simulation and mechanical testing. In addition, the thermal expansion coefficient and thermal conductivity of the spacer are also measured to investigate its operational stability under various thermodynamic conditions. The results show that the geometrical improvements and SiC doping are available to reduce both the normal electric field strength around the central electrode and the tangential electric field strength around the enclosure. The mechanical properties of the concave side of the spacer are slightly lower than those of the regular spacer, while the mechanical properties of the convex side demonstrate the major improvements during both simulations and water pressure-based destructive testing. It is hoped that this work will provide a new approach for the design of spacers for use in HVDC GIS or GIL.

Surface charge accumulation on cone-type spacers restricts the development of dc gas-insulated substations (GISs) and gas-insulated transmission lines (GILs). In previous studies, many surface charge tailoring methods were investigated. However, indirect tailoring of surface charge by temperature field regulation has not been performed. In this paper, the relation between the temperature field distribution and the surface charge accumulation is studied by experiment. And a temperature control method by doping K2O·6TiO2 whiskers in the composite of epoxy resin is introduced and verified by experiment. The results show that the doping of K2O·6TiO2 is able to weaken the surface charge accumulation in both air and SF6.

To study the surface charge distribution of high-voltage direct-current (HVDC) spacers for gas insulated lines (GIL), a surface charge inversion algorithm based on surface potential measurements is necessary. However, previous studies on inversion calculation of surface charge density only considered the surface potential of one side, neglecting the effect of residual surface charge on the other side of the spacer; this leads to an inaccurate inverted surface charge distribution. In this paper, the inversion algorithm methods used to determine surface charges over the past few decades were summarized, and their advantages and disadvantages were discussed. Based on the previous charge inversion algorithms, an improved surface charge inversion algorithm considering the surface potential of both sides of a cone-type model spacer was developed. The improved method was verified experimentally using a cone-type model spacer. Compared with that obtained by previous charge inversion algorithms, the charge distribution derived from the improved algorithm was closer to the situation determined from theoretical analysis. This work has practical relevance for the surface charge measurement of cone-type spacers and offers a new orientation in the surface charge analytical algorithm.

A theoretical model considering the dynamic change of surface charge accumulation in transient and steady states is proposed. The surface charge accumulation under transient and steady states are simulated using three model spacers with different volume conductivities. The results show that on the convex surface of a spacer, when the volume conductivity is lower than 2 × 10−18 S/m, the hetero-polarity charge accumulated on the spacer surface is dominant, while the homo-polarity charge accumulated on the surface is dominant when the conductivity is higher than 6 × 10−17 S/m. The surface charge density is non-uniformly distributed, and the time duration of the transient process of the surface charge density at different positions on the spacer surface is different, due to the distribution of electric field lines. It is hopefully that the content of this paper can provide reference for the mechanism study of gas-volume interface charge transport and the preparation of HVDC spacers.

The spacer is a challenging part of high voltage direct current (HVDC) gas-insulated switchgear (GIS) and gas- insulated transmission lines (GILs). Based on the charge adaptively controlling strategy verified by our previously published papers, this paper serves as an important step towards further industrialization of charge adaptively controlling spacers. The insulation compounding scheme based on previous researches is focused in this paper. Spacers composed of insulating regions and charge adaptive control regions with different mass fractions of nonlinear materials were prepared. The mechanical properties, as well as the DC and AC surface flashover performance of these spacers were investigated. The results show that the DC surface flashover voltage is greatly reduced using spacers with a pure insulating region and with a doped charge adaptive region. As the doping ratio of the nonlinear material in the insulating region increases, the surface flashover voltage increases remarkably. However, the mechanical stress decreases dramatically when the mass ratio of nonlinear material is beyond 35%. The different doping ratios of nonlinear materials does not make a difference in AC surface flashover voltage. However, these surface flashover values obtained by bowl shaped spacers are much more stable and higher than that of the values measured from traditional cone type spacers at AC. The results in this paper can be a key step and are helpful in further determining the preferred option of the industrial spacer that can be potentially used in HVDC GIS/GILs. Meanwhile, based on the advanced performance, the idea of the novel bowl shape has potentially possibility in the application field of AC GIS/GILs.

Surface charge accumulation of high voltage direct current (HVDC) spacers can influence the surface flashover voltage and affects the reliability of the insulation systems. The cone type model spacer was manufactured based on the industrial 550 kV spacer as the prototype. The model spacers were fluorinated and tested in case of AC, DC and polarity reversal test. Experimental results of model spacers, including regular spacers and fluorinated spacers are provided. The effect of surface charges on triggering the surface flashover was discussed. The results show that the surface flashover voltage increases for the fluorinated spacer under application of both AC and DC stresses. In addition, the surface breakdown voltage of fluorinated spacers increases up to 250 kV level, compared with that of the regular spacer whose surface breakdown voltage remains at only 230 kV level.

For high voltage direct current (HVDC) gas-insulated substations (GIS) and gas-insulated transmission lines (GILs), the cone-type spacer is a challenging part. The surface charge accumulation on spacers is one of the most unfavorable factors that could result in unstable surface conditions. In this study, a way to dissipate the partially concentrated surface charge with nonlinear material SiC is verified by electrical simulation. The results show that the electric field distributes more evenly and the surface charge density becomes lower due to the doping of SiC. As the doping concentration increases, the surface charge density becomes lower. It is hoped that this work could bring a new perspective for the manufacture of cone-type spacers to reduce surface charge accumulation.

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.

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...

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.

From a realistic n-body potential of the Ni-Ti system, the critical concentrations of the Ni- and Ti-rich solid solutions were determined by molecular dynamics (MD) simulation to be 38 at% Ti and 15 at% Ni, respectively, beyond which a disordered atomic configuration was more stable than the respective crystalline solid solutions. It follows that the central composition range bounded by the critical solubilities, i.e. within 38-85 at% of Ti, can be considered as the glass-forming range of the system, which was confirmed by room temperature 200 keV xenon ion mixing of alternately deposited Ni-Ti multilayered films. Moreover, MD simulation of a Ni-Ti bilayer revealed that during the solid-state amorphization reaction, the growth of the amorphous interlayer followed exactly a t 1/2 law and grew faster towards the Ti lattice than to the Ni side. The physical origin of such an asymmetric behaviour was found to be due to a difference in critical solid solubilty of the constituent metals.

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.