Yingyao Zhang

Influence of High-Frequency High-Voltage Impulse Conditioning on Back-to-Back Capacitor Bank Switching Performance of Vacuum Interrupters

The objective of this paper is to investigate an influence of high-frequency high-voltage impulse conditioning on the back-to-back capacitor bank switching performance of vacuum interrupters (VIs). Nine identical 7.2-kV VIs were tested by 80 CO operations after the proposed conditioning. Test inrush currents were set to 0-, 2-, and 5-kA peak. The experimental results showed that this conditioning technology could make an impact on both prestrike and restrike characteristics of VIs. The complementary cumulative distribution function (CCDF) of the prestrike gap dpre followed complementary Weibull distribution. In contrast to VIs without conditioning, three typical prestrike gaps, i.e., CCDF 10% prestrike gap d1̅0̅ CCDF 50% prestrike gap d5̅0̅, and CCDF 90% prestrike gap d9̅0̅, decreased by 39%, 19%, and 4% at most, respectively. Besides, the scattering of dpre decreased by 52% at most. Furthermore, the conditioning technology had an influence on the feature of restrike waveform through variation of field emission current. It could also reduce the restrike probability by 22.5% at most and made the entire restrikes occur between 0.5T and 5T during recovery voltage period (T = 20 ms).

Surface potential decay measurements on fluorinated polymeric insulation for high voltage DC applications

Space charge accumulation in polymeric insulation is an increasingly important area in high voltage DC transmission as its presence distorts the local electric field, leading to premature failure. By using fluorination process, the surface of polymeric insulation is chemically treated and consequently modifies charge transport characteristics of the material. In doing so, excellent surface properties similar to fluoropolymers can be obtained without compromising the bulk characteristics of the polymeric insulation. The modifications in chemical components at the surface of polymeric insulation should lead to corresponding modifications in electrical properties as well. Different surface fluorinating conditions were investigated and the fluorinated samples were electrically characterised and tested, so an optimal processing condition can be achieved to meet practical requirements for high voltage DC insulating material. This paper will focus on the surface potential decay from negative corona discharge and PEA measurements on fluorinated epoxy samples of different fluorination times in a controlled environment. It has been found that there is a significant change in surface potential decay characteristics with the introduction of surface fluorinated layer. The effects of fluorination process on surface potential decay mechanisms responsible for the observed phenomena were thoroughly discussed.

Influence of fluorination time on surface flashover of polymeric insulation

Charge build up under high voltage DC is a big concern in transmission system as its presence distorts the local electric field. By chemically treat polymeric insulation via fluorination process, the charge transport characteristics of the material can be modified. In doing so, excellent surface properties of fluoropolymers can be attained without compromising the bulk properties of the original polymeric insulation. The change in chemical components at the surface of polymeric insulation should lead to corresponding change in electrical properties at the surface and consequently suppress charge build up. The purpose of this paper is to review recent research into the fluorination of epoxy resins in order to suppress charge accumulation and improve surface DC flashover strength. Surface DC flashover test using a pair of finger electrodes had been conducted. Modelling and simulation of electric field and current density distribution had also been carried out for the fluorinated polymeric insulator. It has been found that the introduction of fluorinated surface layer on epoxy resins plays an important role in improving the surface dielectric properties as evident from experimental and simulation results. The influences of fluorination process on surface flashover mechanisms were thoroughly discussed.

Effect of high-frequency high-voltage impulse conditioning on inrush current interruption of vacuum interrupters

The objective of this paper is to investigate the effect of a high-frequency high-voltage impulse conditioning technique on the inrush current interruption phenomena. In this paper, the inrush current interruption phenomena referred to the actual interruption and the currentless period following interruption. First, Six 7.2 kV vacuum interrupters (VIs) were conditioned by 400 batches of voltage impulses. The duration of each batch were set to 0.1 s. The peak value of each impulse could reach up to 100 kV. The repetition frequency of the impulse was set to 1000 Hz. The contact gap was 0.8 mm. The VIs were tested for the inrush current interruption performance with inrush current set to 2 kA and 5 kA, with a frequency of about 3800 Hz. The experimental results showed that there were two different types of inrush current interruption depending on high-frequency high-voltage impulse conditioning. The inrush current interruption type I referred to the inrush current interruption during the prestrike process before the contacts touched. This type inrush current interruption could be caused by the erosion of prestrike arc. Moreover, the high frequency voltage impulse conditioning influenced the occurrence frequency, the number of interruptions and the duration of the inrush current interruption type I. The inrush current interruption type II referred to the inrush current interruption during the contact bouncing process after the contacts touched. The inrush current interruption type II occurred only in the case of the VIs after conditioning. Furthermore, the occurrence frequency of the inrush current type II decreased with increasing inrush current.

Mechanisms for surface potential decay on fluorinated epoxy in high voltage DC applications

Epoxy resin has been extensively used for decades as an insulation material in high voltage transmission systems. However, this insulation material does suffer from bulk and surface charging when used as insulating spacer, mainly in high voltage DC applications. By applying fluorination treatment, the surface of polymeric insulation is chemically treated and so modifies charge transport characteristics of the material. In doing so, excellent surface properties can be obtained without compromising the bulk characteristics of the polymeric insulation. In this paper, the authors investigate the surface potential decay performance of non-fluorinated and fluorinated epoxy resin samples. The surface decay performance of insulating material is a crucial parameter in dissipating accumulation of surface and bulk charge that can lead to premature breakdown of the insulating material. The epoxy samples were characterised by Energy Dispersive X-Ray (EDX) analysis to determine the changes in chemical composition of the samples before and after fluorination treatment. Surface potential decay measurement using positive corona discharging was then performed, followed by bulk DC conductivity measurement to further explain the mechanisms which govern the surface potential decay. The existence of surface-fluorinated layer on the treated samples had been found to play a major role in dictating the movement of charges away from the surface during the decay process. The influence of fluorination treatment on the decay mechanisms was discussed.

Fractal-based electric field enhancement modeling of vacuum gap electrodes

The objective of this paper is to study the characteristics of the microscopic electric field enhancement at the micro-projections on the contact surface based on the fractal modeling of the micro-projections in vacuum interrupters. In this paper, the distortions of the microscopic electric field caused by the micro-projections were studied in the case of the applied voltage 60 kV and the contact gap 2 mm. Moreover, the microscopic enhancement factor βm for the various micro-projections with the height-radius ratio being less than or equal to 50 were investigated. First, the basic Weirstrass-Mandelbrot (W-M) fractal function was introduced and simplified into the modeling of the various micro-projections with the height-radius ratio 0.2, 0.4, 1, 2, 10 and 50 respectively. Then, the electric field distributions were analyzed based on these micro-projection fractal models. The simulation results showed that the micro-projections would cause a distortion of the microscopic electric field at the micro-projections on the contact surface, especially in the case of the higher height-radius ratio. In this paper, with the height-radius ratio of the micro-projections increasing from 0.2 to 50, the microscopic enhancement factor βm based on the fractal models would increase from 16 to 3667. In comparison with the idealized ellipsoid model, to the same height-radius ratio of the micro-projection, the microscopic enhancement factor βm based on the fractal models was about four times higher than that based on the ellipsoid models. Furthermore, the microscopic electric field at the micro-projections would have a significant influence on the micro-particles impact phenomena and other microscopic physical phenomena which could give rise to the vacuum breakdown. The results could provide some useful information to understand the physical mechanism to trigger the vacuum breakdown.

Moisture effect on surface fluorinated epoxy resin for high-voltage DC applications

Charge accumulation under high-voltage DC is a major concern in the transmission system as its presence distorts the local electric field. By performing chemical treatment on the polymeric insulation via fluorination process, the charge dynamics of the treated material can be modified. Surface fluorination treatment has been proven to be an effective tool to enhance the dielectric properties. The treatment slightly improves the surface conductivity value and enhances the surface potential decay rate, which helps to prevent the accumulation of surface charge and consequently improve the breakdown strength of epoxy samples. However, the authors suspected the introduction of a fluorinated layer on top of the epoxy sample may have the capacity to absorb moisture from the environment. The increase in the surface conductivity by the fluorination treatment may not come from the fluorine layer itself, but rather from the absorbed moisture in the surface layer. The loss of moisture from the surface may lead to a dip in dielectric performance of the treated materials. Hence, a study on the moisture effect on the surface fluorinated epoxy resin was designed. The fluorinated samples were characterised by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDX) to analyse the morphology as well as the concentration of elements in the treated materials. The samples then were dried in vacuum oven at 105°C and/or inside nitrogen gas chamber at room temperature for 24 hours to forcefully dry out the absorbed surface moisture. The measurements of surface DC conductivity and surface potential decay were taken before and after the drying process. It has been found that the fluorination treatment on epoxy resins did give rise to high moisture content on the surface which plays an important contribution towards the dielectric enhancement properties.

Micro-particle impact phenomena on contact surface under different applied voltages in vacuum interrupters

According to the vacuum breakdown model based on Cranberg “clump” hypothesis, micro-particles on contact surface can be detached from the contact surface and accelerated across the vacuum gap towards to the opposite contact under an applied voltage in vacuum interrupters (VIs). Finally they can impact on the target contact surface with high velocities, which probably triggers a breakdown in the vacuum gap. Thus, the mechanism of the impact phenomena may be the key to understanding the mechanism of the vacuum breakdown initiated by the micro-particles. Moreover, the detailed nature of the dynamic impact process is a subject of the hydro-dynamical theories on the projectile impact. The objective of this paper is to study the micro-particles impact phenomena in VIs by introducing a mesh-free numerical calculation method Smoothed Particle Hydrodynamics (SPH) method which is an efficient method of the projectile impact studies. In this paper, the materials of the micro-particle and the target contact were assumed to be copper and stainless-steel, respectively. The radius of the micro-particle was assumed to be 0.1 µm. The characteristics of the deformations occurring during the high-velocity impact processes are investigated under the different applied voltages (10 to 60 kV). As a result, it is found that the deformations of the micro-particles and the target contacts are severer with the increase of applied voltages, due to the increase of the impact velocities. With the applied voltages 50 kV and even to 60 kV (the impact velocity 526 and 631 m/s), there can be damages generated on the contact surface accompanying by a large number of high-velocity secondary particles. Furthermore, the influencing factors of the micro-particles impact phenomena in VIs, such as the size of the micro-particles, the microscopic electric field distribution, the contact materials and so on, have also be considered. The result of this paper may provide some useful information to understand the vacuum breakdown initiated by the micro-particles.

Dc flashover performance and surface discharge resistance of the fluorinated epoxy insulator in nitrogen gas

The surface of the epoxy insulator was modified by direct fluorination in a laboratory vessel using a F2/N2 mixture with 12.5% F2 by volume at 55 °C and 0.1 MPa for 30 min. The modified surface was examined by ATR-FTIR and SEM techniques. ATR-FTIR results show substantial changes in chemical composition and structure of the surface layer by the fluorination, and SEM observations reveal the fluorinated surface layer having a thickness of 0.76 μm and a roughed surface. Dc flashover performance and surface discharge resistance of the fluorinated surface layer in nitrogen gas were simultaneously evaluated using a simple sample/finger electrode arrangement in a stainless steel chamber at room temperature. The test results indicate a definite improvement in dc flashover voltage and a great increase in tracking resistance to the flashover discharge. The former is mainly attributed to easy leakage and dispersion of the charge from the cathode triple junction on the fluorinated surface due to high conductivity of the fluorinated layer, and the latter is due to the formation of C-F bonds and the disappearance of conjugated double bonds in the surface layer by the substitution and addition of fluorine atoms.

Study of microscopic electric field on contact surface in vacuum interrupters based on fractal theory

The objective of this paper is to study the microscopic electric field on contact surface in vacuum interrupters based on fractal theory, with considering the contact surface roughness. Firstly, the fractal theory is introduced into the application of vacuum breakdown studies. The micro-protrusion on the contact surface is modelled based on the W-M fractal function, with the different contact surface roughness 1.6 um and 3.2 um. Then, the electric field distribution in the contact gap is simulated. With considering the amount of calculation time and the accuracy of calculation, multi-zone mesh generation is used to improve the accuracy of the simulation. The results can provide some useful information to study vacuum breakdown phenomena.