Yong Liu

Bio: Yong Liu is an academic researcher from Tianjin University. The author has contributed to research in topics: Insulator (electricity) & Dielectric. The author has an hindex of 11, co-authored 61 publications receiving 352 citations.

Pattern analysis of discharge characteristics for hydrophobicity evaluation of polymer insulator

Abstract: Hydrophobicity is a prominent characteristic of silicone rubber for polymer insulators. However, due to the environmental influence, the decrease even the loss of hydrophobicity will take place, which causes the essential concern on the electrical accidents induced by the ageing of polymer insulators. The accurate evaluation on the insulator hydrophobicity is increasingly required for the long-term performance of outdoor insulators in service. In this paper, based on dynamic dropping test (DDT), pattern characteristic of surface discharges generated by the dynamic behavior of water droplet was investigated to evaluate the hydrophobic properties of silicone rubber insulator at different ageing levels. The specimens were prepared by using the method of corona degradation and determined by HC levels from HC1 to HC6. Both the dynamic behavior of water droplet and the induced discharge phenomena were captured by using a high-speed camera fixed vertically to the sample surface. In order to quantify the surface discharges, methods of image processing and fractal dimension were employed to establish the relationship between the pattern characteristic of discharge light and the HC levels. The morphology of discharge region and the distribution of luminous brightness in relationship with the lapse time can provide an optical evaluation method for the insulator hydrophobicity. It is found that the number and duration of discharge light increase with the increase of HC level. The increasing tendency in brightness intensity and fractal dimension of discharge pattern can reveal the decrease of hydrophobicity. Therefore, the results obtained indicate that the pattern characteristic of discharge light in DDT method is sensitive to the hydrophobic properties, which can be applied as a non-contact method for hydrophobicity evaluation of polymer insulators.

Frequency Distribution of Leakage Current on Silicone Rubber Insulator in Salt-Fog Environments

Abstract: This paper presents an analysis result of using the frequency distribution of leakage current to monitor insulator performance in salt-fog environments. Experiments have been carried out on silicone rubber insulator by injecting salt fog into an artificial fog chamber with an ultrasonic vibration salt-fog generator and applying an AC voltage of 30 kVrms at 50 Hz as the test voltage. Based on the frequency distribution extracted by a wavelet transform technique, the measured leakage current is separated into the low-, intermediate-, and high-frequency components. Cumulative charge at each frequency component is obtained by a time-integral method and found to correlate the frequency components and the insulator performance. Obtained results show that high-frequency components are more effective than low-frequency components on monitoring insulator performance in salt-fog environments.

Surface Layer Fluorination-Modulated Space Charge Behaviors in HVDC Cable Accessory.

Abstract: Space charges tend to accumulate on the surface and at the interface of ethylene–propylene–diene terpolymer (EPDM), serving as high voltage direct current (HVDC) cable accessory insulation, which likely induces electrical field distortion and dielectric breakdown. Direct fluorination is an effective method to modify the surface characteristics of the EPDM without altering the bulk properties too much. In this paper, the surface morphology, hydrophobic properties, relative permittivity, and DC conductivity of the EPDM before and after fluorination treatment were tested. Furthermore, the surface and interface charge behaviors in the HVDC cable accessory were investigated by the pulsed electroacoustic (PEA) method, and explained from the point of view of trap distribution. The results show that fluorination helps the EPDM polymer obtain lower surface energy and relative permittivity, which is beneficial to the interface match in composite insulation systems. The lowest degree of space charge accumulation occurs in EPDM with 30 min of fluorination. After analyzing the results of the 3D potentials and the density of states (DOS) behaviors in EPDM before and after fluorination, it can be found that fluorination treatment introduces shallower electron traps, and the special electrostatic potential after fluorination can significantly suppress the space charge accumulation at the interface in the HVDC cable accessory.

Recurrent Plot Analysis of Leakage Current in Dynamic Drop Test for Hydrophobicity Evaluation of Silicone Rubber Insulator

Abstract: In order to reduce electrical accidents caused by aging of polymer insulators, hydrophobicity evaluation is one of the essential methods to monitor the practical performance of insulators. This paper presents recurrent plot (RP) analysis of leakage current (LC) to evaluate the hydrophobicity of the silicone rubber insulator. Experiments were carried out according to the dynamic drop test method. Dynamic behaviors of water droplets and the induced discharge phenomena were captured by using a high-speed camera fixed vertically to the specimen surface. LC was measured and extended to m-dimensional phase space by using a phase-space reconstructed method. The RP of LC was obtained to reflect the underlying mechanism of surface discharges in relation to the insulator hydrophobicity. The results indicate that the RP technique can give a visual method for the hydrophobicity evaluation. The increasing tendency in RP indicators can reveal the decrease of hydrophobicity.

Effect of concentration on tracking failure of epoxy/TiO 2 nanocomposites under dc voltage

Abstract: Epoxy has become an integral insulating material for electric and electronic devices due to its excellent mechanical and electrical performance. However, tracking failure on the epoxy surface is an unavoidable phenomenon in the increasing application. Based on the viewpoint of material modification, the properties of composite materials can be changed by the nano-particles. Whether the tracking resistance of epoxy nanocomposites can be improved needs better understanding for the reliable application. In this paper, nano-TiO2 particles were mixed into pure epoxy to obtain the epoxy/TiO2 nanocomposites with the weight ratios of 1, 3, 5, 7 wt%, respectively. Tracking experiments were carried out to investigate the effects of nano-TiO2 concentration on the tracking failure by applying dc voltage. The dc resistance to tracking failure, tracking process and discharge current were measured in relation with the concentration of TiO2 and the applied voltage. Obtained results indicate that with the concentration from 0 to 5 wt%, the number of drops to tracking failure shows the increasing tendency and the erosion depth shows the decreasing tendency. But they show the opposite varying tendency with the concentration from 5 to 7 wt%. Additionally, the fractal dimension method was applied to quantitatively reveal the nonlinear characteristics of both scintillation discharge distribution and tracking pattern for the purpose of identifying the concentration effects on the process of tracking failure. The recurrent plot technique was employed to reveal the nonlinear characteristics of discharge currents, which can reflect the underlying mechanism in the tracking failure affected by the nano-TiO2 particles. Therefore, the experimental results suggest that the tracking resistance of epoxy can be improved by nano-TiO2 particles, especially mixed with the concentration of 5 wt%.

Study on the Thermal and Dielectric Properties of SrTiO 3 /Epoxy Nanocomposites

Abstract: SrTiO 3 /epoxy nanocomposites are prepared using the facile solution-processing technique by incorporating SrTiO 3 nanoparticles with different weight fractions into the epoxy resin host. The morphology of the nanoparticles and composites, as well as the thermal conduction characteristics and electrical properties of the composites were investigated via conventional testing methods. The thermal conductivity increased along with the SrTiO 3 weight fractions, and the thermal conductivity of the SrTiO 3 /epoxy composite with 40 wt % weight fraction increased to 0.52 W/mK. The dielectric constant increased along with the weight fractions and decreased along with frequency, thereby suggesting that the interfacial and dipole polarization do not follow the changes in the electrical field direction at high frequency. The dielectric constants at 1 kHz frequency increased along with temperature. Surface breakdown tests illustrated further improvements in the thermal and electrical properties of the composites. In the same time span of 40 s, the 40 wt % nanocomposite demonstrated a rapid temperature decline rate of 6.77 °C/s, which was 47% faster than that of the pure epoxy sample. The surface breakdown voltage also increased along with the weight fractions. The functional composites can solve the key problem in the intelligentization, miniaturization, and high-efficiency of the gas-insulated switchgear, which warrants further research.

Promising functional graded materials for compact gaseous insulated switchgears/pipelines

Abstract: The electric field distributions along gas-solid interfaces determine the reliability, lifetimes and sizes of gaseous insulated switchgears/pipelines (GIS/GIL), which also affect the reliability of power systems. In this study, the characteristics of steady and transient electric field distributions were first introduced, followed by the concept of functionally graded materials (FGMs). The development histories of FGM applied in electrical engineering and the related optimisation methods were described in detail. The field regulation effect and fabrication technology of different FGM insulators were also compared. To overcome the limitations of traditional FGM insulators, the design of surface FGM (SFGM) was proposed, together with their preparation methods and electrical performance. Furthermore, the future development prospects of FGM and SFGM insulators for compact GIS/GIL were summarised.

웨이블릿 이론과 응용 = Wavelet theory and Its applications

Abstract: This is the final report of a three-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). We investigated the theory of wavelet transforms and their relation to Laboratory applications. The investigators have had considerable success in the past applying wavelet techniques to the numerical solution of optimal control problems for distributed-parameter systems, nonlinear signal estimation, and compression of digital imagery and multidimensional data. Wavelet theory involves ideas from the fields of harmonic analysis, numerical linear algebra, digital signal processing, approximation theory, and numerical analysis, and the new computational tools arising from wavelet theory are proving to be ideal for many Laboratory applications. 1. Background And Research Objectives The theory of wavelet transforms is a recent development drawing contributions from a number of different fields, including harmonic analysis, digital signal processing, approximation theory, and numerical analysis. A wavelet transform provides a linear representation of functions in terms of dilates and translates of a suitable basis function-the mother wavelet. The basis vectors are localized spatially by the translation parameter and lie on exponentially scaled intervals in the frequency domain, so wavelet expansions enjoy simultaneous time/frequency localization that is lacking in Fourier expansions, plus an octavescaled frequency resolution that makes them particularly economical for analyzing processes whose spectrum is biased towards lower frequencies, including much high-resolution sensor data. Wavelet expansions also exist for digital signals and are known as discrete wavelet transforms (DWTs). Such transforms are instances of multirate digital filter banks having the special property that, in the infinite sampling rate limit, they converge to a decomposition of the corresponding continuous-time signal in terms of a basis of continuous wavelets. This means that there is a duality for wavelet transforms analogous to the relationship between the Principal investigator, e-mail: vxf@lanl.gov *

Effect of micro-Si3N4-nano-Al2O3 co-filled particles on thermal conductivity, dielectric and mechanical properties of silicone rubber composites

Abstract: Silicone rubber (SR) is widely used on the distribution and transmission lines of power systems owing to its excellent insulating properties, extraordinary hydrophobicity and high tensile strength. However, the thermal conductivity of pure silicone rubber is very low which restricts its application for long time due to heat formation under electric field. This research focuses on the effect of micro- and nano-sized filler mixture on thermal conductivity, dielectric and mechanical properties of the SR composites. The micro-Si3N4 and nano-Al2O3 cofilled SR composites (MNCSR) with different volume ratios of Si3N4 and Al2O3 were fabricated by simple blending and subsequently hotmolding technique. With the addition of micro-Si3N4??nano-Al2O3 mixture at 30 vol. % (Si3N4/Al2O3=26/4), the composites showed high thermal conductivity of ~ 1.6 W m-1 k-1, low relative dielectric permittivity of ~ 5.3 and high breakdown strength of ~ 85 MV/m. Meanwhile, the introduction of micro-nano-sized particles resulted in improved elongation at beak and tensile strength. Besides, the MNCSR composites demonstrated good hydrophobicity with the static contact angle over 110°. The combination of these outstanding performances makes the MNCSR composites attractive in the field of insulating materials.

Nonlinear conductivity and interface charge behaviors between LDPE and EPDM/SiC composite for HVDC cable accessory

Abstract: Interface charges are easy to accumulate between two different dielectrics with various characteristics, which may cause accelerated degradation of insulation systems. Ethylene-propylene-diene terpolymer (EPDM) is used mainly for HVDC cable joint, which is the most vulnerable part of the cable system because of the interface. Particles with nonlinear conductivity can be doped into the polymer matrix to modify the interface charge behaviors through altering the conductivity under combined stresses. In this paper, silicon carbide (SiC) particles were dispersed into EPDM with 0, 10, 30 and 50 wt% respectively. The space charge behaviors at the interface between LDPE and EPDM filled with SiC particles was measured under 15 and 30 kV/mm. Besides, dielectric constant, dc conduction and trap distribution were introduced to elaborate the suppression mechanism with SiC doping. The SEM results show that the particles are well distributed in the EPDM. The permittivity increases with the fillgrade and the dc conductivity shows an obvious nonlinear trend under various electrical fields. SiC doping can effectively suppress the interface charge accumulation under different stresses. The suppression mechanism is attributed to the nonlinear conductivity and more shallow traps of EPDM/SiC composite. As a consequence, the approximate SiC doped EPDM can availably suppress the interface charge accumulation and offers a possible method for the improvement of cable accessory performance.