Zhicheng Guan

Numeral analysis model for shielding failure of transmission line under lightning stroke

The shielding failure is the occurrence of a lightning stroke that bypasses the overhead ground wires and terminates on the phase conductors. The correct design to improve the shielding effect of transmission line to lightning is one of the key problems of transmission line design. A numerical analysis model for shielding failure of transmission line based on electromagnetic field theory is proposed in this paper. The electrical field of the system including lightning leader, transmission line and ground is solved, the range of lightning current striking the phase conductor is obtained, and the shielding failure probability of transmission line is calculated. The lightning leader model and the critical breakdown electrical field intensity of long gap are discussed. The analyzed results by numerical analysis model are compared with those by the electrogeometric model.

Observation of the transition from a Townsend discharge to a glow discharge in helium at atmospheric pressure

Dielectric barrier discharge in helium at atmospheric pressure was studied by taking fast images of the discharge during one current pulse using an intensified charge couple device. It was observed that there appears a weakly luminous layer close to the anode at the very beginning of the discharge, then the luminous area gradually expands into the entire gap as the anode layer moves toward the cathode, and finally a highly luminous layer forms close to the cathode at the time around the maximum of the current pulse. The evolution of the discharge pattern indicates a transition from Townsend discharge to glow discharge.

Effect of electric field distribution uniformity on electrospinning

Recent experiments demonstrated that the essential electrospinning mechanism is a rapidly whipping dielectric liquid jet in an electric field. The effects of three kinds of electric field distributions including a very nonuniform electric field, a slightly nonuniform electric field, and a uniform electric field were studied to analyze the influence of the electric field distribution on the electrospinning process and the fiber morphology. The average electric field strength Eav and the electric field nonuniformity coefficient f were used to describe the electric field. The results show that the length range of the straight jet path for stable electrospinning process was a function of the electric field distribution. The length of the jet straight part increased with the decreasing f. The initial angle of the looping envelope and the helical pitch decreased with decreasing f. The results also show that higher voltages can be used at the nozzle for a stable electrospinning when f is small. A higher voltage ...

Experimental investigation of the governing parameters in the electrospinning of polyethylene oxide solution

Electrospinning is an old but yet immature process which is now used to form nanoscale polymer fibers. This paper describes the use of SEM and measurements of jet currents to investigate the effects of key parameters on fiber formation during electrospinning of a polyethylene oxide solution (PEO). The ambient humidity and voltage polarity are key factors influencing fiber formation, but this process has not been carefully analyzed. The influence of the ambient humidity can be overcome by choosing a solvent with a larger difference between the saturated vapor pressure and the actual vapor pressure in air or by lowering the ambient humidity when water is used as the solvent. The process works with both negative and positive voltages, but the polarity influences the stability and the fiber diameters. Variations of the current were found to correspond with variations of the fiber diameters. Short target distances were found to prevent fiber formation while long distances resulted in a larger distribution of fiber diameters. The experiments also showed that ultra fine fibers were found between the ends of broken fibers due to a strong electrical force on fibers with little solvent

Laboratory investigation of impulse characteristics of transmission tower grounding devices

Reducing grounding resistance of a transmission tower grounding device is an important method to improve performance of transmission lines against lightning. Principles of impulse characteristics of grounding devices were discussed. Impulse experiments using grounding device models were systematically performed to analyze the influence of different factors on the impulse characteristics of grounding devices. During the experiments, soil resistivity was changed in the range of 100 to 5103 /spl Omega//spl middot/m. Effects of different parameters on impulse grounding resistance and impulse coefficients of different transmission tower grounding devices were discussed. Formulae to calculate impulse coefficients and power frequency grounding resistance of different grounding devices were obtained.

A shield ring enhanced equilateral hexagon distributed multi-needle electrospinning spinneret

The multi-needle electrospinning system is a convenient way to produce fibers with special structures such as core-shell morphologies at a high production rate. In this paper, a specially designed multi-needle electrospinning system is presented. The spinnerets were built-up with an equilateral hexagon array. Each set of 3 needles of the spinnerets were distributed as an equilateral triangle. A coaxial shield ring was used to create an approximate uniform electric field near the tips of the needles and to restrict the collection area. The simulation results also show that the outside needles can help to create a more uniform electric field near the inside tips of the needles and restrict the path of the inside jets, which works almost the same as the additional shield ring. Based on the simulation results, several multi-needle systems were tested. A 7 cm diameter shield ring was used in a 7 needle system, a 9 cm diameter shield ring was used in a 19 needle system and a 10.5 cm diameter shield ring was used in a 37 needle system. Polyethylene Oxide (PEO) aqueous solution was used as the test solution in experiments. The electrospinning results demonstrated that the use of multi-needle spinnerets is robust and that uniform nanofibers can be produced. The more needles used, the smaller the mean fiber diameter for larger mean electric field strengths. These distributions of needles show the scale up possibility of special structure electrospun nanofiber manufacturing.

Influence of aged conductor surface conditions on AC corona discharge with a corona cage

Relatively little investigation has previously been done on the effects of aging on conductor surface conditions, or on the ac corona discharges from these conductors. In the present work these were investigated by means of comparing corona on typical aged conductors with that on new ones. The aged conductors from the ac power transmission lines had been in service for 12, 15 and 20 years. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis were employed to characterize the surface morphology and composition of the conductors; and a non-contact surface profiler was used to measure the surface roughness of the six samples. As a result of corrosion and the adherence of various substances to the conductors, their surfaces were far rougher than the new ones. The ac corona discharge characteristics of these conductors were studied in a small corona cage using an ultraviolet imaging detector. As a result of the rough surfaces, the corona discharge intensity for the aged conductors was found to be much stronger than that for the new ones; and the corona inception voltages for the aged conductors were 10 to 25% lower than the new ones.

Positive corona inception voltages and corona currents for air at various pressures and humidities

The variation of positive dc corona characteristics with air pressure and humidity using a point/plane electrode system was studied in a perspex chamber allowing the pressure and humidity to be varied. A method of calculation was developed to determine the corona current for humid air over a range of pressures and humidities in which the effective ionization coefficient was calculated as a linear interpolation of the effective ionization coefficients for dry air and water vapor. The calculation method involved the solution of partial differential equations using a particle-in-cell algorithm. This also allowed the determination of the corona inception voltages for a range of pressures and humidities as the voltage at which the corona current first appeared. The validity of these calculations was confirmed by a series of experimental determinations of the corona inception voltages and of the variation of the corona current with the applied voltage for the same range of pressures and humidities. It was found that the corona current increased when the air pressure or humidity decreased (at constant applied voltage) mainly because of the rise in the space charge density or the positive ion mobility, respectively. The coefficient C, in the relationship between the corona current, the applied dc voltage and the corona inception voltage, declined with the increase of humidity. The rate of decrease of C with humidity became larger as the pressure was reduced. An approximately inversely proportional relationship was found to exist between C and the relative air density at constant humidity. This variation of C was principally associated with the variation in the positive ion mobility.

Study on hydrophobicity transfer of RTV coatings based on a modification of absorption and cohesion theory

Room temperature vulcanized (RTV) silicone rubber coatings are used increasingly on outdoor high voltage (HV) insulators subjected to heavy pollution. An important property of the RTV coatings is the ability to restore the surface hydrophobicity after a pollutant layer has built up on the surface, which can suppress the development of leakage currents, dry band arcing and flashover. The present study analyzes the hydrophobicity and the hydrophobicity transfer of RTV coatings and the factors that affected the hydrophobicity. A modification of absorption and cohesion theory was used to analyze the hydrophobicity transfer mechanisms and the influences of temperature, polydimethylsiloxane (PDMS) molecular weight, degree of cross-linking, extent of soluble and non-soluble components, and the pollutant properties. The hydrophobicity transfer of RTV coatings was also investigated with various conditions of contaminations and various factors, which affect the hydrophobicity transfer, including the composition of RTV coatings and the outside conditions. Results show that the hydrophobicity transfer of RTV coatings is not only dependent on compositions but also on factors of circumstance. The analysis also explains the obvious difference between the hydrophobicity transfer of RTV coatings to kieselguhr and kaolin based on a particle and surface analysis using the Brunauer, Emmett and Teller (BET) method, which explains the relationship between a BET surface area and the hydrophobicity transfer of the RTV coatings

Electric field analysis of water drop corona

Water drops on surface of polymer insulation materials may cause corona because they can enhance the electric field nearby. Water drop corona plays an important negative role in the long-term performance of composite insulators. In this paper, based on a model with two parallel electrodes water drop corona is studied. The relationship between initial corona voltage and different sets of water drops is obtained from experiments. Then several factors, which may affect the electric field analysis of water drop corona, are studied by numerical calculation on computer. Calculation is mainly on a 2-D model. However, a 3-D model is also used to study the effect of water drops distributed in space.