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Collecting the fiber mats with uniform characteristics is a current technological bottleneck to industrial application (e.g. filter media, nano-sensors, separators of Li-ion batteries etc.). This paper provides a promising industrial method for collecting uniform fiber mats in large area. The simulation results demonstrated that different collection targets had different surface electric field distribution. The experiments gave a result that thinner and more uniform fibers were collected when using a specific spherical-hat target uniform surface electric field distribution, and the thickness of fiber mats was more uniform as well. The paper presents that the collector's surface electric field has great effect on the electrospinning fibers arrived at the collector, and higher electric field intensity can guide the orientation of the deposited electrospinning fibers. Index Terms--Electrospinning, fiber mats, collection target, surface electric field, electrostatic.

Recent experiments demonstrated that the essential electrospinning mechanism is a rapidly whipping dielectric liquid jet in an electric field. There are generally two main methods to control the jet, one is to control the solution and the other is to control the flight of the electrospinning jet through manipulation of the electric field. In this paper, a modified electric field distribution, providing a stagnation surface, is discussed in order to know the effect on the electrospinning jet. The results illustrate that if the bending starting point happened below the stagnation point, the jet movement could be random to any direction as no electric force on the jet at the stagnation point and if the bending starting point happened before the jet reached the stagnation point, the jet movement could be bending as the modified jet path. The results also show that the modified electric field distributions could change the jet path and control the fiber morphology. Index Terms--Electrospinning, stagnation point, electric field distribution, PEO.

When large generators become operational, the stator bars in the slots are subjected to the electromagnetic force during normal operation. After the generators have been operating for a long time, the electromagnetic force may cause the stator bars to vibrate due to the existence of loose stator slot wedges in large generators. This leads to mechanical abrasion of the groundwall insulation and accelerates the deterioration of the generator, and can even cause generators to fail if the slot wedges are not treated. In order to ensure large generators continue working reliably, it is essential to detect loose slot wedges in the stator slots. This paper describes the modeling of slot wedges with different fixing styles in large generators to study the process by which slot wedges become loose, and also presents an analysis of the looseness of slot wedges based on their vibration characteristics corresponding to the loosening process of the stator slot wedge. Moreover, a series of tests were performed with a nondestructive detection system developed based on this analysis to check the tightness of slot wedges in the laboratory. The results showed that loose stator slot wedges of large generators could be detected either by their vibration frequency or by their displacement amplitude. This verified the correctness of the slot wedge loosening process and the validity of the detection system. Index Terms--Large generator, stator, slot wedge, looseness detection, vibration.

This paper presents an analysis of the arcing and flashover performance of contaminated dc insulators of various suspension patterns at high altitudes (1970 m). The experiments tested two types of dc insulators, a bell-type insulator and a tri-shed suspension insulator. The experimental results show that dc arcs play an important role in the flashover performance since they levitate more easily from the porcelain surfaces at high altitudes than at sea level. For heavily contaminated insulators, the relationship between the insulator length and the flashover voltage per insulator unit is affected by the arc development path, and is lower for shorter insulator strings (10 to 20 insulator units). With a non-uniform contamination distribution between the top and bottom insulator surfaces, the arcs propagate close to the insulator surface, resulting in a higher flashover voltage than with uniform contamination. The Y-string configuration appears to lead to much lower flashover voltages than in I- and V-string insulators. With the latter, the arc propagation path is greatly influenced by the angle between the two insulator strings with the flashover voltages for both bell-type and tri-shed insulators increasing with the angle but apparently due to different mechanisms. Experimental results indicate that the bell-type insulator has better flashover characteristics than the tri-shed insulator. Index Terms--Porcelain insulators, flashover, suspension pattern, arc.

According to hydrophobicity and hydrophobicity transfer studies on silicone rubber surfaces, although the silicone rubber can transfer its hydrophobicity to various contaminants deposited on its surface, the hydrophobic states of water droplets on clean silicone rubber surfaces and on various contaminated silicone rubber surfaces were found to be different based on evaluation by contact angle measurement. Furthermore, surface discharge and aging of materials decreases the hydrophobicity of the surface. Focused on the difference of hydrophobicity and the effect of water droplets on the hydrophobic contaminated surfaces, some contrastive computations and tests of water droplets on various surfaces were carried out in this study. The results showed that the water droplets existing on the surface of silicone rubber materials might change the electric field and voltage distribution along the surface, and even reduced the initial voltage of the corona discharge, which was verified with a comparison test on a high voltage side of polymer insulator monitored by a photomultiplier tube (PMT) system. Since the leakage current provides much useful information to diagnose the state of outdoor insulation, this study investigates the time and frequency distributions of surface discharges on ceramic insulators, new room temperature vulcanized (RTV) silicone rubber coated insulators and aging RTV insulators with various hydrophobic states with the short time Fourier transform and Gabor transform methods. Then a correlation was found between the distribution of the spectrum of surface discharges and hydrophobicity via the joint time and frequency analysis of discharges on various hydrophobic surfaces. Index Terms--Hydrophobicity, silicone rubber, discharge, leakage current, electric field distribution

The dynamics of positive streamer in air under different pressure and humidity conditions was studied. The widely used three-electrode system was used to obtain the streamer propagation probability and average propagation velocity as a function of background field. A simulation was performed based on the 1.5D fluid model. Simulated intrinsic stability field for streamer propagation and corresponding average propagation velocity were in agreement with those deriving from experimental results. The intrinsic stability fields obtained with the empirical equations proposed by the author and other investigators were compared. It was analyzed that the effect of the amplitude and duration of voltage pulse applied on the needle. The statistical distribution of propagation field was explained by the dispersion of discharge time delay. The decrease of intrinsic stability field with the reduction of pressure and/or humidity was due to the increase of the conductivity of the streamer channel as a result of the weakening of attachment and/or electron-ion recombination effect. Index Terms--Fluid model, humidity, pressure, propagation field, propagation velocity, streamer.

The control of fiber deposition in electrospinning, a technological bottleneck, has received a wide attention. This paper describes the use of polyvinyl chloride (PVC) insulating tubes to modify the electric field distribution and to produce a more uniform and concentrated electric field. The electric field distribution modified by the PVC tube plays an important role in controlling the swinging straight part of the jet and the spiral angle of the bending process. The fiber morphology was affected by the polarity of the applied voltage and finer fibers were produced by negative voltages than by positive voltage due to the ability of the charges caught and kept on the insulating material. Fibers without beads were produced in an 18 mm diameter area using a 7 cm diameter, 15 cm long PVC tube with 17.5 kV applied to the needle, 30 cm from the collector. The production rate was 13.89 g/h.[m.sup.2]. Index Terms--Electrospinning, nanofiber, insulator, electric field distribution.

Ice accumulation on high voltage insulators can cause accidents by flashover across the ice-covered insulators. The ice accumulation can be reduced by changing the insulator surface properties by application of a semiconducting RTV silicone coating. This method is based on the hydrophobicity and semiconductivity properties of the coating. The mechanism for reducing ice accretion is analyzed theoretically. Experimental results are presented to validate the effectiveness of the semiconducting RTV coating at reducing ice on insulator strings. The results show that the coating on the insulators delays ice formation and reduces the amount of ice. Index Terms--Ice, insulator, semiconducting, coating, RTV, flashover, hydrophobicity.

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. Index Terms--RTV silicone rubber, hydrophobicity, absorption and cohesion.

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. Index Terms--Electrospinning, humidity, voltage polarity, fibers.