Ruobing Zhang

Calculation of DC and AC flashover voltage of polluted insulators

The phenomena and the propagation processes of DC and AC arcs on the surface of polluted insulators were observed and investigated by means of a high-speed camera. Time-resolved photographs indicate that there is an obvious distinction between DC and AC flashover processes. Based on the alternating character of an AC arc on the polluted surface, the relationship between its reignition and recovery conditions is analyzed. The most important factor is the recovery rather than the reignition condition. A formula expressing the resistance of the pollution layer on the insulator is derived. From this, the flashover voltage of a polluted insulator of complex shape can be calculated. A computer program has been developed to calculate the flashover under both DC and AC. Some arc phenomena, such as arc bridging between the sheds or ribs of insulators, the drift of the arc from the insulator surface, and the number of arcs in series before flashover, are taken into account. Much experimentation has been done to determine the flashover voltages of polluted insulators with complex shapes under both DC and AC. Experimental data are in good agreement with the calculated results. >

Bacterial Decontamination of Water by Bipolar Pulsed Discharge in a Gas–Liquid–Solid Three-Phase Discharge Reactor

Inactivation of Escherichia coli by bipolar pulsed discharge in an air-liquid-solid three-phase discharge reactor has been investigated. The effect of several operating parameters on the inactivation efficiency has also been studied. Experimental results showed that bipolar pulsed discharge in the three-phase discharge plasma reactor had good performance in the inactivation of E. coli. Power intensity and gas flow rate had a positive effect on the inactivation, while the initial solution conductivity had little effect. An alkaline solution condition was favorable for the inactivation of E. coli in the reactor. In addition, possible mechanisms of the inactivation were also discussed

Inactivation Effects of PEF on Horseradish Peroxidase (HRP) and Pectinesterase (PE)

The effect of pulsed electric fields (PEFs) applied on the enzymes horseradish peroxidase (HRP) and pectinesterase (PE), which widely exist in vegetable and fruit juices, was investigated using chambers with paralleled plane electrode and coaxial cylinder electrode, respectively. The experimental results show that the relative activity of HRP and PE decreased after PEF treatment. Inactivation effect of HRP and PE increased with the increase of the applied electric strength and the number pulses. HRP activity decreased 14.4% and 16.7% in the parallel-plane chamber and the coaxial cylinder chamber, respectively, at 207 pulses of 25 kV/cm, while a 17 (coaxial) and 16.46 (plane) reduction, respectively, was attained for PE under the same experimental condition. Inactivation effect of HRP and PE in the chamber with coaxial-cylinder-electrode configuration was better than the parallel-plane electrode. The circular-dichroism analysis showed that PEF treatment caused a loss of alpha-helix and change of the secondary-conformation structure of HRP enzyme. The fluorescence intensity of both enzymes also changed after PEF treatment, indicating that conformation changes occur in the local structure of the enzyme molecule

Measurement of dynamic potential distribution during the propagation of a local arc along a polluted surface

A new method for measuring the dynamic potential distribution during the propagation of a local arc along a polluted surface is described. In order to study the role of electric stress during the pollution flashover processes, a flat polluted glass plate model was designed. The measuring electrodes made with a strip of aluminum foil were stuck on the bottom of the plate, opposite to the side on which the local arc propagated. The capacitance between the polluted surface and the measuring probe, which is in series with an additional capacitor, was used to construct a potential divider. By means of the dividers and a magnetic tape data recorder, the dynamic potential distribution, the position of local arc, and its propagation velocity were measured. The results show that the velocity of arc propagation along a polluted surface is 0.2 to 1 m/s and that the maximum average potential gradient in the pollution layer is approximately=0.5 kV/cm. >

DC flashover characteristics of the contaminated insulators under low atmospheric pressure

A simple test arrangement for the investigation of DC pollution flashover characteristics of insulators under low pressure and a DC power source meeting the needs of pollution testing are discussed. The DC pollution flashover performances of two types of insulators with various surface salt deposit densities were investigated under pressures of 48-101 kPa. The pollution discharge phenomena and the influence of pressure on the pollution flashover voltages are described. The difference between AC and DC pollution flashover characteristics under low pressure is discussed.<<ETX>>

Influence of Pulse Rise Time on the Inactivation of Staphylococcus Aureus by Pulsed Electric Fields

In this paper, the effect of rise time (200 ns and 2 μs, respectively) of rectangular pulse on the inactivation of Staphylococcus aureus has been studied. Apple juice with three different conductivities (1.5 mS/cm, 2.0 mS/cm, and 2.5 mS/cm) was inactivated 75 μs at an electric field strength of 25 kV/cm, 30 kV/cm, and 35 kV/cm, respectively. The experiment results showed that the pulse with shorter rise time had higher effect of inactivation and lower solution temperature increases after treatment, and the difference between te inactivation effects was more than 0.5 logs in general. Fast Fourier transform analysis showed that the pulse with shorter rise time had higher amplitude of low frequencies and more high-frequency components, which resulted in a higher transmembrane potential of Staphylococcus aureus cells and higher effect on intracellular material.

Investigation on Shelf Life of Carrot Juice Processed by Pulse Electric Field

Effects of PEF processing (25 kV/cm for 200 mus at 1 Hz) on enzymes and quality-related parameters of carrot juice were investigated during 28 days storage at 4degC and compared with effects of traditional heat pasteurization at 95degC for 15 sec in this paper. PEF treatment inactivated 37.35% and 52.14% of the activity of PME and PPO in carrot juice respectively, and the Activity of enzymes remained significantly constant during 28 days storage at 4degC (p > 0.05). PEF-treated carrot juice retained better turbidity than thermal-pasteurized juice, accompanying with insignificant changes in color indices (L, a, b, DeltaE and browning index) throughout storage. PEF treatment decreased the non-enzymatic browning reaction, and kept browning index almost stable throughout storage. The pH value of juice had no significant change after PEF treatment, and remained almost constant during storage time. PEF treatment caused an 8% decrease of the carotene content, and another 6.2% reduction of carotene content was observed after stored at 4degC for 28 days.

Configuration effect on DC flashover on polluted insulators

DC pollution discharge phenomena and flashover voltages of models made of glass plates, simulating a real insulator of the cap and pin type, were observed and measured. The influence of the main dimensions, such as the distance between sheds, the height of ribs, the width of ditches and the diameter of the disk, on the DC pollution flashover voltages of insulators is investigated and discussed. Some useful results are obtained relating to the prevention of bridging over clearances by local arcs and the improvement of the withstand voltage per unit leakage distance. Based on these results, suggestions for the rational design of DC insulators are presented. >

Hydrophobicity improvement of contaminated HTV silicone rubber by atmospheric plasma jet treatment

Hydrophobicity improvement of contaminated high temperature vulcanization (HTV) silicone rubber is of great significance to avoid risk of pollution flashover during wetting events. In the present investigation, low-temperature plasma jet produced by dielectric barrier discharge (DBD) under atmospheric pressure was used to quickly increase the hydrophobicity of contaminated HTV surface. Effects of the processing time, non-soluble deposit density (NSDD) and transfer time on the hydrophobicity improvement were studied and the long term stability of hydrophobicity was investigated. Experimental results proved that: hydrophobicity of contaminated HTV silicone rubber was significantly improved after a quick exposure to low-temperature plasma jet; and with the increase of exposure time, hydrophobicity transfer increased and a larger contact angle was obtained for the same transfer time. For the low discharge power plasma treatment, NSDD had a negative effect on hydrophobicity improvement and with the increase of the NSDD, increment of the contact angle decreased. In addition, hydrophobicity improvement had long term stability. Control experiment proved that interaction of plasma jet with silicone rubber played an important role in accelerating the transfer of low molecular weight (LMW) silane chains from the silicone rubber to the pollution layer, which finally resulted in the hydrophobicity improvement.

Influencing Factors of Dielectric Breakdown in the PEF Treatment Chamber

Electrical breakdown of the treatment chambers under repetitive high-intensity pulses impedes the industrial application of pulsed electric field (PEF) technology. In this paper, phenomenon and laws of electric discharges in the PEF treatment chambers were studied. The experimental results indicate that temperature increase and formation of gas bubbles under high electric field strength reduces insulation of water solution. Air dissolved in the samples is only a small part of the bubble and hardly affects the size of bubble formed. The majority part of the gas in the bubble comes from the vaporization of liquid. Fluctuations in the temperature curve may be a direct indication of the beginning of bubble formation. Partial electrical discharges occur inside the gas bubble and are the precursor of dielectric breakdown of PEF treatment chambers. Solution temperature and conductivity have significant effect on electrical discharges. Electric field strength needed for the electrical discharge decreases as the initial temperature increases. As the solution conductivity increases, the lowest electric field strength required for electrical discharge occurrence decreases. Findings in the present investigation reveal that measures that help to reduce the bubble formation are helpful to safe operation of the PEF system.