Effect Of Voltage Polarity Research Articles

Effect of voltage type and polarity on streamer dynamics in transformer oil

Mineral oil is one of the most important and widely used liquid as an insulation and cooling medium in large power transformers. Hence, maintaining the insulation strength of oil is the highest goal of all transformer manufacturers. The breakdown in liquid dielectrics involves various pre-breakdown stages. The electrical streamer is the main factor in these pre-breakdown processes. The positive streamer creates greater risk to oil insulated high voltage electrical equipment due to its high propagation velocity. A 2D-axisymmetry fluid model has been developed to investigate the phenomenon before the breakdown in transformer oil by applying the impulse voltage and direct current high voltage using needle-plane geometry with 1 mm gap. The electric field dependent molecular ionization technique is used to represent the streamer dynamics. The effect of voltage polarity and its type on streamer dynamics have been studied.

Effect of voltage polarity and supply frequency on the properties of plasma contacting liquid electrodes and gold nanoparticle synthesis

Plasma contacting with liquid (PCL) provides many charged particles and reactive species into the liquid. The difficulty in controlling or selecting each specific species has significantly limited its applications in industry. Here, we present a study on using voltage polarity to regulate the type of charged particles absorbing from the plasma into the liquid. A detailed understanding of the processes at the plasma–liquid interface and electrolysis due to switching in voltage polarity was determined via a visual pH observation, measuring the concentration of H2O2 and solvated electrons. The results indicated that changes in voltage polarity strongly affect the plasma properties, chemical properties and electrolysis process in liquid, and also in the types of reducing species for gold nanoparticle (GNP) synthesis. The results also showed that using a suitable frequency could improve the efficiency of absorption of H2O2 from plasma into the bulk liquid as well as the yield in the production of GNPs. The results provide a way to select desired species from the plasma to be transferred into the liquid for a distinct purpose and to accompany other properties in the system of PCL.

An Experimental Study on the Mobility Characteristics of Corona Ions Produced by a HVDC Test Line

This paper reports experimental results of the continuous mobility spectrum and mean mobility of corona ions by using a Gerdien tube and the integral equation method. The effects of voltage polarity, nominal electric field on the conductor surface, temperature, humidity and airborne suspended particles on the mobility spectrum and mean mobility are studied. The results show that the positive and negative mean ion mobilities are 1.06 -1.20 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> and 1.74-2.04 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> , respectively, when the temperature is 25 °C and the relative humidity (RH) is 20%. The electric field on conductor surface has little impact on the mean ion mobility in the range of +23.1-+28.2 kV/cm and -20.5–25.6 kV/cm, while increasing temperature and humidity leads to a decrease of the negative mean ion mobility. The results also show that airborne suspended particles have impacts on the mean ion mobility and mobility spectrum, and the impacts are density reliant, which are ascribable to the capture of ions by particles, leading to the variation of the weights of all ion species in the spectrum.

Current polarity effects on laboratory plasma jets

Plasma jets produced by a pulsed power machine were investigated using Thomson scattering and other diagnostics in order to make detailed comparisons to simulations. These jets were produced from a 15 μm thick disk of Al foil on a 1.2 MA, 100 ns rise time, pulsed power machine. Experiments were performed with both a radially inward and a radially outward current flow in the Al foil to investigate the effects of voltage polarity in the experiments and determine how extended magnetohydrodynamic (XMHD) effects, such as the Hall effect, change the formation of the jet. We recorded Thomson scattering spectra with a low enough laser energy to not perturb the plasma while providing a high enough signal to noise ratio to resolve the scattered features. This enabled the measurement of the electron temperature in the jet region of the plasma, 15.5±4 eV for both current polarities. Jets with a radially outward current flow were heated more from inverse bremsstrahlung when 10 J of laser energy was used, implying that these jets are denser than the ones with a radially inward current. This higher density was confirmed by interferometry measurements. Experimental results were compared with XMHD computer simulations, which predicted electron temperatures 1.5–3 σ above those measured, and significantly higher density than experiments in both polarities. Possible sources of this discrepancy are discussed.

The effect of DC voltage polarity on ionic wind in ambient air for cooling purposes

Gas flows can be induced by gas discharges like DC coronas because neutral molecules gain momentum by ion-neutral collisions. This can be used for active cooling and has advantages over mechanical fans. We investigate ionic wind by a DC corona discharge under different conditions with an emphasis on the effects of voltage polarity and the transition between different discharge regimes. We also consider the gas temperature of a DC corona which is important when it is to be used for cooling purposes. Although DC coronas are usually characterized as low temperature plasmas, gas heating can have a significant impact on flow generation, especially at higher operating voltages. In this paper, a 5–20 kV DC voltage of positive and negative polarity is applied to a needle–cylinder electrode. The ionic wind velocity at the exit of the cylinder electrode is measured by hot wire anemometry and the emission spectrum is used to study the gas temperature. It is found that the flow velocity induced by positive coronas is higher than that by negative coronas for voltages above 10–15 kV, which is also demonstrated by a phenomenological EHD force model. Furthermore, a heated column is observed by Schlieren technique for both voltage polarities. An improved self-consistent ionic wind model considering heat transfer is built to study the temperature distribution. The simulation results indicate that the gas flow velocity is lower on the symmetry axis when the temperature gradient is taken into account, something which is usually ignored in ionic wind simulations.

Ion source for IMS based on wire-to-plate corona discharge

In this paper, an ion source based on wire-to-plate corona is developed for Ion Mobility Spectrometer (IMS). The characteristics of the corona discharge and the ion current detected on Faraday plate are investigated under different electrode spacing and voltage. The effect of voltage polarity is also studied. The features of this new designed ion source are compared with that of point-to-plate corona. The results show that the present IMS prototype machine can provide a much larger value of ion current connected by Faraday plate than the point-to-plate corona and/or the traditional 63Ni source. The corona configuration can also act as a good electromagnetic shielding to defense the electromagnetic emission from the corona discharge.

Numerical and experimental study on atmospheric pressure ionization waves propagating through a U-shape channel

A 2D computational study of ionization waves propagating in U-shape channels at atmospheric pressure was performed, with emphasis on the effect of voltage polarity and the curvature of the bend. The discharge was ignited by a HV needle electrode inside the channel, and power was applied in the form of a trapezoidal pulse lasting 2 µs. We have shown that behavior of ionization waves propagating in U-shape channels was quite different with that in straight tubes. For positive polarity of applied voltage, the ionization waves tended to propagate along one side of walls rather than filling the channel. The propagation velocity of ionization waves predicted by the simulation was in good agreement with the experiment results; the velocity was first increasing rapidly in the vicinity of the needle tip and then decreasing with the increment of propagation distance. Then we have studied the influence of voltage polarity on discharge characteristics. For negative polarity, the ionization waves tended to propagate along the opposite side of the wall, while the discharge was more diffusive and volume-filling compared with the positive case. It was found that the propagation velocity for the negative ionization wave was higher than that for the positive one. Meanwhile, the propagation of the negative ionization wave depended less on the pre-ionization level than the positive ionization wave. Finally, the effect of the radius of curvature was studied. Simulations have shown that the propagation speeds were sensitive to the radii of the curvature of the channels for both polarities. Higher radii of curvature tended to have higher speed and longer length of plasma. The simulation results were supported by experimental observations under similar discharge conditions.

Effect of voltage polarity on the plasma-liquid interactions

This letter reports on how voltage polarity affects the plasma-liquid interactions. Unique electrical characteristics are observed because of the conductive and dielectric nature of the electrolyte, which may control the accumulation and dispersion of surface charges over the liquid. Depending on the voltage polarity, distinct spatio-temporal images of the optical emissions are observed; subject to a positive voltage, sodium emissions after evaporation are observed only at the interface, while the sodium emissions are gradually transported to the discharge region because of the motion of the sodium cations in the direction of the electric field during the negative voltage phase. Changing the duty ratio clearly shows that the phenomenon is affected by the voltage polarity.

Measurement of optical signals as a plasma propagation in the atmospheric pressure plasma jet columns

The propagation of plasma jets with argon gas is characterized in terms of two factors, the effect of electric field distribution along the tube and the effect of voltage polarity, from the observed results of optical signals along the entire column of plasma. The optical signal of plasma propagates from the high electric-field region of high-voltage electrode toward the low field region of the open air-space, regardless of the polarity of the voltage. The optical intensity and the propagation velocity are higher for the positive voltage than for the negative voltage. Moreover, the length of plasma plume exited from the end of the glass tube into the open air is shorter for the negative voltage. When the optical intensity is strong enough, a secondary peak signal follows the primary peak. In the plasma column on the inside of the glass tube, the optical intensity and the propagation velocity depend on the strength of the electric field; they are both high at the high-field region of voltage terminal and decay toward the end of the tube. The velocity is as fast as 104 m/s at the high-field region and slows down to 103 m/s at the low-field region of the glass-tube end. However, the plasma accelerates drastically to be (104–105) m/s after exiting the glass tube toward open air, even though the electric field is a quite low and thus the optical signal decays low before fading out. The experimental observations present in this report are explained with the propagation of the plasma diffusion waves.

On OH Density of an Atmospheric Pressure Plasma Jet by Laser-Induced Fluorescence

For room-temperature atmospheric pressure plasma jets (RT-APPJs), because of the gas flow, the OH density at a given position is different from the case without the gas flow. In this paper, an OH density decay model, which includes the main loss processes of OH and gas flow effect, is present. Based on the model, the simulation results have a very good agreement with the OH decay behavior measured by laser-induced fluorescence. The absolute OH density of the RT-APPJ is obtained when the best fit is achieved, which is about $2.4\times 10^{13}~{\rm cm}^{-3}$ at 5 mm away from the plasma jet nozzle and 1 $\mu{\rm s}$ after the discharge. In addition, to control the OH density, the effect of voltage polarity, applied voltage magnitude, pulse frequency, pulsewidth on the OH density is also investigated and discussed.

Effect of voltage polarity on oxidation-reduction potential by plasma in water

Use of plasma in water for water treatment and medical treatment is growing and raises expectations of finding advanced functions such as an increase of biological compatibility. In the present study with a focus on the variation of oxidation-reduction potential (ORP), relationships between the electrode polarities of plasma in water and the change of water quality such as conductivity, H2O2 concentration, dissolved hydrogen concentration, pH and ORP were revealed. Similar line spectra of radiation at the electrode tip were observed for each case of positive and negative electrode polarity. The emission intensities of OH (309 nm), Hα (656 nm), and OI (777 nm) for the positive discharge were significantly higher than those for the negative one, though the energy consumption during the discharge period of both cases was nearly the same. Positive electrode polarity was found to be more suitable than negative electrode polarity for increasing dissolved hydrogen gas and hydrogen peroxide. The ORP for the positive polarity decreased from 460 to 45 mV and that for the negative polarity decreased from 460 to 183 mV, although the pH and conductivity were not significantly changed.

Effect of voltage polarity and amplitude on electroforming of TiO2 based memristive devices

Pt/TiO2/Pt/Ti memristive devices were electrically formed to either the ON or OFF state using voltages of the same polarity but with different amplitudes. The forming step dictated the subsequent switching behaviour. A qualitative model based on the creation and migration of oxygen vacancies was proposed to explain the experimental results.

Coupling of methane under pulse corona plasma (I)

At normal temperature and pressure, pulse corona plasma was used as a new method for the dehydrogenative coupling of methane in the absence of oxygen. The effects of voltage polarity and input energy on the dehydrogenative coupling of methane were investigated. The parameter “energy efficiency” was introduced to examine the coupling of the input energy and the dehydrogenative coupling of methane. The experimental results show that positive corona gives higher energy efficiency than negative corona. When the positive corona was chosen, C2 yield per pass was 31.6% and acetylene yield per pass was 30.1% with 44.6% methane conversion at an input energy density of 1788kJ/mol and a pulse repetition frequency of 66Hz. The function of input energy density towards methane conversion may be expressed as a formula of-In(1-X) =k (PIF). In the range of input energy employed, C2 yield is proportional to input energy density, but energy efficiency drops off with increasing input energy density.

The Effect of Voltage Polarity on Ice Accretions on Short String Insulators

The present study investigated the effect of voltage polarity on ice deposits accumulated on a short-string of porcelain insulators from 85 μm supercooled droplets in a cold room maintained at a temperature of −12°C. The air velocity and the liquid water content were 3.3 m/s and 2.0 g/m3, respectively. Under these growth conditions, the type of ice accretion was hard rime with a density of 0.87 g/cm3 and the highest probability of flashover occurrence. With an applied voltage of 15 kV per insulator unit, corresponding to the operational voltage in service, there were no major changes in weight, density or the texture of the ice grown on the surface of the insulators.

Effects of voltage polarity, electric current, external resistance, number of sparkings, supply frequency, and addition of hydrogen and air on electrical breakdown in vacuum

Effects of voltage polarity, electric current, external resistance, number of sparkings, supply frequency, and addition of hydrogen and air on electrical breakdown in vacuum

Effects of voltage polarity, electric current, external resistance, number of sparkings, supply frequency, and addition of hydrogen and air on electrical breakdown in vacuum

The breakdown potential of a vacuum gap is measured using parallel planar electrodes made of sterling silver as a function of gap separations, number of sparkings, external resistance, and variation of hydrogen and air pressures using dc and ac (50 Hz) applied voltages. It is found that the dc and ac vacuum breakdown voltages become equal after full conditioning by repeated sparkings of both electrodes either with ac or with both polarities of the dc applied voltage. The vacuum breakdown voltage is found to be independent of supply frequency up to 250 Hz. The vacuum breakdown voltage decreases with decreasing the external circuit resistance in series with the gap in the range below 103 Ω. This decrease in the breakdown potential is attributed to the very short discharge time of the stray capacitive energy of the connecting leads into the gap compared to the time necessary for the current growth leading to breakdown. The minimum energy necessary to decondition (degrade) a vacuum gap is measured and the degree of deconditioning is determined as a function of energy discharged into the gap. The dc dielectric strength of the vacuum gap always recovers to its original value after a few sparkings.