Negative DC Voltage Research Articles

Simulations of hybrid direct current radiofrequency (dc/rf) capacitively coupled plasmas

Effects of applying a high negative direct current (dc) voltage in argon capacitive coupled plasmas on the flux of secondary electrons (SEs) to a radiofrequency (rf) biased electrode and plasma uniformity were investigated using particle-in-cell Monte Carlo collision simulation and results were compared with experimental results. High energy SEs originating at electrodes with high negative dc voltage due to ion-impact as well as electron-impact were accelerated out of the sheath through the sheath voltage drop. These SEs gain energy equal to the applied dc voltage and either travel to the wafer electrode where they are lost (dumped) or are trapped between electrodes depending on the voltage on bottom electrode. Trapping and dumping of ballistic electrons depends on the magnitudes of the voltages (dc and rf) applied to each electrode. High energy SEs alter the electron energy distribution function at the wafer, high energy electron flux to the wafer and the plasma density profile. With the application of negative dc voltage, wafer receives high energy electron flux with energies up to applied dc voltage. For the range of conditions examined in this paper, the plasma density increases with the application of the negative dc voltage due to ionization by trapping of high energy SEs. The electrons that originate at the negative dc electrode minimize the impact of enhanced radial electric fields at the rf electrode edge allowing dc/rf plasmas to be center peaked even at low (13.56 MHz) rf frequencies.

Contribution of surface charges on high‐voltage DC silicon rubber insulators to DC flashover performance

In this study, the results of measurement for DC flashover characteristics of silicon rubber insulators in the presence of surface charges and in terms of geometric characteristics have been examined and analysed. In the carried out experimental tests, the surface of different insulators was charged by an external corona source and both the metal end fittings were kept grounded while the needles have been connected to a certain applied negative and positive DC voltages and then surface electric charge measured. A series of flashover tests have been carried out on the charged insulators under positive DC voltages to investigate the effects of surface charges. The experimental results revealed that positive electric charges reduced flashover performance while negative charges increased the flashover voltage level. Also, the experimental modelling of DC flashover of charged insulators regarding the ratio of shed spacing to shed depth and specific leakage distance in terms of surface electric charges, revealed that flashover voltage gradient of the charged insulator is most affected by specific leakage.

Improved Performance for Toluene Abatement in a Continuous-Flow Pulsed Sliding Discharge Reactor Based on Three-Electrode Configuration

The degradation of toluene by non-thermal plasma has been evaluated in a continuous-flow sliding dielectric barrier discharge (SLDBD) reactor based on three-electrode configuration and compared with a traditional surface dielectric barrier discharge reactor. In order to optimize the electrical and geometry parameters of the SLDBD reactor, the effects of positive pulsed high-voltage (U+pulse), negative DC voltage (U−DC), pulse-forming capacitance (Cp)), inter-electrode gap, discharge length, and dielectric material have been systematically investigated. Morphological characterizations demonstrate that the steamer channels can propagate more homogeneously along the dielectric surface when a sufficient U−DC is applied under the condition of slight increase in energy. The average discharge power of the SLDBD reactor mainly depends on U+pulse, while which is less affected by U−DC. Unexpectedly, both toluene degradation efficiency and energy yield using the SLDBD increase significantly as U−DC, indicating that VOC degradation is not only determined by the energy primarily provided by U+pulse, but also depends on the drift of the ionized species induced by U−DC. Increasing Cp enhances the energy injected into the SLDBD reactor and leads to a higher toluene degradation efficiency, but lowers the energy yield when the other parameters remains unchanged. The optimal Cp is 0.67 nF. Shorter inter-electrode gap and longer discharge length appear to be more advantageous in terms of toluene degradation and energy yield. Quartz plate exhibits remarkably better degradation and energy performance than ceramic and polytef ones, leading to the maximum toluene degradation efficiency of 58% and energy yield of 0.85 g/kWh in this work.

Streamer characteristic and breakdown in a mineral oil and a synthetic ester liquid under DC voltage

This paper presents experimental studies of streamer propagation and breakdown in a mineral oil and a synthetic ester liquid under both positive and negative DC voltages. A point-plane electrode configuration with a point tip radius of 10 μm was used. Characteristics of streamer including length, propagation velocity and shape were analysed based on shadowgraph images obtained at a gap distance of 10 mm using a multi-channel ultra-high speed camera. Under positive polarity, the streamers in both liquids propagate in the 2nd mode; the stopping length of the synthetic ester is longer than that of the mineral oil at the same applied voltage level. However there is no obvious streamer propagation (less than 10% of the gap distance) under negative polarity even when the applied voltage approaches to the breakdown voltage; the streamers in both liquids stay in the 1st mode. Breakdown tests at various gaps of 2 mm, 5 mm, 10 mm, 20 mm and 30 mm were also investigated. 50% breakdown voltages of the synthetic ester liquid are lower than those of the mineral oil at all the gap distances investigated under both polarities.

A novel electrical method to measure wire tensions for time projection chambers

We present a novel electrical technique to measure the tension of wires in multi-wire drift chambers. We create alternating electric fields by biasing adjacent wires on both sides of a test wire with a superposition of positive and negative DC voltages on an AC signal (VAC±VDC). The resulting oscillations of the wire will display a resonance at its natural frequency, and the corresponding change of the capacitance will lead to a measurable current. This scheme is scalable to multiple wires and therefore enables us to precisely measure the tension of a large number of wires in a short time. This technique can also be applied at cryogenic temperatures making it an attractive solution for future large time-projection chambers such as the DUNE detector. We present the concept, an example implementation and its performance in a real-world scenario and discuss the limitations of the sensitivity of the system in terms of voltage and wire length.

Analysis of the production mechanism of H2O2 in water treated by helium DC plasma jets

In this paper, helium plasma jets are used for water activation to explore the production mechanism of H2O2 in the water. Either positive or negative DC voltage is used for the plasma excitation. It is found that the concentration of aqueous H2O2 induced by the positive plasma jet is ~4 times larger than that by the negative plasma jet with similar average discharge current. Three production pathways of aqueous H2O2 are considered important in literature, including the electrolysis, the dissolution of gaseous H2O2, and the combination of aqueous OH. However, the first two pathways are found to have similar contributions on H2O2 production for the positive and negative plasma jets. It is deduced that the OH combination in the surface layer of the plasma-activated water is most responsible for the concentration difference of aqueous H2O2 between the two plasma jets.

Influence of moisture on the interface charge of oil–pressboard composite insulation under DC voltage

The distortion of the electric field in the oil–pressboard composite insulation caused by the accumulation of the interface charge is detrimental to both the insulation design and operation of converter transformers. The influence of moisture content on the surface charge accumulation of oil–pressboard insulation under DC voltage was studied in this study. In accordance with the Kerr electro-optic effect, the electric field strengths in transformer oil and the surface charge density were acquired after applying the positive and negative DC voltages in three oil–pressboard insulation models with different moisture content, respectively. The resistivities of the oil and pressboard in three models, namely Model 1# with 3.8–4.2 ppm moisture in oil and 0.35–0.37% moisture in pressboard, Model 2# with 7.6–7.9 ppm moisture in oil and 0.79–0.82% moisture in pressboard and Model 3# with 14.9–15.4 ppm moisture in oil and 1.39–1.42% moisture in pressboard, was also measured. The results indicate that: (i) as negative charges in oil accumulated on the pressboard surface in a much greater speed than the positive ones, the electric field in transformer oil under negative DC voltage decreases more rapidly with time than that under positive DC voltage; (ii) the increase of the moisture content in both oil and pressboard, under either positive or negative DC voltage, leads to the decrease of both the electric field strength in transformer oil and the charge density with time; and (iii) the increase of moisture content could not only decrease the resistivity of both oil and pressboard, but also the ratio of the resistivity between the pressboard and the oil. On the basis of the Maxwell–Wagner theory, the decrease of the ratio between the pressboard and oil could lead to the decrease of the interfacial charge density, leading to the slow transient process of the electric field in transformer oil under DC voltage.

The different stages of PRPD pattern for positive point to plane corona driven by a DC voltage containing ripple

This paper presents the different patterns, which can be observed by means of a phase resolved partial discharge analysis, when a negative dc voltage is applied to a point to plane arrangement in air. The required phase relation is obtained due to the ripple being present on the dc voltage generated by a half-wave rectification. Phase resolved partial discharge patterns are given from corona onset to glow inception. Discharge pulses appearing at the test object are used to support physical explanations of the pattern occurrence. It is shown that five clearly identifiable patterns accompany the evolution of corona discharges when a dc voltage with a ripple is used. The uniqueness of the different patterns and the accompanied reduction in test effort for dc partial discharges is emphasized.

Correlation analysis between SF6 decomposed components and negative DC partial discharge strength initiated by needle‐plate defect

To obtain and understand the decomposition characteristics and decomposition process of SF6 gas under different negative DC voltages, a physical model of the needle‐plate defect is used to conduct decomposition experiments of SF6 gas. Experimental results show that, under different negative DC voltages, the partial discharge (PD) caused by the needle‐plate defect decomposes SF6 gas and generates five stable decomposed components, namely CF4, CO2, SO2F2, SOF2, and SO2. The concentration and effective formation rates (RRMS) of these five components can be associated with the mean discharge magnitude per second (Qsec), and the effective concentration ratios (CRRMS) of CF4/CO2 and SO2F2/(SOF2 + SO2) correlate well with Qsec. The concentration, RRMS, and CRRMS of SF6 decomposed components can be used as characteristic quantities of the PD strength under a negative DC voltage. Based on these characteristic quantities, we establish two evaluation tables to assess the PD magnitude under negative DC voltage. That is, the range of PD magnitude in the gas chamber can be deduced by the RRMS and CRRMS of SF6 decomposed components. These tables provide a practical method for assessing the insulation status of the gas‐insulated equipment under a negative DC voltage. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Corona discharge activity in nanoparticle dispersed transformer oil under composite voltages

Titania (TiO 2 ) nanoparticles with cetyl trimethyl ammonium bromide (CTAB) surfactant dispersed in transformer oil has higher corona inception voltage than the normal transformer oil, under AC, DC, and composite AC and DC voltages. It is observed that corona inception voltage is lower under composite voltages compared to AC and DC voltages. The corona activity radiates ultra high frequency (UHF) signals with its dominant frequency near 1 GHz. Operating a spectrum analyzer in zero span mode, the UHF signals generated from the corona activity, the number of discharges is observed to be more under composite voltages than under AC voltages, both at inception and at higher voltages. A reduction in interfacial tension and an improvement in flash point is observed in nano-titania with surfactant dispersed transformer oil. The addition of surfactant reduces the turbidity of the nanofluid. Phase Resolved Partial Discharge (PRPD) analysis with UHF signals measured, indicates that corona discharge activity occurs around the peak and the pre-peak rising portions of the supply voltage. It is observed that more discharges occur in the positive half cycle for AC superimposed with positive DC voltage and in the negative half cycle for AC superimposed with negative DC voltage. The magnitude of UHF signals formed due to corona activity is less with nanofluid, irrespective of voltage profiles.

Electrical and optical characteristics of surface plasma actuator based on a three-electrode geometry excited by nanosecond-pulse and DC sources

Plasma actuator based on surface dielectric barrier discharge for active flow control has been widely investigated due to broad application prospects. In order to obtain extensive surface plasma, a plasma actuator based on a three-electrode structure sustained by nanosecond-pulse and DC sources is investigated. In this paper, the correlation between actuator parameters (especially in DC source polarity) and discharge characteristics is presented. The experimental results show that the plasma length is extended up to the third electrode due to a sliding discharge induced by a negative DC voltage. The negative DC voltage has a significant effect on the discharge current and propagation velocity of the slide discharge. Compared to the negative DC voltage, the plasma actuator with a positive DC voltage is difficult to induce a sliding discharge. Moreover, the spectrum characteristics, pressure wave, and vortex characteristics are investigated by means of time-resolved spectrum and schlieren visualization, respectively. The vortex characteristics indicate that the sliding discharge at a negative DC voltage induces two jets in opposite directions. However, the vortex characteristics with a positive DC voltage show that the air flow can be induced closer to the dielectric surface than those with a negative DC voltage. It is because a larger perturbation region in the case of positive DC voltage is beneficial to jet acceleration and thus enhances the flow control capability.

External characteristics of long gap AC and DC streamer discharges under low pressure

For the development of space power application, the research on discharge characteristics in vacuum is required. This paper carries out laboratory investigations of AC and DC discharge characteristics for 200–600 mm rod-plane gap within 2–60 kPa. The relationship between discharge voltage U and pressure P and the profile of discharge could clearly describe the characteristics of AC and DC discharges. The U-P curves under AC voltage have obvious saturation region which is caused by partially-developed streamer discharges. The external characteristics such as luminance, tortuosity and branching of discharge channels under different voltage modes are significantly different. Under negative DC voltage at 2 kPa, the self-organized ionization layer plays a decisive role in the development process of streamer which develops from the plane to the rod and has obvious dark space.

Understanding the physico-chemical properties of thermally aged natural ester oil adopting fluorescent technique

The molecular level changes in natural ester oil due to thermal aging are studied. Accelerated thermal aging of natural ester oil at 180°C is performed and the subsequent effect on oil matrix is studied using corona inception voltage (CIV) variation under AC and DC voltage. Various physico-chemical diagnostic techniques such as UV-Vis absorbance, FT-IR, fluorescence microscope, DLS, and TEM were used to study the thermally aged natural ester oil. Thermal aging leads to the reduction in corona inception voltage as observed in the present work. The CIV is much higher under DC voltage than AC voltage but its variation is not much under positive/Negative DC voltage. Spectrum analyzer measurement indicates that burst type discharges occur under negative DC voltages. Thermal aging induces degradation of natural ester oil, resulting in the formation of carbonaceous particles. As these carbonaceous particles are fluorescent in nature, a facile fluorescence based analysis is developed to understand and monitor its degradation. 3D excitation emission matrix (EEM) fluorescence data are modeled with parallel factor analysis (PARAFAC).

Experimental investigation of surface charge accumulation behaviors on PTFE insulator under DC and impulse voltage in vacuum

Surface charge accumulated on insulators plays an important role in flashover process in vacuum, which has not yet been fully understood. In this paper, charging behaviors on insulator surface were experimentally examined under negative DC and impulse voltage. For the case of DC voltage, two charging stages were observed, i.e., the area near cathode was at first negatively charged under lower voltage (stage I) and then followed by a positively charged cathode-anode surface (stage II). Surface charging at stage I is considered to be caused by Schottky injection of homo-charges from electrode into sample surface layer. While at stage II secondary electron emission (SEE) process is responsible to the positively charged anode-cathode surface. However, charging at stage I is quite sensitive to pre-stress process, as well as the electrode-insulator interface conditions. The ongoing injection of electrons from cathode can lead to gradually decreasing in surface charge accumulation as the voltage duration increases. For the case of impulse voltage, Schottky injection of homo-charges has little influence on surface charging as the time is too short for charge injection and migration in surface layer. The SEE process will govern surface charging due to its extremely fast establishment process. A good linear relationship is obtained between saturated surface charge density and applied voltage, which is generally consistent with the numerical calculation result based on secondary electron emission avalanche theory. At last, it is confirmed that the ionization of desorbed gas from insulator surface and subsequent neutralization of electrons with positive surface charge is the main cause to eliminate surface charge in flashover.

A Microbeam Resonator With Partial Electrodes for Logic and Memory Elements

We demonstrate logic and memory elements based on an in-plane clamped–clamped microbeam resonator. The microresonator is electrostatically actuated through a drive electrode and the motional signal is capacitively sensed at a sense electrode, while the resonance characteristics are modulated by dc voltage pulses provided at two separate partial electrodes, independent of the drive/sense electrodes. For the logic applications, we use two separate electrodes to provide dc voltages defined as the logic inputs. The high (low) motional signal at on-resonance (off-resonance) state is defined as the logic output state “1” (“0”). For the memory operation, two stable vibrational states, high and low, within the hysteretic regime are defined as the memory states, “1” and “0,” respectively. We take advantage of the split electrode configuration to provide positive and negative dc voltage pulses selectively to set/reset the memory states (“1”/“0”) without affecting the driving and sensing terminals. Excluding the energy cost for supporting electronics, these devices consume energy in tens of picojoules per logic/memory operation. Furthermore, the devices are fabricated using silicon-on-insulator wafers, have the potential for on-chip integration, and operate at moderate pressure (~1 Torr) and room temperature.

PIV Analysis of Particle Behavior in Electrostatic Precipitator

SUMMARYAn electrostatic precipitator (ESP) is a means to collect particulate matter (PM). They are characterized by a high total particle collection efficiency with a low pressure drop. The particle behavior in ESP depends on the particle characteristic, electric field, space charge, and flow pattern. It is important to understand the particle behavior in ESP. Particle image velocimetry (PIV) was used to grasp the particle behavior in ESP. In this paper, results of PIV of the particle flow velocity fields in a wire‐to‐plate type ESP model are presented. The ESP was an acrylic parallelepiped with a wire electrode and plate collecting electrode. The positive and negative dc voltages were applied to the wire electrode, placed at right angles to the main gas flow. The obtained results show that the particle flow velocity was increased with increasing discharge current. This is attributed to increase in ionic wind. The particle velocity right under of the wire electrode was the fastest.

Surface charge distribution on GIS insulator under DC/AC voltage

The Gas Insulated Switchgear (GIS) equipment observes surface charge accumulation under DC or AC voltage in real operation, which could distort the electric field and incur flashover eventually. With a 3D high-resolution surface charge measurement system established on actual 220 kV GIS basin insulator, the present research captured the surface charge distribution characteristics of the insulator under DC and AC voltage through high-precision probe scanning on the insulator surface. The test results show that: 1) Surface charge demonstrates obvious polarity effect under DC voltage. Under positive DC voltage, there accumulated mainly positive surface charges. Under negative DC voltage, the polarity of accumulated surface charges is all negative. Regardless of charge polarities, the surface charge density increases as the voltage amplitudes mounts and the voltage application duration prolongs. 2) Surface charge accumulation also takes place under AC voltage application. All charges are negative and distribute in a uniformed manner but in smaller density comparing to the DC scenario. As the voltage amplitudes increase, the surface charge density first increases, then decreases and finally reaches a near saturation state. As voltage application goes on, the surface charge accumulation becomes more obvious. Above all, the insulation issues in relation to surface charge accumulation require due attention in the application and maintenance of GIS equipment, and the present research attempts to provide useful reference for the optimal design of GIS/GIL insulators.

A self-consistent model of ionic wind generation by negative corona discharges in air with experimental validation

Ionic wind is produced by a corona discharge when gaseous ions are accelerated in the electric field and transfer their momentum to neutral molecules by collisions. This technique is promising because a gas flow can be generated without the need for moving parts and can be easily miniaturized. The basic theory of ionic wind sounds simple but the details are far from clear. In our experiment, a negative DC voltage is applied to a needle-cylinder electrode geometry. Hot wire anemometry is used to measure the flow velocity at the downstream exit of the cylinder. The flow velocity fluctuates but the average velocity increases with the voltage. The current consists of a regular train of pulses with short rise time, the well-known Trichel pulses. To reveal the ionic wind mechanism in the Trichel pulse stage, a three-species corona model coupled with gas dynamics is built. The drift-diffusion equations of the plasma together with the Navier–Stokes equations of the flow are solved in COMSOL Multiphysics. The electric field, net number density of charged species, electrohydrodynamic (EHD) body force and flow velocity are calculated in detail by a self-consistent model. Multiple time scales are employed: hundreds of microseconds for the plasma characteristics and longer time scales (∼1 s) for the flow behavior. We found that the flow velocity as well as the EHD body force have opposite directions in the ionization region close to the tip and the ion drift region further away from the tip. The calculated mean current, Trichel pulse frequency and flow velocity are very close to our experimental results. Furthermore, in our simulations we were able to reproduce the mushroom-like minijets observed in experiments.

Numerical simulation transient electron distribution of direct current negative corona discharge in air

In order to explore the characteristics of electron in DC negative corona discharge, microcosmic process of negative corona discharge in air is simulated in this paper. The numerical computation is established with a bar-plate electrode configuration with an inter-electrode gap of 3.3 mm, the negative DC voltage applied to the bar is 5.0 kV, the pressure in air discharge is fixed at 1.0 atm, and the gas temperature is assumed to be a constant (300 K). By solution the system of electron conservation equation, the electron mean energy conservation, the heavy species multi-component diffusion transport equation, and the Poisson’s equation, characteristics of electrons (electron mean energy, electron density, and generation and dissipation performances of electrons) at 6 representative time points during a pulse are obtained and then discussed emphatically.

Physical Mechanism of Tri-Electrode Plasma Actuator with Direct-Current High Voltage

Tri-electrode plasma actuator (TED-PA), which has a third electrode with DC high voltage, can generate stronger jet than that of conventional two-electrode plasma actuator. However, the behavior of the jet in TED-PA is totally different from that in a conventional plasma actuator; the induced jet is deflected upward. To clarify the jet deflection mechanism and performance improvement mechanism of TED-PA, the discharge plasma and flow field are numerically simulated. The results show that the jet deflection appears in both cases of negative and positive DC voltage; a rightward jet from the AC electrode and a leftward jet from the DC electrode are generated, and the collision of the two jets generates the upward jet. The jet becomes stronger by the DC high voltage due to two factors: negative body force generation around the DC electrode and positive body force enhancement around the AC electrode. The negative body force generation results from the drift motion of the positive ions in the positive DC voltage case, and, on the other hand, that of the negative ions in the negative DC voltage case. The positive force enhancement is due to the electric field intensification by the DC voltage.