Negative DC Voltage Research Articles

Design and first tests of the trapped electrons experiment T-REX.

Gyrotrons are essential for electron cyclotron resonance heating in fusion reactors, making efficient operation crucial for advancing fusion energy. Past experiments revealed instability issues due to trapped electrons in the magnetron injection gun (MIG) region, causing undesired currents and operational failures. To address this, tight manufacturing tolerances are required for the MIG geometry [Pagonakis etal., Phys. Plasmas 23, 023105 (2016)]. We present the initial findings of the trapped electrons experiment developed at the Swiss Plasma Center, designed to understand the physics of electron clouds in gyrotron MIGs. T-REX replicates MIG geometries, as well as their typical electric and magnetic fields, and it is supported by 2D particle-in-cell simulations with the FENNECS code [Le Bars etal., Phys. Plasmas 29, 082105 (2022); Le Bars, Ph.D. thesis, EPFL, Lausanne, 2023]. The setup includes two coaxial electrodes in a vacuum chamber atop a superconducting magnet, with a central electrode biased to negative DC voltages and an outer one at the ground, creating a radial electric field (1-2MV/m) and an axial magnetic field (B < 0.4T). This setup mimics Penning-Malmberg traps. We present the experimental device and first findings on current distribution and also a qualitative comparison with FENNECS simulations [Le Bars etal., Comput. Phys. Commun. 303, 109268 (2024)]. Planned diagnostics include optical emission spectroscopy, phosphor screen imaging, streak camera imaging, and potentially electric field distribution via the Stark effect. This research aims to enhance gyrotron performance and reliability in fusion energy systems.

DC fault current clearance coordinated control strategy for DC grid with hybrid MMC

For DC grid based on the hybrid modular multilevel converter (MMC), the traditional DC fault current clearance scheme takes a long time and greatly affects the active power transmission. A novel DC fault current clearance coordinated control strategy is proposed, which can quickly interrupt the fault current and reduce the impact of DC fault on the DC grid and AC system. For fault-line connected MMC (FLMMC), a negative DC voltage control is employed, which can improve the current attenuation speed and ensure reliable fault isolation. For non-fault-line connected MMC (NFLMMC), the active current-limiting control (ACLC) based on the virtual reactor is adopted, which can reduce the current flow to the fault location and further shorten fault isolation time. To reduce the impact on the AC system during the DC fault, a short-time active power support control is designed. Finally, a four-terminal DC grid simulation model is built based on the RT-LAB OP5600 real-time digital simulation platform. The simulation results show that the proposed coordinated control strategy for the DC grid can quickly clear DC fault current, shorten DC fault isolation time, and strengthen active power support capability.

Negative Medium-Voltage Direct Current Discharges in Air under Simulated Sub-Atmospheric Pressures for All-Electric Aircraft

The increase in the global temperature due to greenhouse gas emissions is a major concern to the world. To achieve the goal of zero emissions by 2050 in the USA the practical realization of all-electric vehicles, particularly all-electric aircraft (AEA), is important. For the design of electrical power systems (EPSs) in all-electric aircraft, a bipolar medium-voltage direct current (MVDC) system of ±5 kV is being investigated. However, several challenges manifest when using such voltages in a low-pressure environment. One of the main challenges is the partial discharge (PD) behavior of the insulation. It is important to study the PD behavior of the insulation by simulating the aviation environment in the lab. This work aimed to study the partial discharge behavior of air under a negative DC voltage in a needle-to-plane electrode geometry by simulating the aviation pressures in the lab. The partial discharge inception voltage (PDIV) and the breakdown voltage (BDV) show an obvious pressure-dependent variation. Regression analysis was performed to better understand the relationship between the PDIV and pressures. Plots were drawn for the average discharge current at each voltage step until breakdown. This paper’s findings can provide valuable insight into the design of EPS for an AEA. To the best of our knowledge, such a study has not been carried out to date.

Solid Product Analysis on the Epoxy Resin Surface After Flashovers in C4F7N-Based Gas Mixtures Under Negative DC Voltage

Solid Product Analysis on the Epoxy Resin Surface After Flashovers in C₄F₇N-Based Gas Mixtures Under Negative DC Voltage

Voltage Impact on Oil Pipelines from High Voltage Underground Cables: Relocation of Overhead Lines at Kelanitissa

When a large-scale urban construction project requires the relocation of an electrical overhead transmission line, going underground becomes the only feasible solution. However, due consideration must be given to the interaction with underground steel pipelines already laid as the life of metallic pipelines is highly susceptible to corrosion when buried underground. Corrosion occurs because of an electrochemical reaction, with Impressed Current Cathodic protection, with an impressed negative dc voltage of the order of 850 mV, usually employed in many metal pipelines to mitigate the deterioration of the surface. This paper scrutinizes the impact of high voltage underground cables on the cathodic protection of buried metal pipelines. The induced voltage due to the addition of high voltage underground cables crossing the pipelines perpendicularly has been analysed using Ansys Maxwell’s electromagnetic software using actual dimensions and parameters. The introduction of two 220 kV underground cables from Biyagama - Kelanitissa line and two 132 kV underground cable circuits from Kelanitissa - Kolonnawa running perpendicular to six underground petroleum carrying steel pipelines has been investigated to study its influence on existing impressed current cathodic protection. The study has shown that the maximum induced voltage is less than 1.5 mV which has no adverse effect on any existing impressed current cathodic protection. The high voltage cables have been in fact installed and no adverse effects have been detected.

The effects of magnetic field and negative DC voltage on the capacitive argon discharge

A one-dimensional implicit particle-in-cell/Monte Carlo collision simulation was employed to study the effects of a uniform weak magnetic field and negative direct-current (DC) voltage on a radio frequency capacitively coupled plasma (CCP). The simulation results indicate that the application of a magnetic field to RF/DC hybrid power-driven CCP discharge can increase the plasma density and cause it to exhibit an asymmetric distribution. When the magnetic field strength increases, pronounced striations can be observed within the DC sheath in the spatiotemporal plots of an electron heating rate and an ionization rate. This is attributed to the generation of a large number of secondary electrons by the DC electrode. These secondary electrons are accelerated by the sheath voltage and undergo E × B drift motion. When the energy of these electrons reaches the ionization threshold of an argon gas, ionization occurs. At this point, the electrons are still situated within the DC sheath, and hence, they repeatedly undergo this process until they exit the DC sheath. Additionally, the electron energy distribution function reveals that an increase in a magnetic field can cause a transition from stochastic heating to ohmic heating. The simulation results of magnetized CCP discharge under the influence of negative DC voltage show that increasing negative DC voltage can effectively improve plasma density. The application of negative DC voltage and magnetic field strength has similar effects on the heating stripe phenomenon. As the negative DC voltage increases, the striation phenomenon becomes more pronounced.

Surface Flashover Characteristics of Epoxy Resin in HFO-1336mzz(E)/CO2 Gas Mixtures Under Negative DC Voltage

Surface Flashover Characteristics of Epoxy Resin in HFO-1336mzz(E)/CO₂ Gas Mixtures Under Negative DC Voltage

Compact high-efficiency energy harvesting positive and negative DC supplies voltage for battery-less CMOS receiver

In this paper, novel compact high-efficiency multi-band rectifiers that supply positive and negative output voltages are demonstrated for energy harvesting applications. The proposed voltage doubler circuits are used as real DC voltage supplies of radio frequency mm-wave CMOS receivers. Operating multi-band rectifiers have a complicated structure that required more resonance networks to force the rectifier to work in multi-band. Novel series and parallel resonance networks are implemented to force the rectifier to operate in dual-band at frequencies of 850 and 1400 MHz. The proposed resonance network eliminates the Schottky diode impedance variation as the input power or frequency changes and supports the impedance matching and minimizes the insertion loss. A novel high-quality sine-shape micro-strip inductor that obtains a quality factor above 65 over the frequency band from 200 to 1400 MHz and inductance equal to 14 ± 2 nH is designed to improve efficiency and enhance performance at low power levels. The first suggested RF voltage doubler rectifier with series resonance feedback between the input and cathode of the diode and parallel resonance operates at two frequency bands of 850 and 1400 MHz and obtains a peak conversion efficiency of 59%, a saturated output DC voltage is 2.5 V, and the conversion efficiency is 40% at RF-input-power of − 10 dBm. This voltage doubler achieves the required DC supply parameter (1.1 V and 450 uA) for biasing the mm-wave receiver at an RF input power of 0 dBm. Otherwise, the second suggested negative voltage rectifier has a maximum simulated conversion efficiency of 65%, saturated negative DC-voltage is − 3.5 V, and the conversion efficiency is 45% at an RF input power of − 10 dBm. The negative voltage rectifier obtains DC supply parameters (− 0.5 V and no current condition used for a gate bias) at − 10 dBm input power.

Nitrogen Oxidation in a Multi-Pin Plasma System in the Presence and Absence of a Plasma/Liquid Interface

The recent energy crisis revealed that there is a strong need to replace hydrocarbon-fueled industrial nitrogen fixation processes by alternative, more sustainable methods. In light of this, plasma-based nitrogen fixation remains one of the most promising options, considering both theoretical and experimental aspects. Lately, plasma interacting with water has received considerable attention in nitrogen fixation applications as it can trigger a unique gas- and liquid-phase chemistry. Within this context, a critical exploration of plasma-assisted nitrogen fixation with or without water presence is of great interest with an emphasis on energy costs, particularly in plasma reactors which have potential for large-scale industrial application. In this work, the presence of water in a multi-pin plasma system on nitrogen oxidation is experimentally investigated by comparing two pulsed negative DC voltage plasmas in metal–metal and metal–liquid electrode configurations. The plasma setups are designed to create similar plasma properties, including plasma power and discharge regime in both configurations. The system energy cost is calculated, considering nitrogen-containing species generated in gas and liquid phases as measured by a gas analyzer, nitrate sensor, and a colorimetry method. The energy cost profile as a function of specific energy input showed a strong dependency on the plasma operational frequency and the gas flow rate, as a result of different plasma operation regimes and initiated reverse processes. More importantly, the presence of the plasma/liquid interface increased the energy cost up to 14 ± 8%. Overall, the results showed that the presence of water in the reaction zone has a negative impact on the nitrogen fixation process.

Characterization of DC corona discharge current pulses using high-frequency measurement techniques

This paper presents corona discharge current pulse characterization under positive and negative DC voltage excitations. Two electrode systems with three different gap spacings generated corona discharges. Different corona modes in atmospheric air were observed with voltage level changes. Commercial shunt resistors and a high-frequency current transformer (HFCT) captured corona current pulses. The appropriate resistor type and value were determined for the shunt resistor measurement. BNC terminator was proposed as a shunt resistor to detect corona discharge current pulses, and the availability of the shunt resistors and HFCTs for corona pulse detection was discussed. After gathering corona discharge pulse data, features from the pulses were extracted. The best features describing corona discharge types were specified using a recursive feature elimination method based on a decision tree estimator. The corona discharges were discriminated with an accuracy rate of 0.994.

Effect of a central barrier potential on asymmetry-induced transport in a coaxial Malmberg–Penning trap

Experiments are presented measuring the radial particle flux produced when a central barrier potential is applied to the central ring of a coaxial Malmberg–Penning trap at the same time as the voltages producing the main asymmetry potential ϕ1(r) cos (kz) cos (lθ−ωt). When a negative DC voltage is applied to the entire central ring, the peak flux produced by the main asymmetry is reduced by a factor e(Veff/1.2), where Veff is the effective applied potential in volts. This barrier potential also increases the frequency of this peak flux. When asymmetric ± voltages are applied to the two azimuthally divided halves of the central ring, DC voltages increase the radial flux while AC voltages decrease it. It is shown that these asymmetric barrier voltages produce their own transport and that the resulting modification of the plasma may be a factor in changing the observed flux.

Study on Development Characteristics of Partial Discharge in Oil-Pressboard Insulation under Constant DC Voltage

The converter transformer is the core equipment of HVDC transmission system, the valve-side winding of which needs to withstand DC voltage. Partial discharge is one of the main threats to the safe operation of converter transformer, yet the characteristics of partial discharge development of the oil-pressboard insulations under constant DC voltage are insufficiently understood. In order to better understand the partial discharge characteristics of the oil-pressboard insulation under DC voltage and provide deeper theoretical support for insulation diagnosis of converter transformers, development characteristics including the time-varying tendency of discharge magnitude and repetition rate of partial discharge in oil-pressboard insulation under constant positive and negative DC voltage were studied. The results indicate that the development of partial discharge in a needle-plane oil-pressboard insulation model under constant DC voltage has three stages: the intensive discharging stage, the silent-burst stage, and the breakdown stage. Throughout all stages, the partial discharge magnitude and repetition rate first decrease and increase afterwards. At the silent-burst stage, the partial discharge appears in the form of a “cluster” with very large magnitude and repetition rate. Each cluster exists for tens of seconds but with at a very long interval with each other. Further analysis shows that the repeated accumulation and dissipation of free charges on the surface of the pressboard cause the above phenomena. Negative charges are easy to accumulate and difficult to dissipate under the same voltage amplitude compared to positive charges, leading to a weaker actual electric field at the needle tip and thus partial discharges under negative DC voltage with a lower magnitude and longer interval.

Dynamic interactive characteristics between icicle growth and corona discharge on HVDC outdoor insulators during icing accretion

AbstractIn order to improve the stability and reliability of high voltage direct current (HVDC) transmission lines in cold regions, this article investigated the corona discharges and the icicle growth characteristics on outdoor insulators under DC voltages. The experiments were conducted at CIGELE laboratory at University of Quebec. The simulation model of corona discharges and the theoretical model of icicle growth were established. The dynamic process and interactive characteristics between the icicle growth and corona discharges under different HVDC voltages were obtained. The results showed that the discharge activity was strong and the cooling effect of ionic air was weak under negative DC voltage, resulting in a large number of bubbles inside the icicle. More negative ions were produced under negative DC voltage, which enhanced the polarisation of water droplets, leading to a greater density of ice beads outside the icicles. In addition, the longest icicles of each insulator under high humidity could bridge the shed gap. The accumulated charge and leakage current were higher under negative DC voltage, as well as the corona discharge at the top of the icicles lasted longer. Therefore, the negative DC corona discharge more significantly inhibited the growth of the icicle, resulting in a slower growth rate of the icicles.

Low‐Temperature Prepared ZnO Layer with Electron Beam Annealing Process for Enhancing the Environmental, Thermal, and Operational Stability of Organic Photovoltaics

The commercialization of organic photovoltaics (OPVs) requires a high level of stability and a high‐power conversion efficiency (PCE). To satisfy these requirements, inverted OPVs with electron transport layers of ZnO obtained by electron beam annealing (EBA) are fabricated, and their properties are compared with those of a reference device with a thermal‐treated ZnO layer. Electrons are extracted from Ar plasma and accelerated by supplying a negative DC voltage; the ZnO layer is annealed using the energy from the accelerated electrons. The PCE of the OPV with a ZnO layer obtained by the optimal EBA process is 15.6%, which is 11.4% higher than that of the reference device. In addition, the OPV device retains 90.1% of its initial PCE after it is stored at room temperature for 30 days and 70.1% of the initial PCE after 280 h at 90 °C. Further, the operational stability is measured for 500 min at the maximum power point under 1 sun illumination in ambient air. The OPV with the ZnO layer treated under optimal EBA conditions retains 85.0% of the initial PCE and shows outstanding environmental and thermal stability.

Influence of Humidity and Salinity on the Discharge Characteristics of a Short Rod-Plane Gap Under Negative DC Voltage

In this article, the effects of air humidity and salinity on the discharge characteristics of a short rod-plane gap under negative dc voltage are investigated experimentally. The discharge of the rod-plane gap was recorded using a measuring resistance, photomultiplier tube, and ultraviolet imager. The experimental results show that corona discharge of rod electrode can be observed in the rod-plane gap when the relative humidity is lower than 70%, and furthermore, partial discharge of water molecules in the air can also be observed in the rod-plane gap when the relative humidity is higher than 70%. The partial discharge inception voltage (PDIV) decreases with the increase of air humidity and salinity, and the corona discharge inception voltage (CDIV) increases slightly with the increase of humidity. The average discharge current increases with the increase of applied voltage and humidity. The repetition rate of the optical pulse and the Trichel pulse increases with the increase of applied voltage. The discharge repetition rate of water molecules in the air can be estimated from the repetition rate difference between the optical pulse and the Trichel pulse. The amplitude of the Trichel pulse in clean air is higher than that in the salty air, while the repetition rate of the Trichel pulse in clean air is lower than that in the salty air, which is due to the charging of NaCl particles during discharge.

Experimental study on positive corona discharge characteristics in plane-single/double earthed rods air gaps under negative DC voltage

In order to analyze the corona discharge characteristics at the grounding object tip under the thundercloud electric field environment, scaled simulation test has been conducted on corona discharge in the plane-single/double earthed rods air gap. Five gap configurations with three different grounding rods were built during experiments. Factors as rod tip curvature, voltage and gap configuration have been considered in detail. Corona voltage, corona current and the number of ultraviolet photons were all recorded. The grounding rod tip curvature radius can affect corona discharge processes at grounding rod tip. With increasing applied negative DC voltage, five typical corona processes of dark discharge, burst streamer corona, periodic streamer corona, glow corona and brush discharge occurred at conical and spherical grounding rod tip (r=0.75cm), but under d=5cm and d=10cm no glow corona and brush corona appeared at spherical grounding rod tip (r=1.25cm). It is much easier for grounding rod tip with smaller curvature to form glow corona phenomenon. The mean value Iav of periodic streamer corona current pulse amplitude decreases with grounding rod tip curvature radius, while the mean corona current pulse width and pulse interval increases. For grounding rod with conical tip, there is little difference of average current pulse amplitude between periodic streamer corona and brush corona; for grounding rod with spherical tip (r=0.75cm), the mean value Iav of brush corona pulse amplitude increases to 3595.8 μA which has increased by a factor of about 36. The photon number at conical grounding rod tip is larger than that at spherical grounding rod tip at low voltage but the relation reverses as the applied voltage exceeds a critical value. The shielding effect of two grounding rods in plane-double rods air gaps increased with spherical grounding rod curvature radius and decreases with the lateral gap distance between two grounding rods.

Partial Discharge Characteristics of SF6 and SF6/N2 in Nonuniform Electric Field Under Negative DC Voltage

In this study, the partial discharge (PD) characteristics of nonuniform electric fields in SF6 and SF6/N2 gases from PD inception to intermittent breakdown under negative dc voltage were systematically studied. The effects of the gas mixture ratio and electric field distortion on the PD characteristics were analyzed in terms of PD inception voltage (PDIV), PD amplitude, PD repetition frequency, and time-resolved PD (TRPD) patterns. The PDIV of SF6/N2 was higher than that of SF6 within a certain range of gas mixture ratios, which is related to the degree of electric field distortion. The greater the degree of electrical field distortion, the greater the range of gas mixing ratios, in which the PDIV of SF6/N2 was higher than that of SF6. The difference in the value of PDIV also increased with an increase in the degree of electrical field distortion. The PD amplitude and PD repetition frequency in SF6/N2 gradually increased with increasing applied voltage. Moreover, the smaller the SF6 content, the greater the PD amplitude and repetition frequency. In SF6, the PD amplitude significantly increased and the PD repetition frequency significantly decreased when the applied voltage increased to a high value. The shape features of the TRPD patterns in both SF6 and SF6/N2 changed with an increase in the applied voltage, but the change in SF6 was more obvious than that in SF6/N2.

Study on Space Charge Characteristics in XLPE Under DC Voltage Superimposed by Impulse Voltage

In direct current (dc) transmission lines, power cables are susceptible to the invasion of switching overvoltage or lightning overvoltage. This causes the accumulation of space charge in the insulation due to the higher transient composite electric stress. Space charge is an important factor in dielectric breakdown, especially under high electric stress. However, until now, there are few reports on space charge characteristics under the composite stresses of dc voltage and impulse voltage. In this article, a modified pulsed electroacoustic space charge measurement system used for dc voltage superimposed by impulse voltage is developed by reasonably choosing resistor and capacitor elements in the measuring system. Then, space charge characteristics in cross-linked polyethylene (XLPE) are measured under dc voltage, impulse voltage, and dc voltage superimposed by impulse voltage. The test results show that hetero-charges dominated in the XLPE bulk under a positive dc voltage, whereas homo-charge injection occurred from the semiconductor (SC) electrode under a negative dc voltage. However, a continuous impulse voltage with high amplitude can cause a large degree of homo-charge injection from both electrodes. Moreover, greater homo-charge injection appeared under the negative impulse voltage than that under the positive impulse voltage. Under a dc voltage superimposed by an impulse voltage, the superposition of voltages with the same polarities facilitated homo-charges injection from both electrodes to a greater extent than did the superposition of voltages with opposite polarities. Furthermore, regardless of the polarity of the applied dc voltage, greater space charge accumulation appeared in XLPE when a negative impulse voltage is superimposed.

Ion-Implanted Epoxy With Graded Surface Conductivity: An Approach to Improve Surface Flashover Voltage

The electric field distribution along the insulator surface plays a significant role in improving surface flashover characteristics of the insulation in an HVdc power system. Functionally graded material with conductivity distribution ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sigma $ </tex-math></inline-formula> -FGM) is a promising approach that enables the electric field to be more uniform, which leads to hinder the surface flashover. This article proposes a novel method for graded conductivity materials using surface modification via argon (Ar <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{+}$ </tex-math></inline-formula> ) ion implantation. Epoxy samples with homogeneous and graded surface conductivity are prepared considering different ion implantations to verify the effectiveness of the proposed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sigma $ </tex-math></inline-formula> -FGM method. Flashover tests of the virgin, homogeneously ion-implanted, and graded ion-implanted samples are conducted in air and vacuum at negative dc voltage. Moreover, finite-element method (FEM)-based simulations are carried out using COMSOL Multiphysics software to analyze the electric field relaxation before and after modification. The experimental findings indicate that the flashover characteristics of homogenously implanted samples are improved with the rise in implanted layer conductivity. In particular, the flashover voltage of samples with graded conductivity layers is improved by 27.71% in air and 28.90% in vacuum compared to the virgin samples. The experimental findings and theoretical analysis prove that ion-implantation-based graded conductivity material is a promising solution for the insulation of HVdc applications.

SF6 decomposition behaviour under partial discharge of typical insulation defects in gas‐solid insulated valve‐side bushing of converter transformer

AbstractIn this paper we investigated SF6 decomposition behaviour of typical insulation defects in gas‐solid insulated converter transformer valve‐side bushing under partial discharge (PD). Additionally, we compared the results with that of gas insulated switchgear (GIS). The results indicated that SF6 decomposition behaviour varies with the insulation defects type. For metal protrusion defects, the proportion ranking of CO2 and SO2 can be related to whether the needle tip is wrapped with epoxy resin‐impregnated paper (ERIP). The proportion of carbon‐containing products is much higher under contamination defect than under other defects. Moreover, the proportion of carbon‐containing components is higher under flashover condition than under PD condition and is accompanied by the appearance of H2S and CS2. However, the yield of SO2F2 is always low. Compared to negative DC voltages, positive DC voltages have a higher proportion of carbon‐containing components, mainly in the form of more CO2 and less SOF2. Compared to the PD decomposition behaviour of typical insulation defects in GIS, when PD involves ERIP, the proportion of carbon‐containing products, especially CO2, is significant higher, while the proportion of sulphur containing products, especially SO2F2, is lower.