Positive DC Research Articles

Effect of Gas-Mixture Ratio on the Characteristics of Positive DC Corona Discharge in SF6/N2 Gas Mixtures

The characteristics of positive DC corona discharge in SF6/N2 gas mixtures under different gas-mixture ratios are investigated with a needle-to-plane electrode configuration. With increasing applied voltage, a discharge-stage transition process including the Townsend stage, streamer-pulse stage, brush-pulse stage and breakdown stage is observed. The variation in current-pulse characteristics, including pulse-time parameters, mean pulse amplitude and pulse-repetition rate, is investigated under different gas-mixture ratios. The pulse-time parameters are not affected by gas-mixture ratio. The pulse-repetition rate increases with the increase of SF6 concentration and mean current, ranging from hundreds of kilohertz to thousands of kilohertz. The streamer-pulse amplitude increases with increasing SF6 concentration. With increasing mean current, the small-amplitude brush pulses become dominant, thus the mean pulse amplitude decreases. In order to further explore the influence mechanism of gas-mixture ratio on positive corona discharge, a fluid model of positive corona discharge is developed and relevant simulations show the formation process of current pulse, and verify the validity of the pulse characteristics changing with gas-mixture ratio.

Ultra low-power negative DC voltage generator based on a proposed level shifter and voltage reference

With advances in low power and battery-operated devices, concerns over power-efficient designs for system-on-chip devices is growing. Since some subsystems require to have both positive and negative voltage rails, it is essential to ensure this requirement is satisfied without increasing the cost and power consumption. This paper presents a novel Negative Level Shifter used to generate negative voltage levels from the positive DC power supply line. Additionally, an improved CMOS Voltage Reference generator based on a Resistorless Beta Multiplier is also proposed. The two circuits were tested under various conditions, and results validate the low-power operation of the proposed design. The proposed Negative Level Shifter required only 772 pW and has a delay equal to 4ns with a supply voltage of 0.3V. When assembled, both the reference voltage and the level shifter consume 8.5 nW with the supply voltage of 1V at room temperature.

Effect of sinusoidal waveform and frequency on space charge characteristics of polyimide

The charge phenomena under sine and half-wave sine voltages within the frequency range of 500 Hz are studied here. Based on the pulsed electro-acoustic method, the traditional circuit design under high-frequency voltages is first analysed. It is found that the selection of a 186 pF protection capacitor and a 333 kΩ protection resistor can ensure that the actual voltage applied to the sample is consistent with the expected input. Based on this design, experimental results show that the polarity of the charge accumulated in the depth of the sample is determined by that of the upper electrode. Comparison results under special voltages with different amplitudes and frequencies indicate that the amount of accumulated charge under sine voltages are larger than those under positive and negative half-wave sine and DC conditions, and the samples under lower-frequency conditions show more charge accumulation. The maximum electric field strengths appear at 90 and 270 degrees of the sine voltage with a frequency of 10 Hz, and their values are 68.55 and 81.82 kV/mm, respectively. Therefore, the charge characteristics are easily affected by the voltage’s waveform and frequency parameters. The results obtained here can provide guidance for the application of insulating materials under special voltage environments.

Total loss measurement and simulation in a REBCO coated conductor carrying DC current in perpendicular AC magnetic field at various temperatures

In many high-temperature superconducting (HTS) applications, HTS coated conductors carry DC currents under external AC magnetic fields. There are two AC loss mechanisms in this situation: magnetization loss due to the external magnetic field and dynamic loss due to the interaction between the DC current and the external magnetic field. The sum of these two loss components is referred to as total loss. In this work, the total loss in a 4 mm wide REBCO coated conductor is measured under perpendicular AC magnetic fields up to 105 mT at 77 K, 70 K, and 65 K, with reduced DC current level, i (I dc/I c0), from 0.025 to 0.98, where I dc is the transport DC current value and I c0 is the self-field critical current of the coated conductor at each temperature. The experimental results show a good quantitative agreement with an analytical equation for each loss component, as well as 2D finite element modelling (FEM) results from H -formulation. For any given temperature, we observe that the total loss is mostly dominated by magnetization loss at i< 0.2, while dynamic loss makes a comparable, even greater contribution to total loss at i > 0.5. Electromagnetic analysis from the FEM modelling shows the evolution process of total loss, where the dynamic loss region and magnetization loss region vary across the conductor width at high magnetic fields or high DC current level. The simulation results also reveal the superposition of (positive) DC current and the anti-parallel (negative) shielding current, which occurs at high DC current level. The superposition drives the current density of one conductor edge to subcritical stage, and it leads to one-sided loss generation in each half-cycle. Our results provide a valuable reference for total loss behaviours in REBCO coated conductors.

Non-linear effects and electron heating dynamics in radio-frequency capacitively coupled plasmas with a non-uniform transverse magnetic field

A non-uniform transverse magnetic field is used to increase the plasma density and create an asymmetry in radio frequency capacitively coupled plasmas for plasma sputtering and plasma vapor deposition. Based on one-dimensional particle-in-cell/Monte Carlo collision simulations, the effect of the magnetic field magnitude on the non-linear behavior and the electron heating dynamics is studied for a pure helium plasma at a pressure of 30 mTorr. The results show that increasing the magnetic field magnitude can generate a more positive DC self-bias. As a result, non-linear oscillations of the electron current density and the electric field close to the grounded electrode are enhanced. An electric field reversal is induced when the powered electrode sheath collapses to balance electron and ion fluxes toward this boundary due to the strong confinement of electrons. Anomalous energetic electron beams are observed propagating from the collapsed sheath toward the plasma bulk. It is shown that such beams are reflections of the beams originating from the opposite expanding sheath based on the analysis of single particle motions. We show that energetic electron beams can be reflected by the transverse magnetic field.

Simultaneous identification of linear building dynamic model and disturbance using sparsity-promoting optimization

We propose a method that simultaneously identifies a control-oriented model of a building’s temperature dynamics and a transformed version of the unmeasured disturbance affecting the building. Our method uses ℓ1-regularization to encourage the identified disturbance to be approximately sparse, which is motivated by the slowly-varying nature of occupancy that determines the disturbance. The proposed method involves solving a feasible convex optimization problem that guarantees that the identified black-box model, a linear time-invariant system, possesses known properties of the plant, especially input–output stability and positive DC gains. These features enable one to use the method as part of a self-learning control system in which the model of the building is updated periodically without requiring human intervention. Results from the application of the method on data from a simulated and real building are provided.

Experimental study of a positive DC corona jet working with $$\hbox {Ar/CO}_{2}$$ gaseous mixture

Experimental study of a positive DC corona jet working with $$\hbox {Ar/CO}_{2}$$ gaseous mixture

Unbalanced Voltage Analysis and Suppression Method in a Radial Bipolar DC Distribution Network

The difference between positive and negative pole dc loads in a bipolar dc distribution network causes unbalanced current in the neutral line. This unbalanced current increases the power losses in the network and leads to unbalanced voltage in each node. An automatic commutation switch (ACS)-based method is developed in this study to suppress unbalanced current and voltage. Expressions of the unbalanced voltage and current of a single node and multiple nodes are derived. The equations indicate that the unbalanced power between positive and negative poles can be reduced by adjusting the power supply polarity (PSP) of the dc load. The PSP of the dc load and the corresponding switching command is determined by comparing the amplitude and polarity of the neutral line current. Afterwards, the switching command is implemented through ACS. The topology of ACS and the implementation of the switching command are presented. In addition, a simulation platform of a radial bipolar dc distribution network is built in MATLAB/Simulink. The platform verifies the effectiveness of the unbalanced voltage analysis method and the proposed suppression strategy. The proposed method is also validated using a scaled-down hardware setup in the laboratory.

Quasi‐phase resolved partial discharge pattern analysis of surface discharges arising on the tar polluted porcelain bushing of an electrostatic precipitator unit under high voltage DC

AbstractThis paper investigates on understanding the DC electrical discharges arising on the surface of feed‐through type indoor bushing with tar depositions. Such information not only provides diagnostic status but also characterizes the multitude of species forming tar depositions which is currently unavailable in literatures. The current practice on analyzing surface discharges initiated under DC voltage employs primitive methods that accounts for the individual discharge events over a larger span rendering it less sensitive and time consuming. The usual practice is to count the discharge events initiated under DC voltage as PD pulses and count the events, repetition rates, time of occurrence, and so forth, to estimate the severity of the fault condition. On contrary, synchronizing these discharge pulses to the phase of the AC voltage, input to the rectifier unit, manifests a pattern, which is time independent and is sensitive in revealing diagnostic status of the bushing. These objectives are initially verified on a sample extracted from a tar polluted bushing. A half‐wave bridge rectifier that produces an uncontrolled positive and negative DC voltage is selected to initiate surface discharges. Pertinent pulses are then correlated to the phase of the AC voltage input to the rectifier and a quasi‐phase resolved partial discharge pattern (QPRPD) is developed. Using this QPRPD pattern, the significance of positive and negative DC voltages applied on the surface of the polluted sample is studied and the inception of discharges events and their correlation to the defect conditions are evaluated. Following this, the pertinent findings are validated on an actual bushing installed in an electrostatic precipitator unit applied for cleaning producer gas generated in a biomass gasification plant.

The Association Between Cardiac Illness-Related Distress and Partner Support: The Moderating Role of Dyadic Coping.

Managing cardiac illness is not easy because it dramatically disrupts people’s daily life and both the patient and his/her spouse are at risk for experiencing distress, which, in turn, may affect the support provided by the partner as caregiver. The partner, in fact, is the main source of support, but his/her support may sometimes be inadequate. In addition, dyadic coping (i.e., the way partners cope together against stress and support each other in times of difficulty) could likely be a moderating factor. The main aim of the present study was to examine the role that dyadic coping (DC, in terms of positive, negative, and common dyadic coping responses) plays in moderating the link between patient and partner cardiac illness-related distress (in terms of anxiety and depression) and partner support (in terms of overprotection, hostility, and partner support for patient engagement). The study included 100 married couples faced with cardiac illness who completed a self-report questionnaire. We analyzed our data in PROCESS using multiple regressions in order to assess the moderating effects of DC responses in the relationship between the couple’s cardiac illness-related distress and partner support. With regard to patient distress, results showed that higher levels of patient anxiety and depression were linked with ineffective partner support (i.e., overprotection and hostility). With regard to partner distress, higher levels of partner depression were linked with hostility; higher levels of partner depression and anxiety were associated with less partner support for patient engagement. Moreover, the association between distress and partner support was moderated by the quality of DC. In particular, low positive DC represented a risk factor for both the patient and the partner during a cardiac illness, as low positive DC exacerbated the link between patient and partner distress and less effective partner support styles. Also, higher levels of negative DC were risky for couples: The association between distress and less adequate partner supportive behaviors was stronger in the case of higher negative DC. These results imply a need for psychosocial interventions for couples in cardiac illness, especially for couples lacking relational competences, such as positive dyadic coping.

High Temperature Insulation Materials for DC Cable Insulation—Part II: Partial Discharge Behavior at Elevated Altitudes

Utilizing hybrid electric propulsion systems in the next generation of aircrafts with elevated DC voltage level has raised the concern about the degradation of insulation subjected to partial discharge at high altitude and harsh environment. In this work, the surface discharge behavior of commonly used high-temperature insulation materials in aviation systems (FEP, ETFE, and PEEK) under positive and negative DC and ramp voltage was studied, including the pulse waveform, frequency spectrum, PD count and magnitude. Pressure changes as a significant factor influencing surface discharge phenomenon was thoroughly studied. Dust figure technique was employed after voltage ramping tests for both positive and negative polarities at low pressure to investigate the surface charge accumulation and surface discharge traces, and unveil the involved physical mechanisms at different stages of streamer propagation. The content of this paper provides a reference for surface discharge studies and evaluation of high temperature materials for medium voltage direct current power distribution in the future aviation systems.

Characteristics of Creeping Discharge Caused by a Needle Electrode in Oil-Pressboard Insulation under +DC Voltage

This paper presents an analysis of the creeping discharge characteristics caused by a needle electrode in oil-pressboard insulation under a positive DC voltage. The discharge behavior under a strongly (model I) or weakly (model II) vertical component of the electric field is studied using partial discharge (PD) detections, camera observations, and charge distribution simulations. It is found that the development process of the creeping discharge can be divided into three stages according to the stepwise growth trends of apparent charge amplitude and PD repetition rate. The time-resolved PD pattern and equivalent time-frequency pattern are different in the two models. Most discharge pulses are unimodal with underdamped oscillations, but bimodal pulses can occur in model II. The streamer in model I has a tail caused by the stranded charge. A secondary streamer can occur in model II following the initial streamer, causing a bimodal PD pulse. The study findings can provide suggestions for the optimization of insulation design and the promotion of converter transformer maintenance.

Investigation on Impact of Magnetic Field on the Corona Discharge Activity in Punga Oil Using Fluorescent Fiber and UHF Sensor Techniques

This paper reports the experimental investigation of the corona discharge activity of punga oil under the influence of a local magnetic field and different voltage profiles (AC and DC voltages) using fluorescent fiber sensor as well as UHF sensor technique. The corona inception voltage (CIV) of punga oil is higher under negative DC voltage followed by positive DC and AC voltage, with a marginal reduction in its CIV magnitude observed on the impact of external magnetic field. The dominant frequency of the UHF signal obtained under AC voltage shifted towards lower frequency (0.6 GHz) with the influence of magnetic flux density to about 85 mT. The rise time, pulse width and energy content of the fluorescent signal formed due to corona activity under AC voltage is found to vary under high magnetic fields confirming the inception results. The fluorescent signals formed due to corona discharge under both AC and DC voltages have its dominant frequency at 1 MHz with no shift observed in the presence of magnetic field. Also, there is no variation in the phase resolved partial discharge (PRPD) pattern observed due to corona discharge (with and without magnetic field) signal using both UHF sensor and the fluorescent sensor. The fluorescent fiber-based technique provides a better accuracy on detecting the corona discharges in punga oil at an early stage compared to conventional UHF sensor. The breakdown voltage of punga oil under different voltage profiles with and without the effect of magnetic field follows normal distribution. The dielectric dissipation factor and electrostatic charging tendency (ECT) of punga oil is observed to be higher than the limit set for insulating fluids towards power transformer operation.

Simulation study of a pulsed DBD with an electrode containing charge injector parts

By using a multispecies fluid model, the tunability and controllability of plasma parameters such as distributions of electron density, electron energy, ion density, and electric field in a microdielectric barrier discharge (DBD) with a charge injector electrode and driven by negatively polarized nanosecond pulsed voltage superimposed on a positive DC bias voltage are investigated. To this end, the effects of changing features of pulsed voltage like pulse rise time (10–20 ns), pulse peak width (10–15 ns), and pulse fall time (20–30 ns) on characteristics of argon plasma formed inside the reactor are studied. The results show that with the increase in pulse width and pulse rise time, the density of electron and ion increases, while fall time change does not significantly affect the plasma parameters. Generally, the results of this study explicitly prove the possibility of controlling plasma formed inside DBD reactors driven by negative pulse voltage combined with a positive DC voltage, which is very important in waste gas conversion applications.

Micro gap air discharge based on fractal theory

Micro-gap discharge is a form of gas discharge in which the discharge gap is on the order of sub-millimeters orless. To study the initial path of micro-gap discharge and the mechanism and law of particle density change during discharge, in this paper a micro-gap discharge experiment and discharge image acquisition device under atmospheric pressure is built and the COMSOL simulation software is used to simulate the electron density and space charge in the process of micro-gap air discharge. Furthermore, the MATLAB software is used to calculate the fractal dimension and probability development index of micro-gap discharge. The air discharge phenomena produced by applying positive DC voltage to needle tip at atmospheric pressure and room temperature with gap distance ranging from 50 μm to 150 μm are studied. It is found experimentally that there are twists and turns in the discharge channel, and the number of bifurcations in the discharge process with a short gap is less than that with a long gap. Observation of the micro-gap air discharge process with a gap of 100 μm under atmospheric pressure shows that the discharge process is divided into the following three processes: needle tip corona, corona breakdown streamer, and spark discharge channel. Based on the analyses of these experimental results, it can be concluded that the discharge mechanism follows Thomson's theory, supplemented by the streamer theory. The cathode secondary electron emission (including positive ions colliding with the cathode and photoelectron emission) and the space charge distortion electric field form a secondary electron avalanche to maintain the discharge together. The seed electrons formed by a small amount of space photoionization also form an electron avalanche under the action of the space charge distortion electric field. There are tortuous sections in the discharge channel, but the number of branches is small and the degree of tortuosity is low. Therefore, there are weak streamer forms. The discharge channel is tortuous and branched, but the number of bifurcations is relatively small, and the tortuousness is low. In addition, it is also found that a sheath is formed at the cathode, the distortion of electric field is 3–8 times that of original electric field, and the electron density reaches 2 × 10&lt;sup&gt;21&lt;/sup&gt; m&lt;sup&gt;–3&lt;/sup&gt; during discharge, obtained from the COMSOL simulation. Meanwhile, the fractal theory simulation is used to simulate the micro-gap discharge. In the process of research, the fractal dimension is found to be proportional to the voltage and the gap distance. When the probability development index &lt;i&gt;η&lt;/i&gt; = 1.18–1.3, the fractal dimension of the simulated discharge process is closer to the experimental result. The findings in this paper lay the foundation for further exploring the discharge theory of sub-micro- and nano-scaled gaps.

Effects of Temperature Gradient on Electrical Tree Initiation and Breakdown Phenomenon in XLPE Under Harmonic Superimposed DC Voltage

In this article, the temperature of the needle tip and the ground electrode were changed to obtain different temperature gradients. The harmonic superimposed DC voltage was also simultaneously applied to study the electrical tree initiation, growth and breakdown characteristics in cross-linked polyethylene (XLPE) under the temperature gradient. The tree inception voltage, tree structure, tree length, accumulated damage (AD) and breakdown time were used to analyze the electrical tree degradation process. It was found that the greater the temperature gradient, the easier it is for electrical trees to be initialed which is related to the different charge behaviors affected by the trap distribution indicated by surface potential decay (SPD) results at different temperatures. With the temperature gradient increasing, the electrical tree length increases firstly but then decreases slightly when the temperature of the ground electrode is fixed, but the electrical tree length and AD increase linearly with the temperature gradient increasing when the temperature of the needle tip is fixed. The temperature rise of the ground electrode has a more serious impact on the safety of the insulation compared with the temperature rise of the needle tip. Positive DC bias voltage is more likely to cause damage and breakdown than negative DC bias voltage under the temperature gradient. These results indicate that the temperature gradient and harmonic superimposed DC voltage will promote the electrical growth in cable insulation, so special attention should be paid to the safety of electrical equipment under such special conditions.

Effect of net direct current on the properties of radio frequency sheaths: simulation and cross-code comparison

In order to understand, predict and control ion cyclotron range of frequency interactions with tokamak scrape-off layer plasmas, computational tools which can model radio frequency (RF) sheaths are needed. In particular, models for the effective surface impedance and DC rectified sheath potentials may be coupled with full wave RF simulation codes to predict self-consistent wave fields near surfaces and the resulting power dissipation and plasma–material interactions from ion sputtering. In this study, previous work assuming zero net DC current flow through the sheath is generalized to allow the surface to collect net positive or negative current, as is often observed in experiments. The waveforms, DC potential and RF admittance are investigated by means of analytical theory, nonlinear fluid and particle-in-cell codes. Cross-code comparisons provide detailed model verification and elucidate the roles of ion and electron kinetics. When the sheath draws negative (positive) DC current, the voltage rectification is reduced (increased) compared with the zero-current case, and the magnitude of both the real and imaginary parts of the admittance are increased (reduced). A previous four-input parametrization of the sheath rectification and admittance properties is generalized to include a fifth parameter describing the DC sheath current.

Computational and Experimental Study of Time-Averaged Characteristics of Positive and Negative DC Corona Discharges in Point-Plane Gaps in Atmospheric Air

The use of stationary solvers instead of approximate solution methods or time-dependent solvers, which are standard tools in gas discharge modeling, allows one to develop a very fast and robust numerical model for studying the time-averaged characteristics of dc corona discharges. Such an approach is applied to dc corona discharges in point-plane gaps in ambient air. A wide range of currents of both voltage polarities and various gap lengths are investigated, and the simulation results are validated by comparing the computed current–voltage characteristics and spatial distributions of the radiation intensity with experimental results. Specific features of the numerical and experimental results at both polarities are discussed.

Relaxation of competing electromechanical couplings in murine artery

Piezoelectricity and pyroelectricity in biological tissues, which originate from oriented fibrous proteins with a polar axis, have long been suggested to play important roles in physiological functions. The possible manipulation of their polarity by external mechanisms, however, remains unsettled. We revisit this problem here using piezoresponse force microscopy (PFM) as the tool and the intima layer of murine artery as a model system. By carefully examining first and second harmonic piezoresponses at both selected points and through spatial mapping, we establish that electromechanical coupling probed by PFM is predominantly piezoelectric in the intima layer, while the quadratic effect makes only minor contributions. More importantly, we observe competition between the linear and quadratic effects after removal of DC biases applied to the sample surface, revealing not only interesting relaxation dynamics, but also highly asymmetric piezoresponse. Positive DC rotates dipoles in tropoelastin monomers away with reduced alignment, while negative DC aligns dipoles more leading to enhanced piezoresponse. The electric manipulation of biological polarity is thus demonstrated, with the relaxation time constant determined on the order of 0.1 s, much slower than classical ferroelectrics.

Investigation on the performance of thermally aged natural ester fluid impregnated pressboard material

This paper reports the key findings from an experimental study carried out on thermally aged natural ester fluid impregnated pressboard (EOIP) material. The results show that the surface discharge inception voltage (SDIV) with thermally aged EOIP material is higher under negative DC voltage compared with positive DC and AC voltage. The SDIV increases with AC voltage frequency and a minimal reduction in voltage was observed under harmonic AC voltages with different total harmonic distortion (THD). The rate of change in voltage due to ripple content in DC voltage affects the SDIV of EOIP material, initiating discharges at lower voltages. The UHF signals radiated during the surface discharge process have a bandwidth in the range 0.9 to 2 GHz. Surface potential variation studies indicated the formation of high trap sites and trap density due to thermal aging. A reduction in the oxidation onset temperature and the activation energy of thermal decomposition were observed with thermally aged EOIP material. A pyrolysis study showed high composition of levoglucosan and acidic residue with the thermally aged specimen, which reduces its degree of polymerization. Impedance spectroscopic analysis indicates a non-Debye type of relaxation with thermally aged EOIP material. The mobility and charge transfer characteristics were also inferred from the modulus spectroscopy which showed less change for thermally aged pressboard material.