Positive DC Research Articles

Performance Evaluation of Effective Controller for Buck-Boost DC-DC DSNIBBC

This paper deals about the development of intelligent controllers (ICs) for load voltage regulation of a double switch noninverting buck-boost DSNIBBC operated in pseudo continuous coil current (PCCC). The DSNIBBC is a kind of DC-DC converter, which converts the positive DC source voltage into the positive DC load voltage (buck to boost mode and boost to buck mode). Due to the time changing, ON/OFF characteristics along with its transient and dynamic characteristics of DSNIBBC have become nonlinear. Since the classical linear controllers are inappropriate to enhance the transient and dynamic performances of DSNIBBC, ICs are designed to improve the load control of the same DSNIBBC operated in transient and dynamic regions. In this paper, two ICs, namely, fuzzy logic controller (FLC) and neuro controller (NC) incorporated in the load voltage regulation loop are utilized. The performance comparison between the ICs and the classical proportional integral (PI) controller is discussed and validated at various regions through both software and prototype field programmable gate array [FPGA] plants. The responses are depicted to support the designed ICs.

Discontinuous Space Vector Modulation Schemes for Modular Multilevel Converters

In this article, a new, simple, and generalized discontinuous pulse width modulation (PWM)-based submodule capacitor voltage balancing approach is proposed and generated within the carrier-based PWM for modular multilevel converters (MMCs). In this scheme, different variants of zero-sequence voltage (ZSV) component is injected into the pulse width modulator so that each arm of the MMC is virtually clamped to positive or negative dc bus during a specific interval to achieve discontinuous modulation. A simple approach based on the location of the voltage vector mapping to a two-level space-vector diagram is presented to determine various ZSV components. The effectiveness of the proposed scheme with a voltage balancing algorithm is demonstrated through PLECS simulations and experimentally verified using a hardware-in-the-loop-based system. The performance of MMC with different ZSV injections is holistically evaluated to determine the best possible sequence over a wide range of modulation index.

Experimental studies on effects of surface morphologies on corona characteristics of conductors subjected to positive DC voltages

As the surface conditions play a significant role on corona discharge and its related effects of the conductors, the influence of fine particulate matter on positive-polarity, direct-current conductors was studied experimentally in this study. The surface morphologies of the conductor could be discovered from the experiments. The typical morphologies are the parallel chains of particles. To evaluate the surface condition quantitively, the surface roughness of the conductors is measured. It is found that the applied voltage and testing time have a great influence on the surface condition. After that, the corona characteristics of conductors are tested. It reveals that the total ground level electric field and ion flow density increases with the surface roughness growing.

New insight into the association between bile infection and clinically relevant pancreatic fistula in patients undergoing pancreatoduodenectomy.

The association between bacterial infection and clinically relevant postoperative pancreatic fistula (CR-POPF) after pancreatoduodenectomy (PD) has not been fully elucidated. Microbiological data for intraoperative bile culture (BC) and drain culture on postoperative day 4 (DC) were collected. The study population was classified into the following three groups: P1 (positive BC and DC), P2 (positive BC and negative DC), and N (negative BC). A total of 209 patients (P1: 38, P2: 72, and N: 99) who underwent PD between May 2013 and May 2018 met the inclusion criteria of the study. The rate of CR-POPF was significantly higher in the P1 group (34.2%) than in the P2 (12.5%; P=.007) and N groups (14.1%; P=.008). Between P1 and P2 groups, a significant difference was observed in the proportion of Enterococcus faecalis grown in BC (42.1% vs 4.2%; P<.001). Multivariable logistic regression analysis showed that the presence of E faecalis in bile was independently associated with CR-POPF. The detection of both bile and postoperative abdominal fluid infections can be more accurate in predicting CR-POPF. Furthermore, the presence of E faecalis in bile may serve as a novel surrogate marker for CR-POPF.

DC performance of non-uniformly transversal polluted glass insulation under parallel electrical discharges’ effect

The aim of this paper is to quantify the evolution’s effect of parallel electrical discharges on the performance of a plane insulation placed simultaneously under conditions of a non-uniform transversal distribution of pollution and positive DC voltage. The results emanating from this experimental study revealed a maximum increase about 18% of flashover voltage of insulation’s non-uniform transversal distribution of the insulation compared to that obtained under uniform pollution distribution. The existence of a limit value of the electrical conductivities’ ratio of the lightly and the heavily contaminated bands, characterized by an early development of the electrical flashover arc of the insulation in the heavily polluted band, was demonstrated in this investigation. Also, a direct link has been established between the flashover frequency of the insulation’s highly contaminated band, under the effect of a non-uniform transversal distribution of pollution and its electrical resistance. Similarly, an approximation linking the insulation’s length to the effective width of an electrical arc in its last phase of development and consequently the number of electrical arcs that can develop in the two differently polluted bands just before the insulation’s flashover was done in this study.

Experimental Assessment on Air Clearance of Multiple Valve Unit Considering Switching Impulse and DC Superimposed Switching Impulse

Multiple valve unit (MVU), which converts AC to DC and DC to AC, is one of the key elements of high voltage DC (HVDC) transmission. Therefore, the insulation design of MVU against overvoltage should be considered for the stable and reliable operation of HVDC transmission system. Especially, the air clearance of MVU should be calculated based the switching impulse, since it is fatal to MVU in terms of electrical insulation. However, the previous studies were limited to wave front, and the air clearance of the switching impulse is specified only for an ultra-high voltage (UHV) above 750 kV. As a result, it is difficult to calculate the air clearance of MVU which must endure for a switching impulse under 750 kV. In addition, when the switching impulse introduced while the MVU is in normal operation, it is superimposed to DC and creates the most severe situation, but the studies on such subjects are also insufficient. Therefore, as a fundamental step to calculate the air clearance of MVU, the dielectric characteristics of switching impulse and DC superimposed switching impulse in air have been investigated. The experiments on switching impulse showed that the critical flashover voltage was varied according to the curvature of electrode in the gap distance, up to eight times of the electrode radius. However, beyond that gap distance, the critical flashover voltage became similar, regardless of the radius of electrodes. In case of the superimposed experiment, it was performed according to DC pre-stress level and the polarities of switching impulse. The results were most severe when the positive switching impulse was superimposed on the positive DC, and the peak voltage at which flashover occurs was independent of DC pre-stress.

Decomposing Mechanism of SF6 under Positive DC Partial Discharge in the Presence of Trace H2O.

The influence of H2O on SF6 decomposition characteristics under positive DC partial discharge (PD) is significant. To evaluate PD fault severity in DC SF6-insulated equipment using the production characteristics of SF6 decomposition components, the corresponding relationship and mathematical expression between the production of SF6 decomposition components and the H2O content should be identified and achieved. Thus, SF6 decomposition experiments under positive DC PD are performed to reflect the influence of H2O on SF6 decomposition components. Results show that the total discharge quantity and the discharge repetition rate averaged for 1 s decrease slightly when the H2O content increases from 0 to 970 ppmv and then increase when the H2O content increases from 970 to 5120 ppmv. The effective production rates of SO2F2, SOF2, and SO2 increase with the H2O content, whereas that of SOF4 decreases. Finally, the corresponding relationship and mathematical expression between the characteristic ratio (c(SO2F2) + c(SOF4))/(c(SOF2) + c(SO2)) of components and the H2O content have been achieved, which can afford references for PD fault diagnosis in DC SF6 gas-insulated equipment.

An experimental study on the effect of DC electric field on impinging flame

This work presents an experimental study the effects of DC electric field on a laminar premixed flame which impinges on a plate. Positive DC and ground levels of voltage are applied to the plate and burner, respectively. A reverse application of the electric field is also tested. Detailed measurements are undertaken to understand the ionic wind effect on flame impingement heat transfer.Testing shows a favorable effect of positive field on flame stability, as the flame stands leaner combustion. When a sufficiently high voltage is applied, discharge occurs which breaks down the electric field. The results of thermal field testing reveal that the temperature change observed is due to flame shape distortion caused by ionic wind. It is found that ionic wind is majorly physical in nature, inducing the changes in flame stability and temperature through a force. Further, it is mainly the post-combustion part of the flame, instead of reaction zone, bears such force.Under positive field, the burner nozzle is rapidly heated by the flame. However, flame impingement heat transfer has no significant change under either positive or reverse field. It is because heat loss from the post-combustion flame to plate is less altered by DC fields. To further study the ionic wind effect, a co-flow is introduced to the flame, and the standard deviation data of flame temperature indicate that ionic wind insignificantly changes the mixing between flame and ambient air. It confirms that ionic wind is mainly a force effect, with negligible chemical effect in the scope of this work.

Novel flexible HVDC transmission converter station topology with DC fault blocking capability

The conventional half-bridge submodule (HBSM)-based multilevel modular converter (MMC) cannot block DC faults. To solve this problem, a novel flexible overhead-line high-voltage direct current (HVDC) transmission converter station topology is proposed in this study, which provides DC fault blocking capability. By adding blocking submodules (SMs) onto the positive and negative DC buses of a conventional HBSM converter station, a rapid fault current blocking can be achieved in the case of a DC bus short circuit. The DC fault blocking principle of the proposed topology and the rapid fault blocking capability of the blocking SM are analyzed. The quantity of required major components is also calculated. In addition, the IGBT, the system loss and the control complexity quantities are comprehensively compared with those of existing topologies. The validity of the proposed topology is demonstrated based on simulation and experimental studies.

Effect on Resistive Switching by Inserting TiO&lt;sub&gt;x&lt;/sub&gt; Thin Layer in SiO&lt;sub&gt;x&lt;/sub&gt;: Ag-Based Memristor

An approach to design a memristor by inserting a TiOx thin layer in Pt-Ag/SiOx:Ag/TiOx/P++-Si memristor in order to exhibit analog resistive switching has been proposed. The device shows continuous resistance change under positive and negative DC sweeping bias, and the device conductance can also be modulated by consecutive potentiating and depressing pulse programming. These primitive results are beneficial to realize the learning and computing in such kind of memristor devices. High-resolution transmission electron microscopy observations demonstrate a clear interface between the thin layers of Ag nanoclusters embedded SiOx and the amorphous TiOx. The I-V analysis of Pt-Ag/SiOx:Ag/TiOx/P++-Si memristor confirms that the presence of TiOx thin layer controls the formation/rupture of Ag-filament across the Pt-Ag and P++-Si electrodes, realizing the gradual conductance modulation, which is essential to emulate the bio-synaptic characteristics.

Pin-to-plane self-pulsing discharge in transversal airflow: interaction with a substrate of plasma filaments blown out from the discharge zone

The pin-to-plane electrode system for the multi-sectioned discharge in the transversal airflow at atmospheric pressure has been developed. The airflow is directed perpendicularly to the electric current. As it was revealed, all sections of the multi-sectioned discharge operate identically and independently to each other. For this reason, the spatial-temporal behavior of the discharge excited only in a single section was investigated. The pin-to-plane discharge (PPD) was powered by a positive polarity DC voltage that induces the spontaneously repeating streamer-spark breakdowns in a discharge gap. Each breakdown forms the plasma filament (PF) which is being further blown out of the discharge zone and stretched by the airflow. The PF impacts a dielectric plate located in the path of the airflow at different distances away from the discharge zone. Two modes in the behavior of the blown-out PF have been revealed. When the plate is close to the discharge zone, the blown-out PF is tightly pressed by the flow to the plate. In this case, both the brightness and electric current of the PF are periodically pulsing with a high frequency up to the plasma filament breaking up due to its strong elongation by the flow. If the plate is far away, the quasistationary regime happens. During the PF elongation in this regime, plasma filament keeps both the brightness and current approximately changeless till the filament breaking up. At In this regime, the blown-out PF is being stretched in parallel to the surface without contacting it. The release of energy into the PF continues also in the course of its blowing out until its impinging the object located in the path of airflow. Due to that, the PPD pulsing mode generates a lot of reactive species inside the PF up to its impinging the object to be treated. This is a reason why the self-pulsing PPD can be highly effective for surface processing by non-thermal plasma.

± 180° Discontinuous PWM for Single-Phase PWM Converter of High-Speed Railway Propulsion System

As high-capacity alternating current/direct current (ac/dc) power conversion systems, single-phase pulse-width modulation (PWM) converters used in high-speed railway propulsion systems adopt high-voltage Insulated-Gate Bipolar Transistors (IGBTs) as switching elements. Due to their high breakdown voltage characteristics, the switching dynamics are inferior to those of low-voltage IGBTs and switching losses are more dominant than conduction losses despite operating at relatively low switching frequencies of hundreds to several kHz. To solve this problem, this paper proposes ± 180° discontinuous PWM (DPWM) suitable for a single-phase circuit. With the simple addition of offset voltages, the proposed DPWM method can be implemented easily and switching losses can be reduced by half by clamping the switching legs of the H-bridge converter to the positive or negative dc rail during every half cycle. In addition, temperature deviation between the power stacks can be minimized by using selective application of clamping modes. The validity and effectiveness of the proposed DPWM are verified through simulations and experiments of a prototype converter.

Electrification of Sonoluminescing Single Bubble.

The charge of microbubbles in water is considered to be negative at the liquid-bubble interface. We investigated, for the first time, the charge of a single bubble sonoluminescence (SBSL bubble) that exhibits spatiotemporally stable light emission. When negative DC voltage was applied, the SBSL bubble was attracted to a hot electrode. Conversely, the SBSL bubble was repelled by the hot electrode when positive DC voltage was applied. The translation of the SBSL bubble under an electric field suggests that it is positively charged, and the bubble moved to an equilibrium position to balance the primary Bjerknes force and the electrostatic force. The amount of bubble translation under an electric field depended on the elapsed time of sonoluminescence, suggesting that the products generated inside the SBSL bubble affect the mechanism of bubble charging. Furthermore, we measured the electric field effects on bubble expansion and contraction by a light scattering technique. Applying a positive voltage decreased the maximum bubble diameter and also the intensity of the SBSL. Conversely, applying a negative voltage increased the maximum bubble diameter and also the intensity of the SBSL. The present study revealed that a SBSL bubble is positively charged.

Synaptic learning and memory functions in SiO2:Ag/TiO2 based memristor devices

In this study, a TiO2 thin layer is deliberately inserted in the Pt–Ag/SiO2:Ag/p++-Si memristor to achieve enhanced resistive switching performance in terms of retention and cyclic endurance characteristics. The memristor device exhibits a bidirectional analog switching behavior corresponding to a wide dynamic range of synaptic weight modulation as potentiation and depression under the consecutive positive or negative DC bias sweeps and pulse trains. Moreover, the learning and memory functions, including paired-pulse facilitation and pulse repetition-induced short-term memory to long-term memory transition, are also demonstrated. The relation of the memory stability to the repeated input pulse stimulus and the device conductance with time have also been revealed by the stretched exponential relaxation model. It is shown that the conductance of our newly fabricated Pt–Ag/SiO2:Ag/TiO2/p++-Si memristor device can be modulated by successive Ag-filament growth and dissolution in the SiO2:Ag/TiO2 bilayer oxide structure, which is essential for potential applications in memory devices and artificial neural systems.

Ionic wind produced by a DC needle-to-plate corona discharge with a gap of 15 mm

The goal of the present study is to better understand the electrohydrodynamic (EHD) phenomena occurring in 15 mm gap point-to-plate corona discharges supplied by positive and negative DC voltages, and to link them to the discharge regime. For that, discharge current measurements have been conducted and a 20 kHz Particle Image Velocimetry system has been used to characterize the ionic wind produced by the discharges. The main results are as follows: (a) the Townsend’s law can always correctly interpolate the experimental I–V characteristics of a negative corona, even in presence of Trichel pulses, (b) in the case of a positive streamer discharge, the current does not follow the Townsend’s law as it evolves in Vk with k = 4 in the present study, (c) Indeed, the discharge current becomes higher than the Townsend’s law when the glow-to-streamer regime transition occurs, (d) the ionic wind is unsteady, more especially in the case of a positive corona for which it seems that its velocity is pulsed at the same frequency than the streamer one, (e) when the positive high voltage is switched on, a strong streamer occurs at the end of the voltage rising (t = 25 µs for +6 kV), resulting in an over-velocity region upstream the head of the ionic wind jet that progresses in quiescent air toward the plate, (f) the topology of the time-averaged ionic wind is fully different compared to the one observed in a previous study where the gap was equal to 25 mm, (g) in the case of a negative corona, the ionic wind velocity is nearly constant in the electrode gap, (h) in the case of the positive corona, the results are even more surprising since the velocity is minimum at the tip and increases when one approaches the grounded plate, indicating that there is a significant space charge remained by the streamers in the second half of the inter-electrode region, (i) generally speaking, this study highlights that the spatio-temporal characteristics of the EHD force and the resulting ionic wind depend on the distribution of the space charge between both electrodes, the latter being linked to the voltage polarity, the discharge regime and the electrode gap.

The effect of electrode surface roughness on the breakdown characteristics of C3F7CN/CO2 gas mixtures

C3F7CN/CO2 gas mixtures have been widely studied in recent years. And they are considered a potential alternative for replacing SF6 in high voltage power equipment. In this paper, the DC breakdown characteristics of 6%C3F7CN/94%CO2 mixtures in a uniform electric field were studied at electrode surface roughness levels of 0.4 μm, 1.2 μm, 2.1 μm and 3.8 μm. At the same time, in order to provide reference for the power engineering, the gas pressure was set between 0.3 MPa and 0.7 MPa. Meanwhile, the results at 0.4 MPa gas pressure were compared with it of SF6. The growth rates of positive and negative DC breakdown field strengths of C3F7CN/CO2 mixtures decreased gradually with the increasing of gas pressure, and the decreasing trend of growth was more obvious with the increase of electrode surface roughness. At higher levels of surface roughness, the polarity effect of the C3F7CN/CO2 mixture was more significant. At 0.4 MPa gas pressure, as the surface roughness increasing from 0.4 μm to 3.8 μm, the decline degree of positive and negative DC breakdown field strengths of C3F7CN/CO2 mixtures were significantly lower than that of SF6 gas between 0.4 μm and 3.8 μm. The critical breakdown field strength, (E/N)crit, and coefficient, β, of 6%C3F7CN/94%CO2 mixture and SF6 were obtained by Steady State Townsend method(SST), and (Nh)crit values were calculated to characterize the sensitivity of these gases to surface roughness. The (Nh)crit value of the 6%C3F7CN/94%CO2 mixture was higher than SF6. It showed that the 6%C3F7CN/94%CO2 was less sensitive to roughness than that of SF6.

Positive‐negative DC‐DC converter using amorphous‐InGaZnO TFTs

SummaryThis paper presents two DC‐DC converters capable of generating either positive or negative output DC voltage using amorphous‐indium gallium zinc oxide (a‐IGZO) thin‐film transistors (TFTs). While one uses conventional diode‐connected cross‐coupled (design‐1), the other employs a bootstrapped cross‐coupled (design‐2) DC‐DC converter, which overcomes the limitations of design‐1 but requires boosted clock pulses. Therefore, an on‐chip clock booster is proposed to generate all the required signals without any external clock pulses. The performance of the proposed circuits (DC‐DC converters and clock booster) has been demonstrated using in‐house a‐IGZO TFT models in the Cadence environment. The simulation results have shown that design‐1 (design‐2) generates negative or positive output DC voltage of 7.25 V (7.8 V) or 12.7 V (15.2 V), respectively, for a supply voltage of 8 V, using a single DC‐DC converter. Therefore, the output DC voltage of design‐2 is close to the theoretical value, and it ensures robust performance against different load currents when compared with design‐1. Besides, design‐2 has shown a maximum power efficiency of 90%, superior compared with design‐1, where power efficiency is 60.7%. Therefore, this circuit finds potential applications in the next generation flexible displays.

Microalgae Filtration Using an Electrochemically Reactive Ceramic Membrane: Filtration Performances, Fouling Kinetics, and Foulant Layer Characteristics.

Electrochemical membrane filtration has proven to be successful for microbial removal and separation from water. In addition, membrane fouling could be mitigated by electrochemical reactions and electrostatic repulsion on a reactive membrane surface. This study assessed the filtration performances and fouling characteristics of electrochemically reactive ceramic membranes (a Magneli phase suboxide of TiO2) when filtering algal suspension under different dc currents to achieve anodic or cathodic polarization. The critical flux results indicate that when applying positive or negative dc currents (e.g., 1.25-2.5 mA·cm-2) to the membrane, both significantly mitigated membrane fouling and thus maintained higher critical fluxes (up to 14.6 × 10-5·m3·m-2·s-1 or 526 LMH) compared to the critical flux without dc currents. Moreover, applying dc currents also enhanced membrane defouling processes and recovered high permeate flux better than hydraulic and chemical backwash methods. Moreover, fouling kinetics and the cake layer formation were further analyzed with a resistance-in-series model that revealed many important but underexamined parameters (e.g., cake layer resistance and cake layer thickness). The cake layer structures (e.g., compressibility) were shown to vary with the electrochemical activity, which provide new insight into the biofouling mechanisms. Finally, the algogenic odor, geosmin, was shown to be effectively removed by this reactive membrane under positive dc currents (2.5 mA·cm-2), which highlights the multifunctional capabilities of electrochemically reactive membrane filtration in biomass separation, fouling prevention, and pollutant degradation.

Relaxation characteristics of ion jet formed across the short air gap in the cone–sphere electrode under positive polarity DC voltage

The research into the development of discharge under positive polarity voltage across the short air gap is of great significance to improve insulation of electrical equipment. By establishing a photoelectric joint detection system, this study observed phenomena in short air gaps before, and the period of breakdown. By utilising a photomultiplier tube and a Rogowski coil, the optical and current signals were obtained. Moreover, through z-type Schlieren system and high-speed digital camera, relaxation images of ion jet formed across cone–sphere electrodes were obtained. Through comparison and analysis, the discharge of short air gap was divided into three stages: initial corona, strong corona and streamer breakdown. On the basis of this, the relationship between optical and current signals was compared by assessing characteristics of the ion jet in each stage. Also, the relaxation characteristics of an ion jet were analysed. The results showed that ion jets appeared in the initial corona stage, while relaxation channel of the ion jet was formed and the channel fluctuated in the strong corona stage. In the streamer breakdown stage, the process of expansion, constant fluctuation and breakdown appeared; the amplitudes of corona current signals and optical signals in these three stages constantly increased.

Design and analysis of space vector pulse width modulation techniques for Z-source inverter

This paper presents the design, analysis and digital implementation of space vector PWM-based various switching sequences for the Z-source inverter. Three main type of switching sequences have been taken, these are continuous switching sequence, basic bus clamping, and advanced bus clamping. Each of these sequences is designed in order to achieve symmetry in output line voltage. Two main control techniques, maximum boost, and maximum constant boost have been used to analyse each sequence in terms of switching transition per sample and duration of positive dc bus clamping. The novelty of the paper is the design of all switching sequence considering the conditions of symmetry and the implementation of advanced bus clamping techniques in the area of Z-source inverters. Simulation and experimental results have been presented to verify the design.