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In this paper, a pin-cylinder charger was employed to investigate the electrostatic field characteristic and its influence on submicron particle charging in positive and negative voltage polarity, respectively. The submicron particle's charging characteristics were also investigated thoroughly, including polarity, gas velocity, applied voltage, and injection position. Results showed that the increase of the applied voltage was proved a useful approach to enhance particle charging, especially for positive polarity. A 0.4 μm diameter particle achieved about a 60% increase from 5 kV to 8 kV. The negative polarity had advantages on particle charging, whose superiority could reach approximately 15% for 0.1 μm diameter particle in 8 kV voltage. The particle trajectory was a helpful bond to connect the particle charging with the electrostatic field, which decided the charging number and charging rate through the different distribution and intensity in two polarity conditions.

This article introduces a powerful methodology to synthesize high gain step-up dc–dc converters from differential connections of two converters. The two selected converters may be either equal or different, and they can use different kinds of gain cells. Both only have to present a common reference between the input and output voltages and the same output voltage polarity. The proposed differential connection supplies high step-up converters and mainly the method can be applied to synthesize different topologies from different well-known converters. Besides the derivation and analysis of some converters in continuous conduction mode, the article also discusses the power processing, reduction of components, addition of gain cells to further increase the gain, different kinds of modulations, and addresses a different point of view for some topologies previously published in literature. The proposed methodology is verified through thirteen prototypes derived from basic converters considering input voltage of 20 V, duty cycle of 0.75, switching frequency of 50 kHz and rated power of 100 W. Moreover, three new ultrahigh gain dc–dc converters are still derived from the insertion of gain cells in the differential converters herein proposed. These three converters are experimentally evaluated under input voltages of 20 and 30 V, rated power of 200 W and, switching frequency of 50 kHz, providing static gains from 15 up to 30, and peak efficiency of 96.78%.

Epoxy resin has been used to cast the core of the dry‐type high voltage direct current bushings and can accumulate space charge under high electric fields at high temperatures, which is believed to be a potential threat to the safe operation of dry bushings, especially under polarity reversal. In this study, results of a study of the evolution of space charge and electric field distribution in epoxy resin, under polarity reversal at 10 and 20 kV/mm, and at temperatures between 40 and 100°C, are presented. The results show that the space charge dynamics in epoxy resin during the test were different at various temperatures. At temperatures no more than 60°C, space charge accumulation within the samples was not obvious. Instead, a low‐frequency dielectric relaxation process maybe occurs during the polarity reversal, which made the charge peaks on the electrodes after the reversal were slightly smaller than those before the reversal. At temperatures no less than 80°C, considering that the applied electric field has little effect on the barrier reductions of the traps within the samples, the space charge dynamics were mainly governed by a thermally activated process, and the depths of the deepest occupied traps were obtain by analysing the transient processes.

In this study, a simulation model of surface charge accumulation has been established. The model considers three accumulation ways, i.e. electrical conduction within the gas, through insulator volume and along the insulator surface. The generation, diffusion, drift and recombination of charge carriers are also taken into account. Based on it, the influence of polarity reversal, reversal time on surface charge and electric field distribution on a basin-type insulator are studied. The polarity of the surface charges and the direction of the electric field change after the voltage polarity reversal. When the preload voltage is equal to reversal voltage, the surface charge and the electric field distributions at steady state before and after voltage polarity reversal are all the same with opposite sign, and not affected by the reversal time. However, the time to reach the steady state varies with different reversal time. The steady-state surface charge and electric field increased with the rise of reversal voltage. The transient normal and tangential electric field would not exceed the value of the steady state, which means voltage polarity reversal has no additional influence on insulation performance. This research can provide guidance to the design and manufacture of DC GIS/GIL.

Recently, the high-performance converter with wide range and high gain has been widely used in these cases such as PV power generation. On the other hand, there are more highly variable stresses on switch and diode in high-performance converter than the traditional DC-DC converters due to the wide range and high gain. Therefore, it is necessary to diagnose switch and diode faults in high-performance converter. In this paper, the switch and diode fault are analyzed and compared with traditional DC-DC converters. Then, an online diagnosis technique based on the inductor voltage polarity of the DC-DC converter is proposed. This technique only uses the inductor voltage polarity and the switch gate drive signal as signatures to diagnose short-circuit or open-circuit fault of the switch and diode. The technique is cost-effective and simple because it uses simple auxiliary windings to sense the inductor voltage polar and uses some logic circuit to generate indicators. The details of the technique are discussed through an example of the quadratic Boost converter. Experiments illustrate the correctness of the proposed technique and show its capability for switch and diode fault diagnosis.

In this study, the impulse breakdown of conductive water (350 μS/cm) was carried out under 25~1500 μs pulses and with a gap distance of 0.5~100 mm. The influence of pulse duration, gap distance and voltage polarity on the breakdown characteristics are shown. Martin's equation and streamer linear propagation (SLP) model for breakdown voltage calculation are compared, and their application conditions are indicated. The results show the pulse duration and the gap distance on the breakdown characteristics are opposite; however, their influence on voltage polarity are similar. Martin's equation is only appropriate for short impulse breakdown (nanosecond) while the SLP model is shown to be suitable for long impulse breakdown (microsecond or longer).

The design and operation of DC insulation systems is more complex than for AC sinusoidal supply. The most subtle issues are how the electric field in DC can vary with operation and affect the partial discharge inception voltage and partial discharge phenomenology. During operation, a DC insulation system is indeed subjected to voltage and load transients, the former due to energization and voltage polarity inversion. This paper sheds some lights on such issues, going through another fundamental problem, that is, how to carry out effective partial discharge detection under DC supply, both in steady state and during voltage transients. An algorithm that can enable automatic and unsupervised partial discharge measurements in DC electrical apparatus is presented and validated based on partial discharge monitoring performed on test objects representing defective insulation systems. Identification of the source generating partial discharges, which is the basis for diagnostic and condition maintenance, is achieved without the support of experts. The impact of voltage transients on the life and reliability of DC insulation systems is modelled in the presence and absence of partial discharges. It is shown that, depending on the repetition rate, the presence of partial discharges, even if only during voltage transients, can dramatically affect the insulation life and cause premature insulation breakdown.

The corona discharge of insulators has an important influence on the electromagnetic environment. In order to study the influence of different voltage polarities on the corona discharge characteristics of insulators, this paper is mainly based on the fluid-chemical reaction equation to establish a two-dimensional simulation model of insulators, which includes a variety of chemical reactions and substances. According to the above mentioned numerical analysis model, the electric field and the particle density of the insulators under different voltage polarities are given through the simulation calculation. Finally, the discharge parameters under different voltage polarities are compared, and the relationship between corona discharge parameters and voltage polarity is obtained.

Design and operation of DC insulation systems are faced with challenges related to the electric field distribution in DC, which can vary with operation and affect the partial discharge inception voltage and partial discharge phenomenology. Energization and voltage polarity inversions in DC insulation systems result in electric field transients that may last many hours, and load fluctuations can generate variable temperature profile transients. Inception of partial discharge during such repeated transients can significantly reduce insulation life. This paper addresses the behavior of electric field and partial discharge during voltage transients, the influence of such transients on life and how to measure partial discharges during transients and DC steady state. The effect of voltage transients on life of DC insulation systems is modelled with and without partial discharges, and it is shown that the presence of partial discharges can affect dramatically the life, even if it is restricted only during voltage transients. The issues related to measurement of partial discharges in DC insulation systems are also discussed, and an automatic and unsupervised partial discharge measurement approach is presented.

Quality control, commissioning and operation of DC cable systems (and, in general, electrical apparatus) require PD testing and monitoring technologies that are effective in rejecting noise and identifying partial discharges and their source, both under DC and AC power supply. Such technologies are not fully available at present, so that this paper presents a contribution to make them feasible. A new algorithm is suggested that may be effective to separate partial discharges from noise both under steady-state DC and during voltage transients, through an automatic and unsupervised approach which looks very promising for laboratory and on-field testing and monitoring of partial discharges in DC cable systems.

This paper deals with a particular problem in the conventional finite set model predictive control (FS-MPC) method in VSI drives. When FS-MPC is experimentally implemented, the controller chooses any one of the eight available voltage vectors. Therefore, if the controller selects any of the far states, simultaneous switching events among the inverter legs occur. This causes line-line voltage polarity reversal (LLVPR) with the voltage change twice the value of the DC-link voltage. The fast LLVPR combined with the reflected wave by the motor cable can damage the insulation of the stator winding. In this letter, the problem of LLVPR in electric drives with FS-MPC is described, and the solution to this issue is proposed. Experimental results to validate the proposed method are shown.

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

Generally, single-stage buck–boost inverters consist of distinct circuits operating individually in buck/boost or positive/negative modes, the latter definitely leading to crossover distortion in the output current. This paper proposes a switching strategy that changes the bimodal operation of a single-stage buck–boost topology to a single-mode inverter. This is due to the consequent voltage gain that ensures that the output voltage polarity, apart from its magnitude, is an exclusive function of the duty ratio. Operational principle and salient hardware design are explained with the help of equivalent circuits. Details about the dynamic model are presented, based on which controller design is carried out for both stand-alone and grid-connected operation. Experimental results obtained from a 300-W, 110-V laboratory prototype are presented to validate the performance of the proposed switching strategy.

The anode interface effects on the resistive switching characteristics of Pt/HfO 2 /Pt resistors are investigated by changing the forming and switching polarity. Resistive switching properties are evaluated and compared with the polarity operation procedures, such as the reset voltage (V r ), set voltage (V s ), and current levels at low and high resistance states. When the same forming and switching voltage polarity are applied to the resistor, their switching parameters are widely distributed. However, the opposite forming and switching voltage polarity procedures enhance the uniformity of the switching parameters. In particular, the V s distribution is strongly affected by the voltage polarity variation. A model is proposed based on cone-shaped filament formation through the insulator and the cone diameter at the anode interface to explain the improved resistive switching characteristics under opposite polarity operation. The filament cone is thinner near the anode interface during the forming process; hence, the anode is altered by the application of a switching voltage with opposite polarity to the forming voltage polarity and the converted anode interface becomes the thicker part of the cone. The more uniform and stable switching behavior is attributed to control over the formation and rupture of the cone-shaped filaments at their thicker parts.

In this article, the electrical tree initiation behavior in silicone rubber (SIR) under DC voltage was studied. Tests were conducted to reveal the effects of temperature (20–120 °C), defect type and voltage polarity on electrical tree initiation. In samples with an undamaged needle-tip (N samples), we find that the average tree initiation voltage (Fa«) decreases exponentially with increasing temperature by 42%. Single-branch trees and multiple-branch trees are observed. The probability of generating multiple-branch trees decreases with increasing temperature. In samples where an air gap or crack is in the needle (AN samples), Vati is 55.7% of that in N samples at 20 °C. In AN samples, Vati and tree shape do not change monotonically and Vati varies in a small range as temperature rises. Vati for N samples under negative voltage is much larger than that under positive voltage. This polarity effect is reversed in AN samples. Gas discharge theory and Frohlich's theory of dielectric breakdown explain electrical tree initiation behavior at different temperatures. Based on differential scanning calorimetry (DSC) results, we believe that partial segment relaxation of SIR at high temperature leads to Vati decreasing. Electrical trees previously generated inside the cable or an air gap created due to mechanical damage cause Vati to of a subsequently generated tree to drop to nearly half of its typical level. This information is useful for the design of HVDC cable insulation.

Investigation of space charge accumulation processes in a chemically cross-linked polyethylene for ac usage (XLPE) and a modified XLPE for dc usage (DC-XLPE) at voltage polarity reversal was carried out using pulsed electro-acoustic (PEA) measurement system. We found that a huge amount of so called packet-like charge, some of which partially increased the electric field in the material to more than twice of the average applied stress, generated in XLPE at the polarity reversal of DC stress. It enhanced the electric stress locally in the sample by almost twice of the applied average stress. On the other hand, we also found that the packet-like charge was not observed in DC-XLPE under the same experimental condition.

The digital resistive switching with abrupt state change is suitable for the applications of information storage and logical operation, while the analog resistive switching with gradual state change is required in the neuromorphic computing system. This paper reports the Set voltage polarity dependent analog and digital Reset processes coexisted in Pt/CuO/Pt memristive devices. After a negative Set process, the device exhibited a digital Reset process when applying negative bias. In contrast, an analog Reset process could be achieved, following a positive Set process. These digital and analog Reset processes can convert to each other by controlling the Set bias polarity. It is proposed that the Joule-heating effect induced filament rupture is responsible for the digital Reset process, while the analog Reset process can be attributed to the filament rupture dominated by the ion migration under the external electric field.

Gulf War veterans meeting criteria for Haley Syndrome 2 of Gulf War illness endorse a particular constellation of symptoms that include difficulty with processing information, word-finding, and confusion. To explore the neural basis of their word-finding difficulty, we assessed event-related potentials (ERPs) associated with semantic memory retrieval in 22 veterans classified as Syndrome 2 and 28 veterans who served as controls. We recorded EEGs while subjects judged whether pairs of words that represented object features combined to elicit a retrieval of an object memory or no retrieval. Syndrome 2 subjects' responses were significantly slower, and those participants were less accurate than controls on the retrieval trials, but they performed similarly on the nonretrieval trials. Analysis of the ERPs revealed a difference between retrievals and nonretrievals that has previously been detected around 750 ms at the left temporal region was present in both the Syndrome 2 patients and controls. However, the Syndrome 2 patients also showed an ERP difference between retrievals and nonretrievals at the midline parietal region that had a scalp voltage polarity opposite from that recorded at the left temporal area. We hypothesize that the similarities between task performance and ERP patterns in Syndrome 2 veterans and in patients with amnestic mild cognitive impairment reflect disordered thalamic cholinergic neural activity, possibly in the dorsomedial nucleus.

This paper presents an analysis of the influence of different parameters on energy efficiency of a spray, point-to-point, non-thermal plasma (NTP) reactor in plasma activated water generation. The polarity of the DC pulse power supply is assessed for different input parameters such the frequency (60, 150 and 250 Hz), water flowrate (5, 10, 15 and 20 mL/min) and gas flowrate (1 and 2 L/min Ar). There are also presented the values of important electrical parameters of the plasma such the average power or discharge energy. The concentration of hydrogen peroxide generated into plasma activated water was considered as indicator of the energy efficiency.

Many researchers are investigating the feasibility of using polypropylene in HVDC cables. To understand the basic electrical properties of nanocomposite polypropylene, this work investigates the influence of nanostructuring on DC breakdown strength, life under periodic voltage polarity reversals and space charge accumulation properties. Preliminary results show that unfilled samples feature higher charge accumulation, lower breakdown strength, and shorter life than the nanocomposite counterpart. Based on these results, nanostructured dielectrics can give an important improvement in electrical insulation properties.

The dual active bridge (DAB) converter is widely used in different fields, such as electric vehicles, distributed power generation system, energy networks. it also can be introduced in railway applications to implement power electronic transformers, bidirectional battery chargers etc. The DAB converter is usually designed combining with series resonant to reduce the switching off loss. Nevertheless this will make it hard to achieve the zero voltage switching on of the primary power semiconductor at light load, especially when voltage conversion ratio is far away from one. Besides, the dead-time has great effect to the soft switching implementation. The voltage polarity reversal and voltage sag phenomena etc. are caused by the dead-time effect. This paper provides a precise mathematical transmission power model considering the resonant and dead-time effect respectively. A comprehensive theoretical analysis of the dead-time effect on the series resonant DAB converter is presented. By improving the dead-time, the soft switching rang is increased and the efficiency will be improved at light load. The experimental results are provided to verify the theoretical analysis.

International audience; This paper deals with creeping discharges propagating over solid insulator samples in presence of a fluoronitrile mixture consisting of 10% of fluoronitrile and 90% of CO2, under positive and negative standard lightning impulse voltages, in point-plane geometry. The solid samples used are disks made of filled epoxy resin used for real GIS insulators. The experimental results are compared with those obtained with SF6 at pressure ranging 0.1 to 0.3 MPa, while the pressure of fluoronitrile mixture is chosen to match an equivalent dielectric strength of SF6. Different characteristic parameters are investigated namely the maximum length of creeping discharges versus the gas/mixture pressure and voltage polarity. In SF6, two regimes of discharges that depend on the voltage polarity are evidenced. Under positive impulse, the mixture presents similar properties as SF6. However, under negative polarity the maximum discharge length in SF6 is much longer than in the fluoronitrile mixture.

Pilot protection is indispensable in multi-terminal HVDC grid to ensure its safe operation. This paper proposes a pilot protection method based on voltage polarities of current-limiting reactors at the ends of DC lines. Analysis shows that the voltage polarities of terminal reactors at the two ends of DC lines are both positive when internal fault occurs, while they are opposite for external faults. Based on the aforementioned characteristic difference, a pilot protection criterion is built to distinguish internal fault from external fault. To avoid mal operation of the healthy pole generated by the coupling effect, the fault voltage of current-limiting reactor is integrated, and its polarity can be identified based on the integration value. At last, a four-terminal modular multilevel converter (MMC)-based HVDC grid simulation model is built in PSCAD/EMTDC, and the simulation results verified the effectiveness of the protection method.

It is well known that the performance of streamers in transformer liquids are closely related to the test conditions, e.g. liquid nature, voltage polarity, voltage level and field configuration. The differences in streamer phenomena under various test conditions can be reflected by the evolution of streamer shapes. However, the conventional parameters such as stopping length, average propagation velocity and apparent area cannot quantitatively describe the degree of structural complexity of streamers. In this paper, multi-fractal analyses were performed for 2-D projected streamer images captured in a mineral oil under impulse voltages. The multifractal spectrum shape, spectrum shift alone vertical axis and maximum value on the spectrum curve were estimated in order to describe the changing trend of streamer main branches and side branches, the distribution of streamer branches and the complexity of overall streamer morphology, respectively. Variations of these parameters with changes in test conditions including voltage polarity and voltage level were evaluated.

HVDC cables connected to Current Source Converters are subjected to polarity reversal when the HVDC intertie changes the power flow. Mass Impregnated Non-Draining cables had been using since the dawn of HVDC transmission with Current Source Converters and over the years they have shown their own ability to withstand the overstressing electric field values that arise during a polarity reversal. Cable manufacturers give restrictions on the number of polarity reversal that a cable can sustain and furthermore they give procedures to pursue in order to preserve cable life under such conditions. A phenomenological ageing model under fast polarity reversal is illustrated in this paper giving some information about its parameters. Besides, a space charge measurement test method to be applied during laboratory tests on full sized cables is illustrated in line with the IEEE 1732 standard. Aim of this paper is to merge the two approaches since the outcomes of space charge measurements on full sized cables can help in the parameters inference of the polarity reversal ageing model.

The working transformer oil in practical converter transformers stays flowing state to cool the windings. In this paper, the partial discharge (PD) characteristics of metal particles in flowing transformer oil under DC voltage was investigated based on the time-resolved-partial-discharge (TRPD) patterns. On one hand, it was found that the voltage polarity had no obvious influence on the characteristics at any flow rate. On the other hand, with the flow rate increasing, the discharge number had a decreasing trend and the distribution of PDs in the patterns became more and more scattered. Besides, the PDs with a shorter time interval and lower discharge magnitude were the majority, regard less of the flow rate. The total discharge number was extracted and it significantly decreased with the flow rate. To explain the variation, the particle trajectories were simulated based on a two-phase flow model. The simulation results showed that as the flow rate increased, the collisions between particles and electrodes decreased, thereby reducing the discharge number.

Recent advances in energy-harvesting techniques have allowed wearable and IoT devices to operate without batteries. A thermoelectric generator (TEG) is one energy source that can provide a bipolar voltage that is proportional to the temperature difference between its two sides. Most of the prior TEG-harvesting systems could handle the positive input voltage [1–3]; however, the TEG’s input voltage polarity may reverse due to environmental changes. Only a few prior publications provide a method to deal with the bipolar input voltage [4–6]. To harvest bipolar input energy, a switch-matrix-based boost converter is proposed in [4], but it cannot self-start and dynamically track the maximum power point. [5] and [6] provide methods for bipolar self-startup with an ultra-low input voltage, but both of them suffer from a low conversion efficiency. Hence, there is no complete system suitable for bipolar-input TEG energy harvesting. A solution must address three challenges: (1) the transfer of energy from a bipolar input voltage via a high-efficiency converter; (2) system self-start-up with a reasonably low bipolar input voltage; (3) a maximum power point tracking (MPPT) method for bipolar input voltage. This paper proposes a bipolar-input boost/flyback hybrid converter (BFHC) with an on-chip cold start-up for TEG energy harvesters.

In this paper, a new buck-boost dc-dc converter, which is applicable to renewable energy sources, is proposed. The proposed converter is capable of delivering the desired output voltage over a wide range of input voltage variations. Depending on the load requirements, by changing the duty cycle of the switches, the proposed converter can operate as a buck-boost converter. In this converter, the output voltage polarity is the same with the input voltage polarity, and the input has a common ground with the output. The voltage gain of the proposed converter is high compared to similar structures. The proposed converter is thoroughly analysed under CCM operation and simulated in the PSCAD/EMTDC software environment to confirm the mathematical analysis.

The reversal of voltage polarity is essential in HVDC cable systems with Current Source Converters (CSC), since it enables to revert the direction of the power flow. In a previous paper by the authors a life model for HVDC cables subjected to voltage polarity reversal was developed and applied for predicting the life of HVDC cables subjected to a voltage polarity reversal test. In that model, a heuristic procedure was developed for determining the relaxation time constant for the redistribution of space charges within the insulation after voltage polarity reversal. In this paper the model is refined according to a novel approach based on manufacturers’ experience with MIND HVDC cables.