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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.

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.

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.

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.

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.

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.

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.

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.

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.

As large as 4607 cm−1 gain coefficient in two beam coupling experiment was obtained by introducing PVK:C60 film to ZnSe assisted liquid crystal system. As short as 5.0 ms holographic recording time was reached when probing the grating formation process, showing great potential in real time applications. Systematical two beam coupling and grating probing experiments were performed in studying the mechanism behind the high photorefractive (PR) performance. Unusual energy transfer direction change and gain coefficient fluctuation were observed when the voltage polarity and incidence side were altered in the related two wave coupling experiments.

This paper concerns the studies on streamer propagation in mineral oil, synthetic ester and natural ester at standard lightning impulse inception voltage in small point-plane gaps. The inception voltages of both polarities were measured using step method and then analysed statistically. Simultaneously, streamer characteristics such as shape, emitted light and propagation velocity were compared between the liquids tested. The obtained results showed that the inception voltages were close to each other for all the three liquids tested. Concerning streamer characteristics, it was seen that the spatial shapes of the streamers were similar taking into consideration the given voltage polarity. The same may be said about the propagation velocities, indicating that the velocities of the positive streamers were twice as high as the negative ones. In the case of registered time courses of emitted light, the frequencies of the light pulses corresponding to the propagation of the streamers in both esters were a little bit higher than the pulses emitted by the streamers, which developed in mineral oil. In general, in all three liquids the streamers at inception voltages developed as so-called `stopping length' streamers.

Plasma technology is a highly promising and advantageous technology for liquid processing. In air in-contact with water, plasma produces highly reactive species (ions, electrons, radicals, photons, etc) that diffuse into the water volume and initiate physical and chemical phenomena of interest, e.g. organic and inorganic pollutant degradation. In this study, we investigate the influence of basic parameters, such as voltage polarity, voltage amplitude, plasma lifetime, and air-gap distance, on the properties of a discharge in air in-contact with deionized water. Specifically, we analyze the electrical characteristics, the plasma behaviour at the water surface, water acidity and conductivity, and the decoloration rate of a standard organic dye (methylene blue). The concentration of the main reactive oxygen and nitrogen species produced in water is also reported. Compared to positive polarity conditions, the negative polarity voltages enhance the decolaration rate of methylene blue. For instance, under negative polarity voltages and while applying 4 and 6 kV, the decoloration rate is relatively low (

This paper describes experimental studies on streamer propagation in synthetic ester and mineral oil under lightning impulse voltage. These studies are concentrated around the small electrode gaps (15 and 20 mm) of the point-plane electrode arrangement. The spatial shapes of the developing streamers, oscillograms of light emitted and propagation velocities were analyzed and compared. Two streamer propagation modes were observed during the studies performed. Slow 2nd mode streamers developed below the so-called acceleration voltage while fast 3rd mode streamers developed at acceleration voltage and above. Independently of the liquid used the registered streamers shapes corresponding to a given voltage polarity were identical. Concerning the light oscillograms, higher frequencies of light pulses were observed in the case of synthetic ester. A significant difference between both the considered liquids was however noticed in the threshold value of the testing voltage at which the 3rd mode streamers appeared. Fast positive streamers started to develop in synthetic ester at lower testing voltage (of about 26 kV for 20 mm gap and 20 kV for 15 mm gap) than in the case of mineral oil. For negative polarity of lightning impulse, within the applied testing voltages, 3rd mode streamers were observed only in synthetic ester. For both electrode gaps acceleration voltage for negative polarity was 2.2 of estimated inception voltage. This allowed forming the conclusion that synthetic esters have a lower ability of preventing the development of fast and energetic streamers, even at small (15 and 20 mm) electrode gaps.

In this study, investigation of the unipolar resistive switching (RS) of InGaZnO film indicates that the reset voltage polarity can strongly affect the RS region. The dependence of the RS characteristics on the top electrode (Pt, Al, or Cu) was investigated. Asymmetrical electrodes (Cu/InGaZnO/Al) were chosen and the dependence of the RS parameters (e.g., high and low resistance states, set and reset voltages) on these two diverse electrodes can provide two indicators to trace the location of the RS region. The RS region is usually located near the anode when the applied set and reset voltages have the same polarity. In comparison, when the reset voltage has the opposite polarity to the set voltage, the RS region prefers to be located near the anode of the reset process rather than the cathode (which is used as the anode in the set process), indicating the movement of the RS region during the reset process. With the aid of joule heating, oxygen ions can overcome the energy barrier during the reset process under small voltage, resulting in the CFs restructuring and having the inverse shape, which is responsible for the drift mechanism of the RS region.

The active-forced-commutated (AFC) bridge employs a symmetrical thyristor-bridge with auxiliary self-commutated full-bridge chain-link (FB-CL) circuit to assist its soft transition and forced commutation. This combination can form a thyristor based voltage source converter (VSC) with significantly reduced on-state losses and dc-fault blocking capability. Due to the full topological symmetry of the AFC-bridge, either current direction or dc-link voltage polarity can be reversed for power flow reversal as for the full-bridge modular multilevel converter (FB-MMC). Thus, the AFC-bridge is compatible with both line-commutated-converter (LCC) and VSC terminals in a multi-terminal high voltage direct current (MT-HVDC) network. This paper investigates its front-to-front (F2F) dc-dc application for matching the regional dc grids in a LCC and VSC hybrid HVDC network. Simulation studies are carried out to demonstrate its potentials as a high efficiency multi-functional solution for dc-dc conversion.

We demonstrate the coexistence of the unipolar and bipolar switching behaviors in a single oxide resistive device composed of Pt-CuO0.8-W structure and we reveal the switching mechanism by the comprehensive analyses of both unipolar and bipolar switching behaviors. The observations suggest that the RESET (turning OFF) mechanism of unipolar and bipolar switching modes can be explained by a unified scheme. The results also elucidate that the SET (turning ON) process in bipolar mode is originated from by the drift of ions though charged vacancies, while the SET process in unipolar mode is explained by electron tunneling through vacancies. The proposed model successfully explains the observations of resistive switching of our device regardless of input voltage polarity and, more importantly, we propose a way to selectively choose one specific operation mode.

The effect control voltage polarity has on the diffraction characteristics of photonic structures formed in polymer dispersed liquid crystals (PDLCs) is discussed. The possibility of controlling these characteristics with an alternating electric field is demonstrated.

The switch mode power conversion circuits buck, boost, and buck-boost incorporate a power inductor as an energy storage device. If the power inductor in these fundamental circuits is replaced with a transmission line, new power conversion circuits will emerge. By introducing microwave properties, such as propagation delay and characteristic impedance, new functions may be feasible in the area of power conversion. Examples of such functions could be inverting and noninverting voltage polarity abilities or circuits, which share switch components between multiple output voltages reducing the number of semiconductors needed. Alternatively, the buck-boost power converter circuit may give rise to new high-efficiency radio circuits in the area of microwave technology.

ThefilamentarygrowthandswitchingmechanismofSiOx-basedresistiveswitchingrandomaccessmemorieshavebeeninvestigated. In test structures with two devices connected in series, it has been found that the applied voltage in the first electroforming sweep equally stressed the SiOx in both devices. Therefore, it is concluded that a conducting filament (CF) grows in both devices. The resistance switching is induced by the rupture and recovery in a weak spot of the CF. The weak spot formation is a random process in the CF and could be anywhere along the CF. Once the weak spot is established, the rupture and recovery is localized in that particular region and independent of applied voltage polarity.

We demonstrate strong, analog, reversible, and nonvolatile memcapacitance in a Si-based MOS (metal-oxide-semiconductor) memcapacitor with an ITO (In-Sn-O)/HfOx/Si structure. Both accumulation and depletion capacitances change sequentially and reversibly upon repeating voltage application with respect to voltage polarity. This memcapacitance is thought to be induced by oxygen ions' migration between ITO and HfOx layers, which changes the HfOx permittivity and the depletion states in Si and ITO. The Si-based memcapacitor has potential to be applied to the gate stack of the MOS field-effect-transistor for nonvolatile memory and synaptic transistors through modulating drain current determined by the capacitance change of the MOS gate stack.

Breakdown and corona are investigated in clean and dust-polluted rod-rod air gaps varying gap length, rod radius and voltage polarity. It is shown that similar to previous results under impulse voltages, dust pollution can significantly reduce the DC breakdown voltage values of such gaps which depends upon electrode radius, gap length and voltage polarity. These factors should therefore be considered prior to the design and application of air insulation in desert regions, where sand and dust storms can occur frequently. Possible mechanisms which control the observed behaviour are explained in detail.

This paper is a first step towards the evaluation of the life and reliability of HVDC cable systems in the presence of voltage polarity reversals. It combines a physical-phenomenological life model for HVDC cables subjected to voltage polarity with a recently-developed life model for HVDC cables subjected to load cycles. Under some meaningful approximations, a quite powerful model that accounts both for load cycles and voltage polarity reversal is eventually attained. Such model can be applied in order to predict the life of HVDC cables subjected to voltage polarity reversal tests with load cycles superimposed, performed in line with CIGRE Recommendations after Electra 189, 2000. By doing so, results in agreement with the experience gained by Terna (the Italian TSO) in testing HVDC cables - in particular MIND cables - are attained.

The sparkover behaviour of sphere-rod gaps under standard impulse voltages is investigated by assessing the influence of gap spacing, impulse voltage waveshape and polarity, and atmospheric conditions on the distribution of the sparkover voltage. At sparkover the growth of streamers of both polarities can be estimated since a value of 2 MV/m is found to be characteristic for the average negative streamer gradient. The IEC atmospheric conditions correction is discussed and compared with a new optimised correction procedure. The feasibility of using the sphere-rod gap as substandard for measuring impulse voltages, replacing the conventional more expensive sphere gap, is examined.

Evaluation of insulation strength for lightning surge that actually enters into substations is important in estimating insulation reliability of gas-insulated equipment. The standard lightning impulse voltage (1.2/50 µs) is used for factory tests. However, the actual lightning surge waveforms in substations are complex and are usually superimposed with various oscillations. Insulation characteristics of SF6 gas as a function of such complex voltages have not been sufficiently clarified. This paper deals with gap breakdown characteristics in SF6 gas under submicrosecond pulses. Breakdown voltages are lower under a polarity reversal condition than under a monopolarity condition. The cause of this difference is discussed while observing discharge propagation using an image converter camera. The electrode size effect is also discussed. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 146(4): 18–25, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10246

A theoretical model of the contact metal topography of a photoconductive cell and a thermal model of an optoelectronic switch of the voltage polarity are considered. Expressions for photo and thermal emf of photoconductive cells with arbitrary contact metal topographies and distributions of the illumination and temperature at the cell surface are obtained. The regularities of minimization of parasitic emf of photoconductive cells are revealed. An optoelectronic switch of the voltage polarity with low levels of temperature (

This paper describes the molecular alignment behavior of polymer networks with switching of a ferroelectric liquid crystal (FLC) in a molecularly aligned FLC/polymer composite film. The polymer alignment in the composite film, which was slowly formed by photopolymerization-induced phase separation of a heated nematic-phase solution of FLC and monomers, was observed by polarization Raman spectral microscopy. Raman peak intensities originating from the polymers were changed with those from the FLC, when the applied voltage polarity was changed. The trace patterns of the Raman peak intensity with in-plane rotation of the composite film indicated that the formed flexible polymers can follow FLC molecular switching.

This paper presents the results of the studies concerning propagation of the streamers in natural and synthetic ester under lightning impulse 1.2/50 μs at inception voltage. The results are presented in the form of comparison with the results obtained for mineral oil at the same conditions. For the different interelectrode gaps changed from 5 to 20 mm, the inception voltage and propagation velocity were measured and analyzed. Additionally, the oscillograms of light emitted during discharge development under inception voltage and shapes of discharge forms recorded photographically for all three dielectric liquids, were compared. The obtained results showed that at the inception voltage both the light courses and shapes of discharge forms are very similar in all three cases, depending on voltage polarity. Identical values are also observed in the case of propagation velocity of the streamers and in the case of inception voltages determined statistically using maximum likelihood method.

Pt/TiO2/Pt/Ti memristive devices were electrically formed to either the ON or OFF state using voltages of the same polarity but with different amplitudes. The forming step dictated the subsequent switching behaviour. A qualitative model based on the creation and migration of oxygen vacancies was proposed to explain the experimental results.

To better understand the mechanisms of creation and rupture of conductive filaments in resistive switching devices such as Cu/TaOx/Pt, with Cu and Pt being the active and inert electrodes, respectively, a device with limited supply of active metal electrode has been manufactured and electrically characterized. The limited supply of active metal has been realized by depositing a thin (delta) Cu layer (δ-Cu), 6 nm and 12 nm thick, on TaOx, resulting in a Pt/δ-Cu/TaOx/Pt device structure. The limited active metal supply i) has a direct impact on the onresistance (Ron) of the Cu bridge, and leads, after several conventional set-reset cycles, to ii) pulsating behavior, when device turns on and off repeatedly, to iii) symmetric switching behavior with respect to applied voltage polarity, when the device can be set and reset both at positive and negative bias, and to iv) volatile switching behavior.

This paper reports on systematic measurements of voltage-time curves of a 25/70-mm coaxial-cylinder SF6 gap. Tests were carried out with 0.5/50-μs and 1.2/50-μs voltage impulses, for SF6 pressures in the range 0.1–0.3 MPa. Times to breakdown were measured in the range of 0.2–7.0 μs, paying due attention to the random nature of breakdown delay and its principle components, namely the statistical and formative time lags. Gas pressure, voltage polarity and field concentration due to the presence of spherical particle attached to the high-voltage electrode were systematically investigated with respect to their effects on breakdown delays. The results of this study could be significant from the point of view of the insulation coordination of gas-insulated systems.

A new working principle for electrochemical micropump with the gas termination time as short as 100 microseconds is presented. It is based on water electrolysis with a fast change of voltage polarity. A simple electrochemical micropump is designed to demonstrate this pumping principle. The device consists of a working chamber with metallic electrodes, inlet and outlet diffusers, and channels for liquid. The chamber and the channels are filled with the electrolyte that plays a role of the pumped liquid. The pump was tested in different regimes. One of these regimes related to formation and termination of short-lived microbubbles is especially promising. Long time stability of the electrodes is demonstrated.

In this paper, a novel repetitive frequency voltage booster (named repetitive Fitch booster by the authors) based on magnetic switches and Fitch generators is proposed. The principle of operation is to charge capacitors in parallel when magnetic switches (MSs) are unsaturated and reverse voltage polarity of every other capacitor when MSs saturate. With the principle, circuit topology of a 4-stage repetitive Fitch booster (RFB) is presented. Simulation as well as experiment shows its feasibility in boosting voltage and compressing rise-time. In simulation, the input voltage of 100 V is boosted to 372 V, while test stand yields output voltage with frequency of 1 kHz, amplitude of 19 kV with each capacitor charged to about 5.6 kV, and rise-time compression from 7.3 μs to 700 ns. Meanwhile, calculations show that the 4-stage RFB effectively reduces core volume by about half, from 1093.5 cm(3) to 585.2 cm(3). Furthermore, design rules are proposed so that topologies of RFBs with stages other than four can be conveniently derived. As an example, an 8-stage RFB is proposed and verified with circuit simulation, which shows an output voltage of 759 V with the input voltage of 100 V.

Responsivity and thermionic dark current are studied in asymmetric stepped GaAs/AlGaAs multiquantum well infrared detectors with a single bound state. Although attributed to different transport properties, both the photoexcited and the thermally emitted carriers exhibit asymmetric transport properties with respect to the voltage polarity. The experimental characteristics are explained by the minibands-quasi continuum energy structure above the barrier.

We show that bias polarity can greatly accelerate device failure in GST- based (GeSbTe) PCM devices, and trace this effect to elemental segregation, initially driven by bias across the melt but then enhanced during the crystallization process. Implications include device, pulse, and materials design for high endurance.

This paper is an experimental study of a pulsed filamentary plasma discharge inside liquid water in pin-to-plane electrode configuration. Time-resolved electrical and imaging diagnostics have been performed. The initiation and the propagation of the discharge have been studied for several experimental parameters. The propagation is continuous and is followed by reilluminations at low water conductivity. The measured propagation velocity of the plasma discharge is 30 km s−1for the secondary positive mode. This velocity was found to be surprisingly constant whatever the experimental parameters and especially as a function of the water conductivity.

Development of negative and positive streamers along an ice surface in short rod-plane gaps has been investigated experimentally and compared. Major characteristics of streamers including inception voltage, propagation velocity, breakdown voltage, time to breakdown and light emitted during their development are studied. Results showed that these characteristics are largely influenced by the voltage polarity under same experimental conditions. In negative polarity, it was found that the propagation velocity and the time to breakdown are significantly higher and the voltage required to initiate the streamer is lower. It was also found that the influence of the ice surface properties is more intense in negative polarity. High-speed camera recordings also showed different processes in the discharge development for negative and positive polarities.

This paper describes the experimental results on the creeping streamer progressing the surface of a bakelite board immersed in transformer oil under the application of a standard lightning impulse voltage with peak values of plusmn100~350 kV. The growth of streamers is significantly affected by the thickness of bakelite board and the streamers are extended longer exponentially for the applied voltage more than ~plusmn200 kV. These streamers leave the remarkable discharge scars on the bakelite surface. The discharge scars are different by the voltage polarity which influences strongly to the streamer growth. The mean streamer velocities in both polarities have also been examined.

Results are presented concerning prebreakdown phenomena and breakdown in short rod-plane gaps in air under lightning impulse voltages of both polarities. Breakdown probability distributions were obtained and the discharge parameters such as first and second corona inception time and voltage, dark period, time and voltage required for the discharge to arrive at the earthed plane and for breakdown were measured as a function of gap length and impulse voltage polarity. Breakdown under negative impulse voltages is closely associated with the prebreakdown phenomena since there is a close correlation between coronas inception and breakdown voltages; this is not always the case under positive impulse voltages. For both polarities there is a tight correlation between the breakdown and discharge arrival voltages. At 50% breakdown probability, the mean electric field required for the discharge to arrive at the plane is roughly two times higher under negative than positive impulse voltages and for both polarities it decreases with gap length according to a power law. These explain the similar dependence of the 50% breakdown voltage on polarity and gap length.

Looking to reliability of high-k dielectrics, their behavior under electrical stress is very often determined by traps and charges. During the past years a strong focus of the reliability investigation was given to HfO2. However, especially LSTP as well as memory industry is going to use a silicon doped hafnium oxide (HfSiOx). Three different cases of current vs. time dependence are observed with constant voltage stress for HfSiOx. Often the current decreases for short stress times because of charging. This period can be followed by either barrier lowering due to electron detrapping or stress induced leakage current (SILC) (Duschl, 2006). Both mechanisms raise the current level at constant voltage. In the SILC case the increase of leakage can go step wise, whereas for barrier lowering the leakage increases gradually even on small areas. However, although barrier lowering and charging are considered to be reversible by alternating voltage polarity SILC is often not reversible because it introduces damage and high resistance break down spots in the dielectric. During measurement SILC can even remain undetected if barrier lowering and/or charging are dominant. Recently, it was shown that barrier lowering for these HfSiOx is the major contribution to leakage increase over stress time (Duschl, 2006). In the following paper the authors focus on the question if a small SILC contribution behind the dominant measured barrier lowering can become a reliability concern. In a first part major effects of SILC in HfSiOx with EOT>1.5nm and TiN gate are discussed. In a second part we analyze whether SILC damage can be hidden by barrier lowering or not. Finally we are coming to the conclusion, that a sample with leakage increase due to barrier lowering does not exhibit severe SILC in parallel

An additional control scheme has been proposed in this paper for a dynamic voltage restorer (DVR), to protect it against load short circuit conditions. When overcurrents occur in the distribution system, under the proposed scheme the DVR reverses its injected voltage polarity so as to minimise the current flow. The detection method is based on impedance measurement feedback. The advantage of the scheme is that no additional over current device or protection is required for the DVR and it is easy to implement. The proposed control scheme has been validated through simulation.

A comprehensive study has been carried out into the breakdown characteristics (jitter, delay, dV/dt) of pulsed charged triggered spark gaps filled with SF/sub 6/ gas mixtures, for voltages up to 200kV. The influence of the combination of trigger voltage polarity and main gap voltage polarity has been investigated and the results are discussed with reference to the mechanism of discharge initiation and arc formation. A similar study has been carried out with mixtures of SF/sub 6/ and air, N/sub 2/ and He. The results emphasise the importance of trigatron voltage polarity and also reveal that several SF/sub 6/ gas mixtures possess improved switching characteristics compared to those of SF/sub 6/, without a reduction in withstand voltage. The delay time to breakdown results for SF/sub 6/ and SF/sub 6/ gas mixtures have been recorded at several pressures and compare well with predicted results according to Martin (1).

Based on the tested results carried out on iced 10 kV DC composite insulators in the artificial climate chamber of the High Voltage Laboratory of Chongqing University, in this paper it was obtained that the relations among the flashover voltage of iced 10 kV DC composite insulator and atmospheric pressure, ice amount accreted. As well as the effects of voltage polarity on flashover voltage were gained in colligated condition of low atmospheric pressure and icing at high altitude districts. The hydrophobicity of 10 kV composite insulator under three different conditions has been analyzed.

The use of high voltage triggered vacuum switches (TVS) in a crowbar system for the load protection is described. TVSs can most sufficiently satisfy the main switch demands in a crowbar. New generation of compact TVSs with up to 500 kA and 50 kV switching current and voltage is presented. Two ways of increasing the hold off voltage is studied: one-gap device construction and connection of tubes in series. New effective methods of several sequentially connected TVS operation is discussed. Dependent triggering is used but time delay does not exceed two switching-on time. Applied voltage polarity does not matter. The design and test results of 80 kV and 130 kV crowbar systems based on two and three series TVS are presented. To simplify the crowbar utilization the system is supplied with current sensor and triggering unit.

This paper is aimed at the influence of the hydrostatic pressure on the discharge propagating at the liquid/solid insulating interfaces under AC and DC voltages using a point-plane electrode arrangement. It is shown that different shapes of creepage discharges and associated currents can be initiated for the same experimental conditions indicating the existence of different propagation modes of discharges. On the other hand, the final length and the number of creepage discharge branches, the corresponding currents and the light emitted are reduced when a hydrostatic pressure is applied; this indicates the implication of gaseous processes in the creepage discharge phenomena. The final length of discharges increases quasi-linearly with the voltage, whatever the type (DC or AC) and polarity of the voltage, and decreases quasi-linearly when the pressure increases.

This work presents a physically-based model for anode hole injection which explains both the voltage polarity asymmetry and sub-threshold behavior of Fowler-Nordheim (FN) stress generated oxide damage down to low V/sub G/. Also, we have developed an n-well bias technique for PFETs which clearly establishes that this FN stress-induced damage is due to anode hole injection.

A creepage failure which may generate at the wire supporting points of the distribution lines by the invasion of a lightning surge was simulated in the laboratory. The properties of creepage discharges which develop along the wire surface have been studied using the impulse voltage with various duration of wave fronts Tf. The effects of the voltage polarity and Tf on the developing length and aspect of the creepage discharge are shown in the present paper. The properties of positive creepage discharges were not affected by Tf, but a remarkable effect of Tf was found when the negative creepage discharges developed along the wire surface. These results indicated that the lengths of discharges were closely related to the aspects of them.

This paper reports the results of a laboratory study on the first visible discharges developed on ice surfaces with micro air-gaps, using ultra high-speed photography. Two metallic hemispherical capped rods, 12 mm in diameter and half submerged in ice, were used. A relatively short air-gap was made at different locations in the ice, between the electrodes. The development of visible discharges, under lightning impulse voltage as well as the 50% impulse lightning breakdown voltage were determined as a function of air-gap position and voltage polarity. The results found will contribute to a better understanding of combined air-gap and ice surface flashover, and could help for the design of insulators destined to cold climate regions.