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In this paper, initiation behaviors of DC electrical tree in silicone rubber were studied under different voltage polarities and temperature gradients (temperature of needle and ground electrodes ranged from 20 to 120 °C). Electrical tree initiation probability, tree shape, and tree length were obtained via the statistics. The results show that polarity effects, where the positive DC voltage is easier for trees to initiate, appear evidently under various temperature situations. As for the temperature gradient, tree initiation voltage decreases first and then returns to a larger value with the increasing needle temperature when the ground electrode temperature is fixed at 20 °C. Both the probability of multiple-branch tree and corresponding tree length increase and subsequently decrease. However, tree initiation voltage decreases alone and tree length increases with the rise of ground electrode temperature when the needle temperature is fixed at 20 °C. Homo charge injection becomes easier with increasing needle temperature following the Schottky-Richardson approximation under ramped DC voltages. In addition, electric field strength at the tip decreases when the needle temperature is higher than ground temperature, since the conductivity is sensitive to the local temperature and electric field. It is believed that the needle temperature and the temperature gradient dominate tree initiation behaviors, where the former affects the collisional ionization and partial relaxation of silicone rubber chains, and the latter decides electric field distribution in the bulk. [ABSTRACT FROM AUTHOR]

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 Fr?hlich'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. [ABSTRACT FROM PUBLISHER]

Heretofore, unlike HVAC transformers, there is not much experience in both insulation design guides and test methods for ultra HV converter transformers. This special insulation phenomenon is very prominent due to the complication of voltage applied on valve-side windings of converter transformers. There is a severe lack of empirical research on partial discharge in oil-paper insulation under the combined ACDC voltage. The study in this paper provides a timely research into the issues. Nine possible configurations, which intent to simulate different defects in the transformer, have been investigated. The PD inception voltages varied ratios of combined AC-DC voltages were conducted. The discharge magnitude and its relation with the applied voltage for selected four common configurations were obtained during voltage increasing and decreasing respectively, which presented the PD initiating process and also reflected the insulation behavior of dielectrics. Besides, the Finite Element Analysis Soft was used to propose the effect of morphology of electrode surface (3 dimensional roughness is approximately 0.03 mm) on PD characteristics, which is very lacking in the previous references. The roles of DC and AC component in PD characteristics were discussed in the context of the relevant work in literatures. [ABSTRACT FROM AUTHOR]

In this paper, a high sensitivity electrometer is adopted to measure DC conduction currents across the oil gap. The effect of morphology of electrode surface (3 dimensional roughness is approximately 0.03 mm) on conduction current is considered adequately, which is very lacking in the previous references. Based on the Fower-Nordheim theory, three stages will be clearly distinguished with the increase of the electric field strength. At low electric field stage (E<0.44 kV/mm), the current through the oil I is directly proportional to the electric field E, which meets the Ohm's law. Oil conductivity mainly depends on impurity ions with only low mobility. At medium electric field stage (0.44 kV/mm1.33 kV/mm), the obtained I-V2 characteristics is linear, which can be explained by the space charge limited current. Moreover, main factors affecting the conduction current, including oil gap, electrode geometry and materials, polarity effect, temperature, hydrostatic pressure and moisture, have been analyzed and the causes are interpreted as well. [ABSTRACT FROM AUTHOR]

The presence of space charge is a serious threat to the reliability of insulation material under voltage polarity reversal. The results of space charge evolution in oil-paper insulation during external voltage polarity inverse at room temperature were presented in this paper. A mirror image effect charge was observed in the steady state, but the formation of it was affected by the first polarizing, which could cause a change of electrical property. The results showed that the mirror image effect was independent of the involved nature, type and dynamics of charge. Experiments of different reversal polarities and periods indicated that the slow decay of heterocharge retained from previous homocharge injection was the cause of electric field enhancement. And the reversal period therefore had an effect on voltage polarity reversal, namely the shorter the reverse period, the higher the stress enhancement. The reliability of oil-paper insulation could be influenced by the polarity reversal because charge injection and distribution were unpredictable, and space charge modified and in return was affected by the field as well. [ABSTRACT FROM AUTHOR]

Atmospheric pressure low-temperature plasmas produced by dielectric barrier discharge (DBD) provide a promising approach for civilian application of pulsed power technology. In this paper, repetitive nanosecond pulses were generated using a magnetic compression solid-state pulsed power generator, and the rise time and pulse duration of the nanosecond pulse are \sim30 and 70 ns, respectively. The DBD in open air is created using two kinds of electrodes, i.e., water and metal electrodes. The electrical, luminous, and optical characteristics of the DBDs under these two electrodes are studied and compared. The experimental results show that no filaments are observed and the discharge is homogeneous when water electrodes are used. The DBD still behaves in a filamentary mode when the discharge gap is extended to 4 cm in the case of metal electrodes. The results are validated by fast images taken by an intensified charge-coupled device camera. In addition, some discussion about the experimental results is presented. Improvement of discharge uniformity is due to the effect of resistive stabilization using water electrodes. [ABSTRACT FROM PUBLISHER]

Abstract: Space charge is a key insulation problem of HVDC equipment. In this paper, the influence of voltage polarity and electrode material were studied by the pulse electroacoustic (PEA) method. It had been found that the type of space charge accumulating at the interface was the same with the voltage polarity. With different electrode materials, the space charge injection was consequently different, but not evident. The interface made the drift velocity slower in two-layer samples. It suggested that the interface had a significant impact on space charge distribution and the contact state was complex, so a modified Maxwell–Wagner model was proposed. [Copyright &y& Elsevier]

Pulsed power can provide instantaneously high power density to generate atmospheric nonthermal plasma in open air. In this paper, using two power supplies including a \(\mu \) s-pulse generator with a rise time of 300 ns and a pulsewidth of 5 \(\mu \) s and a ns-pulse generator of a rise time of 70 ns and a pulsewidth of 100 ns, pulsed gliding discharges are created with point-to-point electrodes in open air. Discharge images with these two generators under different flow rates are presented for a brief comparison of \(\mu \) s- and ns-pulse gliding discharges in air flow. [ABSTRACT FROM PUBLISHER]

Dielectric-barrier discharge is an important method to produce nonthermal plasma and can be produced by an alternating-current (ac) or pulsed power source. In this paper, by using two power supplies including a 40-kHz ac power source and a 1-kHz unipolar nanosecond-pulsed power of a rise time of 40 ns and a duration of 70 ns, the discharges in open air are created using two liquid electrodes. Images under different conditions are presented for a brief comparison of ac and nanosecond-pulsed excitations. [ABSTRACT FROM AUTHOR]

Nanosecond-pulse dielectric barrier discharge (DBD) is a promising method to produce a homogeneous plasma in atmospheric air. With an excitation of unipolar repetitive pulses with 40-ns rise time and 70-ns duration, the discharge in open air is created using two liquid electrodes. Images taken by a high-speed ICCD camera with an exposure time of 2 ns show that the nanosecond-pulse DBD can generate a homogeneous atmospheric-pressure plasma in air. [ABSTRACT FROM AUTHOR]

In this paper, gliding discharges with a point-to-point electrode geometry were produced by a repetitively pulsed power supply with a rise time of ∼100 ns and a full-width at half-maximum of ∼200 ns. The characteristics of such discharges were investigated by measuring their voltage–current waveforms and taking photographs of their discharge images. Experimental results showed that once the breakdown occurred, the nanosecond-pulse gliding discharges went into a stable stage at all air gaps, behaving in a mode of repetitive sparks. Under certain conditions, a non-stable stage would appear some time after the discharge went into the stable stage, in which the gliding discharges transitioned from repetitive sparks to diffuse discharges. Furthermore, several factors (gap spacing, pulse repetition frequency (PRF) and gas flow rate) influencing the discharge characteristics were investigated. It was observed that both the breakdown voltage and ignition voltage increased with the gap spacing, and a diffuse discharge was absent when the gap spacing was less than 6 mm. The breakdown voltage decreased with the increase in the PRF and its decrease ratio was larger in large gap spacing than in small gap spacing. Discharges would transit from repetitive sparks to diffuse discharges as the flow rate increased. Furthermore, a comparison of nanosecond-pulse and ac gliding discharges was conducted with respect to the power supply. The consumption and energy, the relationship between the power supply and the load, and the time interval between two pulses were three main factors which could lead to different characteristics between the nanosecond-pulse and ac gliding discharges. [ABSTRACT FROM AUTHOR]

Dielectric barrier discharge (DBD) excitation by unipolar high voltage pulses is a promising approach for producing non-thermal plasma at atmospheric pressure. In this study, a magnetic compression solid-state pulsed power generator was used to produce repetitive nanosecond pulses for the excitation. The DBD is created using two liquid electrodes. The electrical characteristics of the discharge voltage and current are illustrated under different experimental conditions. The nanosecond-pulse discharge current is of the order of tens of amperes. This differs from common DBD current excitated by high-voltage ac sources. Compared with the characteristics of two current pulses corresponding to two discharges for unipolar pulsed-excitation, the secondary discharge in this study is minor owing to the pulsed power and discharge configuration. Under the experimental conditions, the luminous emissions from the front and side views of the liquid electrodes show that no filament is observed and the discharge is homogeneous and diffuse in the whole discharge region. The effects of applied voltage amplitude, repetition rate, and air gap spacing on the discharge characteristic are investigated. The discharge mode does not change with the variation of the investigated parameters. A comparison of high voltage ac and nanosecond-pulse excitation is also presented. In addition, discussion of the experimental results is presented. [ABSTRACT FROM AUTHOR]