Needle-plate Electrode Research Articles

Temperature-dependent electrical tree in silicone rubber under repetitive pulse voltage

Silicone rubber (SiR), which is employed as the main insulation materials in the cable accessories, faces the challenge of high temperature during the overload running process. Pulse voltage is inevitable during the switch operation process of the converters or lighting, which aggravates the operation state of the insulation materials. To understand the electrical treeing process at the high temperature with repetitive pulse voltage considering the actual operating state, the needle-plate electrode system was stressed in the experiment. In the test, the ambient temperature (Tamb) was set to 30, 60, 90, 120 and 150 °C, while the pulse frequency was 5, 20, 100, 200 and 1000 Hz. Electrical tree shapes, tree length, fractal dimension (FD), tree inception and tree breakdown probability were studied to investigate electrical tree growth characteristics. The results indicate that the tree shapes are depended not only the temperature environment but also the pulse frequency. Higher pulse frequency and higher temperature facilitate bush tree shape to be generated. With the pulse frequency increasing, the tree length becomes longer, while FD tends to be larger. However, the tree length decreases as the FD increases with the temperature elevating at the same pulse amplitude. As more hot electrons to collide the molecular chain with pulse frequency increasing, the tree inception and breakdown probability become higher. Nevertheless, the tree inception probability tends to be larger as the mechanical property of silicone rubber drops with the temperature elevating, while breakdown probability becomes lower as the shorter tree length grows for a given pulse number.

Effect of Cryogenic Temperature on Tree Characteristics in Silicone Rubber/SiO2 Nanocomposites Under Repetitive Pulse Voltage

Silicone rubber (SiR) is an important insulating material in high-voltage cable accessories. Electrical tree is a typical electrical aging inside SiR insulation and will cause breakdown when it reaches the ground. The tree characteristics may be changed by the cryogenic temperature because of the changing physicochemical properties. Besides, at temperatures above 273 K, addition of nanoparticles into SiR has been proved to have the effect of improving insulating properties. However, the effect of temperature and nanoparticles from 273 to 77 K under repetitive pulse voltage has not been researched. In this paper, the samples were prepared by mixing nano-SiO2 into liquid SiR and the proportion was set to be 0 wt%, 0.5 wt%, 1.0 wt%, 1.5 wt%, and 2.0 wt%, respectively. The experiment temperature was set to be 243, 213, 183, and 77 K. Repetitive pulse voltage was employed to generate electrical trees between the needle-plate electrodes. The growing process of electrical tree was recorded by a microscope system and pictures were captured. The experiment results indicated that both the cryogenic temperature and nanoparticles influence the treeing process in SiR/SiO2 nanocomposites.

Effect of Magnetic Field on Electrical Treeing Behavior of Silicone Rubber at Low Temperature

The development of high temperature superconducting (HTS) materials promotes the development of HTS cable. With the development of HTS cable technology, superconducting insulating materials under magnetic field have been taken seriously. This paper presents the study of the relationship between magnetic field and growth characteristics of the electrical tree in silicone rubber. The test was done under -30 °C, -60 °C, and liquid nitrogen temperature. Needle-plate electrode was employed in this paper to generate electrical tree, and silver needle was employed to avoid stress generated by the magnetic field. Samples were applied pulse voltage of +12 kV and +15 kV. 0, 200, and 400 mT magnetic field density (MFD) was applied, respectively. The direction of the magnetic field and the electric field was vertical. Using a digital microscope captured the morphology of the tree. The fractal dimension and the accumulated damage were measured. It is observed that the magnetic field promotes the increase of accumulated damage and tree width increase rapidly with the increase of MFD. The magnetic field will increase the probability of the electrical tree in liquid nitrogen. This shows that the partial discharge activity in the tree channel is influenced by the Lorentz force, the treeing behavior will be different as a result.

Hydroxyl Radicals and Hydrogen Peroxide Formation at Nonthermal Plasma–Water Interface

This paper investigated hydroxyl radicals and hydrogen peroxide formation under a needle-plate electrode configuration using positive-polarity dc discharges generated in air, nitrogen, and helium. The discharge mode in air and nitrogen was found to change above ultrapure water; initially, a nanosecond pulse discharge was observed, transitioning to a diffuse discharge due to the increasing conductivity of the water. The discharge in helium was a nanosecond pulse discharge and the repetition rate increased with increasing water conductivity. It was found that hydroxyl radicals contribute to 7%, 78%, and 70% of hydrogen peroxide formation when using the ultrapure water in air, nitrogen, and helium, respectively. It is suggested that hydroxyl radicals are formed by water reactions with energetic positive ions and the neutral particles, such as N2+, He+, O, H, and HO2. Part of hydrogen peroxide is directly formed from atoms and radical reactions with water in nitrogen and helium, while oxygen reactions are heavily involved for hydrogen peroxide formation in air. A fluorophotometry method, using terephthalic acid, was used to directly quantify the formation of hydroxyl radicals and compared with the tert-butanol method.

Numerical Study of Pulsed Dielectric Barrier Discharge at Atmospheric Pressure Under the Needle-Plate Electrode Configuration**supported by National Natural Science Foundation of China (No. 11405022)

In this paper, we study the characteristics of atmospheric-pressure pulsed dielectric barrier discharge (DBD) under the needle-plate electrode configuration using a one-dimensional self-consistent fluid model. The results show that, the DBDs driven by positive pulse, negative pulse and bipolar pulse possess different behaviors. Moreover, the two discharges appearing at the rising and the falling phases of per voltage pulse also have different discharge regimes. For the case of the positive pulse, the breakdown field is much lower than that of the negative pulse, and its propagation characteristic is different from the negative pulse DBD. When the DBD is driven by a bipolar pulse voltage, there exists the interaction between the positive and negative pulses, resulting in the decrease of the breakdown field of the negative pulse DBD and causing the change of the discharge behaviors. In addition, the effects of the discharge parameters on the behaviors of pulsed DBD in the needle-plate electrode configuration are also studied.

Study on law of negative corona discharge in microparticle-air two-phase flow media

To study the basic law of negative corona discharge in solid particle-air two-phase flow, corona discharge experiments in a needle-plate electrode system at different voltage levels and different wind speed were carried out in the wind tunnel. In this paper, the change law of average current and current waveform were analyzed, and the observed phenomena were systematically explained from the perspectives of airflow, particle charging, and particle motion with the help of PIV (particle image velocity) measurements and ultraviolet observations.

Effect of pulse frequency on tree characteristics in epoxy resin under low temperature

The superconducting magnet system is the key part of international thermonuclear experimental reactor (ITER) device which has attracted much attention in many countries. Due to its excellent performance in many aspects, epoxy resin has been selected as the base material of superconducting magnet insulation. Considering the unique operating environment, epoxy resin faces the challenge of liquid helium temperature as well as pulse voltage. The influence of pulse frequency and polarity on the electrical tree characteristics in epoxy resin under low temperature is investigated in this paper. Samples made of epoxy resin were stressed with the different frequencies in the needle-plate geometry electrodes. The frequency of pulse voltage varied from 100 to 500 Hz with positive and negative polarities. The experimental temperature was set to -30, -90 and -196 °C. Accumulated damage and expansion coefficient (D/L) are employed to characterize the electrical tree. The experimental results indicate that low temperature, pulse frequency and polarity have an influence on treeing process. The typical tree structures in epoxy resin are obviously different with the variation of low temperature and pulse frequency. It is revealed that the low temperature is resistant to the growth of electrical tree and there are more small branches generating from the main branches with the decrease of temperature. However, when under the same low temperature, the tree initiation and propagation processes are accelerated by the higher pulse frequency. The density of the tree branches increases with the increase of pulse frequency. The accumulated damage presents a similar trend corresponding to branches density. Meanwhile, electrical tree tends to grow faster with the positive pulse than negative pulse under the low temperature. Moreover, the higher value of D/L at lower temperature implies that tree branches extend at a wide range direction.

Study on discharge phenomena of short-air-gap in needle-plate electrode based on fractal theory

The process of gas discharge is very complicated and experimental observations indicate that streamers in short gap under non-uniform electric fields always exhibit irregularity and self-similarity, so a dielectric breakdown model, which is the combination of the random fractal method and the traditional streamer theory, can simulate this phenomenon.In this paper, a stochastic model with the growth probability index at any point proportional to the power of the electric field is utilized to quantify the channel tortuosity, and the space charge effect is taken into account as well. The potential distribution is solved by the Poisson's equation which is calculated iteratively by finite difference method; and the box counting method is used to characterize the channel tortuosity and estimate the fractal dimensions of the discharge channels. Based on this, an idea is proposed that the analysis of the experimental results, which in turn provide the appropriate parameters for the model, can better elucidate this phenomenon.The growth probability index can always get from the previous data, but the range of the will change under different experimental condition and there will exist differences in simulation results on fractal dimensions for different , so the limitation of the previous studies is its possible lack of generalizability. In order to define the range of the growth probability index in this model, the bifurcation phenomenon of plasma channels generated by the discharge, affected by HVDC (high-voltage direct current) of short-air-gap in a needle-plate electrode, is captured by ICCD. Before estimating the fractal dimensions of discharge channels, experimental images are saved as a binarized (black and white) image, and the gray-level transformation and boundary identification algorithm will be conducted to remove the apparent thickness of the discharge channel caused by the magnitude of the flowing currents through different branches. Experimental results show that the range of fractal dimensions in the box counting method for the discharge channel is 1.40-1.55. Under the same condition that other factors remain the same but the adjusted growth probability index in this simulation model should accord with the experimental results, all the facts demonstrate that the value of must lie between 0.04 and 0.05.

Polarity Effects of Propylene Carbonate on Breakdown Strength in Microsecond Range

We investigate the polarity effects of the propylene carbonate on the breakdown voltage using the needle-plate electrodes with gaps of 0.5, 1.0, and 2.0 mm. The devices used in this study involve a compact capacitive-energy-storage pulse power source with charging time varying from 5 ms to 20 ms and a test cell with the needle-plate electrodes. The breakdown voltage is recorded by a digital oscilloscope for each gap. The results of these three groups indicate that the positive breakdown voltage is higher than the negative one and the breakdown voltage of the PC increases with the ascending electrode gap. In addition, a simulation is conducted to support this experiment. Some explanations about the polarity effect of the PC are also given.

Electrical tree characteristics of XLPE under repetitive pulse voltage in low temperature

Cross-linked polyethylene (XLPE) may be operated under repetitive pulse voltage and low temperature environment. In recent years, many influencing factors to the treeing process, which is a main reason for the aging of XLPE have been researched. However, the tree behavior under repetitive pulse voltage and low temperature in XLPE is still not clear. In this paper, the tree behavior under repetitive pulse voltage and low temperature was investigated and analyzed. A positive pulse voltage with the amplitude of 12 kV and the frequency of 300 and 400 Hz was applied on the needle-plate electrodes. The experimental temperature was set to 30, -30, -90 and -196 °C. Four typical morphologies of the electrical trees are observed, including bush, branch, branch-pine and stagnated tree. It is revealed that low temperature has an obvious effect on tree structures and it can reduce the tree growth rate and extend the time to breakdown. It is also revealed that the tree inception probability becomes lower with the decrease of temperature. With the increase of pulse frequency, the tree growth rate, the fractal dimension and the number of tree channels increases and the tree inception probability becomes higher, but the morphologies of electrical trees stay the same.

Tree initiation characteristics of epoxy resin in Ln<sub>2</sub> for superconducting magnet insulation

The superconducting magnet system is the key part of international thermonuclear experimental reactor (ITER) device which ensures the feasibility of fusion energy application. Epoxy resin has been used as the adhesives and electrical insulation in the magnet system. Since ITER device operates in the liquid nitrogen environment and is driven by a pulse power, the insulation made of epoxy resin faces the challenges of extremely low temperature and pulse voltage. This paper investigated the influence of low temperature on the electrical tree characteristics in epoxy resin. Positive pulse voltage with the frequency of 400 Hz was applied on the needle-plate geometry electrodes. The amplitudes of the pulse voltage were 10, 12 and 14 kV. In order to simulate the practical defects, a metal needle was inserted into the samples and the distance between the tip and plane was 2 mm. The experimental temperature ranged from 30 to -196 °C. The experimental results indicate that the low temperature have a significant influence on the electrical tree characteristics. Different tree structures in epoxy resin are observed with the changing temperatures. The density of the tree branches increases as the temperature decreases, while the tree growth rate presents an opposite trend. It is also revealed that the time to breakdown become long under the low temperature. However, once the breakdown occurs, a wide breakdown channels will be left in epoxy resin, which means great destruction to the electrical insulation. In addition, when the samples are under the same ambient temperature, the probability of tree initiation shows a significant increase and electrical trees grow faster in the following period with application of higher pulse voltage. The higher-energy charges may be responsible for the high growth rate.

Experimence Study of Trace Water and Oxygen Impact on SF6Decomposition Characteristics Under Partial Discharge

It is common practice to identify the insulation faults of GIS through monitor the contents of SF₆ decomposed components. Partial discharges (PD) could lead to the decomposition of SF₆ dielectric, so new reactions usually occur in the mixture of the newly decomposed components including traces of H₂O and O₂. The new reactions also cause the decomposed components to differ due to the different amounts of H₂O and O₂ even under the same strength of PD. Thus, the accuracy of assessing the insulation faults is definitely influenced when using the concentration and corresponding change of decomposed components. In the present research, a needle-plate electrode was employed to simulate the PD event of a metal protrusion insulation fault for two main characteristic components SO₂F₂ and SOF₂, and to carry out influence analysis of trace H₂O and O₂ on the characteristic components. The research shows that trace H₂O has the capability of catching an F atom, which inhibits low-sulfide SF_x from recombining into high-sulfide SF₆. Thus, the amount of SOF₂ strongly correlates to the amount of trace H₂O, whereas the amount of SO₂F₂ is weakly related to trace H₂O. Furthermore, the dilution effect of trace O₂ on SOF₂ obviously exceeds that of SO₂F₂.

Electrical tree characteristics in silicone rubber under repetitive pulse voltage

Electrical tree in silicone rubber (SiR) is a threat to the power system. Bedsides ac and dc voltage, there are pulse voltage in power system and the electrical tree behavior in SiR under pulse voltage has not been reported. In this paper, electrical treeing process in SiR under pulse voltage was investigated and analyzed. The voltage was applied to a needle-plate electrode to initiate electrical trees. The frequency of pulse voltage was 100, 200 and 300 Hz and the amplitude ranges from 8 to 14 kV. Aspects such as the patterns of electrical tree, tree length, accumulated damage, fractal dimension and tree breakdown characteristics were studied. Results indicate that the pulse amplitude plays the leading role in electrical tree initiation, propagation and breakdown processes. With the increase of pulse amplitude, the tree structure changes and the breakdown possibility increases obviously. However, the tree length is not sensitive to pulse frequency and higher pulse frequency promotes the increase of tree density. It also has been found that tree structure is closely related to pulse polarity and the inception voltage is lower with positive pulse than that with negative.

A miniaturized optical emission spectrometry based on a needle-plate electrode discharge as a light source for room temperature hydrogen sensing with Pt/CNT nanocomposites

A miniaturized and simple optical emission spectrometry was developed using needle-plate electrodes discharge as a light source. Based on Pt/carbon nanotube (CNT) nanocomposites-decorated fluorine-doped tin oxide (FTO) electrode, the system can be applied to sensing hydrogen at room temperature with improved analytical performance using an alternating current voltage of 1.32kV with the power consumption of 13W in nitrogen (N2) carrier. The emission line of hydrogen at 656nm served as the analytical line. Under the optimum condition, the emission intensity was linearly correlated to the hydrogen concentration in the range of 0.17–4.0% (v/v). The developed system also tested oxygen and sodium determination, showing flexibility of this method.

Effects of low temperature and nanoparticles on electrical trees in RTV silicone rubber

The operating temperature of power cable system has a great influence on the electrical performance of room temperature vulcanizing (RTV) silicone rubber. This paper investigated the effects of low temperature and nanoparticles on electrical tree in RTV silicone rubber. Samples were prepared by mixing SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoparticles into RTV silicone rubber, with the content of 0, 0.5, 1, 1.5 and 2 wt% respectively. The experiment temperature was ranged from - 30 to -90 °C. AC voltage with a frequency of 50 Hz was applied between a needle-plate electrode. The electrical tree patterns, growth rate, expansion coefficient, fractal dimension and proportion of accumulated damage were studied. The results reveal that both nanoparticles and low temperature environment have a significant impact on the electrical tree growth. The tree patterns are branch tree or bush tree when the temperature is higher than -60 °C; however, there is only pine tree when the temperature is lower than -90 °C. Compared with - 30 °C, some tree channels are easier to propagate at - 60 °C in silicone rubber. It is also suggested that the temperature affects the expansion coefficient of electrical tree. The fractal dimension of the electrical tree increases with the nanoparticle content, while the accumulated damage of treeing area shows the opposite tendency.

Experimental analysis of the feasibility of CF3I/CO2 substituting SF6 as insulation medium using needle-plate electrodes

CF 3 I gas has low greenhouse effect and good insulation properties, so it can replace SF 6 gas that has high greenhouse effect. In this study, the partial discharge inception voltage (PDIV) of CF 3 I/CO 2 mixed gas is measured by power frequency test under different gas pressures, mixing ratios, and gap distances. The influence of these conditions on the partial discharge performance of CF 3 I/CO 2 mixed gas is investigated. The PDIV of the CF 3 I/CO 2 mixed gas is also comparatively analyzed with that of SF 6 /CO 2 mixed gas. The results show that the PDIV of the CF 3 I/CO 2 mixed gas is 1-1.2 times higher than that of SF 6 /CO 2 mixed gas,when the mixture ratio is between 10-30% and the gas pressure is between 0.1-0.3 MPa, which means the insulating ability of the CF 3 I/CO 2 mixed gas is better than that of the SF 6 /CO 2 mixed gas. By contrast, the PDIV of these two mixed gases exhibit similar variation trend with gas pressure when the mixing ratio is more than 20%. There is positive synergistic effect on the PDIV of CF 3 I and CO 2 . These results indicate that CF 3 I/CO 2 (20 to 30%) mixed gas may be used to replace SF 6 gas for gas-insulated equipment applications.

Effect of low temperature on tree characteristics in silicone rubber with different power frequency

Electrical tree is one of the main reasons for long-term degradation of SiR cable accessories used in high voltage ac applications. In this paper, an investigation of electrical tree growth characteristics in SiR samples is reported. Ac voltage with frequency ranging from 50 to 200 Hz was applied between a pair of needle-plate electrodes to initiate electrical trees at temperature ranging from 30 to -100 °C. Both the tree structures and growth characteristics have been observed by using a digital microscope system. Three typical profiles of electrical trees, namely branch, bushbranch and pine branch tree, have been observed within the samples. The growth rate and accumulated damage have been employed to analyze the treeing process. It is observed that the high power frequency will lead to the occurrence of bush-branch tree and the low temperature will lead to the occurrence of pine branch tree. Meanwhile, the tree inception probability increases from 30 to -60 °C and then decreases fast. The high frequency will improve the tree inception probability under the same temperature. During the experiment, breakdown has occurred in some samples and the width of breakdown channels is related to the frequency.

Influence of montmorillonite on electrical treeing and breakdown characteristics of low-density polyethylene

Electrical treeing is an electrical aging phenomenon that severely threatens the operation safety of power equipment, in which polymer is used as primary insulation material. A type of low-density polyethylene-montmorillonite (LDPE/MMT) nanocomposite was prepared using melt method to study the impact of MMT nanoparticles on the growth of the electrical tree and on the breakdown properties of the LDPE material. The distribution characteristic of space charges of the specimens under DC voltage of –50 kV/mm and short-circuit conditions was measured using pulsed electro-acoustic method. The electrical tree aging characteristics of specimens under AC voltage of power frequency were studied using needle-plate electrode structure test platform. The starting voltage of initiation, the triggering rate, and the growth rate of the electrical trees, as well as the breakdown performance of the specimens under AC voltage of power frequency were measured. The inhibitory mechanism of MMT nano-particles on the growth of electrical trees in nano-composites, and the mechanism behind the improvement of the breakdown strength of the LDPE/MMT specimens were illustrated. Experimental results showed that, the addition of MMT can effectively reduce the amount of space charges injected from electrodes and the accumulation amount in the center of samples. Moreover, space charges dissipated more slowly during short-circuit conditions. The speed and the scale of the growth of the electrical tree declined as the initiation conditions of electrical trees growing in LDPE/MMT nanocomposites increased; meanwhile, the breakdown strength increased substantially. Here are the reasons: MMT can reduce the mobility of carriers in LDPE material, thereby contributing to the improvement of the distribution of space charges. In the meanwhile, MMT changed the development path of electrical trees, and extended the discharge distance. Consequently, MMT postponed the occurrence of penetrable breakdown in the LDPE/MMT specimen, and ultimately improved the breakdown strength of the specimen.

Effect of Reactive Diluents on the AC Electrical Treeing in Epoxy/Nanosilicate Systems

The effect of reactive diluents on the ac electrical treeing in epoxy/nanosilicate systems was studied, in a needle-plate electrode geometry. Diglycidyl ether of bisphenol A (DGEBA) type epoxy was used as a base resin, and layered silicate was used as a nano-sized filler. Polyglycol (PG) or 1,4-butanediol diglycidyl ether (BDGE) was introduced as a reactive diluent to the DGEBA/nanosilicate system, in order to decrease the viscosity of the nanocomposite system. PG acted as a flexibilizer, and BDGE acted as a chain extender, after the curing reaction. To measure the treeing propagation rate, a constant alternating current (ac) of 10 kV/4.2 mm (60 Hz) was applied to the specimen, in a needle-plate electrode arrangement, at <TEX>$30^{\circ}C$</TEX> of insulating oil bath. When 10 kV/4.2 mm (60 Hz) was applied, the treeing propagate rate in the DGEBA system was <TEX>$1.10{\times}10^{-3}$</TEX> mm/min, and that in the DGEBA/PG system was <TEX>$1.05{\times}10^{-3}$</TEX> mm/min. As 1.5 wt% of nanosilicate was added to the DGEGA/PG system, the propagation rate was <TEX>$0.33{\times}10^{-3}$</TEX> mm/min. This meant that the nano-sized layered silicates would act as good barriers to treeing propagation. The effect of chlorine content was also studied, and it was found that chlorine had a bad effect on the electrical insulation property of the epoxy system.

Effect of Electric Field Frequency on the AC Electrical Treeing Phenomena in an Epoxy/Reactive Diluent/Layered Silicate Nanocomposite

The effects of electric field frequency on the ac electrical treeing phenomena in an epoxy/reactive diluent/layered silicate (1.5 wt%) were carried out, in needle-plate electrode arrangement. A layered silicate was exfoliated in an epoxy base resin, by using our ac electric field apparatus. To measure the treeing propagation rate, constant alternating current (AC) of 10 kV with three different electric field frequencies (60, 500 and 1,000 Hz) was applied to the specimen, in needle-plate electrode arrangement, at 30℃ of insulating oil bath. As the electric field frequency increased, the treeing propagation rate increased. At 500 Hz, the treeing propagation rate of the epoxy/PG/nanosilicate system was0.41×10-3 mm/min, which was 3.4 times slower than that of the epoxy/PG system. The electrical treeing morphology was dense bush type at 60 Hz; however, as the frequency increased, the bush type was changed to branch type, having few branches, with very slow propagation rate.