Positive Impulse Voltage Research Articles

Breakdown and acceleration voltage of biodegradable liquid made in GTL technology at positive lightning impulse voltage

Breakdown and acceleration voltage of biodegradable liquid made in GTL technology at positive lightning impulse voltage

Contribution of Metastable Oxygen Spectra to Fluctuated Waveform Tails after Breakdown Time in Air under Positive and Negative Impulse Voltages

In this study, we explored the correlation between fluctuated waveform tails under both positive and negative impulse voltages and their corresponding spectral lines during millisecond observations of arc discharge. We examined impulse voltages in ±100, ±125, and ±150 kV across 3, 3.5, and 4 cm gaps using spectroscopic analysis focused on oxygen excitations. Six selected spectra in ±100, ±125, and ±150 kV at 3.5 cm and two negative spectra of −100 kV at 3 and 4 cm were analyzed by identifying spectral lines in the wavelength range of 200–900 nm. The results revealed a correlation between the fluctuated waveform tails and spectral lines in positive voltage discharges, which were almost similar, while in negative voltage discharges, this correlation was found only in −100 kV at 3 and 4 cm. We concluded that during the spark phase for both positive and negative voltage discharges, symmetrical fluctuation in the waveform tails was observed after breakdown time, especially above the voltage level of the recombination phase. This suggested the presence of energetic oxygen excited states in the 200–400 nm range, with higher peak intensity than the O I line at 777.417 nm, observed in most positive impulse voltage discharges and at −100 kV with 3 and 4 cm gaps, contributing to rapid breakdown.

Breakdown Time Phenomena: Analyzing the Conductive Channel of Positive Impulse Voltage Discharges under Standard Temperature and Pressure Air Conditions

Even though the streamer process can be identified in nanoseconds and microseconds through experimental measurements, the breakdown time of air discharge is still unknown. The instability of electrons is suspected to be an attachment-instability phenomenon of the channel conductivity. We investigated breakdown time across milliseconds to better understand how the oxygen excitations of the 200–400 nm range influence a high-conductivity channel even with a weaker applied voltage. Experiments were performed with positive impulse voltages ranging from +42 to +75 kV in the step of +6 kV at a 3 cm gap between needle-to-plane electrodes in a horizontal configuration. A spectrometer with an integration time of 70 ms was used to capture the spectra during voltage discharge. The shortest breakdown time was found at +60 kV with 77 ns compared to +66, +72, and +75 kV. We conclude that the shorter breakdown time at +60 kV is primarily due to the oxygen-excited state in O IV at 262.999 nm. This state helps maintain electron flow by preventing electron loss, with a decay time of 2.5 µs, while releasing Joule heat at a temperature of 26,003 K, which optimizes conductivity. This process occurs before the recombination of the O I line at 777.417 nm, which has a significantly shorter decay time of 27 ns.

Simulation of breakdown voltage associated with phase-to-phase air gap for ultra high voltage transmission lines under switching impulse voltage

Abstract Knowledge of the breakdown voltage of air gaps when subjected to positive switching impulse voltage is fundamental to the design of insulation coordination for power transmission systems. However, the length of phase-to-phase air gaps in ultra-high voltage power transmission lines has reached more than 30 m. It is very difficult, costly, and impossible to conduct full-scale test experiments to obtain the breakdown voltage of these gaps. Here, the numerical simulation method based on the self-consistent leader inception and propagation model was first used to simulate the air gap discharge process at four different scales and to obtain the gap breakdown voltage. The simulated data was compared with the measured data to verify the validity of the model. Then, the numerical simulation method was used to simulate the discharge process of phase-to-phase air gap for ultra-high voltage transmission lines with 30.3 m in length. The current and the development trajectory of the leader tip were obtained for the discharge process. More importantly, the breakdown voltage, which could not be obtained by full-scale testing, was obtained by simulation.

An Analysis of Positive Switching Impulse Voltage and negative streamer growth in point-sphere gap towards Valve Hall

An Analysis of Positive Switching Impulse Voltage and negative streamer growth in point-sphere gap towards Valve Hall

Mechanisms of charge-induced surface discharge under positive impulse voltages

Abstract Charge-induced surface flashover is a critical factor leading to insulation failures in high-voltage direct current gas-insulated equipment, and the underlying mechanisms are still unclear. In the present study, the typical surface charge distributions are first summarized. Then, the impact of charge polarity and position on surface discharge characteristics in ambient air is studied, and the surface charge dynamics during multiple discharge processes are also focused. Comparative studies of the charge-induced surface discharge are conducted in SF6, and the effect of the gas atmosphere is discussed. The results indicate that the repulsive effect of deposited positive charges significantly inhibits the positive streamer development by reducing the electric field. The acceleration of negative charges on positive streamer propagation is the result of two competitive mechanisms: the enhancement of the electric field and the neutralization with positive charges in the streamer channel. In the multiple discharge process, positive streamers develop along the gap of positive streamer channels from the previous discharge. When the last discharge is intense enough, back discharges may occur along the pattern of the deposited positive channels from the previous discharge. The effects of the deposited charges on the surface discharge process are consistent in air and SF6. These findings will be advantages in improving the insulation reliability of gas-insulated equipment.

Temperature evolution characteristics of stem root during dark period in positive long spark discharge

Gas temperature plays an important role in the characterization of plasma parameters. The temperature evolution of stem roots during the dark period is at the heart of our understanding of the physical mechanism of streamer to the leader transition process in a long spark discharge. The quantitative schlieren system with high spatiotemporal resolution was designed to conduct positive leader discharge experiments with a 1.0 m rod-plate gap at atmospheric pressure, and the amplitude and rise time of positive impulse voltage waveforms were 380 kV and 200 μs, respectively. The time-resolved quantitative schlieren images of the discharge channel near the electrode tip can be captured to gather the temperature data of stem roots for case study. Further statistical tests revealed that due to the dispersion of the first streamer discharge, there were three evolutionary trends of gas temperature at the axis of the stem root in the early dark period. Interestingly, the gas temperature at the axis of the stem root was maintained at 1000–1200 K in regardless of the evolutionary trend in the early dark period. In addition, the statistical results indicate that there is a significant positive correlation between the curvature radius of the discharge electrode tip and the critical charge value Qcrit of the first streamer discharge, which leads to a transition in evolutionary trends of gas temperature at the axis of the stem root. This research has important guiding significance for understanding the physical mechanism of leader inception and the whole process modeling of long air gap discharge.

Rapid Breakdown Time in Positive Impulse Voltages through Spectroscopy Analysis

The air discharge phenomenon, characterized by its rapid and transient nature, is inherently unpredictable, emphasizing the need for a comprehensive understanding of its physical interactions. Our experimental setup involved voltage generators producing both positive and negative impulse voltages (±100 kV, ±125 kV, and ±150 kV) at a 3.5 cm gap distance in a needle-to-plane geometry. This setup facilitated the study of individual spectral lines of impulse voltage discharges, with a specific emphasis on examining oxygen transitions through spectroscopy analysis. To explore the influence of photon emission on the breakdown rate, we examined the correlation between decay time, excitation temperature, and peak intensity during the transition from an upper state to a lower state. Our findings reveal that positive impulse voltage discharges more rapidly than negative impulse voltages. This heightened discharge rate is attributed to the higher peak intensities of O II at 313.421 and 241.162 nm, as well as O IV at 337.806 nm, observed in the excited state, as opposed to O I at 777.417 nm in the combination state. The inference drawn from the larger peak intensity suggests that energetic photon emission plays a pivotal role in initiating and expediting electron discharge in positive voltages.

Improvement of the flashover threshold of TUK pressboard by using FeO3‐based nanofluid of monoesters

AbstractThis article deals with the improvement of the dielectric properties of TUK cellulose paper by impregnation. The experiment was carried out using a nanofluid based on vegetable oil esters and iron nanoparticles (FeO3). The insulating liquids used are palm kernel oil methyl ester (PKOME) and castor oil methyl ester (COME). The evaluation of the improvement is based on the analysis of the flashover voltage of the creeping discharges produced on the immersed paper. The tests were carried out under a positive lightning impulse voltage and for two electrode configurations. The concentrations of nanoparticles used in the experiment are 0.10 wt.%, 0.15 wt.% and 0.20 wt.%. The experimental results show that the 0.10 wt.% concentration gives the best improvement for the two electrode configurations used. The improvements are 53% for the inclined tip and 56.90% for the vertical tip in the case of COME + FeO3. For PKOME + FeO3, the results are 59.90% and 64.60%, respectively, for the two configurations.

Experimental Study of the Electrical and Physiochemical Properties of Different Types of Crude Palm Oils as Dielectric Insulating Fluids in Transformers

This paper gives information on the electrical and physiochemical characteristics of six different types of palm oil compared with traditional mineral oil. We found that natural processed crude palm oil (PO-C) had a higher resistance to AC breakdown voltage than other types of palm oil, including traditional mineral oil. The results of the positive lightning impulse voltage test for PO-C were still the highest compared to other types of palm oil, including traditional mineral oil, at 58.26%. The summarised dissipation factors of all tested crude palm oils were significantly higher than those of mineral oils, which will make the palm oil less insulating, especially in PO-A palm oil (36.197%), where the values were higher than those of other oils, while mineral oil has a slightly increased dispersion factor. For relative permittivity, all palm oils were compared, and it was found that PO-C had a lower relative permittivity than the other oils. In terms of physical and chemical properties, in the moisture content test on all oils, PO-C had the percentile with the highest moisture content decrease of 58.74%. In the case of testing the surface tension value, it was found that traditional mineral oil had the highest value (48.46 m/Nm) when compared to palm oil. On the other hand, the acidity in traditional mineral oil is the lowest (0.03 mg KOH/g) compared to all palm oils. Results from studies demonstrate the possibility of using natural processed crude palm oil, or PO-C, as a replacement for traditional mineral oil. This is consistent with the results of electrical properties that show PO-C is higher than other types of palm oil and includes traditional mineral oil.

Discharge characterization and altitude correction of shielding sphere-plate gap at an altitude of 3400 m

Southeast Tibet in China is rich in clean energy such as water energy, light energy, and wind energy. As the key means of large-scale clean energy transmission in southeast Tibet, the ultra-high voltage (UHV) power transmission projects will be constructed at an altitude of nearly 4000 m. To accurately acquire the external insulation strength of electrical equipment operating at different altitudes, the standard positive switching impulse voltages were applied to a rod electrode and a 1.1 m diameter shielding sphere at the altitude of 3400, 2100, and 80 m. The switching impulse discharge characterization and altitude correction factors of the 1.1 m diameter shielding sphere-plate gap under different clearances at different altitudes were obtained. The discharge voltage of the rod-plate gap at the altitude of 80 m was corrected to the altitude of 3400 m using altitude correction methods recommended by GB 311.1, GB/T 16927.1, and DL/T 2305. Further comparison was made between the calculation results using the three altitude correction methods mentioned above and the test results at an altitude of 3400 m. The discharge characterization and the minimum clearance of the end fittings of the electrical equipment in the valve hall at the altitude of 3800 and 4500 m were obtained by fitting and extrapolating the test data. The research results provide important reference significance for the external insulation configuration of UHV converter stations constructed in areas with an altitude of 3000 m and above.

Experimental study on streamer-leader transition characteristics of sphere-plane gap in valve hall under positive switching impulse voltage

Since a large number of spherical shielding balls are adopted in valve hall, investigating discharge characteristics of sphere-plane gap is of great significance for insulation design in HVDC projects. In order to study the discharge physical process of sphere-plane gap, this paper conducts sphere-plane gap experiment with gap distance set to 1.0 m and 1.5 m and electrode diameter set to 0.2 m, 0.3 m and 0.4 m under positive switching impulse voltage. The experimental comparative analysis of the discharge process of streamer-leader transition, and the physical parameters such as leader velocity and the amount of injected charge are calculated. The results show that the streamer to leader transition process is accelerated as the spherical electrode diameter increases. The leader velocity of sphere-plane gap is almost consistent with that of rod-plane gap. And the charge injected into initial streamer is much high than that of rod-plane gap.

Experimental Analysis of Streamer Currents Under Lightning Impulse Voltage Using Fast Fourier and Discrete Wavelet Transform Methods

This paper presents an experimental comparative study of streamer currents propagating in mineral oil, natural esters (olive oil and rapeseed oil) and synthetic ester (tetra-ester) under both positive and negative lightning impulse voltages. Because of the similarity in the shapes of streamer currents in investigated liquids, it is difficult to differentiate between the current signals corresponding to each liquid. For that purpose, two techniques are used for the analysis of these signals. The first one is the Fast Fourier Transform (FFT) technique, which is based on the frequency spectrum analysis. The second one is the Discrete Wavelet Transform (DWT) method, which has demonstrated its ability for the analysis of the non-stationary signals. The obtained experimental results show spectral signature specific to each liquid, thus making it possible to differentiate the various streamer currents. Overall, the two techniques show good performance. However, the accuracy and effectiveness of the DWT method is better than those of the FFT technique.

Numerical modeling of positive streamer propagation in alternative natural ester and traditional naphthenic oil under sub-microsecond pulsed voltages

In this paper, a two-dimensional axisymmetric numerical model of streamer discharge in alternative natural ester liquid and conventional naphthenic oil under a positive sub-microsecond impulse voltage is proposed. The model is based on the charge drift-diffusion approximation in a divergent electric field. Simulation shows that streamer propagation in naphthenic oil is divided into two stages: stage I with a small streamer velocity of 0–0.91 km s−1 and stage II with a rapidly rising streamer velocity up to 213 km s−1. In natural ester, the streamer velocity gradually slows down from 4.84 km s−1 to 2.11 km s−1. The electric field at the head of the streamer in natural ester ranges from 2.4 × 108 V m−1 to 2.5 × 108 V m−1 during propagation, which is visibly lower but more consistent than that in naphthenic oil (3.5 × 108 V m−1 to 8.4 × 108 V m−1). It is revealed that the different streamer features within the two types of oil are caused by the different competition between the Laplacian electric field and space charge effect at the streamer head. For streamer propagation in naphthenic oil, stage I is dominated by the Laplacian electric field whereas stage II is dominated by the space charge effect. In natural ester, the whole streamer propagation is dominated by the Laplacian electric field. Further, it is indicated that the streamer head consists of two specific discharging regions, i.e. the front region of the streamer head which guides new space charge accumulation and the back region of streamer head which guides density reduction of accumulated space charge. The expansion of the streamer tunnel is caused by the alternating space charges in these two regions. The work in this paper provides a theoretical reference for the practical application of alternative transformer liquids.

A computational discharge model for sphere–plane long air gap under switching impulse voltage

There are lots of sphere–plane air gaps in valve halls in extra-high-voltage and ultra-high-voltage converter stations. Accurate prediction for discharge characteristics of sphere–plane gaps is of great significance for the selection of shielding structure and determining the dielectric strength of the valve hall. In this paper, based on the physical process of corona inception and continuous leader inception, a computational model for calculating the discharge voltage of a sphere–plane air gap under positive switching impulse voltage is proposed, and the leader characteristics and discharge voltage are analyzed. Then, the switching impulse discharge test of the sphere–plane gap with 0.15, 2, and 0.3 m radius spherical electrodes is carried out to verify the correctness of the model. The results show that the discharge voltage calculated by the proposed method is consistent with the test results, and the error is within 7.3%. The leader inception voltage and inception time increase with the increasing spherical electrode radius at the same gap distance, and an identical spherical electrode require higher leader inception voltage and faster leader inception time with the increasing gap distance.

Streamer and leader characterization in HFO1234ze(E) gas, in a divergent electric field

Pre-breakdown phenomena in HFO1234ze-(E) gas, considered as a potential replacement of SF6 for medium voltage insulation, are studied in needle-plane electrode systems versus pressure (0.01 to 0.3 MPa) under positive impulse voltage. Measurements are also carried out in air and SF6 in the same conditions for comparison. At the lowest pressure in HFO, the propagation of fast streamers is observed. Above 0.03 MPa, breakdown is the consequence of the propagation of stepped leaders, with shapes and velocities nearly identical to those observed in SF6. Several leader features (minimum inception voltage, propagation length) show that leader formation and propagation is easier in HFO compared to SF6. In turn, this allow explaining why breakdown voltages in HFO are slightly lower than in SF6.

Study on Space Charge Characteristics in XLPE Under DC Voltage Superimposed by Impulse Voltage

In direct current (dc) transmission lines, power cables are susceptible to the invasion of switching overvoltage or lightning overvoltage. This causes the accumulation of space charge in the insulation due to the higher transient composite electric stress. Space charge is an important factor in dielectric breakdown, especially under high electric stress. However, until now, there are few reports on space charge characteristics under the composite stresses of dc voltage and impulse voltage. In this article, a modified pulsed electroacoustic space charge measurement system used for dc voltage superimposed by impulse voltage is developed by reasonably choosing resistor and capacitor elements in the measuring system. Then, space charge characteristics in cross-linked polyethylene (XLPE) are measured under dc voltage, impulse voltage, and dc voltage superimposed by impulse voltage. The test results show that hetero-charges dominated in the XLPE bulk under a positive dc voltage, whereas homo-charge injection occurred from the semiconductor (SC) electrode under a negative dc voltage. However, a continuous impulse voltage with high amplitude can cause a large degree of homo-charge injection from both electrodes. Moreover, greater homo-charge injection appeared under the negative impulse voltage than that under the positive impulse voltage. Under a dc voltage superimposed by an impulse voltage, the superposition of voltages with the same polarities facilitated homo-charges injection from both electrodes to a greater extent than did the superposition of voltages with opposite polarities. Furthermore, regardless of the polarity of the applied dc voltage, greater space charge accumulation appeared in XLPE when a negative impulse voltage is superimposed.

The surface flashover process under positive lightning impulse voltage: initial stage and evolution

Surface flashover is a crucial issue in the field of electrical insulation, and it involves many complex physical processes. In this paper, the development process of the surface flashover was studied from different methods. The initial process of positive surface streamers in different gas environments (air, N2, CO2) was studied by photoelectric observation. The evolution of positive surface streamers in the air was described based on a 2D fluid model. The influence of the surface trap energy level on flashover development and the relationship between gas adsorption and surface trap energy level was discussed by density functional theory calculation preliminarily. The results showed that the initial ionization process of surface flashover is considered as the collision ionization between the initial electrons and gas molecules and photoionization of high-energy photons. Some of the high-energy photons can not only ionize some gas molecules but also cause the surface of the insulator to emit electrons (photoemission process), which could promote the development of the streamer. Both the ionization of the gas molecules and the photoelectric emission of the insulator surface may determine the initial development of surface flashover. During the process of the flashover, the electron density of the surface streamer (∼1021 m−3) is high and the main streamer tends to develop towards the insulator surface. The attraction of surface streamers changes with the position of the initial electron, and the positive surface charge brings the stronger ionization process, and the negative surface charge has the opposite effect. The band gap of the insulator surface is affected by the adsorption of gas molecules, which is considered as introducing shallow traps on the surface. For impulse voltage, the charge accumulation and internal charge migration may be not evident, the initial photoionization process and initial surface charge distribution affect the flashover process primarily.

Streamer Propagation along Profiled Insulator Surfaces under Positive Impulse Voltages

Controlling discharge growth on insulator surfaces is important in high voltage gaseous insulation systems. In this study, the effect of small-scale surface profiles on streamer discharge propagation is examined experimentally. The experimental test objects were 5x72x150 mm polycarbonate plates with and without machined surface profiles. One test object had a surface with 0.5 mm deep semi-circular corrugations, while the other profile had 0.5 mm deep rectangular corrugations. The semi-circular profile increased the surface area with 20 %, while the rectangular profile increased the area with 110 %. A plain surface was also examined as a reference. Positive impulse voltages were applied to a 1 mm thick disk electrode placed 2 mm above the insulator. The insulator was placed in a grounded aluminium casing. The streamer development was imaged with a light-sensitive high-speed camera. Surface charges left on the surface after the impulse were examined using an electrostatic probe and simulations of saturation charge. The rectangular surface profile reduced the streamer range significantly, which suggests an effect of added surface area. Imaging indicated that the wavelike surface streamers follow the profiles closely. Surface potential measurements showed a saddle-shaped distributions, with values in line with saturation charge computations.

The Influence of Diethylaniline and Toluene on the Streamer Propagation in Cyclohexane between a Point-Plane Gap under Positive Impulse Voltage Stress

Liquid insulation is used in high voltage equipment such as power transformers as both dielectric medium and coolant. Breakdown in liquid insulation tends to be governed either by streamer initiation under more uniform fields, or by streamer propagation under more non-uniform fields. A model streamer propagation study, which screens the effectiveness of additives based on cyclohexane and mixtures with diethylaniline (DEA) and toluene, is presented in this paper. The effect of additives, at different concentrations, on streamer propagation velocity in cyclohexane under an applied lightning impulse voltage of positive polarity is studied. Cyclohexane (ionisation potential 9.88 eV) was chosen because, being a hydrocarbon, it shares similarities with the constituents of common insulating liquids. Previous studies have also shown how, in general, the addition of additives of lower ionization potential than the bulk liquid can slow down streamer propagation in insulating liquids. A point-plane electrode configuration of 70 mm gap with a 5 μm tip radius is used and subjected to an applied positive polarity impulse of 1.2/50 μs. A high velocity imaging system is also used to capture streamer images to validate a Time-To-Breakdown (TTB) measurement approach used in inferring approximate streamer velocity. The DEA (ionisation potential 6.98 eV) was found to be an effective additive to slow down positive polarity streamers in cyclohexane in the applied voltage range (≈220–280 kV peak) in concentrations above approximately 0.33% (by volume). Toluene (ionisation potential 8.82 eV) was found not to significantly slow down streamers in cyclohexane, even at 10% concentration, for the same voltage range. This is postulated to be due to the fact that toluene does not have a low enough ionisation potential (with respect to that of the cyclohexane) to change the streamer branching characteristics sufficiently during propagation.