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Bubbles, as typical defects of oil–paper insulation systems, pose a great threat to the insulation state. In this article, the characteristics and mechanisms of the partial discharge (PD) inception, evolution, and induced breakdown of bubbles embedded between oil-impregnated paper (OIP) layers are investigated. The results show that the PD inception has a significant statistical time delay due to the lack of free electrons. The PD inception time delay (PDTD) follows an exponential distribution and leads to the large dispersion of the PD inception voltage (PDIV), which follows a Weibull distribution. The changes in PD behavior [charge, repetition rate, pulse current waveform, and phase-resolved PD (PRPD)] over time all show two separable stages (stages 1 and 2). The continuous recording of bubble area and gas composition under PD reveals that the evolution of bubble PD is attributed to the change in gas composition rather than the change in surface conditions as believed by researchers. The OIP breakdown induced by bubble PD indicates two breakdown modes, fast and slow, which correspond to direct-induced breakdown in stage 1 and cumulative-damage breakdown in stage 2. The former occurs because of the severe distortion of the electric field by the space charge at the head of the discharge channel inside the bubble, whereas the latter occurs because of the gradual deterioration of the OIP due to the continuous PD action. This study increases our understanding of the mechanisms of bubble discharge and of oil–paper insulation deterioration, and it provides guidance for fault diagnoses. [ABSTRACT FROM AUTHOR]

Under temperature and moisture combined, bubbles will be formed in oil-impregnated paper/pressboard (OIP/OIPB). Although many studies have been done to understand the thermo-induced bubble phenomenon in OIPB, the formation mechanism is not, let alone its effect on discharge characteristics and insulation performance of OIPB. In this article, the phenomena and mechanisms of thermally induced bubbles, their effects and mechanisms on the partial discharge (PD), and breakdown characteristics of OIPB are investigated. The results show that the bubble inception temperature (BIT) decreases significantly with the increase of the moisture of OIPB. There is a growth process of bubbles from small to large, and the heating temperature and moisture content of OIPB affect the gas component inside the bubble. Thermal bubbles significantly reduce the PD inception voltage (PDIV) and breakdown voltage (BV), and the PD evolution and dissolved gas components in the oil show significant differences in the presence/absence of bubbles. Interestingly, PDIV and BV start to decrease in a temperature interval before BIT, and we infer that many microbubbles and gaseous channels exist inside OIPB before observable bubbles are formed on the surface. Considering the fibrous porous structure of OIPB and combining it with the bubble nucleation theory, we propose a new model for interpreting bubble formation and insulation degradation. The model suggests that bubbles (vapors) form first in microcavities and holes inside the OIPB and then spill over to the surface to form observable bubbles. The microcavities and pores in the OIPB are filled with gas (vapor) as the main cause of PD inception and insulation degradation. This study contributes to understanding the mechanisms of bubble formation in oil–paper insulation as well as provides a reference for risk assessment during overload operation and diagnosis of bubble-induced related faults of transformers. [ABSTRACT FROM AUTHOR]

High entrance capacitances of ultrahigh-voltage (UHV) transformers make it difficult to generate impulses with standard front time during the lightning impulse tests. In this paper, we discuss the feasibility of examining the longitudinal insulation design of a transformer via impulses with a front time longer than the standard value. We consider the following features: transient voltage distributions and insulation characteristics of longitudinal insulation. The transient voltages of two UHV windings are measured with a capacitive voltage sensor under impulses with different front times. The results demonstrate that the front time of the applied impulse has little effect on the gradient voltage distributions and the oscillatory component of the transient voltage, but it determines the front time of transient voltages. Therefore, the relationship between breakdown characteristics of the turn-to-turn insulation and the front time is studied. Experimental results revealed that whatever the insulation thickness, an increase in the front time of the applied impulse decreases the breakdown voltage, but this variation is within acceptable tolerance when the front time changes from 1.2 μs to 6 μs. Thus, as long as the front time is restricted within 6 μs, we can ensure the validity of lightning impulse tests on the examination of longitudinal insulation designs. [ABSTRACT FROM AUTHOR]

The breakdown and streamer behavior in double-layer oil-paper insulation under impulses of varying front time were investigated in this study to determine the prebreakdown process in oil-paper insulation. The relationships between front times and breakdown characteristics were investigated and impulse-polarity-dependent variations were observed. The complete impulse breakdown process and precise role of front time were assessed as per streamer initiation and streamer propagation based on emitted light signals and shadow images. The re-initiation of streamers in non-first oil gaps was determined to be the key factor in breakdown as-dominated by applied voltage and space charge accumulation. The initiation and propagation times of streamers were observed to find that the streamers tend to propagate at phases with higher voltage amplitude as front time increases, which facilitates streamer re-initiation and thus decreases the breakdown voltage. Equivalent applied voltages during streamer propagation were further calculated to quantitatively investigate the effect of front time on streamer propagation. [ABSTRACT FROM AUTHOR]

The presence of free conductive particles in gas-insulated transmission lines (GIL) is the most significant threat to their insulation. The non-invasive identification of free conductive particle defects in GILs is extremely challenging. Existing identification methods which use the acoustic amplitude-flight time (AAFT) pattern are not often adopted on site due to their faulty theory, non-robust extraction algorithm, and generally inaccurate calibration. In this study, the theory established by L. E. Lundgaard is expanded by deriving estimation equations related to the particle's moving state. A new method for extracting the state information from the raw data of acoustic signals is established and the robustness of the acoustic identification method is enhanced. A convenient calibration method is also developed by building a standard collision signal generator. The proposed methods are validated in a scale GIL model and a real GIL model. The results of this study may represent a workable reference for GIL condition monitoring. [ABSTRACT FROM AUTHOR]

Lightning impulse (LI) test is an effective means to detect insulation defects in gas-insulated metal-enclosed switchgear (GIS) equipment. However, the LI test wavefront time Tf will exceed the standard value of 1.2 μs ± 30% for large test load capacitance. Whether extending the Tf will influence defect detection effectiveness should be a concern. In this study, a generating system of impulses with different wavefront parameters was established. The insulation characteristics of a coaxial cylinder structure with an SF6 gas gap with conductive protrusion in a bus under impulses with different wavefront parameters were studied. Experimental results show that the voltage–time curve of the gap shows a bathtub trend that has a flat part in the range of 1 μs to 5 μs. With a Tf increase, the 50% breakdown voltage increases. Whether extending the Tf will influence defect detection effectiveness is analyzed from the standpoint of the different meaning of the breakdown voltage in voltage-time curve and 50% breakdown voltage. The essence of detection effectiveness under impulse voltages with different wavefront parameters is a comparison of the probability of detecting the same insulation defect. Based on the discharge probability distribution, a new quantitative method to evaluate the insulation defect detection effectiveness of impulse voltages for GIS equipment is proposed. The method, based on the 90% discharge voltage value of an insulation defect under a standard LI, normalizes the discharge probability under different wavefront parameters. [ABSTRACT FROM AUTHOR]

Corona discharge under super low frequency (30–300 Hz) voltage is common in high-voltage transmission lines and equipment. The phase resolved partial discharge (PRPD) pattern is the most commonly used tool for power equipment condition-monitoring in the electrical engineering field. The present study centers on the correspondence between PRPD patterns and corona discharge modes under a wide range of applied voltage. Electric and optical emission characteristic measurement methods are applied to obtain PRPD patterns and phase resolved discharge images. The transition of corona discharge types and space charge behavior under different applied voltage are investigated. The electric field generated by negative needle electrode is shown to erase residual positive ions, which leads to an abundance of positive streamer bursts when applied voltage is low. Once the negative glow corona emerges under higher applied voltage, dense positive ions in the cathode sheath suppress the positive streamer bursts. The existence of residual positive ions is crucial for the transition from positive streamer discharge to glow corona discharge. A comprehensive understanding of PRPD patterns may help electrical engineers to assess the threat of partial discharge of the power equipment correctly. [ABSTRACT FROM AUTHOR]

Surface charge accumulation on insulators is one of the bottlenecks for the development of DC electric power system. This paper studies a ±200 kV DC basin-type insulator designed for GIL (Gas-insulated Transmission Line). A surface charge measurement device is made consisting of a 3-dimension, 4-axes manipulating device, a HV shielding conductor installation system and data acquisition system. A new scaling method of the probe with an inversion algorithm using Cholesky decomposition method is proposed to obtain charge density distribution. The dynamic process of surface charge accumulation and dissipation are studied under DC voltages with different polarities, amplitudes and time durations. Charge distributions are obtained and theoretically analyzed. A novel gas-side normal electric field model is proposed for regular charge distribution and methods of suppressing surface charge are suggested, including a recommended ratio of surface and volume electric conductivity of the insulator at 1∼10, structure design, and direct fluorination of the surface. [ABSTRACT FROM AUTHOR]

In recent years, SF6/N2 binary mixtures have been used as insulation in second generation gas-insulated metal-enclosed transmission lines mainly because of the greenhouse effect and diseconomy of SF6. Though the synergistic effect of SF6 is known in a uniform electric field, there is little understanding of the synergistic effect phenomenon with parameters in a non-uniform electric field under standard lightning impulse. A series of experiments were designed to increase our knowledge of this and the breakdown characteristics of SF6/N2 under multiple parameters show a negative synergistic effect. Our experimental results with negative lightning impulse demonstrated that the breakdown voltage of N2 increased greater than with both SF6 and its mixtures with increasing gas pressure. When the gas pressure exceeded 0.3MPa, the breakdown voltage of pure N2 was higher than SF6/N2 gas mixtures and even exceeded the value of SF6 at 0.5MPa. A normalization coefficient h was introduced in this research to evaluate the negative synergistic effect. Under negative lightning impulse the analytical results show that the negative synergistic effect increased with the augment of gas pressure. On the other hand, the synergistic effect was reduced with decreasing gas pressure under positive polarity impulse and it appeared to be a negative synergistic effect with the enhancement of field non-uniformity. The analysis of streamer discharge propagation indicates three reasons lead to the negative synergistic effect: similar streamer corona onset voltage, different space charge effect, and dissimilar discharge form of N2 and SF6/N2 gas mixtures. [ABSTRACT FROM AUTHOR]

Contamination of free conducting particles is a major threat to gas insulated systems. In HVDC GIL, free conducting particles can be even more harmful due to the fact that particles can easily cross the gas gap under DC voltage and distort the electric field. In HVDC systems, impulse overvoltage can be generated and superimposed on the DC operating voltage. As a result, GIL will experience superimposed DC and impulse voltage. In this paper, the behavior of free conducting particles in HVDC GIL is observed and breakdown characteristics of particle contaminated GIL under superimposed DC and impulse voltages are studied. Two motion patterns of free conducting particles in HVDC GIL are mainly found: bouncing and firefly motions. Particles tend to firefly motion near the enclosure under positive DC voltage and near the high voltage bus bar under negative DC voltage. With increased DC voltage amplitude, the probability of firefly motion increases while the probability of bouncing motion decreases. It is found that the insulation strength of GIL is greatly reduced by free conducting particles under superimposed DC and impulse voltages. With increasing particle length, the breakdown voltage of GIL under superimposed voltage decreases sharply. Breakdown voltage decreases slightly with the increase of particle radius. Breakdown voltage of GIL is lower under superimposed voltage of negative DC and positive impulse than under negative DC and negative impulse with the same size of particle. The results indicate that superimposition of DC operating voltage and impulse overvoltage can be a critical condition for HVDC GIL if free conducting particles exist. Therefore, a superimposed DC and impulse voltage on-site test can be considered for HVDC GIL to check for insulation performance. [ABSTRACT FROM AUTHOR]

Even for gas-insulated metal-enclosed switchgear (GIS) equipment that has passed an on-site dielectric test, some dielectric failure can occasionally occur without warning. It is necessary to evaluate the effectiveness of on-site dielectric tests of GIS equipment. In this study, two standard test procedures, power-frequency partial discharge (PD) and lighting impulse (LI) withstand test, were carried out in a real model test platform of 550 kV GIS to evaluate the effectiveness of on-site dielectric tests. Through experiments, the minimum detectable defect sizes were determined for different curvature radius values. The results show the limitation of the power-frequency PD test and the need to use of the LI withstand test as supplementary assessment on site. Two calculable constraints of defect detectability were proposed to extend the results to GIS equipment at other voltage levels. The results clarify the cause of dielectric failure without any warning and provide technological guidance for manufacturing and installation. [ABSTRACT FROM AUTHOR]

On-site lightning impulse (LI) test waveform, restricted by the large load capacitance and impulse generator's inherent inductance, has a long wavefront time Tf compared with standard LI, called non-standard LI. The non-standard LI test has been carried out in the field. But its insulation defect detecting effectiveness is not clear. In this paper, a generating system of impulses with different waveform parameters, including double-exponential impulses with different wavefront times in the range of 0.08 ? 23.5 ?s and an oscillating LI with wavefront time around 10 ?s, was established. The insulation characteristics of SF6 gas gap with highly inhomogeneous electric field under impulses with different waveform parameters were investigated. Experimental results show that the insulation defects detecting effectiveness of oscillating LI with long Tf around 10 ?s is lower than that of standard LI. Tf plays an important role in the insulation defects detection. With Tf increases, the 50% breakdown voltages for rod-plane gaps have an increased trend. The V-t curve presents a U-shape. Prospective voltage is introduced to evaluate the insulation defects detecting effectiveness. The prospective voltage-time characteristics confirm the conclusion that with the increase of impulse voltage Tf, the detection effectiveness for GIS insulation is reduced. The gap distance has influence on the shape of V-t curve. With distance increase, the 50% breakdown voltage of the gap is more sensitive to the wavefront time Tf and voltage rise rate dU/dt. Corona stabilization effect is not just in relationship with the rise rate of impulse, but also with electrode structure and gas pressure. The critical dU/dt for corona stabilization is calculated for different rod-plane gaps. [ABSTRACT FROM PUBLISHER]

Bubbling phenomenon usually arises from overheated cellulose insulation in oil, which is known for deleterious effects on the insulation strength of oil-immersed transformers. In this paper, the experiment was conducted on a self-heating conductor wrapped with insulation paper. The inception temperature of bubbling phenomenon negatively correlates to the rising rate of temperature and moisture content of paper, which is in accordance with the results of previous research. Besides, bubbles formed under different conditions could be different in morphology and locations. Thereafter, a method for temperature rise was proposed, based on which the models were heated for component research. It is found that the component changes in oil and blanket gas are significant in a short time interval under bubbling conditions. Comparison trials were then conducted on naked conductors and paper-wrapped conductors. Based on the theory of mass transport, it is revealed that the water vapor and carbon oxides contribute a major part in bubble composition, and their proportions vary with the bubbling temperature. This study concludes that bubbling phenomenon at temperature below 120 ?C can be recognized as the vaporization of moisture in paper. Moreover, bubbles formed at temperatures around 160 ?C can be attributed to the combined effect of water vaporization and gases from cellulose degradation. [ABSTRACT FROM AUTHOR]

The standard lightning impulse (LI), defined as a smooth full lightning impulse voltage having a front time of 1.2 ?s and a time to half-value of 50 ?s, obtained from field observation, has remained unchanged from IEC Ed. 1 in 1962 through to Ed. 3 in 2010. LI test plays an important role in the equipment design and insulation defects detection. In order to evaluate the reasonability of the standard LI waveform from the point of insulation characteristics, breakdown characteristics of GIS under LI with different wavefront times Tt and wavetail times Tt were studied. Those experimental results show that the LI waveform parameters have little influence on the breakdown characteristics of a sound GIS equipment, but have great influence on the GIS equipment with defects. For a highly inhomogeneous electric field, with the wavefront time increasing, the 50% breakdown voltages U50% have remarkable change and the U50%?Tt curve tends to be U-shaped. The 50% breakdown voltages decrease significantly with the increase of impulse wavetail time, especially when wavetail time is in the range of 1.5 to 15 ?s, but the difference of the values decreases after 15 ?s. Ignoring the generating possibility in the field for the large series inductance and load capacitance, a LI with wavefront time in the range of 0.2 to 0.7 ?s is proposed in the field for it may increase the probability of detecting insulation defects. Finally, the polarity reversion phenomenon and critical radius phenomenon, observed from the breakdown characteristics, can be explained by the migration and diffusion of space charges. [ABSTRACT FROM AUTHOR]

For the application of SF6/N2 in power equipment such as GIS and GIL, the breakdown characteristics of SF6/N2 in varying electric field non-uniformity under standard lightning impulse were studied in this paper based on a lightning impulse experimental platform. The research indicates that the reversal of polarity effect appears in SF6/N2 gas mixtures. That is, when the pressure is relatively low, the 50% breakdown voltage under positive polarity is lower than which under negative polarity. When gas pressure reaches the critical pressure, the situation would be opposite. The augment of SF6 mixing ratio or decrease of electric field non-uniformity would both result in the decline of the critical pressure. Meanwhile the change of polarity effect with SF6/N2 is later than that with pure gas with the increase of electric field non-uniformity. In addition, the phenomenon of critical radius of SF6/N2 gas mixtures was found. The critical radius reduces with the increase of gas pressure and is smaller under positive lightning impulse than negative. And the phenomenon of critical radius with pure gas is more obvious than that with gas mixtures. [ABSTRACT FROM AUTHOR]

The aim of this study was to clarify the hydrostatic pressure effect on the initiation and propagation process of oil streamers under the application of pulsed voltage with different polarities. The statistical characteristics of the impulse breakdown voltage and time lag in transformer oil were studied over a wide range of hydrostatic pressure, and the statistical time lag and the formative time lag were calculated by the Laue's pattern. The breakdown voltage distribution can be well fitted by 3 parameter Weibull distribution. The results show that the breakdown voltage increases with the hydrostatic pressure for both positive and negative polarity. However, the increase trend is different, linear for the negative polarity and saturated for the positive polarity. At a fixed voltage, when the pressure is increased, the breakdown time increases slightly for the negative polarity, but remarkably for the positive polarity. The statistical time lag for negative polarity is much larger than positive polarity, and the hydrostatic pressure has greater influence for negative polarity. The characteristics of formative time lag are quite different: for negative polarity, it decreases very slightly with the hydrostatic pressure, but increases remarkably for positive polarity. These differences are due to different initiation and propagation mechanisms, which are also discussed in the paper. [ABSTRACT FROM AUTHOR]

To pre-discover the insulating defects and avoid possible insulation failures, DC withstand voltage test with partial discharge (PD) measurements are routinely conducted before the operation of a convertor transformer. However, a ?downward trend? of PDs under DC voltage studied in this paper indicates the limitation of the current test. According to the study, under DC voltage both PD magnitude and PD repetition rate decrease with voltage applying time. At Updiv=29 kV, the average charge is 105 pC in the first 10 min and 33 pC in the last 10 min. Whereas for 3Updiv=87 kV, the value are 2700 pC and 1561 pC separately. On the other hand, at Updiv, PD numbers decrease from 12 times in the first 10 min to 5 times in the last 10 min. At 3Updiv, the numbers are 102 times and 83 times. When PD magnitude is higher than 1400 pC, a whitish area appears at the surface of the pressboard, accompanied by the decrease of DC withstand voltage after 2 hours' PD exposure. The results raise the question that the current test measures PDs over 2000 pC in the last 30 minutes but may introduce PDs of higher magnitude at the early stage, which will damage the dielectrics. Based on further analysis, an improved test procedure is suggested. [ABSTRACT FROM PUBLISHER]

To investigate partial discharge-induced degradation characteristics of oil-immersed pressboard under DC voltage, an experimental platform with needle-plane oil-pressboard insulation model is established. The surface condition and DC breakdown voltage of pressboard samples after PD pretreatment are observed and measured. In addition, a fitting formula to evaluate the degradation extent of pressboard samples is proposed. The results show that under constant PD pretreatments, surface roughness of pressboard samples decreases first and increases afterwards. The changing rate is positively correlated with PD intensity. During PD pretreatment, a ?whitish-colored circular region? appears at samples' surface and grows with PD intensity. For PD pretreatments of 500 pC and 1000 pC, DC breakdown voltage of pressboard samples remains unchanged. The breakdown spot has no clear carbonization nor creepage trace. For PD pretreatments of 2000 pC and 3000 pC, DC breakdown voltage decreases. The breakdown spot is obviously carbonized and creepage paths emerge around spots. Further analysis indicates that degradation of oil-immersed pressboard samples due to DC PDs is the result of motion activation process induced by electrical and thermal effects under DC electric field. [ABSTRACT FROM PUBLISHER]

Free particles formed in the discharge process or equipment aging severely threaten the operation security of GIS, which draws great attention of researchers. However, seldom research focuses on disc-type particle, which is the majority of shaped particles in practical GIS. In this paper, an elliptic coordination system is built for obtaining the theoretical lift-off electric field of disc-type particles, and experimental system and observation system are established for studying motion behavior and discharge characteristics of metal particles. Results show that for spherical particles, while the distance between two particles d<9rs, the downward force caused by impurities on electrodes can be neglected; for wire-type particles, as lengths of particles increase, lift-off electric fields and probabilities of Firefly tend to be larger. Besides, lift-off electric fields grow exponentially with the increases of radius. When length of particle exceeds 3 mm, the gap between practical values and theoretical values becomes larger with the decrease of radiuses; for disc-type particles, with the increase of radiuses, dispersions of lift-off electric field becomes larger; the difference between practical and theoretical values broadens with the fall of radiuses. Besides, lift-off electric fields of disc-type particles are higher than that of wire-type particles with same qualities and with the increase of radius, probabilities of Firefly increase in an unapparent speed in comparison to wire-type particles; in the end, discharge processes in SF6 gap are illustrated. When particles are close to the bottom electrode, the lower end of particles will breakdown first. Then the top end of particles will form streamer discharge. With the continuous development of streamer, the gap will eventually breakdown. [ABSTRACT FROM PUBLISHER]

Operating experience shows that residual DC voltage remains in GIS bus bar for a long time after opening of disconnectors. This DC voltage will not only cause accumulation of charges and contaminants on the insulator surface, but will also easily lead to motion of free metallic particles in SF6 gas gap, weakening the insulation ability of GIS. At the time of disconnectors reclosing, impulse voltage could be generated in the bus bar and superimpose on the pre-existing DC voltage. As a result, GIS will be under combined voltage of DC and impulse. This paper experimentally studies the insulation characteristics of GIS under combined voltage of DC and lightning impulse. It is found that, for non-contaminated insulators, when DC and LI voltage are of the same polarity, flashover voltage under combined voltage is nearly the same as the flashover voltage under LI alone. However, when they are of the opposite polarity, flashover voltage decreases with increasing DC voltage. In cases there are metallic particle or powder around insulator, pre-stressed DC voltage will cause adhesion of contaminants on the insulator surface and lead to decrease of flashover voltage. Specially, for the particle-contaminated insulator, it is found that the surface charge accumulation caused by DC voltage will have big influence on the flashover voltage, and the most critical condition is when DC and LI voltage are of the opposite polarity. In addition, the breakdown characteristics of SF6 gas gap with free metallic particle are investigated. It is found that under pre-stressed DC voltage, particle will do standing motion or bouncing motion. And breakdown voltage becomes lowest when particle is doing standing motion. The results of the experiments show that residual DC voltage in GIS could deteriorate its insulation ability and the combined voltage of DC and impulse is found to be a critical condition for GIS insulation. Moreover, the combined voltage is shown to be more sensitive to detect some insulation defects in GIS than applying impulse voltage alone, especially for free metallic particles, which are commonly found in GIS but up to now no effective detection method is proposed. Therefore combined voltage of DC and impulse could be considered to serve as a supplement of field test for GIS. [ABSTRACT FROM PUBLISHER]

Experimental investigation of transient characteristics is an important way to design transformer winding insulation structures. Of the previous measuring methods, there has been the problem of direct electrical connection with the winding, which may influence the transient characteristics. This paper presents a new non-invasive method for measuring the transient voltage distribution in transformer windings based on our previous study. The measuring method is based on the capacitive sensor with the stray capacitance between sensing electrode and transformer discs as the high-voltage arm and the film capacitance as low-voltage arm. An RC integrator and impedance adapter have been designed into the measurement circuit to enhance the sensor worked in the integrating mode and broaden its bandwidth respectively. The final measurement circuit can satisfy the need for measuring various kinds of waveforms, with 0.64 Hz lower cut-off frequency and 76 MHz higher cut-off frequency, respectively. The sensor array has been set up to measure the transient voltage distribution in a transformer winding. In this case, a mathematical conversion is put forward to decouple the influence of the adjacent discs on the corresponding disc to get the voltage distribution of different discs of the transformer winding. [ABSTRACT FROM AUTHOR]

Discharges in liquid obey time effect, pressure effect and scale effect. Traditional formulas are usually applied in a narrow range to keep the formula form consistent with Martin?s formula. In this paper, we have developed a new empirical formula for the electric strength of transformer oil depending on pulse duration, hydrostatic pressure and gap geometry in form Εbr = κe at-beff Ρ/,sub>spl gamma/ A>sub>-Β d-ϵ to be applied in a wider range. In this formula, the time dependent pressure effect is represented by constant γ, which can be expressed as γ=ape -bpt-c>peff . Time effect, area effect and distance effect are represented by e at-beff, Β and ϵ, respectively. For transformer oil, κ=0.42, a=0.53, b=0.23, ap=0.14, bp=0.93, cp=0.41, Β=0.1 and ?=0.25?0.06, with Ebr, teff, P, A and d in MV/cm, μs, MPa, cm2 and cm, respectively. [ABSTRACT FROM AUTHOR]

Follow Phenomenon, defined as a series of continuous breakdowns or withstands without sufficient time lag of successive applied voltages, is observed during the research of SF6 discharge characteristics under lightning impulse. Follow Phenomenon of SF6 gas plays a decisive role for the insulation recovery time between continuous discharges, but little relevant research has been reported before. In this work, Follow Phenomenon as a function of gas pressure, time lag of applied voltages in quasiuniform and non-uniform field, under positive and negative lightning impulse is studied. It is found that, when the time lag t=1 min, Follow Phenomenon is more obvious under positive lightning than that under negative lightning, and it becomes more pronounced under larger pressure; as time lag t increases, the increasing speed of Follow Phenomenon with pressure becomes lower. When time lag t=5 min, the measure uncertainty influenced by Following Phenomenon could be avoided when P⩽0.2 MPa in quasi-uniform field and P⩽0.3 MPa in non-uniform field. As pressure rises, a longer time lag is needed to eliminate the influence of Follow Phenomenon. At the end, analysis is made which shows that Follow Phenomenon is caused by the detachment of negative ion formed at previous discharge processes and by ?the Sweeping effect? of effective initial electrons. [ABSTRACT FROM AUTHOR]

To investigate the characteristics of micro bridges and their interactions with partial discharge (PD) in oil-paper insulation under DC voltage, an experimental platform with two-copper-electrode models is established. Generation and growth of micro bridge impurities are observed. PDs in experimental process are detected and analyzed. The result shows that the generation and growth of micro bridges have three stages based on their quantity and shape. At first stage PD accelerates the generation of micro bridges. At second stage the grown bridges trigger stronger discharges. At the last stage a so-called "hazardous bridge" that triggers highly intensive PDs emerges, through which breakdown happens. Therefore, bridges and PDs promote each other to cause insulation failure. In addition, impurities at the second stage are observed to transform into pile-like, bridge-like or particle-like shapes. The transformation process is considered to relate with the change of composition force on fiber bundles. Besides, a "cluster discharging" phenomenon is observed at the second stage, which is associated with the generation and disappearance of micro bridge impurities. [ABSTRACT FROM AUTHOR]

Dissolved gas analysis (DGA) of oil is used to detect the incipient fault of power transformers. This paper presents a new approach for transformer fault diagnosis based on selected gas ratios concentrated in oil and support vector machine (SVM). Firstly, based on IEC TC 10 database, the optimal dissolved gas ratios (ODGR) are obtained by genetic algorithm (GA) that is designed for simultaneous DGA ratios selection and SVM parameters optimization. Three traditional methods, namely, DGA data with SVM and back propagation neural network (BPNN), IEC criteria, and IEC three-key gas ratios with SVM and BPNN are employed for effectiveness comparison. The fault diagnosis results of IEC TC 10 database show that the proposed ODGR with SVM may be used as an alternative tool for transformer fault diagnosis. In addition, the robustness and generalization ability of ODGR is confirmed by the diagnosis accuracy of 87.18% of China DGA samples. The obtained results illustrate that it is preferable to apply the proposed ODGR to transformer fault diagnosis with the assistance of SVM. [ABSTRACT FROM AUTHOR]

A new method was introduced to evaluate and analyze synergistic effect including negative synergistic effect of SF₆/N₂ gas mixtures breakdown voltage. This method provides an intuitional comparison among different degree of synergistic effect through a simple equation of the normalization coefficient h. The disadvantages of traditional method, which fails to evaluate negative synergistic effect and would be inaccurate when breakdown voltages of the two gas compositions are close, are overcome via the method. In this paper, a typical example of the above is shown to analyze synergistic effect and abnormal negative synergistic effect of SF₆/N₂ gas mixtures in non-uniform electric field under lightning impulse. Meanwhile the validity of the normalization coefficient h with the synergistic effect of SF₆/N₂ gas mixtures under various voltage conditions and electrodes in references was verified. [ABSTRACT FROM PUBLISHER]

Flashover of contaminated insulators is one of the major problems for the power distribution. Laboratory experiments have shown that the arc characteristics have great influences on the study of flashover performance of contaminated insulators. In order to investigate the characteristics of AC arc discharge on the polluted insulation surface, an experimental system was designed. The emission spectroscopy of arc discharge was obtained by a CCD spectrometer, and lithium chloride was added in the artificial pollution to introduce lithium as the indicator element in the analysis of arc emission spectroscopy. Arc temperature was obtained by employing the relative line intensities method, and the electron density was determined by analyzing the Stark-broadening values of the profiles. It was indicated that the temperature of partial arc increased with the leakage current value, but electron density of partial arc barely changes with leakage current. Special attention was paid to the influence of gas pressure on the characteristics of partial arc. It was found that both the arc temperature and the electron density of the arc channel increased with pressure in the case of same leakage current. [ABSTRACT FROM AUTHOR]

Previous studies indicate that cellulose bridges in oil-pressboard insulation are self-generating, and can influence the characteristics of partial discharges (PDs) under DC voltage. In this letter, further experiments based on an optical-electric experimental platform were conducted to determine the generating sequence between cellulose bridges and PDs, and to provide visual evidence of bridges' effects on PD process. The results show that the generation of cellulose bridges can be divided into three stages, which can be finished at a DC voltage lower than PD inception voltage (Updiv). With the increase of the applied DC voltage, the generating time of the first bridge decreases. In addition, by using shadow photographic method, a bridge-triggered discharge in oil was observed. Further analysis indicates that cellulose bridges generate gaseous paths to discharges in oil due to its high conductivity. [ABSTRACT FROM AUTHOR]

The electrical explosions of copper and aluminum wires in air were researched at different pulsed current rate. The different current rates of $dI/dt~\sim 3$ and 30 A/ns were achieved with two series of circuit parameters. Based on the measurement waveforms of the current and voltage in wire explosion, the three stages of melting, liquid state, and vaporization were divided by the definition of four critical time points, and the deposited energy in the three stages was calculated by mathematical method. The differences of process between the fast and slow explosion and effect of current rate on deposited energy in the three stages mentioned above were analyzed by experiments and calculation, respectively. Although the charging voltage of capacitor and peak power were higher in fast explosion, the peak voltage in the fast explosion was lower than that in the slow explosion. For Cu wire, the deposited energy in the stage of liquid state and vaporization was higher in the fast explosion. Compared with Cu wire, the effect of the current rate on deposited energy for Al wire was explicit in the stage of liquid state but obscure in the vaporization stage. [ABSTRACT FROM PUBLISHER]

Pressure is an important factor for the effect of the medium in the process of the electrical explosion of wire in air. In this paper, the electrical explosions of copper and aluminum wires at different air pressures in the range of subatmospheric pressure were investigated with pulsed voltage in a submicrosecond time scale. Based on the measurements of the current and voltage in wire explosion with a current monitor and a high-voltage probe, respectively, the deposited energy in the stages of melting, liquid state, and vaporization was calculated by mathematical methods. The effect of air pressure on deposited energy in the three stages mentioned above was analyzed by experiments and calculation. The results show that the breakdown field strength varied with air pressure, and plays a vital role in energy deposition and the time of duration in the vaporization stage. In addition, the exothermic reaction of Al microparticles and surrounding air also has an important effect on the energy deposition in the vaporization stage. The deposited energy in the electrical explosion of copper and aluminum wire is higher at the higher air pressure, particularly after the beginning of vaporization. [ABSTRACT FROM PUBLISHER]

In this paper, the expansion characteristics of the plasma generated by electrically exploding single aluminum wire are studied through imaging spectra of plasma self-emission. Energy deposition in 40 \mum and 100 \mum exploding aluminum wires with two different rates of increase of current is presented and discussed. Imaging spectra with radius-resolved distribution in a wavelength range of 284 to 415 nm are used to study the composition and expansion rate of specific spectral lines in the explosion plasma. The plasma is symmetrical about its axis and mainly consists of AL I, Al II, and Al III. The expansion rates of intensities of some spectral lines in the plasma are determined by the specific energy deposited into the wire up to the voltage collapse. High current rate and large radius of the wire are helpful to suppress the plasma around the wire and the shunting of the current that causes the decrease of the deposited specific energy. [ABSTRACT FROM AUTHOR]

In this paper, the plasma generated by the electrical explosion of a single aluminum wire in high vacuum is investigated, through self-emission spectrum in the range of near UV to visible wavelength (347.5–477.6 nm) captured by imaging spectrometer with the duration of the spectrometer time frame of 3 ns. Electron temperature and density of the plasma are determined through the spectrum using Boltzmann Plot and Stark Broadening Effect of isolated lines of Al III. The results show that the electron temperature in the plasma is in the range of 3.7–8.6 eV and the electron density has the order of magnitude about 10^18~cm^-3. Population inversion of Al II is found according to the intensity of the isolated lines in the spectra. [ABSTRACT FROM AUTHOR]

As a new gas-phase synthesis method for the production of nanosize powders, the wire electrical explosion method has the advantages of high energy efficiency and high product purity through production under pure inert gas conditions and has been applied to the continuous industrial production of nanopowders. In this paper, an experimental device based on the electrical explosion of metallic wires for nanopowder production and collection is designed and built. Also, aluminum nanopowders were produced by electrically exploding an aluminum wire and collected by the microporous membrane filter successfully under different pressures of argon gas. Moreover, the influence of the argon gas pressure on the characteristics of the aluminum nanopowders was analyzed by a transmission electron microscope. The results showed that the particle shape, size, and distribution of the aluminum nanopowders could be controlled by the pressure of argon gas. The aluminum nanoparticles produced in the high-pressure argon gas had better spherical particle shape; meanwhile, the count mean diameter of the aluminum nanopowders increased obviously with the rise of the argon gas pressure. A higher pressure of argon gas could broaden the range of the aluminum nanoparticle size distribution evidently. [ABSTRACT FROM AUTHOR]

The properties of the medium have an obvious effect on the process of the electrical explosion of the wire. In this paper, the electrical explosions of copper and aluminum wires were investigated in air and water with pulsed voltage in a time scale of a few microseconds. Based on the measurements of the current and voltage waveforms in wire explosion with a self-integrating Rogowski coil and a voltage divider, respectively, the deposited energy in the stages of melting, liquid state, and vaporization was calculated by mathematical method. The effect of air and water on deposited energy in the three stages previously mentioned was analyzed by experiments and calculation. The results show that the pressure of the medium around the wire generated by compression from wire material expansion is an important factor of the effect of the medium on the energy deposition and the time of duration in the different stages. Additionally, the species of the medium also has an important effect on the formation of plasma discharge channel. The deposited energy in the electrical explosion of copper and aluminum wire in water is more than that in air, particularly after the beginning of vaporization. [ABSTRACT FROM PUBLISHER]

Pulsed power technology is one of the technological foundations in high and new technology research, which has extremely broad development and application prospect. Gas switch is one of the key elements in pulsed power devices, and electrode erosion is a key restrictive factor in high-power gas switch development and application. According to the 1-D equation of heat conduction and thermal equilibrium equation near the electrode surface, this paper researches the electrode heat conduction mechanism, establishes the electrode erosion calculation model, calculates electrode erosion heat fluxes and their peak powers caused under different discharge conditions, and proceeds to calculate the depth of electrode etch pit. Calculation results indicate that arc joule heat is the main reason for electrode erosion when discharge current is not too high. In the light of different discharge parameters, that is, peak current, electric quantity, and current waveform, as well as single discharge electrode erosion experiments, the calculation formula of etch pit depth is fitted. The results indicate that model and fitted calculation results are relatively approximate to the experiment results. [ABSTRACT FROM PUBLISHER]

The physical processes that occur during the initial stages of nanosecond explosion of thin wires play an important role in the development of plasma for nanoparticle preparation. The transition of the wire material from the condensed state to conducting plasma has not yet been fully understood. In this paper, underwater electrical explosions of copper wires were investigated with pulsed voltage in the timescale of a few microseconds. A self-integrating Rogowski coil and a voltage divider were used for the measurements of the current and the voltage on the wire load, respectively. The current rise rate was adjusted by changing the circuit parameters, including the discharge voltage and circuit inductance, and the copper wire properties consisting of the length and the diameter. The effect of the current rise rate on the phase transitions involving melting from solid to liquid and vaporization from superheated liquid to the vapor–drop mixture was obtained by the analysis of the current and voltage waveforms of copper wire explosion. The energy of melting and vaporization was calculated based on experimental waveforms of current and voltage. In addition, the effect of applied voltage, circuit inductance, and the length and diameter of copper wire was acquired on the electrical explosion characteristics of copper wires. [ABSTRACT FROM PUBLISHER]

This study explores the breakdown properties of three C 4 F 7 N binary gas mixtures under lightning impulse and the reduction of dielectric strength in the presence of insulation defects. The 50% breakdown voltage (U 50%) of three C 4 F 7 N binary gas mixtures in a quasi-uniform electric field is observed to determine the basic dielectric strength, which has the ranking C 4 F 7 N/Air ≈ C 4 F 7 N/CO 2 > C 4 F 7 N/N 2. A coefficient S E is introduced to quantitatively describe the sensitivity of dielectric strength to electric field inhomogeneity. The S E of different gas medium can be ranked as SF 6 < C 4 F 7 N/CO 2 < C 4 F 7 N/Air < C 4 F 7 N/N 2 , indicating that C 4 F 7 N mixed with nitrogen, carbon dioxide, or dry air is less capable of tolerating insulation defects than SF 6 , which merits consideration in insulation designs. [ABSTRACT FROM AUTHOR]

Partial discharges (PDs) initiated by conductive particles attached to the insulator surface in gas-insulated metal-enclosed switchgear (GIS) may occur at different particle tips with varying spatial information. PD modes also differ depending on the spatial position and voltage phase. In this study, PDs were investigated as per phase-space joint resolved characteristics using two photomultiplier tubes and phase-resolved discharge images. PDs were observed to occur mainly at the negative particle tip regardless of the half-cycle. A sudden increase in apparent PD charge is attributed to the corona-creepage transition. The PD mode transition mechanism and effectiveness of the on-site dielectric test for defect detection were also determined. The results presented here may provide theoretical support for GIS defect identification and threat assessment. [ABSTRACT FROM AUTHOR]

Icicles on the ice-covered insulator string will shorten the air gap between the adjacent sheds of the insulator and make the discharge propagate along the air gap easier. The processes and the leakage current waveforms of the discharge propagation along the air gap are recorded and investigated under two different experimental conditions. The common and different discharge phenomena are shown and compared. [ABSTRACT FROM AUTHOR]

The nanosecond single-pulse voltage was employed here to generate a large-volume discharge between the wire-plane electrodes in nitrogen. In order to find out the effect of pulse polarity on the propagation of a large-volume discharge, both the positive and negative pulses were applied to the wire-plane electrodes, respectively. Images presented here reveal that the discharge channels are more homogeneous under the negative pulse, and the propagation speed of the discharge channels is faster under the positive pulse. [ABSTRACT FROM AUTHOR]

This paper illustrates a unipolar repetitive nanosecond surface discharge in the subsonic airflows at different velocities. The plasma images presented here indicate that the discharge modes vary with the changes in flow condition. The discharge is promoted in the airflows at a lower velocity. The discharge tends to be either inhibited or even extinguished when the flow velocity accelerates. However, the inhibited mode disappears probably due to the strong memory effect at high repetitive frequency of the pulse. [ABSTRACT FROM AUTHOR]

Very fast transient overvoltage (VFTO) in gas insulated switchgear (GIS) is of great concern at the higher rated voltages because the difference between the rated lightning impulse (LI) and the VFTO withstand voltage decreases. In order to clarify VFTO withstand level of 1100 kV GIS, a simulation system was designed for generation of VFTO. The breakdown characteristics of sphere-plane gap and rod-plane gap in SF6 under VFTO and LI were investigated for different gas pressures. Experimental results show that the breakdown voltages in slightly inhomogeneous field gaps under VFTO are higher than that under LI. However, for the rod-plane gaps, the breakdown voltages and voltage-time curves under VFTO fall below that of LI at higher pressure in the case of negative polarity. The polarity effect of rod-plane gaps under LI is reversed at a certain pressure. Based on the analyses, it is concluded that the different breakdown characteristics of SF6 gaps may be attribute to the difference between space charge behaviors during the discharge development under the impulse with different wave front and oscillation tail of VFTO and LI. [ABSTRACT FROM PUBLISHER]

Gas-insulated switchgear (GIS) might withstand ac/lightning impulse (LI) superimposed voltage caused by lightning intrusion or switching operation during operation. When there are insulation defects, the breakdown probability is greatly increased, which threatens the safety of GIS and needs to be further studied. In this study, the discharge characteristics of two typical insulation defects, including a metallic protrusion on bus bar and foreign matter attached to insulator, under ac/LI superimposed voltage were studied, and the influence of superimposed phase, polarity, and ratio was investigated. It was found that, under a quasi-uniform electric field, the superimposed breakdown voltage (${U}_{s}$) was the same as the LI breakdown voltage (${U}_{\text {LI}}$). However, when there were insulation defects, ${U}_{s}$ was significantly affected by the superimposed phase and polarity. For the metallic protrusion, the most critical condition was an −LI superimposed at $\varphi =90$ , with ${U}_{s}$ decreasing by 11% relative to ${U}_{\text {LI}}$. For the metallic particle attached to insulator, when an +LI was superimposed at $\varphi = 0^{\circ }$ or −LI at $\varphi = 180^{\circ }$ , ${U}_{s}$ decreased most, by as much as 19.6% relative to ${U}_{\text {LI}}$. Finally, the validation test was performed in a 110-kV GIS with insulation defects and ${U}_{s}$ was 7%–14% lower than ${U}_{\text {LI}}$. The results showed that, under a proper superimposed phase and polarity, ${U}_{s}$ could be significantly lower than ${U}_{\text {LI}}$ of GIS with insulation defects, indicating the great potential of the ac/LI superimposed voltage test for the high-sensitive detection of GIS insulation defects. [ABSTRACT FROM AUTHOR]

Epoxy resin (EP) is a typical and widely used insulating material for medium-frequency transformers (MFTs). The dielectric loss of EP is one of the key variables in the design and operation of MFT. In this article, six types of epoxy/anhydride systems are prepared, and the dielectric loss of MFT is calculated by finite element method (FEM) analysis based on broadband dielectric spectroscopy measurements. The material dependence of the dielectric loss of MFT is investigated using dielectric spectroscopy and molecular dynamics simulations over a wide temperature and frequency range. The results show that the variation of dielectric loss with output power is attributed to the temperature characteristics of the imaginary part ($\varepsilon ^{\prime \prime }$) of complex permittivity. Compared to the dielectric loss at no-load state, the one at rated power is reduced by 5–10 W. The anhydride hardener containing benzene ring or a methyl substituent has an inhibitory effect on dielectric loss, and the EP cured by methyl nadic anhydride (MeNA) has the lowest dielectric loss (16.2 W) at rated power. The dielectric loss and overload characteristics of MFT are closely related to the $\beta $ -relaxation and $\alpha $ -relaxation of EP, respectively. The acid anhydride curing agent containing benzene ring or a methyl substituent reduces the temperature corresponding to the $\beta $ -relaxation peak by more than 50 °C compared to the one without. The increased rigidity of the chain segment of EP makes the $\beta $ -relaxation move to the low-temperature side, which is beneficial to suppress the dielectric loss. This article not only serves as a bridge among dielectric loss of MFT, dielectric spectroscopy, and molecular structure of EP but also lays a solid foundation for the selection of insulating materials for MFT. [ABSTRACT FROM AUTHOR]

In this paper, research studies on the breakdown progresses of multigap pseudospark discharge under nanosecond pulsed voltages are reported. The experimental results show great differences from dc voltages. The discharge mechanism of the pseudospark under nanosecond pulsed voltages has been successfully explained by the fast electron ionization effect. In each gap, collision ionizations caused by the fast electron start step by step, and development progresses are almost the same and approximately simultaneous until the main gap breaks down. [ABSTRACT FROM PUBLISHER]

When an external short circuit occurs, the axial electromagnetic force increases dozens of times. Under the effect of the axial short-circuit electromagnetic force, the transformer windings vibrate violently. The spatial distribution of the disks is constantly changing during vibration, which can change the temporal and spatial distributions of the leakage magnetic field and the axial electromagnetic force. This process is called the strong coupling phenomenon of the structural field and the leakage magnetic field. In this article, a strong coupled magnetic-structural model is proposed by using the analysis method. The strong coupling equations, where the independent variables are the axial position of the disks, are obtained. The vibration process with strong coupling phenomenon considered can be obtained. The accuracy of the strong coupling model is verified by the iterative method. By comparing the results calculated by the strong coupling model with those calculated by the weak coupling model, the influence of the strong coupling phenomenon on the vibration process is obtained. Due to the influence of the strong coupling phenomenon, the winding vibration intensity increases and the maximum displacement of the disks can be doubled, which implied that the strong coupling phenomenon cannot be ignored when investigating the short-circuit strength of power transformers. [ABSTRACT FROM AUTHOR]

Free metallic particles are the main factor inducing flashover incidents and threatening the safety of gas-insulated switchgear (GIS) and gas-insulated transmission line (GIL) devices. Acoustic diagnosis is widely used to detect free metal particles due to its high sensitivity, but it generally fails under multiple particle conditions because of the overlap of acoustic signals. This paper proposes a new method of acoustically recognizing multiple free metallic particles. The pattern of Δt-Normalized N that can be used to distinguish single/multi-particles is determined first. Second, each particle's neighboring collision signals are extracted from raw data by the established physical relation between adjacent collision signals. Wire, lamellar, and spherical free metal double-particles’ mass values are calculated as per the numerical connection between adjacent collision signals. The geometry of the particles is assumed to be known, so the density of the spherical particles can be effectively estimated. The proposed method is validated in a scale concentric hemisphere platform. The results of this work may provide a workable reference for GIS free metal multi-particle monitoring. [ABSTRACT FROM AUTHOR]

In this paper, breakdown characteristics of multigap pseudospark under nanosecond pulsed voltages were studied. Experimental results reveal that the characteristics under nanosecond pulsed voltages show the trend that the breakdown voltages decrease with the increase of the pressure and gap distance, increase with the increase of the number of gaps. Based on the experimental results, an empirical relationship was obtained. In the end, fast electron ionization effect has been observed, which offers successful explanations of the progress of multigap pseudospark discharge under nanosecond pulsed voltages. [ABSTRACT FROM PUBLISHER]

In this paper, an experimental investigation of pseudospark discharge behaviors was conducted under nanosecond pulsed voltages using a flexible discharge chamber. The breakdown voltages of the pseudospark discharge gap were measured for a wide range of gas pressures, anode-cathode gap distances, and cathode aperture diameters to find the match of gap distance and cathode aperture diameter at a certain gas pressure with the nanosecond pulsed voltage. According to the experimental results, an empirical relationship was obtained. The roles of the parameters in the empirical relationship during the discharge were analyzed. The breakdown characteristics were also consistent with the Paschen’s law in the pseudospark under nanosecond pulsed voltages, when the gap distance was replaced by the effective gap distance. In the end, a possible reason was put forward to explain the difference between the empirical relationships obtained under dc voltages and nanosecond pulsed voltages. [ABSTRACT FROM PUBLISHER]

High frequency voltage oscillations will be generated when transient overvoltages intrude into power transformers. These oscillations may cause damage to the insulation of transformers. In this paper, a capacitive coupling sensor array was presented for measuring voltage distributions and voltage gradient distributions in low-voltage winding of a 1000 kV power transformer. A variable impedance transmission line between impulse generator and winding was also designed to avoid distortion of the applied impulses. The results indicate that under the lightning impulse voltages, voltage distributions have been restrained below applied voltage at sandwich-interleaved section but exceed the applied voltage in continuous coils in the first half of the winding and lead to a very nonuniform voltage distribution. Longer duration time will result in a more nonuniform distribution. The amplitude of the transient voltage under steep front square wave reduces faster downward the winding than that under lightning impulse voltages. A shorter rise time will lead to a much worse distribution. In addition, a dominated oscillation of 63 kHz is found under both impulse voltages and steep front square waves. [ABSTRACT FROM AUTHOR]