Search

Breakdown in dielectrics, the mechanism of which is still not yet fully revealed, exhibits very complicated test phenomena and characteristics. The breakdown voltage of a dielectric is believed to be related not only to the dielectric properties but also to the waveform of the applied voltage. For most dielectrics, the impulse breakdown voltage decreases with increasing wavefront time. However, we found for oil-impregnated paper (OIP) used in power transformers, increasing wavefront time in lightning impulses (at microsecond timescales) may lead to a significant increase in breakdown voltage. In contrast, the variation in the wavetail time produces very little effect on breakdown voltage. Combined with our previous test results on the space charge distributions in OIP, we tried to provide two explanations regarding this interesting phenomenon: First, the wavefront time of the applied impulse voltage affects the movement process of the positive particles in the oil-filled micro cavities in OIP, which is strongly related to the local electric field in the oil filled micro cavities and the breakdown voltage of the OIP. Moreover, the wavefront time is closely related to the trapping and detrapping process of the dielectrics. Therefore the effects of the traps in dielectric on the breakdown characteristics are different when the wavefront time varies. [ABSTRACT FROM AUTHOR]

As striking lightning impulse waveforms in power transformers are quite different from the standard lightning impulse waveform (SLIW) used in the withstand voltage tests, it is essential to identify the breakdown characteristics of oil-impregnated paper (OIP) under non-SLIW and to quantitatively compare them with those under SLIW. On that basis, the impulse insulation levels of power transformers can be rationally determined and then the insulation specifications can be evaluated to maintain high equipment reliability. In this paper, with damped alternating oscillation waveform as a representative basis, the breakdown characteristics of oil-impregnated paper were experimentally obtained by changing the frequency and damping rate of the applied voltage. All experiments were conducted using a column-column electrode model. In addition to the 50% breakdown voltages (U50), Minimum breakdown voltage Umin and V-t characteristics under different circumstances, the influence of various parameters on breakdown characteristics and the possible reasons were discussed. After comparing waveforms and corresponding results, the specific relationship between waveform parameters and U50 were expressed as a single equation. As a result, the breakdown voltage was 1.106 to 1.435 times higher than that under SLIW depending on frequency and damping rate. The steepness of the wave peak and the duration of high voltage around wave crests are considered to be two important factors affecting the breakdown voltage. An effective way was proposed to predict U50 of oil-impregnated paper as a reference for insulation specifications irrespective of wave shape. These results support the rationalization of insulation for actual lightning impulse waveforms in the power transformers. [ABSTRACT FROM AUTHOR]

Oil-paper insulation, as a major insulation mode used in power transformers, is continuously and unavoidably subjected to repeated lightning impulses, which may bring potential risk to the insulation system. Therefore, in this work, accumulative effect of repeated lightning impulses and its damage mechanism in oil-impregnated paper (OIP) are investigated. Firstly a repetitive impulse accumulating test platform was set up, and tests were performed to confirm the existence of accumulative effect of repeated lightning impulses. Then atomic force microscope observation and frequency domain spectroscopy were carried out on OIP samples with different times of lightning impulses, based on which the influence of lightning impulses accumulative effect on surface morphology and dielectric properties of OIP was investigated. The results indicate that repeated lightning impulses produce accumulative damage to the OIP and hence lead to the morphology change and generation of translucent gelatinous substance on the sample surface. Both the surface roughness and dielectric parameters increase with the accumulating of repeated lightning impulses, which can be attributed to the effects of polar products, fiber deformation and the translucent gelatinous substance together. At last, the possible mechanism of the accumulative effect was discussed and relevant tests were carried out to verify the speculation. [ABSTRACT FROM PUBLISHER]

The non-standard lightning impulse waveforms (N-SLIWs) including oscillating lightning impulses, very fast transient voltages (VFTOs) and steep waveforms in the power system gravely endanger the inner insulation of power transformer. To prevent insulation failure and provide guidance for robust transformer design, in this paper, two typical insulation models of oil-immersed transformer were established and their breakdown characteristics under N-SLIWs were investigated. The research mainly focused on the influence of front time and oscillation frequency of N-SLIWs on the breakdown characteristics. The experimental results showed that the breakdown voltage increased with the decrease of front time. For low frequency-oscillating lightning impulse waveforms (LF-OLIWs), breakdown voltage increased with the increase of oscillation frequency and was up to 30% higher than that under standard lightning impulse waveform (SLIW). For VFTOs, with the increase of frequency, breakdown voltages presented a law of inverted "U" type. Insulation model could withstand the highest value of VFTO when the oscillation frequency was around 2.4 MHz. But when the frequency was as high as 9 MHz, breakdown voltage was close to that under SLIW. A very high frequency oscillation was the main reason for the reduction of breakdown voltage as its steepness dV/dt assisted the effective transmission of the sphere?s potential to the head of the streamer channel. [ABSTRACT FROM AUTHOR]

Repeated impulses have long been considered to affect the insulation properties of electrical equipment. This interesting but rarely studied phenomenon may pose great risks to the reliable operation of power equipment. Therefore, in this work, we focus on the accumulative effect of repeated lightning impulses on the oil-impregnated paper (OIP) used in power transformers. Specifically, we study the effect of space charge on the accumulative characteristics of OIP. A repetitive impulse accumulating test platform was set up, and the tests were conducted to obtain the accumulative characteristics of OIP under repeated lightning impulses. The accumulative characteristics of OIP with various interval time were experimentally identified to study the influence of interval time on the accumulative effect. Then, space charge in OIP samples were investigated using the pulsed electro-acoustic technique. Charge behavior in OIP was analyzed under different conditions, i.e., changing the applied times, amplitude and interval time of the applied lightning impulses. Charge injection and transport occurred during lightning impulse accumulation. The increased application of lightning impulses, high applied voltage, and short interval time contributed to the accumulation and transport of space charges. At last, the possible mechanisms of the accumulative effect are discussed in this article based on the experimental results. [ABSTRACT FROM AUTHOR]

On-line monitoring of insulation systems in power components such as power transformer bushings has significant value for their reliability. Dielectric spectroscopy is known as a powerful tool in condition assessment of power components? solid insulation. However, it is difficult to apply it on-line. Therefore, a new approach of intended use for on-line diagnosis of oil-impregnated paper, as a major insulation component of power transformers and bushings, is presented. In this technique, natural transients such as lightning and switching impulses are used as stimuli for on-line dielectric response measurements. The wide frequency range of these transients is their advantage as stimuli for dielectric spectroscopy. The influence of moisture content and temperature on the dielectric response of oil-impregnated paper has been investigated for performance evaluation of the proposed technique. It is shown that this technique can track the changes caused by moisture content and temperature in the dielectric response of oil-impregnated paper. The results of both low and high voltage study cases have been verified by comparison with Frequency Domain Spectroscopy (FDS). According to the well-matched results with FDS, the proposed technique can offer a valuable approach for frequent monitoring of dielectric properties of oil-impregnated paper insulated systems such as those commonly encountered in power transformer bushings. However, the validity range of the results depends on the bandwidth of the applied transients and other discussed measurement considerations. [ABSTRACT FROM AUTHOR]

The electric field uniformity imposes a significant effect on the breakdown mechanisms of dielectric liquids. In general, breakdown is dominated by streamer initiation under uniform field, while it is dominated by streamer propagation under non-uniform field. This paper presents a study on identifying and quantifying the transition field factor boundary, represented by the field enhancement factor ƒ, between the two dominating breakdown mechanisms. The methods for deducing the transition field factor boundary are introduced. The first method from our previous study was developed by measuring and processing both the streamer initiation and breakdown voltages. In this paper, a new method is developed by only processing the breakdown voltage data. Based on the breakdown results reproduced from the literature, the transition field factor boundaries are derived for the mineral oil under both positive and negative lightning impulses. It is found that the negative transition field factor boundary is lower than the positive one. A four-region model is proposed to describe the effect of impulse polarity on breakdown mechanisms of dielectric liquids and to interpret the transition of breakdown mechanisms with respect to the field uniformity. [ABSTRACT FROM AUTHOR]

This paper presents the results of experimental characterization of creeping discharges propagating over pressboard immersed in insulating oils, under AC and standard lightning impulse voltages. For comparison purposes, synthetic ester, rape seeds oil (natural ester) and mineral oil were also investigated. Two experimental arrangements were used: (1) a point-to-plane electrodes system where the pressboard is inserted between the electrodes such as the point is perpendicular to pressboard allowing a radial propagation of discharges; and (2) a point-to-bar electrodes system allowing a propagation of discharges in one direction, tangentially to pressboard. The characteristics of discharges propagation, their shape, stopping length (the maximum extension of creeping discharge) as well as the associated current and electrical charge versus the thickness of pressboard and the type of oil are analyzed. It is shown that the magnitude and polarity of voltage, the type of oil as well as the thickness of pressboard influence the characteristics of discharges and especially the stopping length and density of branches. The stopping length of creeping discharges is shorter for pressboard/naturel esters' interfaces than with pressboard/mineral oil interface. [ABSTRACT FROM AUTHOR]

During the decades-long operation, oil paper insulation in power transformers may suffer the effect of thousands of lightning impulses. Such interesting but rarely studied phenomenon, also known as accumulative effect, may cause irreversible damage to the oil paper insulation, or even premature failure. Therefore, in this work, the influence of repeated lightning impulses on the oil paper insulation, especially the mechanism behind this phenomenon, was experimentally investigated. We determined the accumulative breakdown characteristics of the oil paper insulation and observed the influence of the repeated lightning impulses on the dissolved gas in oil and the degree of polymerization. Specifically, the space charge distribution in the oil paper samples and the relationship between the interval time and accumulative breakdown characteristics were experimentally determined in the end. The test results indicate that the accumulative effect only exists in the cellulose paper rather than the transformer oil. Such effect was considered to be the consequence of electrical degradation and accumulation of space charge. [ABSTRACT FROM AUTHOR]

This paper presents the results of experimental studies about the prebreakdown phenomena in mineral oil under lightning impulse. Experiments were carried out in a non-uniform field setup with HV electrode covered by crepe paper. The results of the experiments were compared with the results of research performed for bare HV electrodes. The onset voltage, propagation velocity, time to initiation, shadowgraph photos of discharges and light emission were respectively measured or recorded during the experiments. Analysis of the research outcomes indicates that the fundamental role of paper insulation is to raise the onset voltage by shoving away the onset sites out of the region of high field stress. The insulation wrapping does not have an influence on the shape of discharge patterns and on the propagation velocity. Some of the results connected with the times to initiation points that the mechanism of discharge initiation at the HV electrode surface does not depend on the fact it is insulated. The source of ?weak points? of the paper-oil insulation system is the oil, not the surface of insulation wrapping or metal of electrode. Observation of the oscillograms of light emitted by discharge channels indicates the difference in the mechanism of discharge propagation. The surface and space charge carriers left in the interelectrode space after discharge flash play an important role. In the case of insulated electrodes, the surface charge remains longer on the surface of electrode, reducing the electric field stress which is responsible for reignition. Simultaneously, the space charge reduces the degree of channel ramification. Both the observations are clearly visible on the oscillograms of light as differences in the frequency of light impulses and their peak values. [ABSTRACT FROM AUTHOR]

This paper is aimed at the variation of AC and lightning impulse (LI) breakdown voltages of samples enriched with fullerene (C 60) nanoparticles (NPs) of natural ester, which stand out as an alternative to mineral oils. C 60 nanofluids with long-term stability and good dielectric properties are examined in this study on five samples for concentrations of NPs ranging 0.05 to 0.4 g/L concentrations. The AC and LI breakdown voltage measurements of each sample are repeated thirty times and the conformity of these measurements of the normal distribution is tested with the Shapiro-Wilk test. Conformity of all samples to normal distribution is accepted by this test and withstand voltages are calculated at 1%, 10% and 50% breakdown probabilities. The AC breakdown voltage performances of 0.3 and 0.4 g/L C 60 nanofluids are increased compared to natural ester. This increase rate is 5.1% and 7.8%, respectively. The withstand voltages at 1% and 10% breakdown probabilities increase by 20.2% and 13.2% compared to natural ester at 0.4 g/L C 60 nanofluid. 0.1 g/L C 60 nanofluid has 8.2% better performance than natural ester at LI breakdown voltages. Withstand voltages of 1% and 10% probability of this nanofluid are 6.5% and 7.7% higher than natural ester, respectively. These measurement results show that C 60 doped nanofluids can meet performance requirements in power system equipment with high power demands. [ABSTRACT FROM AUTHOR]

SF6 is widely used in high voltage equipment because of its outstanding characteristics as insulation medium. However, SF6 is a potent greenhouse gas, so its use and emission are regulated. CF3I has been investigated as a candidate for SF6 alternative gas, whose environmental impact is much less than that of SF6 and other fluorocarbon-based gases. In addition, a flexible gas-insulated transmission line (FGIL) is expected to be used for power transmission from offshore wind farms due to its high flexibility and low capacitance. In this paper, the feasibility of 22 kV class FGIL utilizing atmospheric pressure CF3I gas is evaluated from the viewpoint of withstand voltage with actual lightning impulse tests. The cross-sectional size of FGIL is determined by considering insulation condition, heat transfer condition, sheath electric field condition and spacer stress condition. The impulse tests for FGIL are conducted in straight and bent states, and it is revealed that the 50% breakdown voltage V50 of straight state exceeded lightning impulse withstand voltage (LIWV) for 22 kV class, and that V50 of bent state falls below LIWV. [ABSTRACT FROM AUTHOR]

In this work, effect of the oscillation frequency on the oil surface flashover characteristics under lightning impulse voltage is experimentally studied. Four lightning impulse (OLI) waves with different oscillation frequencies and 1.2/50 µs standard lightning impulse (SLI) wave are used in this study. The experiment results show that flashover voltages under OLI are larger than those under SLI and the flashover time lags under OLI are longer than those under SLI. Additionally, the flashover voltage tends to increase and the time lag tends to be shorter as the oscillation frequency increases. According to the experimental results and the discharge theory of liquids, we believe that the waveform oscillation frequency affects both the effective electron generation time lag and the formative time lag. This work provides a reference for the waveform selection of the on-site withstand voltage test and explains the physical mechanism of the oil flashover discharge from the perspective of impulse waveform oscillation. [ABSTRACT FROM AUTHOR]

This paper discusses the application of the novel technology - functionally graded material (FGM), which combines both spatial permittivity (ε) and conductivity (σ) distributions with the aim to control the electric field around DC-GIS spacer. Some distribution types of ε and/or σ in the spacer bulk, such as U-type and graded to higher conductivity (GHC) are investigated through electric field simulation, in comparison to the uniform spacer with constant ε and σ distributions. The electric field relaxation effect of each distribution type of ε/σ-FGM under DC steady state, DC polarity reversal, DC-on, and lightning impulse voltage superimposed on DC steady state condition are obtained. The results show that ε/σ-FGM with U-type permittivity and GHC-type conductivity distribution of which low σ is applied near the high voltage side of the DC-GIS spacer is the most effective in reducing the maximum electric field under all stated conditions. [ABSTRACT FROM AUTHOR]

In this paper, the synergistic effect of CF3I mixed gas is theoretically analyzed using the Boltzmann method and experimentally studied under different applied voltage, pressure and electric field inhomogeneity. The experimental results show the inversion of synergetic effect, which reflects the influence of external conditions on synergetic effect. The results show that CF3I/N 2 and CF3I/CO2 mixtures under slightly uneven electric field at normal pressure have synergistic effect. CF3I/CO2 mixtures under power frequency voltage and lightning impulse have better synergistic effect than CF3I/N2. The effect of macro parameters on synergetic effect is different for different buffer gases. For CF3I/N2 mixture, the synergistic effect of the mixture can be improved by increasing the pressure and increasing the electrode gap distance. For CF3I/CO2, due to the abnormal breakdown of the mixture, negative synergistic will occur at some pressure and gap distance. [ABSTRACT FROM AUTHOR]

This paper presents an experimental investigation into the breakdown voltage fall time t f of lightning voltage breakdown of 2–4 centimeter vacuum gaps having a long breakdown delay time t d (>30 µs). The results show that the average 50% breakdown voltage fall time t 50 decreases with increasing vacuum gap. It is considered that the lightning impulse breakdown of the centimeter vacuum gaps with a long breakdown delay time may be initiated by the interaction between the cathode thermal processes and the micro-particles phenomena. The increase of the electrode diameter and the decrease of the electrode radius of curvature may contribute to the decrease of the breakdown voltage fall time. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study focusing on the quantitative analyses of spatial characteristics of Mach-cone shaped shockwaves generated by streamers in a mineral oil and a synthetic ester in a fixed 12 mm needle-plane gap under positive lightning impulse. The influences of voltage level and liquid type are investigated. Results showed that Mach-cone shaped shockwaves are always observed ahead of actively propagating streamer channels. In the mineral oil, instantaneous streamer velocities undergo initial bursts before gradually decreasing and stabilizing at around 2 km/s. Shockwave pressures decrease from 6–8 GPa upon the streamer initiation to around 4 GPa as streamer propagates to a longer distance. Compared to the mineral oil, the initial bursts of instantaneous streamer velocities and shockwave pressures are lower in the synthetic ester. Investigated voltage levels from initiation to near breakdown do not impose significant effects on the observed shockwave parameters. In addition, the appearance of Mach-cone shaped shockwave is found to be an effective indicator of actively propagating streamer channel which can be used as a tool to quantify streamer main branches. [ABSTRACT FROM AUTHOR]

In this paper, the influence of surface charges is quantitatively analyzed based on the study of surface charge distribution under DC voltage and flashover characteristics of insulators under superimposed voltage of DC and lightning impulse. It is found that when DC and impulse voltage are in the same polarity, surface charges weaken the electric field along the insulator surface and flashover voltages under superimposed voltage increases. When DC and impulse are in the opposite polarity, surface charges enhance the electric field along the insulator surface, and flashover voltages will decrease. The calculation results of the electric field along the flashover path show that the changing rate of the maximum field strength is approximately equal to the changing rate of flashover voltage, which proves that the influence of surface charges on flashover characteristics of insulators mainly depends on their distortion of the electric field. [ABSTRACT FROM AUTHOR]

According to the IEC Standard, the insulation performances of power apparatus under lightning impulse voltages are evaluated by the withstand test of standard lightning impulse voltage, with the wave front/tail time of 1.2/50 ?s. However, the standard lightning impulse voltage originally obtained from the transmission lines or triggering lightning towers, is quite different from the invading lightning impulse voltage in the power transformers. Hence, in this work, an attempt has been made to obtain the waveform characteristics of invading lightning impulses in power transformers using an overvoltage online monitoring system. Energy Method was then proposed to convert the recorded oscillatory waveform into double-exponential waveform defined by IEC Standard. And the statistical waveform parameters of recorded invading lightning impulses, namely, the non-standard lightning impulses, were determined based on the Energy Method. Current insulation assessment methods, for example, the V-t characteristics, are far from satisfactory to meet the demand of insulation design for the power apparatus, whereas the impulse breakdown voltage-number of voltage applications characteristics (V-N characteristics) could well evaluate the long term performance of insulation system. Therefore, we experimentally identified the V-N characteristics of oil paper insulation under impulse voltage with different waveforms, including non-standard lightning impulse voltage, and then mathematically analyzed the obtained data. At last, the influence of wave front/tail time on the V-N characteristics of oil paper insulation was experimentally studied. The test results indicate that the insulation performances of oil paper insulation under repeated nonstandard lightning impulse voltages and under standard switching impulse voltages were better than that under repeated standard lightning impulse voltages. And the capacity to withstand repeated voltage became increasingly stronger with the increase in wave front time or the decrease in wave tail time. The breakdown voltage, however, was only related to the wave front time of applied impulse voltage. [ABSTRACT FROM AUTHOR]

The focus of this paper is on the effectiveness of lightning impulse (LI) with different parameters for detecting protrusion defects in gas insulated switchgear (GIS). The study examines the 50% (U 50) breakdown probability for typical protrusion defects on the conductor of a 363 kV class GIS. A quantitative method is developed to predict breakdown voltage, calculate minimum detectable length of defect, and express the effectiveness of the LI test. The results show that there is little difference of U 50 between aperiodic and oscillating LI. Corresponding detecting effectiveness decreases for LI with long wavefront, which is expressed quantitatively by detectable area. The critical defect length detected increases with increased wavefront. This study provides a general method to express the effectiveness of LI with different parameters for detecting protrusion defects and it support the LI test for on-site testing of GIS. [ABSTRACT FROM AUTHOR]

The objective of this paper is to propose a method for quantitatively estimating the breakdown voltage of double short vacuum gaps in series under the lightning impulse voltage. The study found a quantitative relationship between the breakdown voltage and the maximum electric field strength at the cathode. This relationship is validated by previous experimental works when the total gap distance is less than 20 mm. [ABSTRACT FROM AUTHOR]

C 4 F 7 N and mixtures have been intensively investigated around the world for the substitution of SF 6 in gas insulated transmission lines (GIL). In the present paper, the authors experimentally investigate the lightning impulse (LI) breakdown voltages of C 4 F 7 N/CO 2 in different electric field uniformities, temperatures and pressures. In a slightly nonuniform electric field, the insulation strength of 9%C 4 F 7 N/91%CO 2 can reach 80% strength of SF 6 at the same pressure. In a very un-uniform electric field, the pressure of 9%C 4 F 7 N/91%CO 2 must be increased to 0.7 MPa to keep 80% strength of the 0.4 MPa SF 6. Also, the polarity effect occurred in different electric fields. The breakdown voltage of 9%C 4 F 7 N/91%CO 2 versus gas pressure shows an N-curve, which may be utilized for the design of GIL. At the pressure of 0.35∼0.7 MPa, 9%C 4 F 7 N/91%CO 2 gas mixtures in ultra-high voltage platform has passed the LI withstand voltage tests. Based on various temperature breakdown tests and the Antoine equation calculation, 9%C 4 F 7 N/91%CO 2 could work at −15 °C. [ABSTRACT FROM AUTHOR]

The insulation performance of an oil-immersed power transformer against lightning surges is verified by applying a lightning impulse (LI) voltage alone. However, the surge voltage generated at the transformer terminal in an actual system is superimposed over the ac operating voltage. Particularly for UHV- and 500 kV-class, where the ratio of ac operating voltage to the LI withstand voltage is high, the insulation characteristics for this LI and ac superimposed voltage are crucial factors to study the insulation design. Accordingly, this paper reviewed the research results of insulation characteristics of transformers for the superimposed voltage. Subsequently, these results were evaluated from perspectives of an insulation coordination and transformer insulation test. The insulation elements of transformers covered were the turn-to-turn and section-to-section insulation, for which the LI test is key for insulation design. As a result, for the section-to-section insulation for a shell-type transformer and the turn-to-turn insulation, the breakdown (BD) voltage for the superimposed voltage remained unchanged from the result when the LI was applied alone. Conversely, for the section-to-section insulation of a continuous disk winding for a coretype transformer, the BD voltage attributable to the superimposed voltage declined by about 10% to 20% from that when the LI was applied alone if the oil-impregnated paper component ratio compared to the oil-gap length was low. This is because the oil gap was primarily responsible for the BD and flaws were generated there when the ac voltage was applied. On the other hand, when this ratio was increased, the oil-impregnated paper part was primarily responsible for BD and the decline in BD voltage could be suppressed. For the section-to-section insulation for the interleaved disk winding, the BD voltage attributable to the superimposed voltage remained constant from that when the LI was applied alone. This is because the BD was determined by the LI partial discharge inception at the turn-to-turn part of the section-to-section insulation, where the superimposed voltage made little impact on BD. Based on the above, the LI test is considered adequate to verify the insulation performance of the section-to-section insulation for a shell-type transformer and for an interleaved disk winding and the turn-to-turn insulation against surge overvoltage. Conversely, for the section-to-section structure of a continuous disk winding, the influence of the ac voltage must be taken into account. These results are key findings for rationalizing transformers while maintaining their insulation reliability. [ABSTRACT FROM AUTHOR]

Field grading materials (FGMs) with nonlinear conductivity, which can form a uniform local field in high-voltage direct current (HVDC) cables, have received considerable attention. This paper focuses on the temperature-dependent charge transport property of an FGM based on high-temperature vulcanized (HTV) silicone rubber (SiR). A nonlinearity in the conductivity is achieved by mixing silicon carbide (SiC) particles with the SiR matrix. The conductivities of the samples are measured at temperatures of 30–90 °C. In addition, the surface potential decay is analyzed. The experimental results show that the nonlinear conductivity is suppressed at high temperatures. Increased temperature can cause a lattice vibration, which scatters carriers and reduces the conductivity. With an increase in the homopolar impulse voltage, all samples experiences a rise in the initial surface potential. The increase in SiC content and the increase in impulse voltage can both accelerate the decay process. The heteropolar impulse voltage not only intensifies the accumulation of surface charge but also slows down the decay process. Neutralization occurs immediately between heterocharges. The positive charges are captured by traps, and the attraction between heterocharges is thought to be the reason for the observed phenomena. [ABSTRACT FROM AUTHOR]

Vegetable oils are considered nowadays as a potential alternative to Mineral Oil (MO) for dielectric insulation fluids in transformers due to its high biodegradability, non-toxicity and high fire resistance. This paper presents an experimental investigation on the streamer propagation and breakdown of Refined, Bleached and Deodorized Palm Oil (RBDPO) under both positive and negative lightning impulse voltages. The streamer shape, stopping length, velocity and breakdown voltage of 2 types of RBDPO were examined. Needle-plane electrodes configuration with 50 mm gap was used in this study and streamer images were captured by a high speed camera. It is observed that the negative streamer shape patterns are thicker than positive streamer for both RBDPO and MO. At the same applied voltage level and polarity, both RBDPO have longer streamer stopping lengths and the streamers propagate faster than MO. [ABSTRACT FROM AUTHOR]

In this paper, the basic lighting impulse breakdown performance of trifluoroiodomethane and nitrogen mixture with a volume ratio of 20% CF3I - 80% N2 under 7 bar absolute gas pressure has been investigated. The 50% lighting impulse breakdown voltages (U50) of three types of electrode gap were measured to investigate the dielectric behavior of the gas mixture. The maximum breakdown field strengths (Emax) have been calculated to evaluate the influence of the electrode gap, which can be used to estimate whether breakdown occurs. At the same time, the characteristics of pure sulphur hexafluoride (SF6) and 20 % SF6 – 80% N2 gas mixture have been studied for a comparison. In order to evaluate the potential of CF3I-N2 mixture to replace SF6 in high voltage equipment, a metal enclosed gas insulated transmission line (GIL) was applied, which was initially designed to be insulated with pure SF6. Also to ensure sufficient safety margin for GIL product, the grounded shield ring embedded in the epoxy disc-insulator was used to reduce the electrical field strength in the gas. [ABSTRACT FROM AUTHOR]

In this paper, power frequency breakdown tests as well as positive and negative lightning impulse breakdown tests have been performed. The breakdown voltages of CF3I/CO2/N2, c-C4F8/CO2/N2, CF3I/c-C4F8/CO2 gas mixtures have been measured in different gas pressures, mixing ratios and electrodes distances in slightly non-uniform electric fields. In addition, the breakdown voltages of CF3I/c-C4F8/CO2 gas mixtures have been comparatively analyzed with SF6/CO2, CF3I/CO2, c-C4F8/CO2 gas mixtures. The results show that CO2 is excellent buffer gas to CF3I and c-C4F8 as it has synergistic effect. The power frequency breakdown voltages of CF3I/c-C4F8/CO2 gas mixtures are basically the same as that of c-C4F8/CO2 gas mixtures, which are higher than that of CF3I/CO2 gas mixtures. Besides, the 50% breakdown voltage of c- C4F8/CF3I/CO2 gas mixtures under lightning impulse are higher than that of CF3I/CO2. Compared with SF6/CO2 gas mixtures, CF3I/c-C4F8/CO2 gas mixtures have the same insulating performance in short electrode distance, and show more excellent insulating ability at an electrode distance more than 20mm. [ABSTRACT FROM AUTHOR]

SF6 gas is widely used in power systems, however, it has a strong greenhouse effect. The search for a new environmentally friendly alternative gas has become an urgent task for the power industry. In this paper, CF3I/CO2 gas mixtures in the ratio of 3:7 was tested based on a lightning impulse experimental platform. The 50% impulse breakdown voltage of CF3I/CO2 gas mixtures was determined under various pressures (0.1~0.3MPa) and gaps (5~30mm) and compared to pure SF6 gas. The results indicate that CF3I/CO2 gas mixtures has good insulation properties, reaching 0.84 times that of SF6 gas under slightly non-uniform electric field and 0.65 times that of SF6 gas in extremely inhomogeneous electric field. The polar effect of CF3I/CO2 gas mixtures was also analyzed and the coefficient K is proposed to describe and compare the differences of 50% breakdown voltages under different electric field. The results show that the polar effect of CF3I/CO2 gas mixtures under extremely inhomogeneous electric field is more obvious than that under slightly non-uniform electric field, and the polar reversal phenomenon occurs only under extremely inhomogeneous electric field. Considering various factors, CF3I/CO2 mixtures has the capability of replacing SF6 gas in power equipment such as GIL. [ABSTRACT FROM AUTHOR]

SF6 is a kind of strong greenhouse gas, it is urgent for researchers to seek new alternative insulating gas. This paper studies the discharge characteristics of CF3I/N2 gas mixture with needle-plane and sphere-plane electrode under standard lightning impulse, which is important for CF3I and its mixtures applied in electrical equipment. The experimental results show that the breakdown strength of CF3I/N2 increases with increasing gas pressure and gap distance. The discharge characteristics of CF3I/N2 gas mixtures present significantly polarity effect both in highly non-uniform electric field and slightly non-uniform electric field. In the highly non-uniform electric field, the 50% breakdown voltage in negative lightning impulse is greater than that under positive polarity. Under slightly non-uniform field, breakdown voltage in negative lightning impulse is lower than that in the positive polarity. When the electrode gap is 5mm, the polarity reversal phenomenon appeared with the increase of gas pressure. The analysis shows that CF3I/N2 gas mixture appears polarity effect and polarity reversal phenomenon in the short gap is due to the migration and interaction of space charge between the electrodes gap. [ABSTRACT FROM AUTHOR]

The operation experiences of power transmission lines show the lightning outage is still the main part of the total outage rate. So it is very important to calculate the lightning overvoltage across the insulators precisely and evaluate the lightning outage of the transmission lines correctly. Lightning flashover criterion of the insulator/gap is one of the most important factors that influence the precise calculation results of the lightning withstand level of transmission lines. In China, several ?800 kV ultra high voltage direct current (UHVDC) power transmission projects have been set up in recent years, and the altitude of the regions that the transmission lines across ranges from 0-3650 m. The paper mainly analyzed the influence of the modelling methods of insulator/gap on the lightning withstand level of ?800 kV UHVDC transmission lines by PSCAD/EMTDC program. The compared lightning flashover criteria of insulator/gap include the leader propagation model recommended by CIGRE, the volt/time curve recommended by IEEE and an improved leader propagation model, which is proposed based on the lightning flashover experiments of the ?800 kV UHVDC line insulators carried out at an altitude of 2100 m. Meanwhile, the influences of lightning current waveform and insulator structure were also considered and compared. The simulation results show that the calculated back-flashover lightning withstand level is much lower when using the improved leader propagation than that using other flashover criterions in the paper, it is lower under the 2.6/50 ?s double exponential lightning current than that under the 2.0/50 ?s Heidler function lightning current, and it is higher when using V-string than that using I-string insulator. Besides, the influences of the lightning current waveform and the insulator structure on the calculation of shielding failure lightning withstand level are little, but the flashover criterion has obvious influences on the shielding failure lightning withstand level. [ABSTRACT FROM AUTHOR]

Models of one or several extremely simple transmission towers (pylons) are usually used for analyzing the influence of lightning strikes on power network. Models of more complex transmission towers with transmission lines, ground wires and even lightning channel are required. In this paper, first of all, the reason why one only focuses on the magnetic field at the observation points is briefly explained. Secondly, a method used to get the time-domain results from the frequency-domain results by Antenna Design Framework Electromagnetic Satellite (ADF?EMS) software is established and its validity is proved. Thirdly, the effects of the lightning channels with different geometries are discussed. Finally, this paper comes up with a complex multi-tower system under the following fault types of lightning strikes, 1) direct lightning strikes to the edge phase wire without flashover, 2) direct lightning strikes to the edge phase wire with flashover, 3) lightning strikes to the top of the tower without flashover, and 4) lightning strikes to the top of the tower with back flashover. The magnetic field waveforms from these fault types are very important for the follow-up activity to achieve the fault type identification of lightning strikes in the observation system automatically. [ABSTRACT FROM PUBLISHER]

During operation transformers are typically subjected to overvoltages with different wave shapes. Even though overvoltage protection systems, such as metal oxide surge arresters, are often employed, the overvoltages generated in power systems can still have a significant impact on the insulation systems of the windings of a power transformer. There are numerous examples of transformer failures as a result of overvoltages occurring during operation, even in cases that standard impulse voltage tests show no indication of potential problems and the transformers are protected by surge arresters. Comparisons of the internal overvoltages generated in transformers during voltage tests as well as those generated during standard operation of electrical networks provides fundamental insights into these overvoltages; insights which in turn can be used to improve the construction of insulation systems and overvoltage protection approaches and ultimately increase the reliability of transformers. In this paper, internal overvoltages occurring in the windings of selected transformer during standard voltage tests are compared with those which occur during typical operation of a transformer in an electrical power system. Overvoltages measured in the windings of the transformer form the basis of this comparison. Measurements were conducted with the transformer being subjected to voltage impulse tests, as well as transient voltages with waveforms simulating overvoltages generated in electrical power systems. In order to investigate the response of the windings to voltage strokes, full and chopped lightning voltage impulses were applied to the transformer. The waveforms of overvoltages generated in the electrical power systems were modelled as a rectangular voltage stroke, sinusoidal voltage, voltage with shape of “sweep sinus” also often referred to as a chirp signal and voltage with shape of “sweep ramp”. [ABSTRACT FROM AUTHOR]

Winding deformation is a critical power transformer issue that needs to be detected as soon as it emerges due to its progressive nature and severe consequences it may lead to. Frequency response analysis (FRA) is the only reliable technique currently used to detect such deformation. However, the technique is conducted offline that may cause interruption to the electricity grid when taking an operating transformer out of service for testing. Moreover, as the current FRA technique is only conducted on suspected units, a mechanical fault may emerge and progress before the utility becomes aware of. To overcome these limitations, this paper presents a practical feasibility study to detect transformer winding deformations online. The technique relies on utilizing transient overvoltage signals that a transformer is subjected to during its normal operation as a natural variable frequency excitation source. The feasibility of utilizing various transient overvoltage signals as excitation source for transformer FRA signature measurement is firstly investigated and optimum signal parameters are recommended based on signal voltage and energy spectrum analyses. Secondly, the proposed technique is verified through simulation analysis by comparing the transformer frequency response signatures when transient overvoltage and conventional sweep frequency voltage are used as excitation sources. Also, experimental testing is conducted to assess the feasibility of the proposed technique to identify various winding deformations such as radial buckling, telescoping and short circuit turns. Custom-made capacitive voltage divider is developed to accurately detect the response of transient overvoltage signals generated due to environmental conditions or power system switching operations. The proposed technique along with the developed capacitive voltage divider is implemented on a 3-phase Wye-Delta, 50 Hz, 31.5 MVA, 110/10.5 kV power transformer to measure the frequency response signatures of the high voltage winding. [ABSTRACT FROM AUTHOR]

The field lightning impulse withstand voltage test is an important step to ensure the safe operation of the gas insulated switchgear (GIS). The oscillating lightning impulse voltage is recommended in field test by IEC standard due to its high generating efficiency and lower impulse ratio. The insulation condition of GIS can be evaluated overall by detecting the partial discharge (PD) when conducting the field impulse withstand voltage test. In this paper, the protrusion defect was set in a 330 kV real GIS to study the PD characteristics under lightning impulse voltages, including SLI (standard lightning impulse) voltage and OLI (oscillating lightning impulse) voltages with different oscillating frequencies. The PD under impulse voltages was effectively detected by using the internal UHF sensor. The 50% probability PD inception voltage (PDIV50), maximum discharge amplitude, and discharge number were analyzed based on the measurement results. It was concluded that the PD number is significantly larger and the PDIV50 of OLI voltage is lower when the oscillating frequency is low by comparing the results of oscillating and standard impulse voltage. The PDIV50 increases and discharge number decreases with the increasing of oscillating frequency of the OLI voltage. [ABSTRACT FROM AUTHOR]

This paper presents an examination on the electrical properties of Palm Oil (PO) and Coconut Oil (CO) under the presence of semi-conductive nanoparticle, Titanium (IV) Oxide (TiO2). The type of PO used in this study was Refined, Bleached and Deodorized Palm Oil (RBDPO) Olein. TiO2 was dispersed into RBDPO and CO at volume concentrations ranging from 0.001% to 0.05%. The effect of surfactant, Cetrimonium Bromide (CTAB) was also examined and the morphology of RBDPO and CO based TiO2 nanofluid was observed through Transmission Electron Microscope (TEM). A number of measurements were carried out including the AC/lightning breakdown voltages, dielectric dissipation factor, relative permittivity and resistivity. Statistical analysis was carried out through Weibull distribution to determine the AC and lightning withstand voltages at 1% probability for RBDPO and CO. It is found that without CTAB, there are only slight improvements on the AC breakdown voltages of the RBDPO and CO with introduction of TiO2 but could decrease with presence of CTAB. The same pattern is found for lightning breakdown voltages of RBDPO and CO under non-uniform field where the highest percentages of increments decrease with introduction of CTAB while the effect is not apparent under quasi-uniform field. Significant effect of CTAB is found on the dielectric dissipation factors and resistivity of RBDPO and CO based TiO2 nanofluid while the relative permittivity are not affected. The statistical analyses reveal that RBDPO and CO experience apparent increments of the AC breakdown voltages at 1% probability after the introduction of TiO2 regardless with or without CTAB. The improvements done by TiO2 on the lightning breakdown voltages at 1% probability for all samples with CTAB under quasi-uniform field are more apparent than non-uniform field. [ABSTRACT FROM AUTHOR]

In this paper, streamer and breakdown phenomena and their mechanisms of an inhibited Gas-to-Liquid (GTL) transformer oil under standard negative lightning impulse voltages were studied. A conventional inhibited mineral oil was also tested as the benchmark. Experiments were carried out in 25 mm and 50 mm point-plane gaps. Streamer and breakdown phenomena of both oils were observed from the streamer inception voltage level up to the voltage level at which fast streamer appears with a velocity of over 10 km/s. The results showed that the GTL oil has a higher breakdown voltage and a higher acceleration voltage compared to the mineral oil. This is due to the fact that the GTL oil has a much lower aromatic content compared to the mineral oil. The streamer investigations showed that the development of fast streamers in both oils is the consequence of transition from dark streamer channels to bright streamer channels. By comparing with the results obtained under positive polarity [1], the differences in the propagation mechanisms between the fast negative streamers and fast positive streamers were explained. [ABSTRACT FROM PUBLISHER]

As the greenhouse effect of SF6 gas, the research on seeking alternative insulating gas to SF6 has become an imperative case. CF3I and its mixtures have been considered as the most promising insulating gas to replace SF6 gas. In this paper, the discharge characteristics of gas gap and insulator in CF3I/N2 gas mixtures under lightning impulse and alternating voltage were investigated. The effect of voltage, pressure as well as decomposition product after flashover were studied. Experimental results showed that in inhomogeneous electric field the breakdown voltages of CF3I/N2 with 30% CF3I gas is lower than SF6/N2 mixtures with 20% SF6. However, the flashover voltage of insulator in CF3I/N2 is higher than that in SF6/N2 mixtures. When compared to CF3I/N2 itself, the 50% breakdown voltage under positive lightning impulse were lower than that under negative lightning impulse. The CF3I/N2 gas mixtures with insulator also had polarity effect and appear polar reversal phenomenon. Under lightning impulse, there is no decomposition product on the surface of insulator after flashover but under alternating voltage, obvious iodine was produced on the surface of insulator. [ABSTRACT FROM PUBLISHER]

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]

Inductive fault current limiters (FCLs) have several advantages, such as significant current limitation, immediate triggering and relatively low losses. Despite these advantages, saturated core FCLs have not been commercialized due to its large size and associated high costs. A major remaining challenge is to reduce the footprint of the device. In this paper, a solution to reduce the overall footprint is proposed and discussed. In arrangements of windings on a core in reactors such as FCLs, the core is conventionally grounded. The electrical insulation distance between high voltage winding and core can be reduced if the core is left at floating potential. This paper shows the results of the investigation carried out on the insulation of such a coil-core assembly. Two experiments were conducted. In the first, the behavior of the apparatus under high voltage conditions was assessed by performing power frequency and lightning impulse tests. In the second experiment, a low voltage test was conducted during which voltages of different frequencies and pulses with varying rise times were applied. A finite element simulation was also carried out for comparison and further investigation. [ABSTRACT FROM AUTHOR]

This paper presents breakdown and withstand strengths of a synthetic ester, a natural ester and a mineral oil in a quasi-uniform sphere-to-sphere electric field under impulse voltages. The influences of impulse waveforms (lightning and switching), voltage polarities (positive and negative) and testing methods (rising-voltage, up-and-down and multiple-level) on the breakdown behaviour were investigated. The results indicated that breakdown voltages of both the ester liquids are comparable with those of the mineral oil under various test conditions. The withstand voltage at 1% breakdown probability was also obtained based on Weibull distribution fitting on the cumulative probability plot, which was built using the ∼1000 impulse shots. It was found that the withstand voltages at 1% breakdown probability of the ester liquids are almost the same as that of the mineral oil. The results in this paper were further compared with other available test data in the literature. By comparing the performances between the ester liquids and the mineral oil, recommendations for insulation design were given to aid the application of ester liquids in high voltage large power transformers. [ABSTRACT FROM PUBLISHER]

This article presents the results of the studies on lightning impulse breakdown voltage measurements performed for two commercial natural ester liquids of different viscosities. These viscosities measured at 100?C were 8.0 mm2/s and 4.6 mm2/s respectively. The dielectric test results presented in this paper concern the measurements performed using the recommendations of the IEC 897 standard where point-to-sphere electrode system is dictated. In addition to measuring the breakdown voltage other experimental techniques were applied such as time to breakdown estimation, light emission registration using the photomultiplier and static photograph collection. The results showed that the differences between the liquids tested concern only positive polarity of lightning impulse voltage. In the case of negative polarity from the standard based calculations and statistical analysis using Weibull distribution the breakdown voltages may be treated as a very similar. In turn the other discharge characteristics such as light waveforms, static photographs and propagation velocities did not showed any differences. In the case of positive lightning breakdown voltage the results were assessed only on the basis of the average values calculated because a deep statistical analysis was not possible due to the small scattering of data. These results indicated on slightly better lightning properties of the natural ester of higher viscosity. Confirmation of this conclusion shall constitute the light waveforms registered where the discharges developing in the low viscosity natural ester are characterized by higher intensity than corresponding discharges developing in the ?classical? natural ester. However the static photos did not indicate any differences between the esters tested. [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]

Gas insulated switchgear (GIS) is used in power system more and more widely. With the increased voltage level of power system, field impulse voltage withstand test becomes increasingly important. It would be beneficial for a comprehensive insulation evaluation to carry out the detection of partial discharge (PD) under impulse voltage withstand test in field. Metal protrusions or particles on HV conductor are typical defects in GIS and are critical at lightning impulse voltages. This paper therefore aims at PD characteristics study on needle-plane defect under oscillating lightning impulse (OLI). PD detection system was established based on ultra-high frequency (UHF) and pulse current detection. Then, the PD of a needle-plane defect under negative OLI was measured. The relation between statistical time lag, PD amplitude, PD times and the applied voltage was analyzed respectively based on UHF detection. The results show that UHF detection has high sensitivity and it is suitable in field impulse voltage withstand test. PDs occur around each wave peak of the applied voltage. With the increase of peak voltage, PD magnitude and PD times in each oscillation cycle both increase linearly. Further analysis indicates that the occurrence and development of PD under negative OLI can be interpreted by electron emission from metal surface, field detachment from negative ions and corona stabilization effect. This approach is valuable for PD measurement and analysis in field impulse voltage withstand test for GIS. [ABSTRACT FROM AUTHOR]

The introduction of the test voltage function or k-factor in IEC 60060-1 Ed. 3 has been an important step to ensure reproducibility and traceability in high voltage tests techniques for lightning impulses with superposed oscillations and overshoots. The standard recommends computed fitting methods to extract a base curve from the oscillating recorded voltage curve and, applying the test voltage function, it is possible to calculate the impulse parameters. Testing external insulations with positive polarity lightning impulses at 50% breakdown voltage or lower level, the disruptive discharge usually arrives after ten microseconds. For these cases the test voltage function and procedures for calculating impulse parameters proposed by the standard may lead to divergent results. In this paper a new approach is proposed for fitting the base curve for metric air gaps based on the relevance of the leader propagation phase, highlighting the influence of the instantaneous voltage between the impulse crest and the time-tobreakdown. A comparison of three calculation methods for test curves with long timeto- breakdown is discussed and a new test voltage function obtained by measurements on a 1 m rod-plane air gap is proposed. Compared to the standard test voltage factor, this approach evidences a negligible influence of oscillations and the relevancy of the impulse tail voltage for testing external insulations with moderately distorted lightning impulses. Finally, a generalization of the voltage test function is proposed for several types of insulation considering mainly the average breakdown times. [ABSTRACT FROM PUBLISHER]

This paper presents experimental studies on streamer and breakdown phenomena and the mechanisms of an inhibited Gas-To-Liquid (GTL) transformer oil under positive lightning impulse voltages. A conventional inhibited mineral oil was also tested as the benchmark. Point-plane configurations with gap distances of 25 mm and 50 mm were used. Streamer and breakdown phenomena of both oils were observed from the streamer inception voltage level up to the voltage level at which fast streamer appears with velocity over 50 km/s. The results showed that the GTL oil has a higher breakdown voltage but a lower acceleration voltage than the mineral oil. This is due to the fact that the GTL oil has a much lower aromatic content compared to the mineral oil. The streamer investigations showed that the development of fast streamers in both oils is the consequence of transition from dark streamer channels to bright streamer channels. Based on detailed characterizations of the transition process, a model considering charge splitting and amplification processes was proposed to explain the underlying mechanisms. [ABSTRACT FROM AUTHOR]

The transient electric field distribution under lightning impulse voltage (LIV) is vital to the reliability of 750 kV composite cross-arm. However, the transient electric field of electrical equipment under impulse voltage has usually been substituted by the equivalent sinusoidal voltage, without considering the variation of permittivity with frequency. Using the frequency decomposition computation method (FDCM) based on finite element method (FEM), this paper presents the study of transient electric field under LIV for a 750 kV composite cross-arm. The LIV waveform has been decomposed via Fourier transformation. The complex dielectric functions of the insulation materials used in composite cross-arm have been investigated, and the computation parameters for the dc and each harmonic component of LIV have been obtained. Then, the transient electric field distribution can be computed through the superposition of responses from dc and each harmonic component. Moreover, the computation results have been compared with that using traditional method. The comparison indicates that the potential distribution of composite cross-arm is similar. The maximum percentages of electric field strength difference on fittings and silicone rubber sheath are 2.9 %, 14 %, respectively. [ABSTRACT FROM AUTHOR]

To lower the insulation specifications (specifically, the lightning impulse withstand voltage) of a gas insulated switchgear (GIS) and thus cut the equipment cost while maintaining the high reliability of its insulation performance, it is necessary to define in an organized way the insulation characteristics for non-standard lightning impulse voltage waveforms that represent actual surge waveforms in the field and compare them with the characteristics for the standard lightning impulse waveform quantitatively. In the preceding paper, lightning surge waveforms and disconnector switching surge waveforms at UHV, 500 kV, and 275 kV substations were analyzed and five to six non-standard lightning surge waveforms with basic frequencies of 0.6 to 5.0 MHz were identified. In this paper, the dielectric breakdown voltage — time characteristics were measured under several different conditions mainly for the quasi-uniform SF6 gas gaps that represent an insulation element of a GIS toward four kinds of non-standard lightning impulse waveforms associated with lightning surges. As a result, in the tested range, the dielectric breakdown values for non-standard lightning impulse waveforms were higher than for the standard lightning impulse waveform by 3% to 32%. [ABSTRACT FROM AUTHOR]

In the design of high-altitude high-voltage transmission systems and in the optimization of the reliability of power sources in aeronautical environments, it is important to have reliable data concerning the effect of air density and humidity on reducing air insulation performance under local conditions. This is helpful also in the modeling of discharges and lightning in mountainous regions. This paper describes a systematic laboratory investigation of the combined effects of humidity and gas density on the breakdown strength of a 0.2 m rod/plane air gap that is subjected to positive-polarity lightning impulses, using a test chamber to reproduce the conditions commonly found in such regions. The results show that there are appreciable deviations between the measured humidity and air density correction factors and those of the 1973 and 1989 I.E.C. Standards when extended to low air density. The paper proposes the adoption of a radically different correction procedure, based upon the influence of density and humidity on streamer propagation field, and including the effect of the high gradient in the anode region. [ABSTRACT FROM AUTHOR]

The paper investigates the applicability of some closed form expressions for the ground impedance and ground admittance of buried horizontal wires (bare and insulated) for lightning or switching transient analyses based on transmission line (TL) theory. In view of the frequency contents that typically characterize such transients, the behavior of the ground impedance and admittance is studied for a wide frequency range up to 10 MHz. Low frequency approximation of the ground impedance is not always appropriate for transient analysis. Sensitivity analyses show that, unlike overhead wires, the ground impedance for buried wires is little sensitive to the ground conductivity. On the other hand, the ground admittance varies strongly with the ground conductivity. The paper also discusses the results of transient analysis of buried cables performed by means of electromagnetic transient programs (EMTP) that neglect the ground admittance. The limits of such an approximation are discussed in order to evaluate the applicability of EMTP-hike programs to the transient analysis of buried conductors. Transient pulse propagation in time domain based on finite difference time domain (FDTD) method of solution of TL equations is also discussed for a future inclusion of non-linear phenomena, like soil ionization and arcing/breakdown mechanisms, in the soil. The analysis presented could be useful in estimating surge propagation characteristics of buried wires for appropriate insulation coordination and transient protection. [ABSTRACT FROM AUTHOR]

To lower the insulation specifications (specifically, the lightning impulse withstand voltage) of oil-immersed transformers and thus reduce equipment cost while maintaining high insulation reliability, it is important to identify the insulation characteristics under non-standard lightning impulse waveforms that are associated with actual surge waveforms in the field besides the standard lightning impulse waveform and quantitatively compare them with the characteristics under the standard lightning impulse waveform. In a preceding paper, field overvoltages in the lightning surge time region were analyzed, and four typical non-standard lightning impulse waveforms were identified. Two of these including pulses were used to measure the breakdown voltages and the partial discharge inception voltages while changing the parameters, on three models of the winding insulation elements of oil-immersed transformers. This paper investigates the insulating properties of the other two types: "damped-oscillation waveform" and "rising-oscillation waveform", and evaluates the resultant average breakdown voltages in terms of the overvoltage duration, permitting their formulation in a unified way. In the examined range, the dielectric breakdown values under non-standard lightning impulse waveforms were up to 50% higher than those under standard lightning impulse waveforms in all cases. Further investigations will be expected for the reduction in insulation specifications. [ABSTRACT FROM AUTHOR]