Lightning Impulse Voltage Research Articles

Flashover discharges dynamic with continuous and discontinuous pollution layer under lightning impulse stress

The paper is aimed at the study of the dynamic of discharges propagating at the surface of flat PVC insulator (i) with uniform continuous pollution layer and (ii) with uniform discontinuous pollution layer of different conductivities, under lightning impulse voltage for both polarities. It is shown that the morphology of discharges, the current and the voltage as well as the discharge velocity depend on the polarity of voltage, the configuration and the conductivity of the pollution layer; and the behavior of discharges is similar to that of discharges in long air gaps.

Measurement of Streamer Propagation Velocity over Solid Insulator Surface in a C4F7N/CO2/O2 Mixture under Lightning Impulse Voltages

For a better understanding of the discharge development over a solid insulator surface, experiments are conducted over epoxy-anhydride resin samples in a C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> N/CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2 </sub> mixture under lightning impulse voltages. A measurement method of streamer propagation velocity using a photomultiplier is presented. It is observed that the propagation velocity greatly depends on the gas nature, the polarity of the voltage, and the high voltage electrode geometry. The involved mechanisms in propagation are also discussed.

Experimental Study on the Effect of Lightning Impulse Oscillation Frequency on Pressboard Flashover in Oil

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.

Interaction Between Partial Discharge and Generated Bubbles Under Repeated Lightning Impulses in Transformers Using a Complex Structure Model

The focus of this research is on the insulation structure where radial and axial spacers connect in a power transformer. The interaction between the occurrence of partial discharge (PD) and the generation of bubbles using a model that is very similar to the real structure in a transformer is studied. The inception voltage of the PD caused by the oil gap created by the axial and radial spacers might lower than that caused by a metallic impurity. However, PD caused by the metallic impurity was more likely to generate bubbles because of its higher discharge energy. It is also found that the residual space charge on the insulation surface prevents the occurrence of PD and also affects the generation of bubbles under repeated lightning impulse voltage. Moreover, it is found that generated bubbles do not affect the occurrence of PD. One reason for this is that the bubbles disappear during the interval time between repeated impulses. The effect of the residual space charge is considered to be another reason.

Electric Field Simulation of Permittivity and Conductivity Graded Materials (ε/σ-FGM) for HVDC GIS Spacers

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.

Drift-diffusion model of corona discharge in coaxial geometry due to negative lightning impulse voltage

Corona discharge at atmospheric pressure in cylindrical geometry is simulated numerically for negative lightning voltage impulse. The simulation is performed by solving the drift-diffusion-reaction equations for electrons, positive and negative ions. Spatial dependence and time evolution of the concentration of charged particles and created space electric field are used for detailed theoretical analysis of corona discharge based on the values of the transport and reaction gaseous coefficients. This exact drift-diffusion model enables clear physical picture of the corona discharge dynamics. It is used for the determination of the charge-voltage curves necessary for taking into account the corona effects in transient overvoltage calculation on transmission lines. In contrary to the previous engineering corona models, it is shown that the corona charge continues to increase although the input voltage pulse decreases after reaching its maximum value. This behavior is confirmed by measurements in a few experimental studies. Secondly, it has been shown that according to this model there are no positive streamers formation departing from central electrode or wire during decreasing edge of the input voltage pulse. The total charge simply commences to decrease when the negative ions neutralize themselves on the central electrode or wire of positive polarity. A sharp peak of corona current is observed immediately after the beginning of the intense ionization. The drift-diffusion model could also be used to check the validity of other common assumptions and predictions of previous engineering corona models.

Impulse Breakdown Characteristics of Transformer Oil with Cellulose Particles in Quasi-Uniform Field

This study presents an experimental comparison of the breakdown characteristics of oil samples with four cellulose particle content levels in a quasi-uniform electric field under lightning impulse voltage. For the experiment to resemble the actual working conditions of transformers and meet the on-site withstand impulse voltage test requirements, both the standard lightning voltage (SLI) and oscillating lightning impulse (OLI) are considered. The experimental results show that an increase in the number of cellulose particles directly weakens the oil insulation strength. At the same particle content level, the oil breakdown voltage under OLI is higher than that under SLI, and the breakdown time lag under OLI is longer than that under SLI. The breakdown time lag is more sensitive to the effect of particle content under SLI. This work is beneficial in deepening the understanding of the impulse breakdown characteristics of transformer oil within cellulose particles and in providing references for the on-site impulse test of oil-immersed power transformers.

Leader Transition Properties of Propagating Partial Discharges in SF&lt;sub&gt;6&lt;/sub&gt; Gas and the Scaling Law under the Negative Lightning Impulse Voltage Application

Leader Transition Properties of Propagating Partial Discharges in SF&lt;sub&gt;6&lt;/sub&gt; Gas and the Scaling Law under the Negative Lightning Impulse Voltage Application

Design and Experimental Study of Traveling Wave Signal Generator for Distribution Network

Aiming at the problem of operation safety and stability of electrical equipment in distribution network caused by impulse voltage, this paper will study and analyze the traveling wave signal generator. Through the study of the standard lightning wave shape characteristics and the mechanism and characteristics of the existing lightning impulse voltage signal generator equipment, the working principle of the corresponding traveling wave signal generator was designed and the relevant parameters were calculated. Finally, the results of the research and analysis were simulated and analyzed with the Multisim simulation software. Test results show that the impulse voltage generated by the full wave form basic satisfy the provisions of this design is expected waveform, and consistent with the actual lightning wave shape, therefore, this thesis studies the traveling wave signal generator unit can meet the normal travelling wave signal of traveling wave positioning system requirements, verify the real effectiveness and practicability of this thesis research.

Breakdown Voltage of Transformer Oil Containing Cellulose Particle Contamination With and Without Bridge Formation Under Lightning Impulse Stress

This study investigates the effect of cellulose bridge formation on insulating oil. Such effect influences the breakdown voltage and strength of mineral oil (MO) under standard lightning impulse (LI) voltage. A cylindrical test vessel fitted with spherical–spherical electrodes with a 2 mm gap distance between them was used to generate a quasi-uniform field and observe bridge formation between the gaps. A positive LI waveform (1.2/ $50~\mu \text{s}$ ) generated by an impulse generator was applied during the breakdown tests. The rising-voltage method was used to measure the breakdown voltage in accordance with the IEC 60897 standard test method. Weibull cumulative breakdown probability was applied to present the breakdown results statistically, and the outcome was compared with clean oil (CO). Commercial cellulose microcrystalline particles were dispersed into the transformer oil to act as bridge. The concentrations of the samples were 0.004%, 0.008% and 0.012% by weight. The breakdown test results showed that the cellulosic particles reduced the LI breakdown voltage (LIBV) of MO by up to 10%. The influence of cellulose particles on LIBV was also more prominent during bridge formation, with a further reduction of 14%. Evidently, the presence of the cellulose bridge exerted a detrimental effect on the breakdown voltage of MO under LI stress, yielding a reduction of 24% and 27% at 0.004 wt% and 0.012 wt%, respectively. Finite element analysis also indicated that the electrical field strength of cellulosic particle contamination of 0.004 wt% and 0.012 wt% without a bridge skeleton exceeded the critical electric field strength of CO by 20% and 61%, respectively. The presence of cellulosic contamination at any concentration, i.e. either with or without a bridge structure, clearly decreased the breakdown strength of transformer oil. However, the breakdown is not occurred instantaneously. The LI breakdown was initiated and established only at a certain impulse stress level, when the electric field strength along the bridge lines exceeded the critical electric field strength of the oil. Thus, this finding potentially contributes to the formulation of guidelines for condition assessment and contamination monitoring to minimise common issues in power transformer failures that are attributable to the insulation system, and consequently, achieve optimum transformer insulation integrity.

친환경 전력케이블용 PP의 백화현상에 의한 절연특성 변화에 대한 연구

Cross linked polyethylene (XLPE) has been widely used for power cables as an insulation material due to its excellent dielectric and mechanical characteristics. However, XLPE can not be recycled after the lifetime of power cables because of its thermosetting property. In many countries, power cables have been developed and installed actively since 1970. It means that the lifetime of power cables is reaching over 50 years. It is known that the lifetime of power cables is around 50 years. Therefore, many power cables in a power grid installed in 1970s should be replaced with new power cables in order to improve the reliability of a power system. In this replacement, there are many studies to change thermosetting materials, XLPE into thermoplastic materials, polypropylene (PP) which can be recycled. This paper deals with the analysis on the dielectric degradation characteristics of PP according to white rust. White rust may occur in power cables due to curvature and scratch which could be caused by bending stress and friction. Especially bending stress is inevitable because of narrow installation area and process of installation. The dielectric degradation characteristics of PP are analyzed under AC as well as lightning impulse voltage test. It is found that the dielectric degradation characteristics of PP according to white rust is more degraded at lightning impulse voltage test than AC voltage test.

Resonance at the Front of Lightning Impulse Voltage Waveforms Caused by the Load Capacitor

During the impulse voltage comparison test, it is important to keep the waveforms smooth for obtaining consistent experiment results, especially for the front time parameter. However, steps or oscillations are often found on the front of the waveforms. The authors propose a theory that these oscillations can be caused by the resistors in the damped capacitive voltage divider, which often serves as the load capacitor in the impulse voltage generation circuit. The theoretical analysis, numerical simulation, and experimental results are performed, and a good agreement has been found in support of the proposed theory. Furthermore, the waveform parameters measured by two resistive reference standard systems were found to be influenced by the shape of the waveform and the oscillation on the front. It is recommended to avoid using a damped capacitive divider as the load capacitor but use an impulse capacitor with low equivalent series resistance instead. This would help to eliminate this common cause of oscillation on the front of the lightning impulse voltage waveform. Waveform uniformity and uncertainty of the result could then be improved during the comparison of reference measuring systems.

Resonance at the front of lightning impulse voltage waveforms caused by the load capacitor

Resonance at the front of lightning impulse voltage waveforms caused by the load capacitor

Reshaping of Micro-morphology Feature in Electrode Surface under Conditioning by Lightning Impulse Voltage in Vacuum Gaps

The objective of this paper is to determine the reshaping of the micro-morphology feature in both the cathode and anode electrode surfaces after conditioning by lightning impulse voltage in vacuum gaps. Materials of the experimental rod-plane electrodes are Cu and stainless steel. The micro-morphology feature in the electrode surface is recorded by a 3D laser microscopy. Experimental results indicate that conditioning by the lightning impulse voltage reshaped the micro-morphology feature in both the cathode and anode electrode surfaces. The BD characteristics are closely related to the micro-morphology feature and roughness in the cathode surface after conditioning. Moreover, the conditioning process takes different effects in the cathode and anode surfaces. For cathode electrode, the main difference of the micro-morphology feature is in the distributing characteristics of the erosion regions. For anode electrode, there aree different patterns of the anodic conditioning mechanism of the micro-morphology feature. In the molten pattern, conditioning melted the anode surface and removed the micro-protrusions effectively. In the crater pattern, with the increasing BD voltage and energy, the material below the anode surface is heating, which is the cause of formation of the many craters in anode surface.

Effects of a Crossarm Brace Application on a 275 kV Fiberglass-Reinforced Polymer Crossarm Subjected to a Lightning Impulse

The crossarm is an important component of transmission towers, providing insulation for transmission lines at different voltage ratings. Recently, composite crossarms were widely used as a composite tower component and were found to be the most favorable choice for replacing old wooden crossarms. Owing to the satisfactory pilot operation and multiple sets of testing, fiberglass-reinforced polymer (FRP) composite crossarms have been used in Malaysia in both 132 and 275 kV transmission lines since the late 1990′s. Since then, some modifications have been proposed to improve the mechanical performance of the crossarm, in order to ensure the reliability of its performance. In this investigation, the effect of a proposed improvement, achieved by installing a brace for the crossarm, was investigated numerically. A simulation study was conducted, with a consideration of the lightning impulse voltage (LIV) and swing angle exhibited by the crossarm. The potential and electric field (E-Field) distribution were analyzed and are presented in this paper. It was found that the potential distribution and E-Field strength for the crossarm and the surrounding air were greatly affected by the installation of the brace.

Dielectric properties of mixed mineral and synthetic ester oil

The dielectric properties of 20% synthetic ester oil mixed with mineral oil were studied for its potential applications as an insulator in transformers. The corona inception voltage of mixed oil under AC and DC voltages was measured by the ultra-high frequency (UHF) technique. A statistical study of the breakdown voltage of mixed oil under AC, DC and standard lightning impulse voltages was carried out to obtain a database which would be useful for designing transformer insulation. Further, the mixture was subjected to continuous breakdown studies under AC voltage with lightning impulse voltages, at regular intervals and characterised through measurement of interfacial tension, flash point, ionic mobility as well as UV–vis spectroscopy and frequency-domain spectroscopy studies to understand the liquid's chemical stability. The results of the study have shown that the mixture was stable even after several breakdowns. The ionic mobility and polarisation current have increased, indicating higher losses in the oil. UHF signal energy analysis and phase-resolved partial discharge analysis have revealed a reduction in discharge activity with mixed oils.

Effective Simulation Approach for Lightning Impulse Voltage Tests of Reactor and Transformer Windings

In this paper, an effective simulation method for lightning impulse voltage tests of reactor and transformer windings is presented. The method is started from the determination of the realized equivalent circuit of the considered winding in the wide frequency range from 10 Hz to 10 MHz. From the determined equivalent circuit and with the use of the circuit simulator, the circuit parameters in the impulse generator circuit are adjusted to obtain the waveform parameters according to the standard requirement. The realized equivalent circuits of windings for impulse voltage tests have been identified. The identification approach starts from equivalent circuit determination based on a vector fitting algorithm. However, the vector fitting algorithm with the equivalent circuit extraction is not guaranteed to obtain the realized equivalent circuit. From the equivalent circuit, it is possible that there are some negative parameters of resistance, inductance, and capacitance. Using such circuit parameters from the vector fitting approach as the beginning circuit parameters, a genetic algorithm is employed for searching equivalent circuit parameters with the constraints of positive values. The realized equivalent circuits of the windings can be determined. The validity of the combined algorithm is confirmed by comparison of the simulated results by the determined circuit model and the experimental results, and good agreement is observed. The proposed approach is very useful in lightning impulse tests on the reactor and transformer windings.

Fast and Effective Technique in Evaluation of Lightning Impulse Voltage Parameters

This paper presents an approach for the waveform parameter evaluation of lightning impulse voltage in high voltage tests according to the IEC standards. Such waveform parameters are composed of peak voltage (Up), front time (T1), time to half (T2), and the overshoot rate (Be). An artificial neural network with a back-propagation learning algorithm was applied to determine a base curve and its parameters from 14 points along the recorded waveform between 20% of the peak voltage on the wave front part to 40% of the peak voltage on the wave tail part. The 29 waveforms recommended by the standard were used in the training process of the development of the network model, and some experimental cases were also utilized for verification of the proposed method. It is found that the waveform parameters evaluated by the proposed approach are in the tolerances of the standard requirements. Maximum absolute deviations of Up, T1, T2, and Be are 0.06%, 2.00%, 0.12%, and 0.79%, respectively. Due to that no iteration process in the proposed approach is required, the efficiency in calculation process is significantly faster than the standard recommended approach.

Breakdown characteristics of transformer oil with cellulose particles in a non-uniform field under lightning impulse voltage

This study presents experimental results on the influence of cellulose particles on the breakdown characteristics of transformer oil in a highly non-uniform field under lightning impulse voltage. The experimental results show that as the particle content increases, the breakdown voltage reduces with little change in the breakdown time lag. At the same cellulose particle content, the oil breakdown voltage under oscillating lightning impulse (OLI) is lower than that under standard lightning impulse (SLI) and the breakdown time lag under OLI is longer than that under SLI. The impulse wave oscillation has a large effect on the accumulation of particles around the high voltage electrode and cellulose particles affect the development of the streamer process. At mid to high particle content the accumulation of particles around the high voltage electrode causes a decrease in the conductivity of the surrounding oil. This work contributes to the understanding of the effect of cellulose particles in oil and breakdown and on oscillating waveforms and provides guidance for lightning impulse tests on oil-immersed power transformers in the field.

Comparison of breakdown voltage of vegetable olive with mineral oil, natural and synthetic ester liquids under DC voltage

This paper deals with a comparative study of the negative and positive DC breakdown voltages of different insulating liquids namely two natural esters (an extra virgin olive oil (OO) without additive and rapeseed oil), two synthetic ester fluids (a methyleolate obtained by transesterification of olive oil and tetraester), and a naphthenic mineral oil (MO); an oils mixture namely 80% OO + 20% MO is also investigated. Statistical analysis of the experimental data is also performed. The obtained results are also compared with those obtained in our previous work under AC and lightning impulse voltages. It is observed that the negative and positive DC breakdown voltages of natural esters are higher than those of mineral oil. On the other hand, the DC breakdown voltage of (80% OO + 20 % MO) mixture shows comparable behavior to that of mineral oil. The DC negative breakdown voltages are higher than the positive ones in the tested liquids. It is also shown that the DC breakdown voltage values of the investigated liquids generally obey the normal distribution.