Wave Tail Time Research Articles

Effects of impulse waveform parameters on the breakdown characteristics of nano-TiO<inf>2</inf> modified transformer oil

Statistical results have shown that the waveforms of impulses invading power transformers differ from those of the standard impulse voltage recommended by the IEC Standard. However, the effect of such differences on the modification effect of nano-modified transformer oil has rarely been studied. The present work therefore experimentally investigated the effects of impulse waveform parameters on the breakdown characteristics of nano-TiO 2 modified transformer oil (NMTO). We determined the breakdown characteristics of NMTO with different nano-particle concentrations, and compared the breakdown characteristics under both standard impulse voltage and non-standard impulse voltage. Specifically, the influence of the wave front time and wave tail time of the applied voltage on the breakdown characteristics of NMTO were experimentally identified. Finally, mechanisms of observed phenomena were analyzed. Test results indicate that both the waveform parameters and polarity have important effects on the modification effect of NMTO. Such effects are attributed to the migration of positive particles and charged nano-particles.

Impact of wavefront time of microsecond impulse on the breakdown voltage of oil impregnated paper: Mechanism analysis

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.

Discussion on lightning impulse test waveform according to breakdown characteristics of SF6 gas gaps

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

Measurement and analysis of transient overvoltage distribution in transformer windings based on reduced-scale model

Non-uniform voltage distribution under transient voltage is a primary cause of breakdown of the major insulation of power transformers. Actual power transformer winding has no tap, and is unable to directly measure impulse voltage distribution. Research on the measurement of impulse voltage distribution is important and practical in engineering. According to the relationship between physical quantities, the present study established a simplified shrinkage ratio criterion. Then, the researchers designed a similar ratio 1/20, typed 10kVA 220V/2400V, tapped transformer reduced-scale model, and used the finite element simulation software to simulate the reduced-scale and the original model of the electromagnetic field. Finally, a test platform was built to measure nanosecond pulse voltage and lightning impulse voltage distribution in transformer winding. The researchers also studied the nanosecond pulse width and lightning wave tail time affecting the winding voltage distribution.

Impact of time parameters of lightning impulse on the breakdown characteristics of oil paper insulation

At present, the insulation performances of electrical apparatus under lightning impulse voltages are always evaluated by the withstand test of standard lightning impulse voltage (1.2/50 µs), which is recommended by IEC Standard. However, the actual time parameters of transformer invading lightning impulse voltage present an obvious dispersibility characteristics. The standard wave shape is only representative of one particular class of lightning waves, which is far from satisfactory to act as the guidance waveform in the design of power equipment insulation. On the basis of the above research background, the relationship between breakdown characteristics of oil paper insulation and time parameters of applied lightning impulse voltage was investigated in this study. The 50% breakdown voltage characteristics of oil paper insulation under lightning impulse voltage with different time parameters were determined. The test results indicate that a short wave front time could lead to relatively low 50% breakdown voltage, while when the wave tail time varied, the 50% breakdown voltage of oil paper insulation barely changed. At last the simulation model of oil paper insulation was developed, and the current flowing through the oil paper insulating system under lightning impulse voltage was calculated. On the basis of the simulating results, the relationship between current and time parameters was analysed.

Study on voltage-number characteristics of transformer insulation under transformer invading non-standard lightning impulses

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 μs. 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.

Analysis on the Lightning Current Model

Through the analysis of characteristic of lightning currents, this paper expounded the lightning current wave-form and the primary parameters of it. The previous lightning current models contain only two parameters, wave tail time constant and the peak current value, which is imperfection. In order to solve this problem, a new lightning current model with the additional parameters such as lightning channel radius and velocity of the return stroke was proposed. On the basis of lightning current parameters and wave-form characteristic, we compared the new lightning current model with the double exponential function model, Hodler model and impulse function model. And then the distinctions among the four different models were received and the wave-form of the new model accorded with the characteristic of really lighting current wave-form well. On the other hand, the advantages and disadvantages about the new lightning current model were explained. This new lightning model provides the theoretical basis for further study of lightning electromagnetic field.

A computational tool to assist the analysis of the transformer behavior related to lightning

This paper proposes the application of computational intelligence techniques to assist complex problems concerning lightning in transformers. In order to estimate the currents related to lightning in a transformer, a neural tool is presented. ATP has generated the training vectors. The input variables used in Artificial Neural Networks (ANN) were the wave front time, the wave tail time, the voltage variation rate and the output variable is the maximum current in the secondary of the transformer. These parameters can define the behavior and severity of lightning. Based on these concepts and from the results obtained, it can be verified that the overvoltages at the secondary of transformer are also affected by the discharge waveform in a similar way to the primary side. By using the tool developed, the high voltage process in the distribution transformers can be mapped and estimated with more precision aiding the transformer project process, minimizing empirics and evaluation errors, and contributing to minimize the failure rate of transformers.

A Neural Approach to Evaluate the Effect of Lightning in Power Transformers

This paper proposes the application of computational intelligence techniques to assist complex problems concerning lightning in transformers. In order to estimate the currents related to lightning in a transformer, a neural tool is presented. ATP has generated the training vectors. The input variables used in Artificial Neural Networks (ANN) were the wave front time, the wave tail time, the voltage variation rate and the output variable is the maximum current in the secondary of the transformer. These parameters can define the behavior and severity of lightning. Based on these concepts and from the results obtained, it can be verified that the overvoltages at the secondary of transformer are also affected by the discharge waveform in a similar way to the primary side. By using the tool developed, the high voltage process in the distribution transformers can be mapped and estimated with more precision aiding the transformer project process, minimizing empirics and evaluation errors, and contributing to minimize the failure rate of transformers.

The Attenuation and Distortion of Short-Tail Traveling Waves on Overhead Power Transmission Lines

This paper describes the results of field investigations of the attenuation of short-tail traveling waves (wave-tail time 1-4 μs) on overhead transmission lines. A theoretical analysis of the methods for calculating this attenuation and distortion is also presented. Short-tail traveling waves can be calculated by harmonic analysis through the use of the attenuation data on high-frequency signals. It is difficult, however, to find an adequate theoretical equation.