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The impulse voltage wave invading a transformer is not the double exponential impulse wave (DEIW) specified by the IEC but a non‐standard waveform with oscillation attenuation characteristics, and this fact leads to doubts about the scientificity and optimality of insulation coordination. A key solution for this issue is the establishment of an equivalent relationship between the measured non‐standard impulse voltage wave and the DEIW. In this study, a test platform of multiparameter impulse voltage was set up. Then, the authors obtained the 50% breakdown voltage (U50%) of the oil‐impregnated paper (OIP) for transformer under the bipolar oscillatory attenuated impulse wave (BOAIW) and the DEIW with different waveform parameters. The effects of waveform parameters on the breakdown characteristics of the OIP were finally revealed. On the basis of experimental data and several mathematical methods, the authors established the U50% prediction mathematical models for the OIP under the BOAIW and DEIW. Based on the prediction models and the microfluctuation characteristics of measured overvoltage's waveform parameters, the authors proposed a method for evaluating the waveform equivalence. The characteristics of voltage amplitude and change rate were regarded as the two key factors for the waveform equivalence of this method. [ABSTRACT FROM AUTHOR]

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. [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]

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]

Insulation failures in dry-type distribution transformers occur frequently and severely when 12-kV vacuum circuit breakers are applied to switch shunt reactors. However, the reason for this phenomenon is unclear. Based on the test platform for the failure circuit of a running 10-kV power system, a series of field tests was conducted to investigate the overvoltage mechanism of this type of failure. Field tests successfully recorded the prestrike and reignition waveforms during the transient processes of switching shunt reactors, based on which transient process and phenomena were analyzed. The prestrike probability was as high as 60% during the transient process of switching on shunt reactors. However, the probability of reignition was significantly lower than that of prestrike. The phase-to-ground and interphase maximum overvoltages during the switching transient processes have approximately the same amplitude. The prestrike caused phase-to-ground overvoltages beyond 6.0 p.u., whereas the reignition caused overvoltage beyond 8.0 p.u. The mechanisms of overvoltage generation of the switching transient processes were analyzed based on circuit analysis. Multiprestrikes and multireignitions, cable length, switching conditions, and system configurations were the main reasons for severe overvoltages. [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 mechanisms by which conductive and dielectric nanoparticles (NPs) trap electrons are explained by the potential well distribution caused by induced or polarized charges on NPs. Thus, the distributions of surface and saturation charges on conductive and dielectric NPs are determined. Given conductive Fe3O4, semiconductive TiO2, and dielectric Al2O3 NPs, insulation performance tests are conducted and ionization models of nanofluids (NFs) based on transformer oil are developed. These models are compared with those of NFs based on pure oil. The NP whose conductivity or permittivity does not match that of the dielectric liquid has a potential well and an increased amount of saturation charges on its interface. This NP influences streamer development strongly and enhances the breakdown of oilbased NF. [ABSTRACT FROM AUTHOR]

The wide variety of lightning stroke characteristics and the modifying effects of power system components result in a diversity of intrusive lightning voltage waveshapes that stress transformers. These are not the traditional standard lightning impulses with a waveshape of 1.2/50 μs. Therefore the applicability of the standard lightning impulse voltage to power transformer testing has been questioned, and the overvoltage used in the test on transformers should be in accordance with the lightning overvoltage measured in the field. In this study, a lightning overvoltage monitoring system was installed in a 110 kV air‐insulated substation, and the lightning overvoltages from 2007 to 2012 have been measured. Analysis was conducted on the measured data, which indicated that bidirectional oscillatory lightning waveforms rose up in the measured overvoltages. Using the energy method, the bidirectional oscillatory lightning waveform was reformulated into double exponential waveform, and the characteristic parameters, such as peak amplitude, front time and tail time, of the measured data were statistically analysed. [ABSTRACT FROM AUTHOR]