Positive Impulse Breakdown Voltage Research Articles

Modification Effect of TiO<sub>2</sub> Hollow Nanoparticles on Insulating Properties of Transformer Oil

With the promoting of transmission voltage level and the continuous fluctuation of operating conditions by connecting renewable energy into the power system, more stringent requirements have been placed on the operation reliability of transformers and other large electrical equipment. Transformer oil, as the main liquid insulating medium in large power equipment, the modification of its insulating performance has always been research focus in dielectric field. Studies have proved that TiO₂ nanomaterials can not only enhance the AC and positive impulse breakdown voltage of transformer oil, but also significantly improve the aging resistance of transformer oil. However, the DC breakdown voltage of transformer oil has not been greatly increased yet, and sometime even weakened. In this paper, TiO₂ hollow nanoparticles were prepared by hard template method to modify transformer oil. AC and DC breakdown voltage and dielectric permittivity of transformer oil before and after modification were measured and analyzed. The results show that the AC breakdown voltage of the transformer oil can be increased by 32.5%. DC breakdown voltage was increased first and then reduced with the increasing of nanoparticle concentration, while no obvious improvemnet was observed. Based on breakdown mechanism and permittivity of transformer oil, we think the hollow structure of the nanoparticles can significantly enlarge the interface between particle and oil, which greatly promotes the capturing capability to fast electrons and inhibit their migration in the oil, so the AC breakdown voltage are significantly increased. Under DC stress, nanoparticles can capture electrons and meanwhile migrate along fixed direction, which increased electrical conductivity of oil and resulted in lowering of DC breakdown voltage with the increase of nanoparticle concentration.

Computation of breakdown voltage of long rod-plane air gaps in large temperature and humidity range under positive standard switching impulse voltage

The 50% breakdown voltage of the air gap is an important basis for the design of the external insulation of the high voltage power systems, and the atmospheric conditions have a large impact on the breakdown voltage of the air gap. The temperature and humidity of the operating environment of the high voltage power systems have a wide range. The breakdown voltages of the gaps obtained in the laboratory are mostly near the standard atmospheric conditions, and there is no good way to calculate the discharge voltage outside the standard atmospheric conditions. This paper established a 50% positive switching impulse breakdown voltage prediction model based on the cuckoo search algorithm optimized support vector regression for the rod-plane air gap. This calculation model is used to predict the selected prediction data. The predicted results are basically consistent with the experimental results, and the effect is better than the atmospheric correction results. Finally, the method was used to calculate the 50% positive impulse breakdown voltage of the rod-plane air gap under the extreme temperature and humidity conditions in Beijing. This method provides a new idea for obtaining the air gap breakdown voltage under different atmospheric conditions.

Investigation of the Lightning Impulse Breakdown Voltage of Mineral Oil based Fe3O4 Nanofluids

The recent interest in the development of nanomaterials has led researchers to the study of their electrical properties and the applications that they may have as insulating materials. One of these applications is the use of nanofluids as electrical insulation of power transformers. It has been reported that the dielectric properties of insulating liquids in which small amounts of nanoparticles have been dispersed are, in some cases, superior to those of the base fluids. Although these materials are promising, and their application could lead to advantages for the transformer design and reliability in the future, more research is necessary to evaluate different combination of materials under a wider range of experimental conditions. In this paper, a research on the lightning impulse breakdown voltage of mineral oil and several Fe3O4-based nanofluids is presented. Fluids prepared with different concentrations of nanoparticles were subjected to impulse lightning voltages considering both positive and negative polarities. As shown in this work the positive impulse breakdown voltage of the liquids showed improvements of up to 50%; in the case of the negative impulses not significant improvements were obtained.

Effect of Nanoparticle Morphology on Pre-Breakdown and Breakdown Properties of Insulating Oil-Based Nanofluids.

Nanoparticles currently in use are challenged in further improving the dielectric strength of insulating oil. There is a great need for a new type of nanoparticle to promote the application of insulating oil-based nanofluids in electric industries. This paper experimentally investigates the effect of nanoparticle morphology on pre-breakdown and breakdown properties of insulating oil-based nanofluids. The positive impulse breakdown voltage of insulating oil can be significantly increased by up to 55.5% by the presence of TiO2 nanorods, up to 1.23 times that of TiO2 nanospheres. Pre-breakdown streamer propagation characteristics reveal that streamer discharge channels turn into a bush-like shape with much denser and shorter branches in the nanofluid with TiO2 nanorods. Moreover, the propagation velocity of streamers is dramatically decreased to 34.7% of that in the insulating oil. The greater improvement of nanorods on the breakdown property can be attributed to the lower distortion of the electric field. Thus, when compared with nanospheres, pre-breakdown streamer propagation of nanofluid is much more suppressed with the addition of nanorods, resulting in a greater breakdown voltage.

Effect of Fe3O4 nanoparticles on space charge distribution in propylene carbonate under impulse voltage

Addition of nanoparticles of the ferromagnetic material Fe3O4 can increase the positive impulse breakdown voltage of propylene carbonate by 11.65%. To further investigate the effect of ferromagnetic nanoparticles on the space charge distribution in the discharge process, the present work set up a Kerr electro-optic field mapping measurement system using an array photodetector to carry out time-continuous measurement of the electric field and space charge distribution in propylene carbonate before and after modification. Test results show that fast electrons can be captured by Fe3O4 nanoparticles and converted into relatively slow, negatively charged particles, inhibiting the generation and transportation of the space charge, especially the negative space charge.

Space charge inhibition effect of nano-Fe3O4 on improvement of impulse breakdown voltage of transformer oil based on improved Kerr optic measurements

Transformer oil-based nanofluids (NFs) with 0.03 g/L Fe3O4 nanoparticle content exhibit 11.2% higher positive impulse breakdown voltage levels than pure transformer oils. To study the effects of the Fe3O4 nanoparticles on the space charge in transformer oil and to explain why the nano-modified transformer oil exhibits improved impulse breakdown voltage characteristics, the traditional Kerr electro-optic field mapping technique is improved by increasing the length of the parallel-plate electrodes and by using a photodetector array as a high light sensitivity device. The space charge distributions of pure transformer oil and of NFs containing Fe3O4 nanoparticles can be measured using the improved Kerr electro-optic field mapping technique. Test results indicate a significant reduction in space charge density in the transformer oil-based NFs with the Fe3O4 nanoparticles. The fast electrons are captured by the nanoparticles and are converted into slow-charged particles in the NFs, which then reduce the space charge density and result in a more uniform electric field distribution. Streamer propagation in the NFs is also obstructed, and the breakdown strengths of the NFs under impulse voltage conditions are also improved.

Electrical characteristics of insulating materials for HTS bushing immersed in <TEX>$LN_2$</TEX>

For the operation of high temperature superconducting (HTS) power equipments, it is necessary to develop insulating materials and high voltage (HV) insulation technology at cryogenic temperature of bushing. In this paper, the surface flashover characteristics of various insulating materials in <TEX>$LN_2$</TEX> are studied. These results are studied at both AC and impulse voltage under a non-uniform field. The negative impulse breakdown voltage of GFRP is slightly higher than the positive impulse breakdown voltage. The use of glass fiber reinforced plastic (GFRP) and polytetrafluoroethylene (PTFE, Teflon) as insulation body for HTS bushing should be much desirable. Especially, GFRP is excellent material not only surface flashover characteristics but also mechanical characteristics at cryogenic temperature. The surface flashover is most serious problem for the shed design in <TEX>$LN_2$</TEX> and operation of superconducting equipment.

Effect of electron shallow trap on breakdown performance of transformer oil-based nanofluids

Transformer oil-based nanofluids with TiO2 semiconductive nanoparticles exhibit substantially higher AC and positive impulse breakdown voltage levels than that of pure transformer oils. Thermally stimulated current method (TSC) and pulse electroacoustic technique (PEA) have been used to measure charge trap and transportation characteristics of pure oils and nanofluids. It is found that electron shallow trap density and charge decay rate are greatly increased in nanofluids, i.e., fast electrons may be converted to slow electrons by electron trapping and de-trapping in shallow traps of nanofluids, resulting in improved breakdown performance compared to that of pure oil.

The role of methylnaphthalene in EHV cable oil as related to oil breakdown and discharge velocity

AbstractThis experimental investigation explores the effect of high voltage lightning surges on the insulating oil used in the oil filled extra high voltage cables that serve electric power networks worldwide. Using non‐uniform electrode configuration, the breakdown voltage and corresponding discharge propagation velocities in oil samples that are doped with trace amounts of 1‐Methylnaphthalene are investigated under both polarities of applied stresses in gap spacing of up to 10 mm. Two distinct ranges of additive concentrations are observed which effect these characteristics. Generally, the positive discharge growth decreases with increase in gap spacing while the negative ones show opposite behaviour. Anomalous behaviour of discharge growth is observed at an optimum concentration of additive, as it attains supersonic velocity which increases with the increase in gap spacing. A correlation between the positive impulse breakdown voltage and the discharge propagation velocity is also established.

Electrical strength of air containing ozone and nitric oxides produced by intensive partial discharges

Internal arcing test results of gapped SiC arresters are presented. During a test in which the arrester is exposed to internal arcing, the positive impulse breakdown voltage increases. Measurements made with a model arrangement indicate that the increase in the electrical strength of gas is higher for positive than for negative impulse polarity. When the internal discharges extinguish, recovery processes occur. Additional phenomena that can cause changes in the electrical strength are discussed.

Effect of injected space charges on the positive impulse breakdown of SF6 in a point-plane gap

Positive and negative polarity charges were injected into a point-plane gap by means of four auxiliary needle electrodes surrounding the gap. Experiments showed that the positive impulse breakdown voltage of SF6 in a 20 mm point-plane gap increased when the needles were positively charged up to a certain voltage, and then decreased with further increase of charging voltage. The prebreakdown corona current was virtually unaffected by the injected positive charge. When negative charges were injected from the needles, the prebreakdown corona activity was greatly increased and the positive impulse breakdown voltage was decreased. The findings reported here are consistent with the mechanism of impulse breakdown recently reported for highly non-uniform field gaps in SF6.

Nonlinearity in Impulse Breakdown of Very Large Air Gaps

Optical and electrical measurements of positive impulse discharges, in a point-to-plane electrode configuration, have been performed for electrode spacings ranging from I to 7 meters. A comprehensive analysis of these results indicates that the breakdown mechanism above 5.2 million volts (MV) differs from the mechanism operating at voltages below this level. A new extrapolation method for breakdown voltages levels above the measurable range has been developed. This method permits the determination of positive impulse breakdown voltage for very large gap spacings up to 14 meters, well above the maximum capabilities of presently available voltage sources. This study provides essential data contributing to a feasibility study of future UHV overhead systems.

Electric breakdown of SF6 in nonuniform fields

SEVERAL INVESTIGATORS have shown that in a nonuniform field the positive direct breakdown voltage versus pressure characteristic of sulfur hexa-fluoride (SF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> ) exhibits a very pronounced maximum. Pure nitrogen (N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> ) does not show such a maximum. This unexpected performance is best illustrated by Fig. 1. Although the positive direct breakdown voltage goes through a maximum, the positive impulse breakdown voltage is much lower over a considerable pressure range. This produces an impulse ratio as low as one half. At the same time, the positive direct voltage required for breakdown may be as high as six times that for nitrogen. In the region of this maximum, corona occurs at a voltage much lower than breakdown. The 60-cycle crest breakdown voltage practically coincides with the positive direct breakdown voltage.