Negative Impulse Breakdown Research Articles

Effect of metal particles on the electrical properties of mineral and natural ester oils

Metal particles, such as copper and iron, not only reduce the breakdown voltage but also accelerate the oxidation of insulating oils. The effects of metal particles on the dielectric properties of mineral and natural ester insulating oils were investigated in this work. The experimental results indicate that the AC and impulse breakdown voltages of the insulating oil decrease with the increase of the size and the quantity of the metal particles. The positive breakdown voltages V b+ of insulating oils decrease moderately in the beginning, while the particles quantity increases to a certain extent, the V b+ decreases significantly. In comparison, the metal particles always reduce the negative impulse breakdown voltage V b− significantly. The effect of the copper particles on the insulating oils is slightly greater than that of the iron particles due to their higher conductivity. The permittivity of contaminated insulating oils increases quasi-linearly with the concentration of copper particles. Improved models considering the polarization of the surface oxidation layer of copper particles were proposed, and the calculated data were in good agreement with the measured permittivity of the contaminated insulating oil.

Trapping and scattering effect of charge carriers on impulse breakdown characteristics of nano‐fluids

Propylene carbonate (PC)-based nano-fluids (NFs), containing only 2 ppm TiO2 nano-particles, exhibit more than 60% higher negative impulse breakdown voltage than that of base liquid. It is found that the resistance of the test gap containing PC remains basically invariable when subjected to high amplitude voltage before the development of electrical breakdown. However, for the NF case, the resistance increases by a factor. Moreover, compared with PC, the streamers in NFs are more complex branched. These experimental results indicate that the charge carriers in NFs can be effectively captured and scattered by nano-particles, which is verified by means of the thermally stimulated current method.

Preparation of Three Types of Transformer Oil-Based Nanofluids and Comparative Study on the Effect of Nanoparticle Concentrations on Insulating Property of Transformer Oil

Nanofluids have the potential to become the alternatives of conventional transformer oil for their exquisite electrical and thermal properties. Three kinds of nanoparticles with distinct conductivities, namely, nonconductive nanoparticle Al2O3, conductive nanoparticle Fe3O4, and semiconductive nanoparticle TiO2, with different concentrations from 5% to 40% w/v were selected and suspended into transformer oil to develop nanofluids. The lightening impulse breakdown strengths of the oil samples with and without nanoparticles were measured according to IEC standard methods. The positive impulse breakdown strength indicated that breakdown strength is first increased up to the maximum value at certain concentration and then starts decreasing. The results of negative impulse breakdown manifested that the breakdown voltages of nanofluids with different concentrations were less than the breakdown voltage of pure transformer oil. Different effect mechanisms of dielectric and conductive nanoparticles were also used to describe the difference among three prepared nanofluids.

Inhibition effect of space charge transportation on impulse breakdown performance of propylene carbonate with Al2O3 nanoparticles

Propylene carbonate-based nanofluids with Al2O3 nanoparticles exhibit 26.42% higher negative impulse breakdown voltage levels than those of pure propylene carbonate. The Kerr effect electro-optic method has been used to measure the space charges and electric field distribution of pure and modified nanofluids of propylene carbonate under impulse voltage. Test results indicate significant inhibition effect of space charge transportation in propylene carbonate-based nanofluids. The fast free charges are captured by nanoparticles and converted to slow-charged particles in nanofluids, which result in the uniform electric field distribution. The streamer propagation in nanofluids is also hindered, and the breakdown strength of nanofluids is improved under impulse conditions.

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.

Impulse Breakdown of Negative Rod-Plane Gaps in Hydrogen and Hydrogen-SF6 Mixtures

Impulse breakdown voltages have been measured for negative rod-plane gaps in hydrogen and in H2/SF6 mixtures for pressures in the range 0.5-3.0 bar and electrode spacings up to 0.25m. In hydrogen, the negative-impulse breakdown voltages are 60-100% higher than those previously reported for positive polarity: However the increase in breakdown voltage with the addition of small quantities (?1%) of SF6 is smaller than for positive-impulse conditions and is confined to pressures below~H2 bar, with the result that the positive and negative impulse strengths of rod-plane gaps in such low SF6-concentration mixtures are almost equal. The results are discussed with reference to observations of the effect of SF6 on the prebreakdown corona activity.

The influence of irradiation on the measurement of impulse voltages with sphere-gaps

The impulse breakdown of short gaps between spheres is greatly affected by the amount of irradiation present, and recommendations are made in the British Standard “Rules for the Measurement of Voltage with Sphere-Gaps”1 that appropriate precautions should be taken when voltages of less than 50 kV (peak) are being measured. These precautions are generally considered to be unnecessary in the measurement of higher voltages, and, in most laboratories, no deliberate steps are taken to provide irradiation.In a previous publication the author has shown that appreciable errors may be incurred if irradiation is not used with spheres of 6.25 cm diameter in the measurement of voltages up to 100 kV (peak). Further measurements have now been made up to 400 kV (peak) with 1/5 and 1/50 impulse voltage waves, of positive and negative polarities, for spheres of 6.25 cm, 12.5 cm and 25 cm diameter. The present results confirm and extend the previous observations.The irradiation provided by the ultra-violet illumination from the impulse generator spark-gaps has a marked influence on the behaviour of the sphere-gap, in the absence of other forms of irradiation, and consequently the position of the sphere-gap relative to the impulse generator is observed to affect the results. This factor does not appear to have been considered in other laboratories, and it may explain some of the differences between the results obtained by various investigators in the calibration of the sphere-gap with impulse voltages.As it is not always convenient to illuminate the gap with the light from the impulse generator spark-gaps, the irradiation provided by the insertion of 0.5 mg of radium in the high-voltage sphere has been examined and is found to give satisfactory results. Measurements of the impulse breakdown of the unirradiated sphere-gap show that there is a gradual transition from the voltage which causes breakdown for 10% of the applied impulses to that which causes 90% breakdown, whereas a sharply defined breakdown voltage is obtained when the gap is irradiated. The mean breakdown voltage of the unirradiated gap exceeds that of the irradiated gap by an amount which varies with the gap length. For a 3 cm gap between 12.5 cm diameter spheres, breakdown of the unirradiated gap takes place at a mean voltage about 13% in excess of that for the irradiated gap. With larger gaps, for which a difference between the breakdown voltages for positive and negative impulses is recorded when one sphere is earthed, the difference is found to be largely influenced by irradiation. The mean breakdown voltage for an unirradiated gap of 9 cm between 12.5 cm diameter spheres is 20% higher than the irradiated value for a positive impulse, and 1.5% higher for a negative impulse. The negative impulse breakdown voltages agree closely with the corresponding values given in the B.S. 358 Tables, whereas the voltages given in the Tables for the positive impulse breakdown are intermediate between those obtained in the present experiments for the unirradiated and the irradiated gaps.The results show that some revision of the Standard Rules for the use of sphere-gaps is necessary, and, in particular, that more definite recommendations should be made concerning irradiation. Further, it is evident that the influence of irradiation is an important factor to be considered in the breakdown of other forms of discharge gaps in which corona is not observed to precede breakdown. Also, the breakdown of discharge gaps on the wavefront may be expected to be governed largely by the amount of irradiation present, and, though his has been realized by many workers for short gaps, no consideration appears to have been given to irradiation in the measurement of wavefront breakdown for longer gaps.