Negative Lightning Impulse Breakdown Voltage Research Articles

The Influence of Nanoparticles’ Conductivity and Charging on Dielectric Properties of Ester Oil Based Nanofluid

This study addresses the effect of nanoparticles’ conductivity and surface charge on the dielectric performance of insulating nanofluids. Dispersions of alumina and silicon carbide nanoparticles of similar size (~50 nm) and concentration (0.004% w/w) were prepared in natural ester oil. The stability of the dispersions was explored by dynamic light scattering. AC, positive and negative lightning impulse breakdown voltage, as well as partial discharge inception voltage of the nanofluid samples were measured and compared with the respective properties of the base oil. The obtained results indicate that the addition of SiC nanoparticles can lead to an increase in AC breakdown voltage and also enhance the resistance of the liquid to the appearance of partial discharge. On the other hand, the induction of positive charge from the Al2O3 nanoparticles could be the main factor leading to an improved positive Lightning Impulse Breakdown Voltage and worse performance at negative polarity.

AC and Negative Lightning Impulse Breakdown Voltage of Mineral Oil with Different Nitrogen Addition Treatment Methods

AC and Negative Lightning Impulse Breakdown Voltage of Mineral Oil with Different Nitrogen Addition Treatment Methods

Lightning impulse breakdown voltage of synthetic and natural ester liquids-based Fe3O4, Al2O3 and SiO2 nanofluids

This article deals with the study the negative lightning impulse breakdown voltage (LI-BDV) of MIDEL 1204 natural and MIDEL 7131 synthetic ester liquids when adding small amounts of specific nanoparticles (NPs); the considered NPs being magnetic (Fe3O4) and insulating (Al2O3 and SiO2) NPs. It is shown that these NPs enhance the LI-BDV of investigated esters. In addition, for a given type of NPs, there is an optimal concentration for which the U50% LI-BDV is the highest. This latter is enhanced by about 16.8% for natural ester with Al2O3 at a concentration of 0.05 g/L; and by about 25.6% for synthetic ester with Fe3O4 at a concentration of 0.05 g/L.

A Promising Nano-Insulating-Oil for Industrial Application: Electrical Properties and Modification Mechanism.

Despite being discovered more than 20 years ago, nanofluids still cannot be used in the power industry. The fundamental reason is that nano-insulating oil has poor stability, and its electrical performance decreases under negative impulse voltage. We found that C60 nanoparticles can maintain long-term stability in insulating oil without surface modification. C60 has strong electronegativity and photon absorption ability, which can comprehensively improve the electrical performance of insulating oil. This finding has great significance for the industrial application of nano-insulating oil. In this study, six concentrations of nano-C60 modified insulating oil (CMIO) were prepared, and their breakdown strength and dielectric properties were tested. The thermally stimulated current (TSC) curves of fresh oil (FO) and CMIO were experimentally determined. The test results indicate that C60 nanoparticles can simultaneously improve the positive and negative lightning impulse and power frequency breakdown voltage of insulating oil, while hardly increasing dielectric loss. At 150 mg/L, the positive and negative lightning impulse breakdown voltages of CMIO increased by 7.51% and 8.33%, respectively, compared with those of FO. The AC average breakdown voltage reached its peak (18.0% higher compared with FO) at a CMIO concentration of 200 mg/L. Based on the test results and the special properties of C60, we believe that changes in the trap parameters, the strong electron capture ability of C60, and the absorption capacity of C60 for photons enhanced the breakdown performance of insulating oil by C60 nanoparticles.

Breakdown characteristics of CF3I/N2/CO2 mixture in power frequency and lightning impulse voltages

Trifluoroiodomethane (CF3I) and its mixtures are believed to be prospective alternatives to sulfur hexafluoride (SF6), which has been included as a greenhouse gas. In this paper, the breakdown properties of a CF3I/N2/CO2 mixture with the volume fraction of CF3I fixed at 10% are investigated under power frequency and lightning impulse voltages. The experimental result shows that N2 possesses higher power frequency and negative lightning impulse breakdown voltages than CO2, but the power frequency and more negative lightning impulse breakdown voltages of the CF3I/N2/CO2 mixture do not increase with the content of N2. For the purpose of explaining this abnormal phenomenon, the ionization energies and excitation energies of CF3I, N2 and CO2 are calculated. The computation results indicate that the ionization energy of CF3I is lower than the first excitation energy of N2, but higher than the lowest excitation energy of CO2, which means that CF3I molecules are easily ionized by metastable N2 molecules. The first excitation energy of N2 is too high, which hinders its application as the buffer gas of CF3I.

Insulation characteristics of triple mixtures of c-C4F8/N2/CO2 under lightning impulse voltage

This paper has researched the insulation characteristics of 10% c-C4F8/N2/CO2 mixtures under lightning impulse voltage by experiment. It is shown that the positive and negative lightning impulse breakdown voltages of 10%c-C4F8/N2/CO2 gas mixtures rise linearly as the electrode gap distance and gas pressure increase and under the same conditions, the positive lightning impulse breakdown voltage of the gas mixtures is always higher than the negative lightning impulse breakdown voltage. As the gas mixtures have a little higher liquefied temperature than SF6 and the comprehensive GWP is about 5% of SF6, and the positive and negative lightning impulse breakdown voltages can both reach 60% of SF6, 10%c-C4F8/N2/CO2 gas mixtures can be applied as insulation gas in electrical equipment such as C-GIS, GIT, GIL and so on.

Characteristics of Negative Lightning Impulse Breakdown Voltage in Pure Water

Characteristics of Negative Lightning Impulse Breakdown Voltage in Pure Water

Enhanced Electrical Insulation and Heat Transfer Performance of Vegetable Oil Based Nanofluids

Nanoparticles enhance the electrical insulation and thermal properties of vegetable oil, and such improvements are desirable for its application as an alternative to traditional insulating oil for power transformers. However, the traditional method of insulating nanofluids typically achieves high electrical insulation but low thermal conductivity. This work reports an environmentally friendly vegetable oil using exfoliated hexagonal boron nitride (h-BN) showing high thermal conductivity and high electrical insulation. Stable nanofluids were prepared by liquid exfoliation of h-BN in isopropyl alcohol. With 0.1 vol.% of the nano-oil, the AC breakdown voltage increased by 18% at 25°C and 15% at 90°C. Both the positive and negative lightning impulse breakdown voltages of the nano-oil were also enhanced compared with those of the pure oil. Moreover, the thermal conductivity of the nano-oil increased by 11.9% at 25°C and 14% at 90°C. Given its high thermal conductivity, the nano-oil exhibited faster heating and cooling effects than the pure oil. Nano-oils with an electric field (either DC or AC) displayed a faster thermal response than that without an electric field. The reason is that h-BN is oriented under the electric field and formed a thermal network to increase the heat transfer.

LIGHTNING IMPULSE BREAKDOWN CHARACTERISTICS AND ELECTRODYNAMIC PROCESS OF INSULATING VEGETABLE OIL-BASED NANOFLUID

Insulating vegetable oils are considered environment-friendly and fire-resistant substitutes for insulating mineral oils. This paper presents the lightning impulse breakdown characteristic of insulating vegetable oil and insulating vegetable oil-based nanofluids. It indicates that Fe 3 O 4 nanoparticles can increase the negative lightning impulse breakdown voltages of insulating vegetable oil by 11.8% and positive lightning impulse breakdown voltages by 37.4%. The propagation velocity of streamer is reduced by the presence of nanoparticles. The propagation velocities of streamer to positive and negative lightning impulse breakdown in the insulating vegetable oil-based nanofluids are 21.2% and 14.4% lesser than those in insulating vegetable oils, respectively. The higher electrical breakdown strength and lower streamer velocity is explained by the charging dynamics of nanoparticles in insulating vegetable oil. Space charge build-up and space charge distorted filed in point-sphere gap is also described. The field strength is reduced at the streamer tip due to the low mobility of negative nanoparticles.

On the possible origin of X-rays in long laboratory sparks

In this paper, based on theoretical estimation of the achievable electric fields during the physical development process of a long spark under different conditions, we show that the encounter of negative and positive streamer fronts just before the final breakdown is one scenario, under which the observed X-ray bursts in long sparks is highly possible. Our calculations show that for example in an 80 cm long rod–sphere air gap at atmospheric pressure with negative lightning impulse breakdown voltage of about 925 kV, electrons are accelerated to values in the range of 100–300 keV during the encounter. Subsequently, these electrons gain more energy moving through the potential gradient of the positive streamer region. The total gain of energy by electrons may reach 300–500 keV. The results also show that negative discharges can produce more energetic electrons than positive. If the suggested mechanism of X-ray production in long sparks is correct, then the X-ray burst may consist of several pulses closely spaced in time. Time resolved photography in simultaneous measurement of X-rays would be able to confirm this prediction.