Pre-breakdown and breakdown phenomena are investigated in liquid Nitrogen (LN 2 ) with a tape-plane electrode geometry. Different voltage waveforms are used: lightning and switching standard ...impulses, step impulse, and DC ramp. The influence of temperature is also investigated, by sub-cooling LN 2 down to 65 K. Breakdown voltage depends on polarity, gap distance, temperature, and HV waveform: the longer waveform, the lower breakdown voltage. Positive pre-breakdown streamers are about 3 decades faster than negative ones. With short duration lightning impulses, the propagation of slow negative streamers is quenched, resulting in higher breakdown voltage. In boiling conditions, slow negative streamers may propagate for long durations. Sub-cooling LN 2 induces a slight increase of streamer initiation voltage, and impedes the propagation of slow streamers due to condensation.
An increasing number of dielectric fluids intended for use in transformers are appearing on the market. Due to the growing awareness of sustainable development and serious environmental risks posed ...by any mineral oil spills, transformer manufacturers are constantly working on alternative dielectric liquids. Therefore, it is crucial to test those liquids thoroughly. Commonly performed tests are also focused on dielectric strength under both lightning impulse breakdown voltages (IEC 60897) and breakdown voltages at power frequency (IEC 60156). In this paper authors propose extension of the standard list of tests by including the measurement of partial discharges at power frequency and acceleration voltage at lightning impulse of both polarities. In the first case a standard IEC TR 61294 is adopted while in the second one the methodology is enlargement of the provisions of IEC 60897 by the mentioned acceleration voltage measurements. In both cases, these additional tests enable a more comprehensive evaluation of dielectric liquids under consideration, especially in the case of newly ester liquids introduced in the market. The results show that proposed approach is worth to be further considered in terms of use a given liquid in power transformer.
This paper reports on an observed phenomenon of increasing breakdown voltage while performing standard up and down lightning impulse (LI) testing in short air-gaps with weakly non-uniform field in an ...enclosed volume. The phenomenon could occur due to accumulation of space charges in the gap, changes in the chemical composition of the insulation gas, and/or changes in electrode surface due to discharges. To investigate the effect of these factors, LI tests are carried out in a 10 mm rod-plane air gap using four different methods: 1) ion flushing by DC voltage application in between LI shots, 2) with opposite polarity LI intertwined, 3) using forced air flow by a fan during the test, and 4) a reference test without any fan or voltage application. Results indicate that the phenomenon is more likely to be caused by formation of electronegative byproducts, e.g., ozone. Ozone was also experimentally detected near the test gap followed by consecutive LI breakdowns. It was found that air circulation rapidly reduces ozone concentration, which may explain why the increasing trend is not observed when a fan is used. Calculation of electron transport coefficients indicates that lack of primary electrons might be the prime reason of the increased impulse breakdown voltage of the air gap in presence of ozone.
The synthesis of insulating nanofluids by integration of nanoparticles in insulating oils has been employed over the last two decades by many researchers in an effort to improve both the dielectric ...strength and the thermal conductivity of dielectric liquids. In this paper, nanofluids with different volume concentrations of the nanoparticles were prepared by integration of semi-conductive TiO2 nanoparticles into natural ester oil. Ultra-sonication and magnetic stirring were used for preparing homogeneous dispersions. AC, Lightning Impulse Breakdown Voltage and Partial Discharge Inception Voltage were measured and compared to the corresponding values of the base oil. Furthermore, the stability of the dispersions was studied ex situ by Transmission Electron Microscopy and in situ by Dynamic Light Scattering. Experimental results reveal the optimal TiO2 nanoparticles' concentration in terms of breakdown voltage maximization, and future extensions of the current research are provided.
•AC BDV of nanofluids is enhanced with highest maximization by 22.4%.•NF2 and NF3 present enhanced LI BDV with the highest maximization being 12.9%.•Paper impregnated to NF3 shows the greatest resistance to appearance of PD by 40%.•DLS shows agglomerates present an abrupt increase against time.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•Establishment of comprehensive Database: through extensive lightning impulse flashover tests on 110 kV insulators, a robust database for U50 % and volt-time characteristics was created, providing a ...solid foundation for analysis.•Superior performance of PSO-RF in U50 % Prediction: comparative analysis revealed that the PSO-RF model exhibited the lowest prediction error and highest accuracy among other models such as BP and SVM. Its exceptionally low eMAPE of 1.17 % makes it the optimal choice for constructing U50 % prediction models, particularly suitable for high-dimensional data with a small sample size.•Innovation in Volt-time characteristics Prediction: this study introduces regression prediction for insulator volt-time characteristics, showcasing the superiority of intelligent models over traditional approaches. Comparison with existing standards and literature validates the efficacy of these models under different operating conditions.•Practical Implications: the predictive models developed in this study offer practical utility in analyzing flashover characteristics of 110 kV insulators under diverse conditions. Their implementation ensures the safe and stable operation of transmission lines, thereby enhancing overall system reliability.
The lightning flashover criteria of insulators vary significantly under different conditions, and it is impractical to exhaustively enumerate all the criteria through extensive tests. Therefore, this paper aims to explore the intelligent prediction of lightning flashover criteria for 110 kV insulators using a data-driven approach. Initially, a test database comprising 4,978 high- and low-altitude test data of 110 kV insulator lightning impulse flashover is established. Subsequently, relevant characteristic values are extracted from the database as inputs for the prediction model. Various machine learning algorithms, such as the BPNN, SVM and RF algorithms, are employed to construct prediction models for the U50% and volt-time characteristics of 110 kV insulators under lightning impulses. The results demonstrate that the RF algorithm yields an average absolute percentage error of merely 1.17 % for the U50% prediction model. Additionally, the RF and BP algorithms achieve the highest prediction accuracies of 10.7 % and 6.5 %, respectively, for the volt-time characteristics at high- and low-altitude. This validates the feasibility of substituting many traditional tests with more accurate flashover criteria predicted through a data-driven approach. This paper provides a novel concept for predicting the lightning impulse flashover criteria of insulators under different working conditions. Reducing the need for repetitive tests is expected to acquire the high-precision intelligent prediction of insulator lightning impulse flashover criteria.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
This study presents experimental results on the influence of cellulose particles on the breakdown characteristics of transformer oil in a highly non-uniform field under lightning impulse voltage. The ...experimental results show that as the particle content increases, the breakdown voltage reduces with little change in the breakdown time lag. At the same cellulose particle content, the oil breakdown voltage under oscillating lightning impulse (OLI) is lower than that under standard lightning impulse (SLI) and the breakdown time lag under OLI is longer than that under SLI. The impulse wave oscillation has a large effect on the accumulation of particles around the high voltage electrode and cellulose particles affect the development of the streamer process. At mid to high particle content the accumulation of particles around the high voltage electrode causes a decrease in the conductivity of the surrounding oil. This work contributes to the understanding of the effect of cellulose particles in oil and breakdown and on oscillating waveforms and provides guidance for lightning impulse tests on oil-immersed power transformers in the field.
This study presents an experimental comparison of the breakdown characteristics of oil samples with four cellulose particle content levels in a quasi-uniform electric field under lightning impulse ...voltage. For the experiment to resemble the actual working conditions of transformers and meet the on-site withstand impulse voltage test requirements, both the standard lightning voltage (SLI) and oscillating lightning impulse (OLI) are considered. The experimental results show that an increase in the number of cellulose particles directly weakens the oil insulation strength. At the same particle content level, the oil breakdown voltage under OLI is higher than that under SLI, and the breakdown time lag under OLI is longer than that under SLI. The breakdown time lag is more sensitive to the effect of particle content under SLI. This work is beneficial in deepening the understanding of the impulse breakdown characteristics of transformer oil within cellulose particles and in providing references for the on-site impulse test of oil-immersed power transformers.
The usage of medium-voltage switchgear is more and more common in high altitude areas. Few relative standards about the selection of switchgear air gap in low air pressure conditions are currently ...available for referring. In this study, four different types of switchgear air gaps, as well as a typical rod-plane gap, are taken as the samples, and their positive and negative lightning strike discharge characteristics under low air pressure are tested in the artificial climate chamber. The effects of the electrode structure, polarity, air pressure, humidity, and gap distance on the discharge performances are analysed as well. Research results indicate that the four switchgear air gaps and typical rod-plane gap differs greatly in lightning impulse discharge performances and polarity effects. In low air pressure conditions, the lightning impulse breakdown voltage of either positive or negative polarity can be corrected by U50 = U50,0 × (P/P0)n. The discharge voltage under positive lightning impulse is more easily affected by air pressure than that under negative lightning impulse. Atmosphere humidity will cause dispersiveness of air-gap lightning impulse discharge voltage, which performs to be non-linear with a gap distance lower under 400 mm. Research results are referable for the design, selection and installation of medium-voltage switchgear in high altitude areas.
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Ester-based nanofluids constitute an effective ecological alternative to petroleum-derived dielectric fluids for electrical engineering industry. Pulse induced dielectric breakdown is a key cause of ...power device failure. In this paper, we deal with the lightning impulse breakdown voltage (LI BDV) in natural and synthetic ester-based (NE and SE) nanofluids with various concentrations of magnetite (Fe3O4) and fullerene (C60) nanoparticles. The nanofluids show a decrement in positive polarity LI+ BDV as compared with the base ester. Negative polarity LI- BDV in the nanofluids is similar to that of base oils. Weibull statistical distributions are applied to analyse the results at different cumulative probabilities of 1 %, 10 %, and 50 %. Natural ester with C60 nanoparticles shows higher positive LI+ BDV values at a low probability level, while the negative LI- BDV reaches higher values at higher probability levels. The decreased LI BDV in NE and SE due to the magnetite nanoparticles are attributed to the nanoparticle clustering and percolation. The tendency of C60 nanoparticles to enhance the LI BDV in NE but not in SE is interpreted in the view of the nanoparticle concentration and oil's viscosity effect on the streamer branching and propagation.
Display omitted
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
This study is focused on exploring the change in the dielectric and cooling efficiency of vegetable oil following the integration of semiconducting SiC and TiO 2 nanoparticles in two different weight ...percentages by comparing the corresponding behavior of nanofluids at the time of their synthesis and after one month of ageing. The thermal response has been evaluated by means of diffusivity and conductivity, while dielectric properties are measured with non-destructive tests (relative permittivity and dissipation factor) and destructive ones in terms of lightning impulse withstand voltage. The results of the latter property measurements are also compared to numerical model findings. The analysis of results proves that the properties under study are worsened with increase in nanoparticles' concentration, probably attributing it to quicker agglomeration, as it is also confirmed by the simulation results.
