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Pre-breakdown Phenomena in Mineral Oil Based Nanofluids
KTH, School of Electrical Engineering and Computer Science (EECS).ORCID iD: 0000-0002-8173-8765
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Mineral oil is a dielectric liquid commonly used in high voltage equipment such as power transformers. Interestingly, it has been experimentally observed that the dielectric strength of the mineral oil is improved when nanoparticles are added. However, the mechanisms behind these improvements are not well understood, hindering the further innovation process of these so-called nanofluids. This thesis aims to contribute to the understanding of the mechanisms explaining the dielectric strength improvement of the base oil when nanoparticles are added.For this, several experiments and numerical simulations are performed in this thesis. The initiation voltage of electric discharges infive different kind of nanofluids was measured. The large data set obtained allowed to cast experimental evidence on the existing hypotheses that are used to explain the effect of nanoparticles. It is found that hydrophilic nanoparticles hinder the electric discharge initiation from anode electrodes. On the other hand, electric discharge initiation from cathode electrodes was hindered by nanoparticles with low charge relaxation time.The electric currents in mineral oil and nanofluids were also measured under intense electric fields (up to 2GV/m). It is found that the addition of certain nanoparticles increases the measured currents. The possible physical mechanisms explaining the measured currents inmineral oil with and without nanoparticles were thoroughly discussed based on results of numerical simulations. Preliminary parameters used in this thesis to model these mechanisms led to a good agreement between the measured and simulated electric currents.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2019. , p. 69
Series
TRITA-EECS-AVL ; 2019:58
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electrical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-255605ISBN: 978-91-7873-241-8 (print)OAI: oai:DiVA.org:kth-255605DiVA, id: diva2:1340023
Public defence
2019-09-06, Kollegiesalen, Brinellvägen 8, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20190802

Available from: 2019-08-02 Created: 2019-08-01 Last updated: 2019-08-02Bibliographically approved
List of papers
1. Streamer Inception from Ultra-Sharp Needles in Mineral Oil Based Nanofluids
Open this publication in new window or tab >>Streamer Inception from Ultra-Sharp Needles in Mineral Oil Based Nanofluids
2018 (English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 11, no 8, article id 2064Article in journal (Refereed) Published
Abstract [en]

Positive and negative streamer inception voltages from ultra-sharp needle tips (with tip radii below 0.5 m) are measured in TiO2, SiO2, Al2O3, ZnO and C-60 nanofluids. The experiments are performed at several concentrations of nanoparticles dispersed in mineral oil. It is found that nanoparticles influence positive and negative streamers in different ways. TiO2, SiO2 and Al2O3 nanoparticles increase the positive streamer inception voltage only, whilst ZnO and C-60 nanoparticles augment the streamer inception voltages in both polarities. Using these results, the main hypotheses explaining the improvement in the dielectric strength of the host oil due to the presence of nanoparticles are analyzed. It is found that the water adsorption hypothesis of nanoparticles is consistent with the increments in the reported positive streamer inception voltages. It is also shown that the hypothesis of nanoparticles reducing the electron velocity by hopping transport mechanisms fails to explain the results obtained for negative streamers. Finally, the hypothesis of nanoparticles attaching electrons according to their charging characteristics is found to be consistent with the results hereby presented on negative streamers.

Place, publisher, year, edition, pages
MDPI, 2018
Keywords
streamer inception, electric discharges, nanofluids, mineral oil
National Category
Other Chemistry Topics
Identifiers
urn:nbn:se:kth:diva-238926 (URN)10.3390/en11082064 (DOI)000446604100143 ()2-s2.0-85052822998 (Scopus ID)
Note

QC 20181114

Available from: 2018-11-14 Created: 2018-11-14 Last updated: 2019-08-01Bibliographically approved
2. On the injection and generation of charge carriers in mineral oil under high electric fields
Open this publication in new window or tab >>On the injection and generation of charge carriers in mineral oil under high electric fields
2019 (English)In: JOURNAL OF PHYSICS COMMUNICATIONS, ISSN 2399-6528, Vol. 3, no 3, article id UNSP 035019Article in journal (Refereed) Published
Abstract [en]

Charge injection and generation mechanisms under intense electric fields (up to 10(9)Vm(-1)) in mineral oil are assessed experimentally and numerically. For this, current-voltage characteristics under positive and negative polarities are measured in a needle-plane configuration using sharp needles (with tip radius R-tip <= 1.1 mu m). In addition, a state of the art electro-hydrodynamic (EHD) model is implemented to calculate the contribution of the different mechanisms on the high-field conduction currents in the liquid. In order to evaluate exclusively the contribution of field emission, experiments are also performed in vacuum. It is found that neither field emission nor field ionisation can explain the conduction currents measured in mineral oil. It is proposed that field molecular ionisation, as described by Zener tunnelling model for solids, and electron impact ionisation are the processes dominating the generation of excess electron-ion pairs in mineral oil under positive and negative polarity, respectively. It is also shown that Zener molecular ionisation alone grossly overestimates the measured currents when parameters previously suggested in the literature for mineral oil are used. Preliminary model parameters for these mechanisms that best fit the conduction currents measured in mineral oil are presented and discussed.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2019
Keywords
electrical conduction, electrohydrodynamics, mineral oil, excess carriers, RENZO SE, 1974, PHYSICAL REVIEW A, V9, P2582 ng N. V., 2012, IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION, V19, P1593, NAT A, 1988, IEEE TRANSACTIONS ON ELECTRICAL INSULATION, V23, P545 ang J. George, 2012, IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION, V19, P162 ten P, 1996, IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION, V3, P1 rrow R, 1999, JOURNAL OF PHYSICS D-APPLIED PHYSICS, V32, PL20 nas, 2006, Nytro 10X Safety Data Sheet, P1, VINS JC, 1981, JOURNAL OF APPLIED PHYSICS, V52, P4531 rbes Richard G., 2007, PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, V463, P2907 nat A., 2006, IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION15th IEEE International Conference on Dielectric Liquids, JUN 26-JUL 01, 2005, Coimbra, PORTUGAL, V13, P518 tcher M, 2006, IEEE TRANSACTIONS ON PLASMA SCIENCE, V34, P467
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-249818 (URN)10.1088/2399-6528/ab0d59 (DOI)000463117800019 ()
Note

QC 20190423

Available from: 2019-04-23 Created: 2019-04-23 Last updated: 2019-08-01Bibliographically approved
3. Electric Conduction in Mineral Oil based ZnONanofluids under Intense Electric Fields
Open this publication in new window or tab >>Electric Conduction in Mineral Oil based ZnONanofluids under Intense Electric Fields
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The electric conduction processes in mineral oil based ZnO–C18 nanofluids under intense electric fields are investigated. For this, conduction currents are measured usinga needle-plane electrode configuration. Furthermore, an electrohydrodynamic (EHD) model is used here to discuss the charge generation mechanisms and the electronic properties of the ZnO–C18 nanofluids. The analysis of the conduction currents shows that ZnO–C18 nanoparticles increase the generation of charge carriers, and at the same time they augment the scavenging of quasi-free electrons compared with the measurements with mineral oil only. It is found that the existing nanoparticle electron scavenging model reported in the literature grossly underestimates the electron scavenging process here reported. A new analytical formulation for the nanoparticle electron scavenging process is proposed. The EHD model is also used to simulate the electric conduction processes just before negative streamer inception in mineral oil and ZnO–C18 nanofluids. It is shown that ZnO–C18 nanoparticles hinder the streamer initiation process by reducing the effective electric field at the tip of the needle. This electric field reduction is caused by the combined effect of the generation of charge carriers and the electron scavenging of ZnO–C18 nanoparticles.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering Nano Technology
Research subject
Electrical Engineering
Identifiers
urn:nbn:se:kth:diva-255604 (URN)
Note

QC 20190802

Available from: 2019-08-01 Created: 2019-08-01 Last updated: 2019-08-02Bibliographically approved
4. Simulation of the electrical conduction of cyclohexane with TiO2 nanoparticles
Open this publication in new window or tab >>Simulation of the electrical conduction of cyclohexane with TiO2 nanoparticles
2014 (English)In: Proceedings of the 2014 IEEE 18th International Conference on Dielectric Liquids, ICDL 2014, IEEE , 2014, p. 6893119-Conference paper, Published paper (Refereed)
Abstract [en]

Nanoparticles mixed with transformer oil can potentially increase the breakdown strength of the base liquid. Unfortunately, the basic physical mechanisms leading to such improvement are still not clear. This paper implements two existing theories to model the electrical conduction of cyclohexane with TiO2 nanoparticles in a needle to plane configuration. The generation and drift of carriers in the liquid are simulated by coupling the continuity equations for electrons, positive ions, negative ions, and nanoparticles with Poisson's equation for the electric field. The current-voltage characteristics are simulated and compared with the case of pure cyclohexane. The nanoparticles are modeled as either absorbers of electrons or as source of shallow traps in the fluid, according to the existing theories. The simulations show that the considered theories predict no significant effect of nanoparticles added to cyclohexane on the conduction current from a negative point electrode in steady state or under transient conditions.

Place, publisher, year, edition, pages
IEEE, 2014
Series
IEEE International Conference on Dielectric Liquids, ISSN 2153-3725
Keywords
conduction process, cyclohexane, Nanofluid, titania
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-157951 (URN)10.1109/ICDL.2014.6893119 (DOI)000360485100050 ()2-s2.0-84907894775 (Scopus ID)978-1-4799-2063-1 (ISBN)
Conference
2014 IEEE 18th International Conference on Dielectric Liquids, ICDL 2014, 29 June 2014 through 3 July 2014, Bled, Slovenia
Funder
StandUp
Note

QC 20150623

Available from: 2014-12-19 Created: 2014-12-18 Last updated: 2019-08-01Bibliographically approved
5. Electrical conduction currents of a mineral oil-based nanofluid in needle-plane configuration
Open this publication in new window or tab >>Electrical conduction currents of a mineral oil-based nanofluid in needle-plane configuration
2016 (English)In: 2016 IEEE CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA (IEEE CEIDP), IEEE conference proceedings, 2016, p. 687-690Conference paper, Published paper (Refereed)
Abstract [en]

We present experiments and simulations on the electrical conduction currents of purified transformer oil with and without surface-modified MgO nanoparticles. Results show that on the injection regime of the voltage-current characteristics, nanoparticles increase the charge production in the fluid. It is also found that the conduction currents in the space-charge-limited regime increased at a lower rate as a function of the voltage in the presence of nanoparticles. The numerical simulations suggest electron attachment is increased due to the nanoparticles, leading to larger accumulation of negative ionic space charge close to the needle in the space-charge-limited regime. It is concluded that electron attachment may be significantly increased with nanoparticles, becoming an important process of electrical conduction in nanofluids.

Place, publisher, year, edition, pages
IEEE conference proceedings, 2016
Series
Conference on Electrical Insulation and Dielectric Phenomena Annual Report, ISSN 0084-9162
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-201297 (URN)10.1109/CEIDP.2016.7785694 (DOI)000391639700142 ()2-s2.0-85009823702 (Scopus ID)978-1-5090-4654-6 (ISBN)
Conference
IEEE Conference on Electrical Insulation and Dielectric Phenomena (IEEE CEIDP), OCT 16-19, 2016, Toronto, CANADA
Note

QC 20170214

Available from: 2017-02-14 Created: 2017-02-14 Last updated: 2019-08-01Bibliographically approved

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