Study of electrohydrodynamic properties in insulating liquids and research of an alternative mixture to mineral oil for energy transformers

doi:10.1016/j.elstat.2022.103684

Journal of Electrostatics

Volume 115, January 2022, 103684

https://doi.org/10.1016/j.elstat.2022.103684 Get rights and content

Highlights

•
Natural ester oils (vegetable oil), instead of mineral oil, are good candidate to be used in energy transformers.
•
The mixture between naphthenic mineral oil and olive oil leads to the reduction of turbulent movements which can participate in the fight against the breakdown of energy transformers.
•
When the percentage decrease, in the amount of naphthenic oil, it's remplaced by the same percentage of one of the vegetables, olive or corn oils leads to the change in the value of intensity of the electric field causing the onset of movement in these mixtures.
•
The O4 mixture (60% MO + 40% OO) presents the best compromise.
•
The O4 mixture has a viscosity closer to that of mineral oil, although the other liquid dielectrics studied have a higher viscosity.

Abstract

The article discusses, the electro-hydrodynamic phenomenon of three types of insulating oil, one mineral and two vegetable (olive oil and corn oil) and their mixtures in terms of aging time. The experimental results, allowing us the determination of the initiation and the development of electro-convection voltage of different insulating oils.

A very important results was obtained, concerning the relaxation time of the insulating liquid after the appearance of its movement under the effect of the electric field. The analyzed physicochemical properties of different insulating liquids were: viscosity, resistivity, conductivity, breakdown voltage and dissipation factor, all carried out and compared by means of measurements. A close correlation between the flexibility of liquid dielectrics in movement in an electric field and their susceptibility to static electrification is approved. It appears from the various tests carried out that the optimal mixture obtained consists of 60% naphthenic mineral oil and 40% olive oil. The respect of these percentages of mixtures has made it possible to improve some characteristics of this insulating liquid and to keep the good properties possessed by naphthenic mineral oil. From the recorded observations, We have found that this mixing percentage also leads to the reduction of turbulent movements which can participate in the fight against the breakdown of energy transformers.

Introduction

During the 1970s many cases of breakdown of high voltage transformers occurred in Japan. Since the 1980s a dozen incidents of this type have been recorded in the United States and Australia [1,2]. At first, the damage to these energetic devices with forced oil flow was attributed solely to the phenomenon of static electrification and this is based on the observation of traces of erosion on the surface of the insulating paper and especially in the upper part. of the transformer where the electric field is moderate [2,13,33]. The serious economic consequences due to the damage to these transformers have led to intensive research focused on the phenomenon of static electrification of insulating oils. The analysis carried out by Thierry et al. [21] shows that the accumulation of charges on the surface of the cardboard makes it possible to estimate the electrostatic risk in power transformers. In 1990 Fleszynski et al. [16] described a hypothetical scenario of damage to these power transformers. They concluded that it is not only the electrostatic (ES) properties of insulating oils that lead to the breakdown of these devices but also that their electro-hydrodynamic (EHD) properties can do the same. Among the foci of electro-hydrodynamics is the existence of a phenomenon of macroscopic movement in the liquid dielectric medium forced by the electric field also called electro-convection. Currently, it is already known that this EHD phenomenon that takes place in transformers can have a remarkable effect on mass transport, heat exchange, charge transport, electrical conductivity and electrical resistance [[5], [6], [8],14,16]. Research carried out over the past dozen years in various scientific research centers [3,10,14,16,27] shows that the phenomenon of electro-convection of insulating oils can play an important role in the mechanism leading to pre-breakdown of energy transformers and that the initiation and development of this phenomenon depends on the type of oil used.

Mineral oils have long been used for the insulation and cooling of energy transformers. Although these oils have several advantages, they also have certain disadvantages, such as the high risk of fire, the low potential for biodegradability and above all the phenomenon of electro-convection which occurs in these liquids in the presence of the electric field and which can create problems for users of these power devices [[9], [10], [11]]. These drawbacks which can lead to the breakdown of these devices therefore call for the search for another ecological insulating liquid which has a high solubility in water, an excellent degree of biodegradability as well as low flexibility in movement in an electric field which can contribute to a decrease in the turbulent movements that can occur inside these devices [22,30,32,35]. Pukel et al. [18], from the company Siemens (Siemens Transformers Austria) made a comparison between different insulating liquids and mentioned above all that esters are more prone to leak breakage than mineral oils. Perrier et al. and Beroual et al. [35,38,39] have devoted much of their research to the study of mixtures of mineral oil and esters as well as to the study of mixtures of nanoparticles and their effect on the breakdown voltage of mineral oil, all with the aim of arriving at an insulating liquid that can replace the oils currently used for the insulation and cooling of energy transformers. They deduced that a mixture of mineral oil with 20% synthetic ester presents a good compromise to obtain a better liquid than mineral oil alone and that this mixture could improve the insulation of the power transformer. Bin Yahya et al. [26], have shown that mixtures of olive oil with naphthenic oil have better performance.

The phenomenon of electro-convection of insulating oils is still a major concern for academic researchers and especially for manufacturers and users of power transformers [3,15,25,28,29,38]. Although many studies have been undertaken to understand this phenomenon, the aim of which is to find a solution that can increase the reliability of these transformers, the problem still persists and indeed today constitutes one of the most important lines of research in the world not only for the users of these energy devices but also for the producers of insulating oils [4,7,8,15]. Despite the advanced research in this area, the main focus has been on the study of the electrostatic and electro-hydrodynamic properties of insulating oils, each studied separately, which in a way has neglected the study of possible correlations between these properties, the degree and duration of turbulent movements that take place within these liquids in relation to their physico-chemical properties [9,12,13,16]. This work therefore attaches central importance to these phenomena and properties that have not received the enough research attention.

Our work therefore consists of implementing a series of experimental studies to assess the influence of various parameters on the lifetime of the transformers in order to better control the latter to guard against such a risk of breakdown. These parameters to be examined include the thickness of the electric double layer that occurs at the solid-liquid interface, the initiation and development of an electro-convective movement within insulating liquids in the presence of an electric field, the physico-chemical properties of these liquid dielectrics, the existing relationship between the susceptibility of insulating oils to static electrification and their flexible mobility under the effect of an electric field.

This work, which aims above all to study the EHD phenomenon which occurs in different types of insulating oil (mineral and vegetable) and to compare it with that which occurs in mixtures of mineral oil with natural esters, consists of three parts.

The first part is devoted to a comparative study of the flexible mobility of these liquid dielectrics in an electric field using a measuring device that allows us the good observation and recording of these movements either with a video camera or with a photographic device. In this part we will also be interested in measuring the time interval during which the movement of these liquids remains continuous just after application and cut-off of the initiating voltage causing their movements, which for the most part has not been taken into consideration by other researchers.

The second part consists of studying the phenomenon of electro-convection according to the aging time as well as according to the degree of humidity of some insulating oils and their mixtures by choosing the best mixture where it exists, which can be an alternative to mineral oil for insulation and cooling of transformers.

Finally, the last part of this document is focused on the study of correlation between the electrostatic (ES) and electro-hydrodynamic (EHD) properties that occur in these insulating liquids studied; it highlights the most significant physico-chemical properties of each of these oil samples; and compares them with one another with the aim of finding a mixture that can help extend the lifetime of power transformers.

Section snippets

Mathematical description and classification of electro-convections

The general model of electro-convention equations in dielectric liquids was first formulated by G. A. Ostroumow [23]. The model includes 8 relationships describing the electric field as well as its effect on the viscous mechanism of liquids: $γ [\frac{d \overline{v}}{d t} + (\overline{v} \nabla) \overline{v}] = - \overline{\nabla} p + \overline{f} + γ \overline{g} + η \nabla \overline{v}$ $\frac{\partial T}{\partial t} + \overline{v} \nabla T = α \nabla^{2} T + \frac{σ E^{2}}{C_{P^{γ}}}$ $\nabla \overline{v} = 0$ $\overline{E} = - \nabla φ$ $\nabla (ε \overline{E}) = ρ$ $\overline{j} = σ \overline{E} + ρ \overline{v} + \frac{\partial (ε \overline{E)}}{\partial t}$ $\nabla \overline{j} = 0$ $\overline{f} = ρ \overline{E} - \frac{1}{2} E^{2} \nabla ε + \frac{1}{2} \nabla [E^{2} γ {(\frac{\partial ε}{\partial_{γ}})}_{T}]$

The Navier-Stokes equation (1) includes not only the density of the pressure forces $p$ , but also inertia $γ \frac{d \overline{v}}{d t}$ , gravitation $γ \overline{g}$ ,

Methods of studying the phenomenon of initiation and development of electro-convection

The electro-hydrodynamic properties of insulating oils can be characterized by the value of the initiating electro-convection voltage. This voltage is applied to two electrodes immersed in an insulating oil spark gap and for which an oil movement begins to appear under well-defined conditions [14,16]. The study of this phenomenon or more precisely the measurements of the values of this initiating voltage requires a highly sensitive measurement device.

The study of this phenomenon, or more

Factors influencing the EHD phenomenon

Published studies [12,14,16,27] show that the phenomenon of initiation and development of electroconvection depends on several external and internal factors.

Among the most interesting external factors are:

-
configuration of the electrode system;
-
voltage type (direct or alternating);
-
Temperature;
-
covering the electrodes with a layer of solid dielectric.

Internal factors are associated with the oil itself such as:

-
type of oils;
-
oil aging;
-
contamination of the oil.

In our study we only consider the first

Choice of electrode system and voltage type

Our choice fell on a direct voltage and on a model of metal electrodes: tip - plate. The chosen distance, which separates the two electrodes during this study, is d = 8 mm. Between two successive measurements, the test cell and the electrodes are cleaned and dried, so that they do not contain characters from the previous measurement.

In addition, it is important to allow 10 min of rest between measurements in order to dissipate the gases which occur during the electric discharge.

Choice of oils and their mixtures

Paraffin-based mineral oil has the defect of freezing quickly and therefore prevents the good operation of transformers, while in a cold climate,naphthenic mineral oil oxidizes easily but in the form of soluble sludge and it has good dielectric properties such as heat transfer and low cost, which has led to its use in most power transformers [9,19,20,28,29]. For this reason, among these two types of mineral oil, we chose naphthenic oil for our study.

The use of vegetable oils or natural esters

Electro-convection initiation voltage

The measurement of the electro-convection initiating voltage of insulating oils in a tip-plate or plate sphere electrode model is an essential factor in the evaluation of the dielectric strength of these oils. In this work, we measure this initiating electro-convection voltage for the already chosen different types of oil and their mixtures before and after their aging. Each measurement must be carried out six (06) times and we adapt their average value. A relaxation time of 15 min between

Physicochemical properties

The results of studies of the electro-hydrodynamic phenomenon of different liquid dielectrics have shown that the O4 mixture (60% MO + 40% OO) has more advantages than that of naphthenic mineral oil and of the C3 mixture (initiating voltage of high electro-convection, movement of the oil less turbulent than that observed for the other dielectrics). To ensure that the flexibility of these mixtures in movement in a high electric field leads to the reduction of the risks of breakdown and to the

Conclusion

The experimental results obtained during the study of the of electro-convection phenomenon of the different types of insulating oil have shown that the movement of olive oil from one electrode to another requires an electro-convection initiation voltage higher than that of corn oil and a little lower than that of mineral oil. The most interesting point to emphasize is that the value of the voltage causing turbulent electro-convective motion in the insulating liquid does not depend on its

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Special thanks to the members of the high voltage laboratory of the Fundamental Institute of Electrical Engineering and Electrotechnology, Wroclaw Polytechnic School - Poland and to the Faculty of Sciences at the University of Gafsa-Tunisia.

References (45)

Qichun Nie et al.
Designing working diagrams for electrohydrodynamic printing
Elsevier Chem. Eng. Sci.
(2021)
F. Song et al.
Molecular dynamics study on the coalescence and break-up behaviors of ionic droplets under DC electric field
J. Mol. Liq.
(2020)
J.A. Cross et al.
Electric stress induced motion in transformer oils under 60 Hz stress
J. Electrost.
(1979)
Rafael Villarroel et al.
Moisture dynamics in natural-ester filled transformers
Electr. Power Energy Syst.
(2021)
Shimisu S. , Murata H., Honda M., : Electrostatic in power transformers, IEEE Trans. Power Apparatus Syst., 1079, 98,...
M. Kozlowski
Static Electrification of Transformers. Research on Physical Phenomena and Computer Simulation of the Effect of Charges on the Distribution of the Electric Field. Seminar of the Great Powers Section of the Electrotechnical Committee of the Polish Academy of Sciences and Department of Electrical Networks and Security
(1995)
Viadimir Chirkiv et al.
The interaction between two electrohydrodynamics phenomena when an electric field affects a two-phase immiscible liquid
Phys. Fluids
(2021)
Lei Wang, Zhouchao Wei, Tianfu Li, Zhenhua Chai, Baochang Shi : A lattice Boltzmann modelling of electrohydrodynamic...
P.S. Casas et al.
Numerical study on electrohydrodynamic multiple droplet interactions
Phys. Rev. E
(2019)
K. Luo et al.
Numerical analysis of two-phase electrohydrodynamic flows in the presence of surface charge convection
Phys. Fluids
(2020)

P.A. Vazquez et al.

Indepth description of electrohydrodynamic conduction pumping of dielectric liquids: physical model and regime analysis

Phys. Fluids

(2019)

G. Theodossiou et al.

The influence of electrohydrodynamic motion on the breakdown of dielectric liquids

J. Physics D.

(1988)

P. Atten et al.

Electroconvection and its effect on heat transfer

IEEE Trans. Electr. Insul.

(1988)

A. Castellanos

Coulomb driven convection in electrohydrodynamics

IEEE Trans. Electr. Insul.

(1991)

P. Atten

Electrohydrodynamic instability and motion induced by injected space charge in insulating liquids

IEEE Trans. Dielectr. Electr. Insul.

(1996)

Zheng-Gang Su et al.

Instability of electroconvection in viscoelastic fluids subjected to unipolar injection

Phys. Fluids

(2020)

J. Flyszynski et al.

Method of analysis for initiation and evolution of electroconvection in dielectric liquid under DC and AC electric field

Mater. Sci.

(1990)

D. Feng, L. Yang, R. Liao and L. Zhou, “Effect of moisture content on the production and partitioning of furfural in...

G.J. Pukel et al.

Environmental friendly insulating liquids – a challenge for power transfoemers, 6th Southern Africa, regional conference

Cigré

(2009)

Ruijin Liao et al.

A comparative study of physicochemical, dielectric and thermal properties of pressboard insulation impregnated with natural ester and mineral oil

Li junYang et al.

Influence of vegetable oil on the thermal aging of transformer paper and its mechanism

IEEE Trans. Dielectr. Electr. Insul.

(2011)

P. Thierry et al.

Electrostatic Hazard in high-power transformers: results of ten Years of experience with P' capacitive sensor

IEEE Trans. Ind. Appl.

(July-Aug. 2013)

Cited by (5)

Molecular ionization potentials of triglycerides and mechanism analysis of streamer discharge in vegetable insulating oil
2023, Journal of Molecular Liquids
With the enhancement of social awareness of environmental protection, scholars have gradually began to focus their research on vegetable-oil-based insulating fluid. In this work, we calculated the ionization potential of triglyceride molecules in vegetable oil (VO), analyzed the mechanism of charge generation, and explained the cause of fast streamer discharge in VO based on the distribution characteristics of ionization potential. Some interesting results were obtained. The position of CC on the triglyceride branch chain can affect the ionization potential of triglycerides. Specially, the CC at the Sn-2 position can significantly reduce the ionization potential of glycerol molecules. The ionization of unsaturated triglycerides is mainly caused by the electron loss of carbon atoms in the CC double bond; the formation of negative ions mainly rely on ester groups and their nearby atoms to capture electrons. In VO, low ionization potential molecules are predominantly present, and their presence results in a lower breakdown voltage and easy formation of the fast streamer discharge. We anticipate that our research can provide theoretical support for the application research of vegetable insulating fluid.
Impact of silver sulfide on rheology and streaming electrification of mineral oil and mixed fluid
2022, Journal of Electrostatics
Citation Excerpt :
Among the different techniques, the spinning disc technique [11] requires a smaller volume of insulating fluid for operation and the centrifugal force governs the formation of electrical double layer at the fluid/pressboard interface. This method is mostly utilized for its evaluation on the transformer oil and other alternative insulating liquids used for transformer applications [12–14]. Also, the dependence of various parameters including viscosity, conductivity and shear rate on the streaming current has been discussed [15].
This study reports the effect of adding silver sulfide to mineral oil and mixed fluid on their static electrification and visco-elastic properties. The flow electrification studies indicate an increase in streaming current for higher silver sulfide concentration with the hysteresis pattern indicating a higher current during the decrement of disc velocity. The transition from loss modulus to storage modulus was visible in the mixed fluid compared to mineral oil. Further, the effect of silver sulfide has reduced the storage modulus with frequency sweep and temperature sweep testing on both mineral oil and mixed fluid.
A New Type of Two-Element Mixed Natural Ester Insulating Oil with Better Fluidity and Oxidative Stability: A Molecular Simulation Study
2024, SSRN
Effect of strong DC Electric Fields on the flow properties of insulating liquids at different temperatures
2023, Proceedings - IEEE International Conference on Dielectric Liquids
Diagnostics of Dielectric Liquid Aging Based on Dynamical Current-voltage Characteristics
2022, Proceedings - IEEE International Conference on Dielectric Liquids

View full text

Study of electrohydrodynamic properties in insulating liquids and research of an alternative mixture to mineral oil for energy transformers

Highlights

Abstract

Introduction

Section snippets

Mathematical description and classification of electro-convections

Methods of studying the phenomenon of initiation and development of electro-convection

Factors influencing the EHD phenomenon

Choice of electrode system and voltage type

Choice of oils and their mixtures

Electro-convection initiation voltage

Physicochemical properties

Conclusion

Declaration of competing interest

Acknowledgments

Elsevier Chem. Eng. Sci.

J. Mol. Liq.

J. Electrost.

Electr. Power Energy Syst.

The interaction between two electrohydrodynamics phenomena when an electric field affects a two-phase immiscible liquid

Phys. Fluids

Numerical study on electrohydrodynamic multiple droplet interactions

Phys. Rev. E

Numerical analysis of two-phase electrohydrodynamic flows in the presence of surface charge convection

Phys. Fluids

Indepth description of electrohydrodynamic conduction pumping of dielectric liquids: physical model and regime analysis

Phys. Fluids

The influence of electrohydrodynamic motion on the breakdown of dielectric liquids

J. Physics D.

Electroconvection and its effect on heat transfer

IEEE Trans. Electr. Insul.

Coulomb driven convection in electrohydrodynamics

IEEE Trans. Electr. Insul.

Electrohydrodynamic instability and motion induced by injected space charge in insulating liquids

IEEE Trans. Dielectr. Electr. Insul.

Instability of electroconvection in viscoelastic fluids subjected to unipolar injection

Phys. Fluids

Method of analysis for initiation and evolution of electroconvection in dielectric liquid under DC and AC electric field

Mater. Sci.

Environmental friendly insulating liquids – a challenge for power transfoemers, 6th Southern Africa, regional conference

Cigré

A comparative study of physicochemical, dielectric and thermal properties of pressboard insulation impregnated with natural ester and mineral oil

Influence of vegetable oil on the thermal aging of transformer paper and its mechanism

IEEE Trans. Dielectr. Electr. Insul.

Electrostatic Hazard in high-power transformers: results of ten Years of experience with P' capacitive sensor

IEEE Trans. Ind. Appl.