Dielectric Liquid Research Articles

Behavior of Nanoparticles in Service Transformer Oils and their Performance on Laboratory Ageing

The transformer is the most critical apparatus in the electric power network. Reliable operation of it is of foremost concern for a proficient power supply. Therefore, it is vital to safeguard the transformer so that it functions at its highest capacity. In order to achieve it, an improvement in the insulation system is necessary, as the majority of faults in transformers are due to the malfunctioning of the insulation system. Petroleum-based oils are extensively used as insulating liquids in transformers. The latterly evolved field of nanotechnology has encouraged studies on Nanofluids (NFs), which are suitable as liquid insulators. In spite of the advantages of NFs in electrical and thermal behavior compared to their base fluids, further studies are needed to establish their long-standing performances as liquid insulators. In this study, three mineral oil samples, new, mid-aged, and extensively aged, were taken. Nanofluids of 0.02% Al2O3 and 0.02% SiO2 of these oils were prepared. The base fluids and nanofluids were characterized by interfacial tension, acidity, dielectric dissipation factor, resistivity, and electric strength. The nanofluids prepared were subjected to laboratory thermal ageing in the presence of copper and kraft paper for 64 hours at 150 °C. The unaged and laboratory-aged nanofluids were characterized for acidity, Interfacial Tension (IFT), Specific Resistance (SR), Tan Delta (TD), and Dielectric Strength (BDV). It was found through this work that the addition of nanoparticles upgraded the properties of oil samples. The long-term applicability and permanence of nanofluids might depend on catalytic effects. These catalytic effects are derived from the internal assembly materials of the transformer.

Contribution of the Liquid Phase on Direct Current Interruption by a Forced Fluid Flow

The rising amount of medium voltage direct-current systems requires novel solutions for DC switching. The interruption of direct-currents is accomplished by enforcing a current zero, which can only be achieved when the arc voltage of the switching device raises above the grid voltage. On way to achieve this, is to force the arc into narrow channels by an imposed fluid flow. The increasing arc voltage than not only depends on increased cooling due to phase change but also on the mechanical elongation of the arc enforced by the fluid stream. Hence, the interaction of the fluid flow and the arc should be studied in more detail. For this, the switching characteristic of selected dielectric liquids are examined. Using a self developed setup, direct-current interruptions at a constant voltage of 10kV were carried out. Our results indicate, that the process of enforcing a current decay strongly depends on the mechanical resistance of the liquids to deform under the pressure of the electric arc.

Intensity Ratio Distribution in Different Dielectric Liquids using Kerr Effect Method

Light intensity is the result from the Kerr effect measurement that can be used in determining the electric field distributions in dielectric liquids as the analysis continues afterwards. One of the most important parameters to consider in designing the Kerr effect experiment is the Kerr constant of the test liquid. This paper aims to study and compare light intensity distribution in different dielectric liquids using Kerr effect method. Propylene carbonate, transformer oil and purified water are used as the test liquids. From the results obtained, the light intensities as a function of electric field of the test liquids are compared. With higher Kerr constant, low voltages can be applied to the test liquid but with larger optical signals. Furthermore, the length of the electrodes can be designed accordingly to suit the experimental setup.

Temperature Dependency of Ion Mobility in Synthetic Ester by Dielectric Response Analysis in Time and Frequency Domain

Synthetic ester is a promising alternative to mineral oil for emerging applications relying on high-power static conversion, such as solid-state transformers (SST). The highly compact components used in SSTs must fulfill MV-DC insulation requirements to guarantee the safe operation of the system, therefore, their design relies on the characterization of the dielectric stress caused by the electric field. The prediction of the electric field induced by DC voltages requires analyzing the conduction phenomena in the liquid, including the effect of charge transport due to ion drift. In this scenario, one of the critical parameters that determine the transport of charge and the resulting electric field distribution is the mobility of ions μ; the value of μ in synthetic ester is available only at room temperature, however, investigations on other dielectric liquids indicate a significant temperature dependency, which therefore needs to be characterized. In this paper, the temperature dependency of μ in synthetic ester liquid is determined by comparing alternative methodologies based on the analysis of the dielectric response in the time and frequency domain, and the viscosity of the fluid. The characteristics obtained by these methodologies are consistent; the temperature dependency of μ obeys the Arrhenius law and an increase of one order of magnitude starting from 1.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">–10</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs is observed in the 303K–353K temperature range.

The Effect of Nanoadditives in Natural Ester Dielectric Liquids: A Comprehensive Review on Stability and Thermal Properties

Numerous experimental investigations were carried out by several researchers exploring vegetable oils as substitutes for transformer oil, since these vegetable oils are biodegradable. Also, their usage may reduce the negative environmental impact of other traditional oils. Additionally, since today’s mineral oils are made from petroleum-based materials, they will ultimately run out in the near future. Vegetable oils might therefore be a contender to take the place of mineral oils in transformer applications. Vegetable oils are natural esters and are triglycerides of fatty acids that can be extracted from a wide variety of plants. Natural esters are more cost-effective than synthetic esters and they also abundant in many countries. This paper provides a comprehensive review on natural esters-based nanofluids. First, the necessity, benefits, and drawbacks of natural esters are all discussed. Methods used to improve the effectiveness of natural esters and overcome their disadvantages are presented. Then, the concept of nanofluids is introduced, and recent experimental research on dielectric nanofluids is reviewed in details, including preparation methodology, stability inspection, and thermal properties. Finally, all physical mechanisms behind the obtained results are discussed and clarified.

The Effect of Nano-Additives in Natural Ester Dielectric Liquids: A Comprehensive Review on Dielectric Properties

Many experimental studies were conducted by various researchers using vegetable oils as transformer oil alternatives. The use of these vegetable oils as dielectric oils for transformers has the potential to minimize the environmental effect of other conventional oils since vegetable oils are biodegradable. In addition, present-day mineral oils are derived from petroleum products; thus, mineral oils will inevitably run out in the future, and major shortages may occur. So, vegetable oils can be the potential candidates to replace mineral oils in the future for transformer applications. Also, these can be potential candidates in heat transfer applications, capacitors, cables, circuit breakers, and tap changers. Vegetable oils are mainly natural esters extracted from natural products. Natural esters are more economical than synthetic esters and sometimes exist naturally in large quantities in certain countries. To get enhanced dielectric properties of natural ester dielectric liquids, many studies propose using nano-additives to form what are called nanofluids. This comprehensive review presents the effect of nano-additives on the AC, DC, and impulse breakdown voltage, dielectric constant, and dissipation factor of natural ester dielectric liquids. The results obtained by various researchers on the dielectric properties of these oils are discussed, as well as clarified physical mechanisms, along with future research prospects.

Numerical Study of Free-Surface Electro-Hydrodynamic Wave Turbulence

Direct numerical simulation of threedimensional chaotic motion of a dielectric liquid with a free surface under the action of external horizontal electric field is carried out. The numerical model takes into account the effects of surface tension, viscosity, and external isotropic random forcing acting at large scales. A transition from dispersive capillary wave turbulence to quasi-isotropic nondispersive EHD surface turbulence with increase of the external electric field strength is numerically observed for the first time. At the regime of developed EHD wave turbulence, the total electrical energy is found to be much greater than the energy of capillary waves, i.e., electrohydrodynamic effects play a dominant role. At the same time, anisotropic effects are detected that lead to the generation of capillary wave packets traveling perpendicular to the external field direction. Despite the revealed anisotropy, the calculated spectrum of EHD wave turbulence is in very good agreement with the analytical spectrum obtained on the basis of dimensional analysis of weak turbulence spectra.

An Extension of the Standard-Based Approach in the Assessment of Liquid Dielectrics Behavior

An Extension of the Standard-Based Approach in the Assessment of Liquid Dielectrics Behavior

Magnets, Gradients, and RF Coils of MR Scanners

The topic of this paper is the parts of modern MR devices, which contain magnet coils. MR scanner magnets are made of four types of electromagnetic coils: 1) Main magnet, made of superconducting material. The main magnet of an MR (Magnetic Resonance Imaging) scanner creates a strong and uniform magnetic field around the patient being scanned. This magnetic field is typically in the range of 0.5 to 3 Tesla and is used to align the magnetic moments of the hydrogen atoms in the patient's body. The superconductors, which create the main magnetic field, should be cooled with liquid helium and liquid nitrogen. The main magnets made of superconductors should use a cryostat, with cooling vessels with liquid helium and liquid nitrogen, thermal insulation, and other protective elements of the magnet system. 2) The gradient magnetic field is made of three types of coils: x-coils, y-coils, and z-coils. The X coil, made of resistive material, creates a variable magnetic field, horizontally, from left to right, across the scanning tube; 3) The Y coil creates a variable magnetic field, vertically, from bottom to top; 4) The Z coil creates a variable magnetic field, longitudinally, from head to toe, inside the scanning tube. RF coils are used to generate RF pulses to excite the hydrogen protons (spins) in the patient's body and detect the signals emitted by the protons when they return to their equilibrium state after the RF excitation is turned off. The resulting interaction between the magnetic field and the aligned hydrogen atoms produces a signal that is used to generate the images seen in an MRI scan. The main magnetic field is what allows MR imaging to produce detailed anatomical and functional information non-invasively. The structure of the MR scanner magnet is complex. The resonant frequency changes at each point of the field in a controlled manner. Inside the copper core are embedded the windings of the main magnet made of superconducting material in the form of microfibers. A non-linear gradient field is created by coils of conductive material. It adds to the main magnetic field. Thus the resulting magnetic field is obtained. The types of magnets that exist in the basic configurations of MR scanners are analyzed. Scanners in the form of a closed cylindrical cavity generate their magnetic fields by passing current through a solenoid, which is maintained at the temperature of a superconductor. Exclusively used superconductors are niobium-titanium (NbTi), niobium-tin (Nb3Sn), vanadium-gallium (V3Ga), and magnesium-diboride (MgB2). Only magnesium diboride is a high-temperature superconductor, with a critical temperature Tc = 390K. The three remaining superconductors are low temperatures. New high-temperature superconductors have been discovered, as well as superconductors at room temperature. Newly discovered superconducting materials are not used in MR scanners.

Formation mechanism of Fe-based amorphous powders produced by spark erosion

Spark erosion is a convenient, flexible, and low-cost method to quickly produce fine powders of metals, alloys, and semiconductors in size ranging from nearly a hundred micrometers to submicrometer by repetitive spark discharges. Due to the complexity of the powder-forming journey, normally accompanied by high temperature, high pressure, decomposition, diffusion and rapid quenching caused by discharge plasma, the mechanisms of powder formation and possible contaminant infiltration are still controversial, posing a significant challenge to control particle size and chemical composition of the powder produced by this method. In this study, Fe-based amorphous powders in different particle-size distributions with high sphericity were fabricated by spark erosion under different discharge-energy conditions. The maximum particle size of the resultant powders can be correlated with discharge parameters, crater depth, and crater radius, respectively. A multi-ring-breakup model is proposed to reveal the particle-size distribution of the powder formed from the electrode melt under a single-pulse discharge. Furthermore, a dielectric-element infiltration model is provided to quantitatively evaluate the infiltration mass ratio of the contaminant elements, stemming from the decomposed products of dielectric liquid, in the resultant powder with different particle sizes. The models verified through the experimental data are significant for the development of high-performance fine Fe-based amorphous powder with controlled particle size and chemical composition.

Comprehensive and statistical investigation on the impact of alumina nanoparticles and hydrocarbon solvent upon the reliability of transformer liquid insulation composed of moringa oil

Comprehensive and statistical investigation on the impact of alumina nanoparticles and hydrocarbon solvent upon the reliability of transformer liquid insulation composed of moringa oil

The influence of the helical twisting power on the bistable electro-optical performance of the liquid crystalline epoxide/mercaptan /negative-dielectric-anisotropy cholesteric liquid crystal composite films

Bistable electro-optical devices have been successfully prepared by thermal curing of the liquid crystalline epoxide, mercaptan and the cholesteric liquid crystals (N-ChLCs) with negative-dielectric-anisotropy. In order to study the influence of helical twisting power (HTP), an important parameter of the chiral dopant, on the bistable electro-optical properties of the liquid crystalline epoxide/mercaptan/N-ChLC composite films, four samples A1-A4 were prepared. Among them, four different N-ChLCs with the same Bragg reflect wavelength were prepared via blending the negative dielectric anisotropy nematic liquid crystal and the chiral dopant R811, R1011, R2011, as well as R5011, respectively. Results indicated that as the HTP increased, the transmittance in the wavelength range of 400–2500 nm became lower under the low-frequency (LF) electric field and after removing the LF electric field, which resulted from the smaller polymer microspheres. Meanwhile, samples with large HTP showed their large transmittance states under the high-frequency (HF) electric field (10 KHz, 90V) and after removing the HF electric field. Moreover, the sample A4 possessing the chiral dopant R5011 with a large HTP exhibited good bistability, and its large- and low-transmittance states can be maintained for a long time (>30 days) in the absence of electric fields.

Stability and bifurcation of annular electro-thermo-convection

Abstract We numerically investigated the global linear instability and bifurcations in electro-thermo-convection (ETC) of a dielectric liquid confined in a two-dimensional (2-D) concentric annulus subjected to a strong unipolar injection. Seven kinds of solutions exist in this ETC system due to the complex bifurcations, i.e. saddle-node, subcritical and supercritical Hopf bifurcations. These bifurcation routes constitute at most four solution branches. Global linear instability analysis and energy analysis were conducted to explain the instability mechanism and transition of different solutions and to predict the local instability regions. The linearized lattice Boltzmann method (LLBM) for global linear instability analysis, first proposed by Pérez et al. (Theor. Comput. Fluid Dyn., vol. 31, 2017, pp. 643–664) to analyse incompressible flows, was extended here to solve the whole set of coupled linear equations, including the linear Navier–Stokes equations, the linear energy equation, Poisson's equation and the linear charge conservation equation. A multiscale analysis was also performed to recover the macroscopic linearized Navier–Stokes equations from the four different discrete lattice Boltzmann equations (LBEs). The LLBM was validated by calculating the linear critical value of 2-D natural convection; it has an error of 1.39% compared with the spectral method. Instability with global travelling wave behaviour is a unique behaviour in the annulus configuration electrothermohydrodynamic system, which may be caused by the baroclinity. Finally, the chaotic behaviour was quantitatively analysed through calculation of the fractal dimension and Lyapunov exponent.

Numerical analyses of wire-plate electrohydrodynamic flows

We present numerical analyses of two-dimensional electrohydrodynamic (EHD) flows of a dielectric liquid between a wire electrode and two plate electrodes with a Poiseuille flow, using direct numerical simulation and global stability analysis. Both conduction and injection mechanisms for charge generation are considered. In this work we focused on the intensity of the cross-flow and studied the EHD flows without a cross-flow, with a weak cross-flow and with a strong cross-flow. (1) In the case without a cross-flow, we investigated its nonlinear flow structures and linear dynamics. We found that the flow in the conduction regime is steady, whereas the flow in the injection regime is oscillatory, which can be explained by a global stability analysis. (2) The EHD flow with a weak cross-flow is closely related to the flow phenomena in an electrostatic precipitator (ESP). Our analyses indicate that increasing the cross-flow intensity or the electric Reynolds number leads to a less stable flow. Based on these results, we infer that one should adopt a relatively low voltage and weak cross-flow in the wire-plate EHD flow to avoid flow instability, which may hold practical implications for ESP. (3) The case of strong cross-flow is examined to study the EHD effect on the wake flow. By comparing the conventional cylindrical wake with the EHD wake in linear and nonlinear regimes, we found that the EHD effect brings forward the vortex shedding in wake flows. Besides, the EHD effect reduces the drag coefficient when the cross-flow is weak, but increases it when it is strong.

Glycerol Flow through a Shielded Coil Induces Aggregation and Activity Enhancement of Horseradish Peroxidase

Glycerol has found its applications as a heat-transfer fluid in heat exchangers, and as a component of functional liquids in biosensor analysis. Flowing non-aqueous fluids are known to be able to induce electromagnetic fields due to the triboelectric effect. These triboelectrically generated electromagnetic fields can affect biological macromolecules. Horseradish peroxidase (HRP) is widely employed as a convenient model object for studying how external electric, magnetic, and electromagnetic fields affect enzymes. Herein, we have studied whether the flow of glycerol in a ground-shielded cylindrical coil affects the HRP enzyme incubated at a 2 cm distance near the coil’s side. Atomic force microscopy (AFM) has been employed in order to study the effect of glycerol flow on HRP at the nanoscale. An increased aggregation of HRP on mica has been observed after the incubation of the enzyme near the coil. Moreover, the enzymatic activity of HRP has also been affected. The results reported that their application can be found in biotechnology, food technology and life sciences applications, considering the development of triboelectric generators, enzyme-based biosensors and bioreactors with surface-immobilized enzymes. Our work can also be of interest for scientists studying triboelectric phenomena, representing one more step toward understanding the mechanism of the indirect action of the flow of a dielectric liquid on biological macromolecules.

Feasibility analysis of new green liquid dielectrics for transformers

The insulating ingredient of the traditional transformer is a mineral oil with low combustibility and biodegradability. As a result, it is unsuitable for current and future transformer applications. Furthermore, finding fossil fuels is becoming increasingly difficult. As a result of the aforementioned limitations, there is a need to look for a replacement shielding oil. This study analysed the crucial characteristics of two biodegradable oils – namely, olive and palm oils – against worldwide standards. In addition, a 1 kVA transformer was subjected to a load test using olive and palm oils in place of traditional insulating solutions to assess their practical feasibility. The authors also looked into the technical feasibility of making a percentage-based blend. According to study results, vegetable oil is a choice superior to mineral oil for electrical and fire protection. However, the density of vegetable oil is substantially higher than that of mineral oil. As a result, the cooling tube of a transformer must be carefully considered throughout its construction. Vegetable oils also have a higher concentration of lipids with a higher molecular weight. The load test revealed that palm oil performs better than mineral oil and olive oil as a liquid dielectric. Furthermore, palm oil with a higher concentration of mineral oil provides properties superior to those of others.

Design and optimization of a type-C tank for liquid hydrogen marine transport

As one of the most promising renewable energy sources, hydrogen has the excellent environmental benefit of producing zero emissions. A key technical challenge in using hydrogen across sectors is placed on its storage technology. The storage temperature of liquid hydrogen (20 K, or −253 °C) is close to absolute zero so the storage materials and the insulation layers are subjected to extremely stringent requirements against the cryogenic behaviour of the medium. In this context, this research proposed to design a large liquid hydrogen type-C tank with AISI (American Iron and Steel Institution) type 316 L stainless steel as the metal barrier, using Vapor-Cooled Shield (VCS) and Rigid Polyurethane Foams (RPF) as the insulation layer. A parametric study on the design of the insulation layer was carried out by establishing a thermodynamic model. The effects of VCS location on heat ingress to the liquid hydrogen transport tank and insulation temperature distribution were investigated, and the optimal location of the VCS in the insulation was identified. Research outcomes finally suggest two optimal design schemes: (1) when the thickness of the insulation layer is determined, Self-evaporation Vapor-Cooled Shield (SVCS) and Forced-evaporation Vapor-Cooled Shield (FVCS) can reduce heat transfer by 47.84% and 85.86% respectively; (2) when the liquid hydrogen evaporation capacity is determined, SVCS and FVCS can reduce the thickness of the insulation layer by 50% and 67.93% respectively.

Toward whispering-gallery-mode microdroplet sensing: Surface charge accumulation, microdroplet interface evolution and forward-photovoltaic-jetting on C-cut LiNbO3:Fe crystals

LiNbO3-based photovoltaic jetting can generate dielectric microdroplets with volume down to fL, showing great potential in producing whispering-gallery-mode (WGM) microresonators. However, so far no successful work on the photovoltaic generation of microdroplet resonators has been reported due to the lack of the knowledge about the association of the jetting behaviors with the accumulation and confinement of photovoltaic charges. Here we study the microdroplet interface evolution caused by the central and boundary illumination of a focused laser beam, report that the photovoltaic charges may spread along LiNbO3:Fe surface, and reinforce it through the dependences of photovoltaic current curve on probing position and illumination intensity. By properly configuring LiNbO3:Fe crystals we confine the photovoltaic charges strictly resulting in strong electrodynamic flow to trigger the directional jetting of the dielectric liquid from LiNbO3:Fe surface toward target substrates. Through this forward-photovoltaic-jetting we realize an all-optical, in-situ printing of WGM microdroplets containing fluorescent quantum dots onto substrates without thickness limitation. The WGM modes in the microdroplets are found highly dependent on the microdroplet size, shape, and especially, the contact angles of the microdroplets on modified substrates. Applications of printed WGM microdroplets as anti-counterfeiting spectral barcode and temperature/gas sensors are demonstrated by detecting the change of resonant modes.

Determination of Hydrofluoric Acid Formation During Fire Accidents of Lithium-Ion Batteries with a Direct Cooling System Based on the Refrigeration Liquids

To avoid overheating of the batteries, which could lead to a fire, Lithium-ion batteries are provided with a thermal management system using refrigeration liquids. Since some of the commercial dielectric liquids used as refrigeration liquids contain halogens, their presence will contribute to a formation of hazardous emissions such as hydrofluoric acid during a potential fire. In this study, a simulation of a high temperature accident has been performed for lithium-ion batteries cooled with the direct immersion cooling systems using single-phase dielectric liquids to define their contribution to HF formation. Four commercial refrigeration liquids based on perfluoropolyethers, hydrofluoroether and polyalphaolefin were investigated in this work. By simulation of a fire, it was observed that the refrigeration liquids delayed the smoke formation by a factor of 2 to 2.5 in comparison to the case when the battery was burned without the cooling liquid. By analysis of the fluoride concentration in the washing system, it was determined that without the refrigeration liquid approximately 46.8 mmol/l of [F] was captured after the fire. When refrigeration liquids based on two perfluoropolyethers and hydrofluoroether were applied, the fluoride concentration in the washing system was 259 mmol/l, 173 mmol/l and 145 mmol/l, respectively. This work also proposed the reaction mechanisms of the refrigeration liquid´s decomposition during a fire. It was concluded that the refrigeration liquid based on polyalphaolefin does not contribute to the additional formation of hydrofluoric acid due to the chemical stability and low content of fluoride and can be considered as a more sustainable alternative for a direct cooling system for Lithium-ion batteries.

Biodegradable Electrohydraulic Soft Actuators

Biodegradable materials decompose and return to nature. This functionality can be applied to derive robotic systems that are environmentally friendly. This study presents a fully biodegradable soft actuator, which is one of the key elements in “green” soft robotics. The working of the actuator is based on an electrohydraulic principle, which is similar to that of hydraulically amplified self‐healing electrostatic actuators. The actuator developed in this study consists of a dielectric film made of polylactic acid (PLA) and polybutylene adipate‐co‐terephthalate (PBAT), with soybean oil as the dielectric liquid and electrodes made from a mixture of gelatin, glycerol, and sodium chloride (NaCl). The synthesized biodegradable electrode material exhibits a Young's modulus of 0.06 MPa and resistivity of 258 Ω·m when the mass fraction of NaCl relative to the amount of gelatin and glycerol is 10 wt%. The softness and resistivity of the electrode material results in actuation strain values of 3.2% (at 1 kV, corresponding to 1.2 kV mm−1) and 18.6% (at 10 kV, corresponding to 9.6 kV mm−1) for the linear‐type and circular‐type actuators, respectively. These values obtained for the biodegradable electrohydraulic soft actuators are comparable to those of nonbiodegradable actuators of the same type, representing the successful implementation of the concept.