Showing papers on "Liquid dielectric published in 2022"

Assessing the dielectric performance of Sclerocarya birrea (Marula oil) and mineral oil for eco-friendly power transformer applications

Abstract: The ever increasing power demand has forced the power sector companies to increase their generation, transmission and distribution in turn increasing the number of power transformers in the system where necessary. Depending on the nature of operations, the transformers are chosen as either dry-type or liquid-cooled and the preference is mostly liquid cooled. Naphthenic oils have been the primary choice over the years and alternative solutions in the form of natural esters are researched extensively due to the environmental effects of Mineral Oil (MO). One such ester, Marula is presented in this research. The experiment places its focus on measurement of electrical, physical, chemical and thermal characteristics as per the standards of ASTM and IEC. The results prove the superior dielectric performance of MAO over MO. The antioxidant rich MAO exhibits superior dielectric properties and restores faster than MO after the instances of breakdown phenomena. The antioxidants show stability at both the low and high temperatures, and temperature causes minimum damage to antioxidants. In addition to that the use of such dielectric coolant minimizes the winding damage inside the transformer tank. Thus, it can be said that MAO is an excellent coolant and improves the efficiency of the transformer without the requirements of additional cooling devices.

Bubbles in Transformer Oil: Dynamic Behavior, Internal Discharge, and Triggered Liquid Breakdown

Abstract: It is generally accepted that the presence of bubbles severely degrades the electric strength of liquid dielectrics, but the influencing factors and mechanisms have yet to be elucidated. Dynamic behavior, partial discharges (PDs) inside bubbles, and their effects on the breakdown of transformer oil are investigated experimentally and theoretically in this study. The results show that, under a nonuniform electric field, the presence of bubbles does not significantly affect the PD inception voltage (PDIV) and breakdown voltage (BV) of the liquid, as bubbles migrate to the region with lower field strength under dielectrophoretic force. In a uniform electric field, conversely, bubbles greatly reduce PDIV and BV; an increase in the number of bubbles or a slower voltage rise rate appears to exacerbate this effect. The PDIV observed in the experiment is much higher than the theoretical value calculated by the Townsend theory or streamer criterion, which may be due to the lack of free electrons inside the bubble, thus resulting in scarce internal discharge. The addition of X-rays enhances the radiation ionization and then eliminates the inconsistency between the theoretical and measured values of PDIV, which supports our hypothesis. The bubble-triggered breakdown of liquid dielectrics, namely, once a discharge occurs inside the bubble, likely triggering the breakdown of the liquid gap, is observed. The electric field enhancement caused by surface charges and the instability of the gas–liquid interface may account for this.

Numerical analysis of electro-convection in dielectric liquids with residual conductivity

Abstract: Injection-induced electro-convection (EC) of dielectric liquids is a fundamental problem in electrohydrodynamics. However, most previous studies with this type of EC assume that the liquid is perfectly insulating. By perfectly insulating, we mean an ideal liquid with zero conductivity, and in this situation, the free charges in the bulk liquid originate entirely from the injection of ions. In this study, we perform a numerical analysis with the EC of dielectric liquids with a certain residual conductivity based on a dissociation–injection model. The spatiotemporal distributions of the flow field, electric field, and positive/negative charge density in the parallel plate configuration are solved utilizing the finite volume method. It is found that the residual conductivity inhibits the onset of EC flow, as well as the strength of the flow field. The flow features and bifurcations are studied in various scenarios with three different injection strengths in the strong, medium, and weak regimes. Three distinct bifurcation sequences with abundant features are observed by continually increasing or decreasing the electric Reynolds number. The present study shows that the residual conductivity significantly affects the bifurcation process and the corresponding critical point of EC flows.

Dielectric Design of Ester-Filled Power Transformers: AC Stress Analysis

Abstract: In the last 20 years the use of ester fluids as an alternative to mineral oil for transformer insulation has been an active field of research and development. These liquids have a much lower environmental impact than mineral oils, besides reducing the transformer’s fire risk. Although the use of natural and synthetic esters is nowadays frequent for certain applications, as for distribution transformers in highly populated areas, railway transformers or off-shore windmill transformers, the experience on large and medium sized units is still reduced. One of the critical aspects that must be assessed to use esters as dielectric fluids for large size transformers is the dielectric design of the equipment. The permittivity of esters and ester-impregnated cellulose are different from those of mineral-oil-cellulose systems, what has an impact on the electric field distribution in the transformer. Additionally, in some cases, the dielectric strength of ester fluids differs from that of mineral oil. This paper presents a study about the dielectric design of ester-based power transformers. The insulation system of a 400/138 kV, 280 MVA transformer was modelled using a finite element tool and a comparison of the field distribution under 50 Hz AC power voltage of both insulation systems is provided.

Assessing the dielectric performance of Sclerocarya birrea (Marula oil) and mineral oil for eco-friendly power transformer applications

Abstract: The ever increasing power demand has forced the power sector companies to increase their generation, transmission and distribution in turn increasing the number of power transformers in the system where necessary. Depending on the nature of operations, the transformers are chosen as either dry-type or liquid-cooled and the preference is mostly liquid cooled. Naphthenic oils have been the primary choice over the years and alternative solutions in the form of natural esters are researched extensively due to the environmental effects of Mineral Oil (MO). One such ester, Marula is presented in this research. The experiment places its focus on measurement of electrical, physical, chemical and thermal characteristics as per the standards of ASTM and IEC. The results prove the superior dielectric performance of MAO over MO. The antioxidant rich MAO exhibits superior dielectric properties and restores faster than MO after the instances of breakdown phenomena. The antioxidants show stability at both the low and high temperatures, and temperature causes minimum damage to antioxidants. In addition to that the use of such dielectric coolant minimizes the winding damage inside the transformer tank. Thus, it can be said that MAO is an excellent coolant and improves the efficiency of the transformer without the requirements of additional cooling devices.

Investigation of Effects of Different High Voltage Types on Dielectric Strength of Insulating Liquids

Abstract: Liquid dielectrics are different from each other, but are used to perform the same tasks in high-voltage electrical equipment, especially transformers. In similar conditions, the insulation performance of transformer oils under different types of voltage will provide dielectric resistance. In this study, three different dielectric liquids applied in transformers, namely mineral oil, natural ester and synthetic ester, were tested. Tests under AC and negative DC voltage were performed at electrode gaps of 2.5 mm, 2 mm and 1 mm using disk and VDE type electrodes as per ASTM D1816-84A and ASTM D877-87 standards, respectively. In turn, the impulse voltage tests were performed under an electrode configuration suggested by the IEC 60897 standard. The current data of 500 ms prior to breakdown under AC electrical field stress was decomposed using the empirical mode decomposition (EMD) and variational mode decomposition (VMD) methods. These analyses were conducted before the full electrical breakdown. Although synthetic ester has the highest dielectric strength under AC and negative DC electrical field stress, mineral oil has been assessed to be the most resistant liquid dielectric at lightning impulse voltages. In addition, stabilization of mineral oil under AC and negative DC voltage was also seen to be good with the help of calculated standard deviation values. However, synthetic ester has a significant advantage, especially in terms of dielectric performance, over mineral oil in spite of the stability of mineral oil. This indicates that liquid dielectric selection for transformers must be carried out as a combined evaluation of multiple parameters.

Numerical Study on Heat Transfer Characteristics of Dielectric Fluid Immersion Cooling with Fin Structures for Lithium-Ion Batteries

Abstract: Electric vehicles (EVs) are incorporated with higher energy density batteries to improve the driving range and performance. The lithium-ion batteries with higher energy density generate a larger amount of heat which deteriorates their efficiency and operating life. The currently commercially employed cooling techniques are not able to achieve the effective thermal management of batteries with increasing energy density. Direct liquid cooling offers enhanced thermal management of battery packs at high discharging rates compared to all other cooling techniques. However, the flow distribution of coolant around the battery module needs to be maintained to achieve the superior performance of direct liquid cooling. The objective of the present work is to investigate the heat transfer characteristics of the lithium-ion battery pack with dielectric fluid immersion cooling for different fin structures. The base structure without fins, circular, rectangular and triangular fin structures are compared for heat transfer characteristics of maximum temperature, temperature difference, average temperature, Nusselt number, pressure drop and performance evaluation criteria (PEC). Furthermore, the heat transfer characteristics are evaluated for various fin dimensions of the best fin structure. The heat transfer characteristics of the battery pack with dielectric fluid immersion cooling according to considered fin structures and dimensions are simulated using ANSYS Fluent commercial code. The results reveal that the symmetrical temperature distribution and temperature uniformity of the battery pack are achieved in the case of all fin structures. The maximum temperature of the battery pack is lower by 2.41%, 2.57% and 4.45% for circular, rectangular, and triangular fin structures, respectively, compared to the base structure. The triangular fin structure shows higher values of Nusselt number and pressure drop with a maximum value of PEC compared to other fin structures. The triangular fin structure is the best fin structure with optimum heat transfer characteristics of the battery pack with dielectric fluid immersion cooling. The heat transfer characteristics of a battery pack with dielectric fluid immersion cooling are further improved for triangular fin structures with a base length -to -height ratio (A/B) of 4.304. The research outputs from the present work could be referred to as a database to commercialize the dielectric fluid immersion cooling for the efficient battery thermal management system at fast and higher charging/discharging rates.

Electrophoresis of a highly charged fluid droplet in dilute electrolyte solutions: Analytical Hückel‐type solution

Abstract: An analytical formula is presented here for the electrophoresis of a dielectric or perfectly conducting fluid droplet with arbitrary surface potentials suspended in a very dilute electrolyte solution. In other words, when the Debye length (κ−1) is very large, or κa ≪$\ll $ 1, where κ is the electrolyte strength and a stands for the droplet radius. This formula can be regarded as an extension of the famous Hückel solution valid for weakly charged rigid particles to arbitrarily charged fluid droplets. The formula reduces successfully to the ones obtained by Booth for a dielectric droplet, and Ohshima for a perfectly conducting droplet, both under Debye–Hückel approximation valid for weakly charged droplets. Moreover, the formula is valid for a gas bubble and a rigid solid particle as well. Classic results obtained by Hückel for a rigid particle are reproduced as well. We found that for a dielectric droplet, the more viscous the droplet is, the faster it moves regardless of its surface potential, contrary to the intuition based on the purely hydrodynamic consideration. For a perfectly conducting liquid droplet, on the other hand, the situation is reversed: The less viscous the droplet is, the faster it moves. The presence or absence of the spinning electric driving force tangent to the droplet surface is found to be responsible for it. As a result, an axisymmetric exterior vortex flow surrounding the droplet is always present for a dielectric liquid droplet, and never there for a conducting liquid droplet.

Next Generation Insulating Liquids Prepared by the International working Group of IEEE DEIS Technical Committee on Liquid Dielectrics

Abstract: The IEEE technical committee on liquid dielectrics (TC-LD) activities mostly include study of aging phenomena, condition monitoring, and diagnostic testing of dielectric liquids. Notably, the emphasis is laid on the environmental and sustainability factors of the dielectric liquids. The TC-LD acts in promoting research on biodegradable insulating fluids, vis-à-vis existing literature on mineral insulating fluids for applications in oil-filled electrical apparatus. Thus, to prepare the global transformer technology for the next generation, a working group is planned within the scope and framework of the TC-LD. This working group aims to understand and summarize the intrinsic nature, behavioral aspects, and challenges in using ester liquids for liquid-filled high-voltage apparatuses. The objective of fine-tuning the ester liquids as high-performance insulating liquids with enhanced workability remains the focus of the working group. The present article presents the contemporary situation prevailing with the application of ester liquids in high-voltage transformers. The key concerns to improve the workability of ester liquids are discussed. The present article may be helpful to engineers and researchers interested in alternative insulation technologies, insulation engineers, and transformer owners.

Preliminary analysis of a novel battery thermal management system based on a low boiling dielectric fluid

Abstract: In this paper a novel battery thermal management system was experimentally studied. The batteries are submerged in a low boiling dielectric fluid with the aim to reduce the batteries surface temperature when subjected to high charge or discharge currents. The fluid change of phase allows to create a thermal buffer in case of instantaneous peak of absorbed current. This innovative system was studied on a battery pack composed of 3 cells in series and 3 cells in parallel connection for several discharge currents. For the sake of comparison, two battery packs of same dimensions were investigated: one submerged in the dielectric fluid and the second without the fluid. The cells potential and surface temperature were measured during discharges. Moreover, also the fluid temperature was evaluated in the external region. The results show a significant improvement of the thermal management since the increase of temperature is very restricted. This effect is even more evident when the fluid reaches the boiling point.

Electric Discharge Machining of AZ91 Magnesium Hybrid Composites under Different Dielectric Mediums

Abstract: In this work, an attempt was made to machine the AZ91/5B4C/5Gr hybrid composites in a castor oil electric discharge medium with an objective of attaining green environment. The hybrid composites were produced using stir casting technique in a protective environment. Experiments were conducted by varying dielectric fluid (castor oil and hydrocarbon oil), tool materials (copper, graphite, and brass), polarity, current, pulse-on time, and gap distance in Al2O3 mixed dielectric medium. L36 Taguchi approach was adopted for the design of experiments, and machining performance was accessed in terms of material removal rate (MRR), tool wear rate (TWR), and surface roughness (Ra). Because of castor oil’s high thermal conductivity, high kinematic viscosity, and lower dielectric strength, the specimen exhibits 5% higher MRR than EDM oil. When the electrodes were connected to the negative polarity, the Ra was 1.72 μm and 3.66 μm at positive polarity; however, at negative polarity, the TWR was higher than the MRR. The high density and specific heat of castor oil facilitate flushing and uniform heat distribution; as a result, the composite had a Ra of 2.52 μm compared to 2.86 μm when machined with conventional EDM oil. Surface topography showed the specimen machined with hydrocarbon dielectric medium proffers black spots, which were eliminated in castor dielectric medium. Best parametric combination was selected by the Relative Index Method optimization technique.

Investigation Of Different Solid Insulation Impregnated In Liquid Dielectrics

Abstract: An Electrical life is an important way to assess the insulation properties of dielectric materials. The breakdown voltage of insulating paper is an important electric feature. In this study, the electrical and physical properties of oil-impregnated insulation paper are investigated experimentally. For various periods, the breakdown voltage and time of mineral oil and vegetable oil-impregnated paper insulation were determined. The impact of insulation paper thickness on electrical and mechanical characteristics has been investigated. The results showed that as the time of immersing insulation paper in oil at room temperature increased, the breakdown voltage reduced and vice versa. As the thickness increases, breakdown voltage increases and decreases for a reduced thickness.

Ester-based Dielectric Fluid for Power Transformers: Design and Test Experience under the GreenEst Project

Abstract: Ester-based dielectric fluids have now been on the market for several decades, providing fire-safe and environmentally friendly alternatives to mineral oils, which have traditionally been used in transformers and other electrical equipment. This opens the door to innovation in power transformers. However, the use of esters-based dielectrics in power transformers is still very limited, especially for the higher voltage levels. The usage of these esters-based dielectrics in higher voltage power transformers is not yet consensual. this work present results with the use of natural esters in power distribution transformers. Tests carried out on mineral oil and natural ester oil found that the ester-based dielectric can withstand higher voltage thresholds for AC and Impulses tests, mainly within the specs of destructive tests, e.g., the natural ester was able to withstand a 185kV impulse without registering dielectric rupture while the natural oil registered a dielectric rupture with a 160kV impulse. Heating and mechanical tests demonstrated that ester-based dielectric oils for power transformers lead to a flow reduction between 16,8% and 18,2% in the cooling system that was design for mineral oils but they achieve a higher heat transfer coefficient, between 0,5% to 5% depending on the location of measurement.

Study on cooling system for SR motors by pumpless forced convection boiling equipment with liquid dielectric coolant

Abstract: A forced convection boiling system with liquid dielectric coolant is proposed as a new cooling system for SR motors (switched reluctance motors) for electric vehicles. In a previous study, the superiority of cooling by liquid dielectric coolant was confirmed compared to conventional cooling by epoxy resin. We have improved the cooling method using this liquid dielectric coolant and devised a method to cool the coils by forced convection boiling of the liquid dielectric coolant inside the motor. The liquid dielectric coolant is divided into a "heating room" that is directly heated by coils and a "cooling room" that is cooled by external cooling water, and the liquid dielectric coolant is circulated in the two rooms. This system is named the "Boiling Immersion Cooling System". However, if boiling cannot be controlled, the coils are exposed from the liquid surface and the cooling capacity is lost. In this experiment, this cooling system was verified by experiment. In addition, to reduce the cost of performance estimation and model improvement, a thermo-fluid analysis method was established to reproduce the boiling phenomenon of liquid dielectric coolant in the simulation. Thermo-fluid analysis and experiments confirmed that thermal runaway due to coil exposure does not occur even when the heat generated by the coil is assumed to be 19 kW at the motor's steady-state output.

A Novel Packaging with Direct Dielectric Liquid Cooling for High Voltage Power Electronics

Abstract: This work investigates a packaging solution for high voltage semiconductors (20 kV), allowing for a dramatic reduction in size and complexity of power electronics modules. The standard packaging structures typically introduce a competition between electrical insulation (which requires thick insulating layers) and thermal performance (where thin, high thermal conductivity layers are preferred). Here, we introduce a concept which addresses this competition and is based on direct cooling using dielectric liquid. Single-chip heatsinks are designed, optimized using computational fluid dynamics (CFD), built and tested.

Electric Discharge Machining of Inconel 718 Under a Distinct Dielectric Medium

Abstract: In this research, an attempt was made to machine the Inconel 718 alloy under varied dielectric media with the objective of generating wealth from waste and establishing a green environment. Experiments were conducted to analyze the influence of input variables under EDM oil, pure sunflower oil, used sunflower oil, palm oil, and ground nut oil insulating media. Five different tools, copper, brass, aluminium, copper–graphite, and aluminum–graphite, were utilized for investigation and experimental runs were designed using Taguchi mixed orthogonal array. The results revealed that the characteristics of the dielectric fluid influences the spark generation and machined cycle time. The findings show that dielectric fluid with a high density facilitates flushing, low specific heating distributes heat uniformly, and low carbon content eliminates black spots. An electrode with low thermal conductivity dissipates more heat and low latent heat of fusion erodes excessively. Micro pits, globules, angled and clouded blobs, black spots, material dunes and redeposited material are the various features observed on the machined surface texture.