The mineral or synthetic oil is mostly being used in conjunction with paper as the dielectric medium in most of the high voltage equipment. However, impact on environment, lack of petroleum oil reserves and disposal problems with used oils, have prompted researchers to direct their focus onto biodegradable and renewable insulating materials. The new insulating liquid materials development is guided by multiple factors such as environmental requirements and other safety and economic considerations. Therefore transformers manufacturer have to face new specifications related to these new requirements. The Vegetable-oil based transformer fluids increasingly replacing mineral oil-based products in the market place. They are successful because they perform better than mineral oil products and they provide definite environmental and safety gains. This paper reviews the current status of vegetable oils use as transformer oil, including their production, processing, and characterization. The vegetable oils most used as transformer oils are presented and their main advantages described in comparison with mineral oil. The various experimental work carried out in different countries is described, giving an overview of the current research carried out on the vegetable oils. In addition scope and challenges being faced in this area of research are clearly described.

Use of vegetable oils as transformer oils – a review

As a non-renewable resource, mineral oil has been re-evaluated for its value and suitability as the coolant and insulator in transformers because of its environmental unfriendliness, fire hazards, and operating efficiency dissatisfaction. As a result, alternative dielectric liquids meeting the demands for more reliable, safer, and cleaner energy has been a critical need. Plant-based insulating fluids, non-toxic to the environment and ultimately biodegradable, have been flowing into the mainstream for achieving cost efficiencies, performance advantages, and enhancing safety. With more than 20 years of field experience, plant-based oils are now employed in more than 600,000 transformers worldwide, and have been established an enviable performance track record. Far from being just a mineral oil replacement, but more significantly, plant-based oils have filled a gap in some specific application scenarios where mineral oils fail to satisfy the fire-safety and environmental standards. This article reviews the research status of plant-based insulating fluids for transformer, including both challenges and outlook circa 2020. Plant-based oils have many inherent advantages over mineral oil, but also have numerous and unique challenges.

A critical review of plant-based insulating fluids for transformer: 30-year development

Ester-based dielectric fluids have gained widespread popularity for applications in high voltage apparatus. Synthetic and natural esters have been subjected to research for decades vis-a-vis mineral insulating oils around the world. Although many researchers favor the application of ester fluids, utilities are still uncertain and application of these alternatives remains a challenge. The intent of this survey is to present recent research progress and highlight the state of the art of key aspects that should be emphasized in future research. The contemporary research scenarios pertaining to the performance of ester fluids versus mineral oils, miscibility, and retrofilling of insulating fluids are discussed. In addition, pre-breakdown phenomena, usage of esters in on-load tap changers, environmental and fire resistance properties, and use of esters in cold climates are also discussed. Importantly, challenges and future aspects that should be investigated to improve the existing knowledge of ester dielectric fluids for applications in transformer technology are highlighted.

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Alternative Dielectric Fluids for Transformer Insulation System: Progress, Challenges, and Future Prospects

In November 2017, the first ±1100 kV high-voltage direct-current power transformer in the world, which was made by Siemens in Nurnberg, passed its type test. Meanwhile, in early 2017, a ±1000 kV ultra-high voltage (UHV) substation was officially put into operation in Tianjin, China. These examples illustrate that the era of UHV power transmission is coming. With the rapid increase in power transmission voltage, the performance requirements for the insulation of power transformers are getting higher and higher. The traditional mineral oils used inside power transformers as insulating and cooling agents are thus facing a serious challenge to meet these requirements. In this review, the basic properties of traditional mineral insulating oil are first introduced. Then, the variation of electrical properties such as breakdown strength, permittivity, and conductivity during transformer operation and aging is summarized. Next, the modification of mineral insulating oil is investigated with a focus on the influence of nanoparticles on the electrical properties of nano-modified insulating oil. Recent studies on the performance of mineral oil at molecular and atomic levels by molecular dynamics simulations are then described. Finally, future research hotspots and notable research topics are discussed.

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Review of Research Progress on the Electrical Properties and Modification of Mineral Insulating Oils Used in Power Transformers

To provide a composition for a heat cycle system having favorable lubricating properties and comprising a working fluid for heat cycle which has a low global warming potential and which can replace R410A and a heat cycle system employing the composition. A composition for a heat cycle system comprising a working fluid for heat cycle containing trifluoroethylene, and a refrigerant oil (for example, an ester refrigerant oil, an ether refrigerant oil, a polyglycol refrigerant oil or a hydrocarbon refrigerant oil), and a heat cycle system employing the composition for a heat cycle system.

Composition for heat cycle system, and heat cycle system

The behavior of creeping discharges developing on the pressboard surface was investigated experimentally in palm fatty acid ester oil, natural rapeseed oil and commercial mineral oil under variable ac voltages with 60 Hz, respectively. The oilimpregnated pressboard was used as the solid dielectric and it was immersed completely into the same oil. The tungsten needle electrode was installed in the one side of pressboard surface without the counter electrode as a high voltage electrode and a grounded fine copper rod was adhered tightly in the other side of pressboard as a back side electrode. The discharge shape, streamer extension, streamer velocity and tracking at the oil/pressboard interface were observed as a function of the time and voltage by using a still camera equipped with a night viewer. Simultaneously, the discharge current and dissipated energy were measured. The gas components dissolved in oils by the creeping discharge were also examined by the dissolved gas-in-oil analysis (DGA). In this paper, it is presented that there are distinctive events due to the occurrence of creeping discharges.

Creeping discharge developing on vegetable-based oil / pressboard interface under AC voltage

Today, esters obtained from plant oils are attracting attention as insulating oils from the viewpoints of renewable resources and low environment impact in the event of leakage. We have developed a palm fatty acid ester (PFAE) synthesized from palm/coconut oil for use as an environmentally friendly insulating oil. PFAE is expected to be an alternative to mineral oil because of its high fluidity and high dielectric characteristics. On the other hand, because insulating oil for transformers is used over several decades, high oxidative stability is required. In our study, we investigate oxidative stability with regard to the electro-chemical characteristics of PFAE compared to another vegetable oil. As a result, the breakdown voltage and total acid value of PFAE showed less change than the other oil. We discuss differences in the oxidation stability of PFAE and it superiority over the other vegetable oil.

Study on the oxidative stability of palm fatty acid ester (PFAE) as an insulating oil for transformers

Recently, fatty acid esters synthesized from the natural oil obtained from plants have attracted much attention as an insulating oil from the viewpoint of environmental problems, fluidity, and greater resistance to dielectric breakdown than conventional mineral oil. In the present study, to clarify the reason for the superior moisture tolerance of fatty acid esters, the inter- and intramolecular movements of fatty acid esters and the situation of dissolved water in the esters are investigated using nuclear magnetic resonance (NMR) and infrared (IR) spectroscopic measurements and density functional theory (DFT) calculations; it became clear that water molecules in fatty acid esters are trapped by the ester group of fatty acid esters. This is why fatty acid esters have a higher moisture tolerance against electrical breakdown than mineral oil.

Consideration on the relationship between dielectric breakdown voltage and water content in fatty acid esters

Disclosed is a base agent for electrical insulating oils, which mainly contains an esterified product of glycerin and a linear or branched, saturated or unsaturated fatty acid having 6-14, preferably 8-12 carbon atoms. This base agent for electrical insulating oils is excellent in electrical characteristics, oxidation stability, cooling characteristics, flame retardance and safety. In particular, this agent for electrical insulating oils can meet energy/environmental problems by using an edible oil and fat, which is obtained by using a fatty acid derived from a vegetable oil as a raw material, as the linear or branched, saturated or unsaturated fatty acid having 6-14 carbon atoms.

Base agent for electrical insulating oil

Partial discharge (PD) properties and dissolved gas analysis (DGA) are investigated on three kinds of fatty acid alkyl esters synthesized from palm oil, fatty acid methyl esters synthesized from soybean oil and from rice bran oil with low viscosity to be adopted in transformers. For comparison, the properties of rapeseed oil of vegetable oil having high viscosity are also investigated. The results revealed that the number of PD and average PD charge depended on the molecular structure of the ester oils. It is indicated that the number of PD of large molecular ester oils and ester oils containing double bonds at hydrocarbon part is relatively few. PD in the ester oils with high viscosity were triggered by bubble. DGA showed that there were no remarkable acetylene gases by PD in the ester oils, though hydrogen gas was generated. CO and CO 2 were also generated in the ester oils, especially remarkably generated by PD in ester oils containing double bonds at hydrocarbon part.

Takaaki Kano

Papers

Creeping discharge developing on vegetable-based oil / pressboard interface under AC voltage

Study on the oxidative stability of palm fatty acid ester (PFAE) as an insulating oil for transformers

Consideration on the relationship between dielectric breakdown voltage and water content in fatty acid esters

Base agent for electrical insulating oil

Partial discharge properties of ester oils having different molecular structures