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

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

Natural ester oils began to be implemented in higher voltage of power transformer in 2002. Gases generated by such oils need to be examined in order to anticipate the use of dissolved gas analysis (DGA) method to diagnose the presence of faults in natural ester filled transformer in service. This paper deals with an analysis of the gas generations of natural monoesters of different molecular structures derived from palm and rice bran oils under localized and low thermal faults ranging from 200 to 300 °C. Types of gases and their generation rates at various temperature levels were investigated. An attempt is made to compare the gas generation tendency between tested oils and commercially available natural ester insulation oil reported in the previous literature as well as mineral oil. The phenomenon of ethane generation by fatty acid monoesters having C-C double bond in their hydrocarbon chains and those without C-C double bond are discussed. Due to the expectation of overheating temperature, the possible implementation of CO/CH 4 gas ratio is also evaluated.

Low thermal fault gases of various natural monoesters and comparison with mineral 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.

Partial discharge properties of ester oils having different molecular structures

Partial discharge (PD) properties and dissolved gas analysis (DGA) are investigated on two kinds of fatty acid alkyl esters synthesized from palm oil with additive, namely Pastell 2H-08 and Palm Fatty Acid Ester (PFAE) to be adopted in power transformers. For comparison, the properties of mineral oil which is commercially used transformer oil were also investigated in the research works. The results revealed that PD repetition rate number of the ester oils with additive increase remarkably at above 15 kV rms of voltage application than that of mineral oil. PD charge of ester oils with additive tends to saturate at about 10,000 pC, while PD charge of mineral oil seems to keep increasing with increasing the applied voltage at above 20 kV rms . The amounts of generated hydrocarbon gases in ester oils with the additive are higher than that of mineral oil. It is found that combustible gas pattern of the ester oils with the additive are different from that of conventional mineral oil. Duval analysis of ester oils with additive and mineral oil are classified as high energy electrical discharge, while for mineral oil is classified as low energy electrical discharge.

Takashi Suzuki

Papers

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

Low thermal fault gases of various natural monoesters and comparison with mineral oil

Partial discharge properties of ester oils having different molecular structures

Comparative study of partial discharge characteristics and dissolved gas analysis on palm-based oil as insulating material