Comparative evaluation of alternative fluids for power transformers

ower transformers are one of the most expensive elements in a power system, and their failure is a very costly event. Power transformers are mainly involved in energy transmission and distribution. Unplanned power transformer outages have considerable economic impact on the operation of electric power networks. To have reliable operation of transformers, it is necessary to identify problems at an early stage before a catastrophic failure occurs. In spite of corrective and predictive maintenance, preventive maintenance is gaining due importance in the modern era, and it must be taken into account to obtain the highest reliability of power apparatus such as power transformers. The well-known preventive maintenance techniques such as dissolved gas analysis (DGA), thermal monitoring, partial discharge measurement, capacitance and tan delta measurements, frequency response analysis, etc. are performed on transformers for a specific type of problem. DGA is a very efficient and reliable tool for the earliest detection of inception faults in transformer and other electrical equipment using insulating oil. Condition monitoring of electrical equipment, such as transformers, helps users in many ways such as planning of maintenance schedule, obtaining knowledge of the health of equipment, estimating the remaining service life of equipment, finding areas of further improvement, refining product specification, etc. Kraft paper (cellulose) immersed in mineral oil is used as the insulation system for the copper windings in large power transformers. As the system ages under load, the paper and oil can degrade, potentially leading to catastrophic failure. Partial discharge (PD) is an important tool for improving the reliability of HV insulation systems. It is a very sensitive and nondestructive method of evaluation of the health of the insulation of any HV equipment. PD always is associated with the degradation of insulation systems in HV equipment. Therefore, PDs need to be detected, measured, located, and reduced to a safe value so that the quality of the insulation system is not affected.

Condition Monitoring of Power Transformers - Bibliography Survey

In a long 130 years of its journey in service to mankind, the power transformer has grown in many aspects; ratings, performance, technology, aesthetics and eco-friendliness. Today, the transformer industry is witnessing the era of shifting from petroleum based mineral oils that are depleting, environmentally unfavorable, and proven dangerous to natural esters (vegetable oil based) that are renewable, environmentally friendly, and non-hazardous. The market and regulatory pressures to reduce liability risk exposure of mineral-oil-filled distribution and power transformers are increasing. In addition, there are demands to improve equipment efficiency and adopt more "earth-friendly" options in our power systems. In the last two decades there has been a resurgence of usage of natural ester insulating liquids for their "GREEN" and Safety credentials. These renewable resources have good dielectric properties and are compatible for use in power transformers. This paper evaluates and relates the new natural ester oil with existing mineral oil technology in terms of environmental concern, fire safety, physio-chemical properties, thermal performance, and dielectric performance. It also covers key modifications in design, manufacturing and operation to help assure excellent functionality and long life. In brief, this paper provides transformer designers and users about the advantages and challenges of "Green Insulating Liquid". Due to the voluminous properties that are compared and discussed, this paper is presented in two parts: Part 1 discusses on key properties and standards for the two liquids. Part 2 emphasizes on additional key properties and material compatibility related to electrical design. Based on evidence discussed in both parts, it is concluded that natural esters are acceptable for both new transformers and for retro filling existing units.

A review of critical evaluation of natural ester vis-a-vis mineral oil insulating liquid for use in transformers: Part II

In this study, three different insulating liquids, natural ester, mineral oil which is currently used by the Public Power Corporation of Greece and a nanofluid of surface coated Fe3O4 nanoparticles into a natural ester matrix, were subjected to AC voltage stress and their statistical breakdown voltage was measured and compared. Three different electrode configurations were used during the measurements in order to study the effect of non-uniform fields on the breakdown voltage distribution. Four different statistical distributions were used for the statistical processing of the experimental results. The two of them were the frequently used normal and Weibull distributions; in addition, Gumbel and generalised extreme value (GEV) function distributions were studied. It was shown that, in most cases, the experimental data followed GEV and Weibull functions rather than normal or Gumbel. Therefore, GEV statistics fits indiscriminately better to the experimental results concerning breakdown voltage, for the three different electrodes configuration and the three understudied insulating oils. The experimental results indicated that nanofluid has better performance, in terms of dielectric breakdown voltage, compared with natural ester oil and mineral oil, hence nanofluid is considered as a potential substitute of the conventional dielectric liquids, with enhanced properties and prospects.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-smt.2016.0031

Statistical investigation of AC breakdown voltage of nanofluids compared with mineral and natural ester oil

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.

/pdf/alternative-dielectric-fluids-for-transformer-insulation-3v9zvvk9ix.pdf

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

Mineral oil with high insulation performance and cooling capability is currently used for oil-immersed transformers. However, mineral oil is derived from petroleum and thus is a limited resource. In a search for mineral oil substitutes, we investigated the characteristics of silicone and ester oils. The electrical performance was assessed in terms of the breakdown and streaming electrification characteristics and the volume resistivity. The burning characteristics of each oil type were also investigated. The temperature dependence of the breakdown voltage in an oil unit and the surface breakdown characteristics in a composite insulating system comprising an insulating oil and an oil-immersed insulator were determined. We also investigated the temperature dependence of the charge density in oil. The charge density in silicone and mineral oils increased with temperature, with maximum values at ~ 100°C and ~ 60°C, respectively. By contrast, the charge density in ester oil decreased with increasing temperature and was maximum at ≤30°C.

Electrical characteristics of natural and synthetic insulating fluids

We have investigated the performance of silicone oil as an insulation medium with the aim of developing environmentally-friendly, flame-retardant electric power equipment. In this paper, the insulation characteristics of silicone oil and ester oil, which is the alternative insulating oil candidate, have been investigated and compared. The insulation characteristics examined in this paper are the streaming electrification characteristics including electrification phenomena generated when an insulating oil streams, dielectric breakdown characteristics with increasing temperature and the surface breakdown characteristic when the insulator is placed between electrodes. The range of temperature investigated was 30 to 130 °C. Both kraft and aramid paper were used for the insulator. In the comparison between silicone and ester oil, it was found that the insulation performance of the ester oil was better than that of the silicone oil. We have considered possible mechanisms for these results.

Comparison of the insulation characteristics of environmentally-friendly oils

In a search for mineral oil substitutes, we investigated the characteristics of environmentally friendly silicone oil and ester oil. The electrical performance was assessed in terms of the breakdown, streaming electrification characteristics and the volume resistivity. The burning characteristics of each oil were also investigated. The temperature dependence of the breakdown voltage in an oil unit and the surface breakdown characteristics in a composite insulating system comprising an insulating oil and an immersed insulator were determined. We also investigated the temperature dependence of the charge density in oil. The charge density in silicone and mineral oils increased with temperature, with maximum values at ∼100°C and ∼60°C, respectively. By contrast, the charge density in ester oil decreased with increasing temperature and was maximum at ≤ 30°C.

Electrical characteristics of environmentally friendly insulating oils

In a search for mineral oil substitutes, the authors focus on silicone and ester oils. These insulating oils are fire-resistant and environmentally friendly. In previous studies, the authors investigated streaming electrification and dielectric breakdown phenomena in these fresh oils and found that their performance was similar to or better than that of mineral oil. In the present study, the authors focus on the thermal degradation of insulating oils. Thermal degradation tests in the presence of a solid insulator and copper for carried out 1000 hours. Changes in color and dielectric breakdown characteristics were investigated during thermal degradation of the insulating oils. The authors also investigated the mechanical strength of the solid insulator. The characteristics of aged oils were compared with those of fresh oil.

Satoshi Arazoe

Papers

Electrical characteristics of natural and synthetic insulating fluids

Comparison of the insulation characteristics of environmentally-friendly oils

Electrical characteristics of environmentally friendly insulating oils

Thermal degradation characteristics of insulating paper and oils

Breakdown Characteristics and Streaming Electrification Characteristics of Flame Retardant Silicone Oil