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

In 130 years 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 I discusses the key properties and standards for the two liquids. Part II emphasizes 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.