Ester been used as liquids since the invention of oil-filled transformers in the late natural ester dielectrics were subsequently found to incompatible The because of their chemistry, and were gradually replaced by mineral offering a high fire factor alternative. However, they oils. an questionable. expressed a growina were found lo be environmentally industry has for some time to mineral oil The electrical power transmission and distribution viable alternatives also need for friendly as appeal, they must to have widespread PCBs. However, in order these fluids performance demonstrate that they are safe, economical, and offer a high standard of electrical used fo life. Silicone and symthetic ester dielectrics have been successfully almost 40 years and continue to grow in popularity. In the last 10 years there has been a of usage in the use of natural ester dielectrics because of their obvious 'green credentials. mineral oil help the nature of alternative dielectric fluids and how they differ from this report attempts to explain the fundamental aspects of chemical, physical and dielectric properties of insulating liquids, and to summarise their in-service experiences of the fluids to mineral oil. For the purposes of this brochure, the alternative fluids under discussion are natural esters, synthetic esters and silicone fluids. overview of the Current Application of Alternative Fluids' 2 Alternative are now widely used in a variety of transformer applications, covering transformers for distribution, power and traction, but not yet including instrument transformers, as these have only recently been introduced as prototypes. Currently the knowledge database of fluids is growing rapidly which may encourage end users to look at these fuids, as more and data become available. Table 2.1 briefly summarizes the up-to-date distribution of the use of insulating liquids in the various applications Table 2.1: Use of insulating liquids silicone synthetic vegetable ester Power transformers A A A Traction transformers A A A Distribution transformers A x x x Instrument transformers (Key: A Largely used, B Used but less common, X Currently not used) Mineral o Mineral oil is the most commonly used insulating liquid for transformer applications, and years of experience with using mineral oils has led to the accumulation of a large knowledge database. However nowadays due to the environmental concerns, the use of a mineral oil subject to additional requirements The choice of a new mineral oil is guided by the IEC 60296 standard. In service evaluation of mineral oils is defined by the standard IEC 60422. 2.2 Silicone fluid Silicone based liquids have been used by transformer manufacturers since the end of 1970's as an alternative to the PCB based liquids, mainly for fire safety applications.