Challenges for the application of natural ester fluids in extra high voltage transformers

Approaching 20 years of commercial application, natural ester insulating liquids are, by far, the most successful ‘alternative fluid’, and have reached a relevant market share positioned beyond a niche product to a mainstream fluid. A significant number of test programs and field applications have shown that natural ester liquids are an effective alternative to mineral oil.

Currently the two initial motivations for its development (improved fire and environmental safety) are increasingly considered as a “bonus”, overshadowed by the life extension of the cellulose based materials immersed in natural ester liquid. The possibility of increasing the operational temperature of transformers by up to 20 degrees, associated with the increased reliability enhanced by the continuous process of water removal from the solid insulation, is the primary motivator for the accelerated adoption of natural ester liquid filled transformers.

Years of validation were required before increasing the range of voltage class, reaching the 245 kV class ten years ago. Five years ago, the application at extra high voltage units, at the level of 420 kV and 550 kV, gained serious consideration. The first extra high voltage transformer, 420 kV/300 MVA, is in operation for over two years. Higher voltage equipment and HVDC are currently receiving attention.

The baseline for dielectric validation has always been side-by-side comparison with mineral oil. However, because of fundamental differences between natural ester liquids and mineral oil, testing procedures may require modifications to achieve reliable results.

This paper presents several studies and tests conducted by renowned institutions around the world. Differences between the liquids and application conditions have been identified, which have an impact on the dielectric design and the insulation system of a transformer. There are no apparent barriers for application of natural ester liquids in extra high voltage equipment, provided that design and testing are conducted properly.