Design considerations for the use of ester-based dielectric liquids in transmission equipment

The use of alternative dielectric liquids is on the rise, with fire safety and environmental concerns at the forefront for the switch from mineral oil. Ester-based dielectric liquids have been in use in the power industry for almost four decades, with synthetic esters having originally been introduced as replacements for harmful PCBs in the late 1970s. Over time the use of esters has grown steadily and the introduction of natural ester-based liquids in the 1990s has further accelerated their adoption, as in addition to being fire safe and biodegradable, these liquids provide users with a very environmentally friendly, sustainable alternative to mineral oil. Natural esters have made some significant inroads into the distribution industry, where they are used extensively for equipment such as pole-mounted transformers. One limiting factor to the use of esters in transmission transformers in the past has been a lack of experience at higher voltages, coupled with some different behavior in standard dielectric tests. However, over ten years of concerted research, effort is changing this. Beginning with the fundamental differences in the physical and chemical properties between esters and mineral oil, this paper aims to provide an overview of the characteristics which influence both thermal and electrical design of transformers. In addition, research from across the industry will be presented along with an examination of the implications of various findings for the design of higher voltage equipment. Furthermore the latest research conducted by M&I Materials in collaboration with Budapest University of Technology and Economics will be presented which provides a comparative design curve between mineral oil and different types of ester-based products, both natural and synthetic organic.

I. Introduction

For many years synthetic ester fluids were mostly deemed suitable for use in niche applications, such as rolling stock transformers, offshore installations and steel plants where fire safety was a prime consideration. However, in more recent times users are realizing that ester-based liquids could offer a more mainstream alternative to mineral oil. Although these liquids are more expensive, the overall project costs can be reduced, after taking into account factors such as reduced fire protection. In some constrained urban environments ester-based liquids may even become the preferred option, with the flammability and potential environmental impact of mineral oil making the design of modern installations extremely challenging. This type of situation has been seen with the latest 400kV projects incorporating both synthetic and natural ester liquids.