Turning Up the Heat With Esters

High-temper­a­ture on-load tap-changers get consid­er­ably better perfor­mance out of ester trans­formers.

Using esters instead of mineral oil in trans­formers is by no means a new concept. Given their biodegrad­able attrib­utes, both synthetic esters (pentaery­thritol tetra fatty acid esters) and their natural coun­ter­parts (tri­glycerides) are the perfect choice for trans­formers in areas prone to flood­ing, nature reserves, loca­tions close to the sea, and offshore wind farms – and some appli­ca­tions in these contexts even stip­u­late that esters have to be used. In 2015, a trans­former malfunc­tion caused mineral oil to spill into the Hudson River in New York state, leading to an expen­sive and labo­rious clean-up oper­a­tion. A growing number of oper­a­tors are keen to prevent any damage that might befall their own facil­i­ties lead­ing to unfore­see­able costs like this—and are switching to trans­formers that operate with esters instead.

Another advan­tage of esters is that they make trans­formers safer to use thanks to a low level of flam­ma­bility – putting them in a cate­gory referred to as class K—with a combus­tion point above 300 degrees Celsius. This means that the fire load of this alter­na­tive insu­lating mate­rial is much lower than other types, and that any fires which do break out can be extin­guished easily. In sharp contrast, trans­former fires involving mineral oil are extremely diffi­cult to quench. As such, esters easily comply with safety requirements—even the very strict ones—that are imposed on high-voltage instal­la­tions in densely popu­lated urban areas and relate to fire preven­tion, toxic fumes, and explo­sion damage in the event of a malfunc­tion.

Since there is already a clear trend toward the use of ester trans­formers, why not make the most of further advan­tages of these insulat­ing liquids? Esters have a higher combus­tion point than mineral oil, allowing for oper­ating temper­a­tures of up to 150 degrees Celsius (as compared with 115 degrees Celsius in trans­formers that use mineral oil). If on-load tap-changers and motor-drive units are also able to with­stand oper­ating temper­a­tures of this magni­tude, then this means that a trans­former will be able to transfer more power but without having to increase in size.


MR has made several devel­op­ments with this in mind: Its VACUTAP® VR® high-temper­a­ture on-load tap-chang­ers, DEETAP® DU high-temper­a­ture de-ener­gized tap-changers, and TAPMOTION® ED motor-drive units allow ester trans­formers to run in a controlled over­load state over several weeks or months. This new oppor­tu­nity is set to benefit trans­former oper­a­tors in megac­i­ties and mobile appli­ca­tions in partic­ular.

A Solu­tion for Megac­i­ties

In large cities, oper­a­tors are repeat­edly confronted with the need to main­tain more infra­struc­ture in a consis­tently small space due to the increasing demand for energy. High-temper­a­ture trans­formers are the ideal solu­tion here. They are able to achieve the same perfor­mance with a reduced size, or higher perfor­mance with the same size. When trans­formers are being replaced, the existing foun­da­tion can normally be used with­out the need to extend it. In many cases, the resulting construc­tion cost savings alone balance out the acqui­si­tion costs of ester insu­lating fluid, which are roughly double compared to mineral oil.

“Not only can trans­formers that operate with esters deliver more power, they are also safer.”

Oper­a­tors of mobile trans­formers can also benefit from a more compact struc­ture that delivers the same level of perfor­mance: A trans­former of this kind can reach its desti­na­tion more quickly in the event of an emer­gency, since obsta­cles such as low bridges no longer pose an issue, and the lower weight makes trans­porta­tion easier too.

What also makes these mobile trans­formers so resilient in emer­gency situ­a­tions is their max­imum over­load capa­bility of 150 degrees Celsius, which they are able to main­tain over an extended period. Addi­tion­ally, it is worth noting that safe­­ty restric­tions in many coun­tries, the USA included, prohibit the trans­porta­tion of trans­formers filled with min­eral oil—so in these cases, ester trans­formers are the go-to solu­tion. With MR equip­ment added to them, they then become high-temper­a­ture trans­formers.

Service Life Put to the Test

On-load tap-changers can also be filled with esters as an insu­lating fluid to make them capable of withstand­ing high temper­a­tures. Synthetic or natural esters are both options, but natural esters are gener­ally preferred due to their carbon-neutral creden­tials, which make them an espe­cially eco-friendly choice.

While esters in trans­formers are only required to cool and elec­tri­cally insu­late active parts and bush­ings, in on-load tap-changers they need to meet four addi­tional criteria in order to ensure that the equip­ment main­tains a long oper­ating life. These criteria relate to lubricity, arc elim­i­na­tion behavior, viscosity, and compat­i­bility with other mate­rials. MR has been inves­ti­gating esters for use in on-load tap-changers and de-ener­gized tap-changers as part of a compre­hen­sive series of tests span­ning more than ten years.

  • Where lubricity is concerned, it has not been possible to iden­tify any notable differ­ence between esters and mineral oil, which has already been tried and tested in this area.
  • The behavior that esters demon­strate in elim­i­nating arcs is not as good as that of mineral oil. This is of little conse­quence to on-load tap-changers that use vacuum-switching tech­nology, how­ever, as any switching arcs that do occur are fully encap­su­lated within the hermet­i­cally sealed vacuum inter­rupters. Addi­tion­ally, the only kind of wear that esters undergo in vacuum switches is thermal aging, which means that there is usually no need to change the insu­lating fluid at any point during the tap changer’s service life. Using esters in de-ener­gized tap-changers some­times requires a higher level of dielec­tric strength, although this can be resolved through actions such as opting for a higher voltage range (in Um).
  • Esters are much more viscous than mineral oil, but this has only had a moderate impact on switching behavior in cold condi­tions during testing, where it has been shown that the lower temper­a­ture limit of esters is slightly above that of mineral oil.
  • In mate­rial compat­i­bility tests, we have iden­ti­fied that seals with certain rubber formu­la­tions become hard and brittle in synthetic esters, but swollen and soft in natural ones. In appli­ca­tions involving esters, MR there­fore uses only Viton seals as they have proven them­selves to be fully compat­ible with this kind of fluid. While the plas­tics used in on-load tap-changers do not react with esters, they still need to with­stand the high­er temper­a­tures at which the equip­ment is run… To achieve this, MR uses special high-tech plas­tics that undergo neither elastic nor plastic defor­ma­tion even when they are exposed to excep­tion­ally high oper­ating temper­a­tures.

Following exten­sive rounds of test­ing and several hundred instal­la­tions of ester-based on-load tap-changers in the field, including special appli­ca­tions such as trac­tion trans­formers and test-field trans­formers, MR can say with certainty that oper­a­tors will get much better perfor­mance out of their ester trans­formers if they use high-temper­a­ture on-load tap-changers—and will benefit from safe oper­a­tion throughout the equipment’s service life.


Are you looking for a high-temper­a­ture trans­former?
Sebas­tian Rehkopf, Tech­nical Product Manager, is here to help: