This website uses 'cookies' to give you the best, most relevant experience. Using this website means this is Ok with you. To find out more click 'how we use cookies'.x

Select your language

Specialist Materials for Industry and Science



Our synthetic dielectric fluid MIDEL 7131 transformer fluid has been used successfully in power transformers with ratings of up to 238 kV for many years. This includes transformers in high risk areas such as underground.

Recently MIDEL 7131 has been independently tested and consequently specified for 400kV transformers which are due for installation in 2015.

Multiple benefits such as fire safety, environmental protection and safeguarding against corrosive sulphur have combined to create a high performing, reliable product.

We would also recommend our natural ester fluid MIDEL eN for use in power transformer applications. It is produced from renewable vegetable oil, and provides a cost effective, environmentally friendly solution for sealed transformers, ideally in temperate or indoor locations.

In 2013 Siemens produced a 420kV natural ester (vegetable oil) transformer to connect the 380-kV ultra-high voltage level with the 110-kV grid in the Bruchsal-Kändelweg substation plant near Karlsruhe, Germany.

Properties of Midel Transformer Oils

Fire Safety

Midel dielectric oils have a 100% fire safety record, and unlike alternatives such as mineral oil their high fire point makes them ideal for use in power transformer applications, particularly in high risk areas.

For example, Vattenfall use MIDEL 7131 in a high voltage transformer (151kV) in Storuman to reduce the risk of fires in tunnels.

Both MIDEL 7131 and MIDEL eN are FM Approved Less-Flammable Transformer Fluids and have gained the globally recognised UL® classification - EOUV dielectric medium. For more information about these Approvals please click FM® or UL®.

Environmentally Friendly

Midel insulating liquids are also environmentally friendly, and in contrast to mineral oil offer a readily biodegradable and non-water hazardous option that has proved crucial in many projects

For example, VA Tech EGB manufactured a MIDEL 7131 power transformer to safeguard an ecologically sensitive area near Lake Malgomaj in Sweden.

Protection Against Corrosive Sulphur

Previously, the transformer industry has reported the failure of multiple large units due to the presence of corrosive sulphur.

Midel transformer oils have been tested by independent laboratories and found to be non-corrosive. They were checked using:

  • ASTM D1275 B – Standard test method for detecting corrosive sulphur in electrical insulating oils
  • IEC 62535 – Test method for detection of potentially corrosive sulphur in used and unused insulating oil

It is also important to note that some design alterations may be necessary when ester fluids are used in power transformers. There is evidence from laboratory research that ester fluids have different dielectric behaviour to mineral oil, especially under lightning impulse voltage loads.

Benefits of using Midel Esters in Power Transformers

High Performance

  • Greater moisture tolerance than mineral oil
  • Robust and stable, conducive to lower levels of maintenance
  • Low pour point
  • Can be used in both breathing and sealed transformers (MIDEL 7131)
  • Allows for compact transformer design
  • Self-healing, unlike cast resin transformers
  • Lower no-load losses than cast resin transformers
  • No corrosive sulphur – ASTM D1275 and IEC 62535
  • No sludge formation, unlike mineral oil

Greater Environmental Protection

  • Readily biodegradable according to OECD 301 and fully biodegradable according to IEC 61039
  • Non-toxic and non-water hazardous
  • MIDEL eN is a natural ester from a renewable vegetable oil
  • Less threat to the environment than mineral oil

Excellent Customer Service

  • Multilingual commercial team
  • First rate customer care
  • Continued technical support from our transformer fluid experts

Find out more

If you would like more information on how Midel transformer oils could be used effectively in power transformers, you can contact the technical department who will be happy to answer your questions.

  • Apiezon
  • Metrosil
  • Midel
  • Wolfmet

Products of M&I Materials Ltd.