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Specialist Materials for Industry and Science



MIDEL 7131 is a synthetic ester dielectric liquid designed to provide an alternative to mineral oil, silicone liquid and dry-type transformers. Its high fire point significantly increases the fire safety of transformers and reduces the need for fire protection equipment. MIDEL 7131 is readily biodegradable, avoiding environmental damage should leakage occur and enabling reductions in containment measures. With its extremely low pour point of -56˚C, it is a highly effective solution for colder climate installations. MIDEL 7131 is currently used in a wide range of transformer applications up to 433kV, and is perfectly suited for non-breathing and breathing transformers (due to its excellent oxidation stability) located indoors or outside.

In 2016, MIDEL 7131 was specified and used in a 433kV urban transformer installation – read more here.

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

Our natural ester fluids MIDEL eN 1204 and MIDEL eN 1215 are also suitable for use in power transformer applications. They are produced from renewable vegetable oils, and provide a cost effective, environmentally friendly solution for non free-breathing transformers, ideally in temperate or indoor locations.

Properties of MIDEL Transformer Liquids

Fire Safety

MIDEL dielectric liquids 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 areas where risk mitigation and environmental protection are critical issues.

MIDEL 7131 and the MIDEL eN liquids 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

There are documented instances of the transformer industry reporting the failure of multiple large units due to the presence of corrosive sulphur.

MIDEL transformer liquids 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 non free-breathing and free-breathing 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 1204 is a natural ester made from rapeseed/canola
  • MIDEL eN 1215 is a natural ester made from soybean
  • Less threat to the environment than mineral oil

Excellent Customer Service

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

Find out more

If you would like more information on how MIDEL transformer liquids could be used effectively in power transformers, contact the technical department.

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Products of M&I Materials Ltd.