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Specialist Materials for Industry and Science



One of the main applications of MIDEL 7131 after it was introduced in 1979 was to replace Polychlorinated Biphenyls (PCBs) in existing distribution transformers. Commonly referred to as retrofilling, the focus today has shifted towards capitalising on the benefits of retrofilling mineral oil transformers with Midel ester fluids.

Customers usually retrofill transformers with MIDEL 7131 to improve service life, reduce environmental hazards or increase fire safety. The process is very simple because the thermal expansion characteristics, cooling performance and electrical properties are comparable to those of mineral oil.

With the introduction of natural ester based MIDEL eN fluids, customers now have another option for retrofilling sealed distribution transformers. These fluids are manufactured from renewable materials, further reducing environmental impact.

When retrofilling with MIDEL ester fluids (K-class) you can increase the transformer fluid's fire point from around 170°C to more than 300°C. As well as making the transformer much safer, our customers also report that switching to a K-class fluid such as one of the MIDEL esters can dramatically reduce the cost of their insurance premiums.

Furthermore, retrofilling with MIDEL ester could also improve the service life of a mineral oil transformer that has been negatively effected by exposure to high humidity levels. This is because both MIDEL esters have superior moisture tolerance than mineral oil. This has been demonstrated in field trials where MIDEL 7131 was used to bring transformers with very wet cellulose back into operation by a simple retrofilling operation.

For guidance on retrofilling with MIDEL 7131 please click here. Customer wishing to use MIDEL eN should contact us for further advice. We also provide free MIDEL names plates for your transformers. To find out more or to order your plates please contact us today.

Why Retrofill with MIDEL Ester based fluids?

Increased Fire Safety

High Performance

  • Fully miscible with mineral oil
  • Greater moisture tolerance than mineral oil
  • No corrosive sulphur - ASTM D1275 B and IEC 62535

Greater Environmental Protection

  • Readily biodegradable according to OECD 301 and fully biodegradable according to IEC 61039
  • Non-toxic and non-water hazardous
  • MIDEL eN fluids are manufactured from renewable vegetable oils

Excellent Customer Service

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

For further information please visit our Documents or Contact Us

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  • Metrosil
  • Midel
  • Wolfmet

Products of M&I Materials Ltd.