MIDEL 7131 Dielectric Insulating Fluid

MIDEL 7131 Dielectric Insulating Fluid

MIDEL® 7131 is a dielectric insulating fluid for transformers, tap changers, and electrical control equipment. It is a synthetic ester-based fluid specifically formulated to provide a safe, superior alternative to traditional fluid and dry-type transformers that can be used in both outdoor and indoor locations. MIDEL 7131 offers increased fire safety, greater environmental protection, and improved moisture tolerance. MIDEL 7131 conforms to IEC 61099:1992 “Specifications for Unused Synthetic Organic Esters for Electrical Purposes.” It is classified as type T1, a halogen-free pentaerythritol ester.

Fire Safe

MIDEL® 7131 transformer fluid has a high Fire Point of 317+5°C and a low net calorific value (less than 32 MJ/kg) and is therefore classified as a K3 class liquid according to IEC 61100. This, coupled with a slow heat rate due to its high specific heat and thermal conductivity, gives the product its noted resistance to ignition. Even in the unlikely event of ignition, MIDEL is a self-extinguishing fluid that does not contribute to the fire.

Environmentally Friendly

MIDEL 7131 is a biodegradable, non-toxic to aquatic life, and will not affect the degradation activity of activated sludge in biological sewage treatment plants. It is an environmentally friendly alternative to conventional transformer fluids and coolants that meets the classification criteria as “non-water hazardous,” according to the Federal Department of the Environment, Berlin (UBA No. 52330, data sheet 770). Midel is supplied with a low dissolved gas content and moisture level.


MIDEL 7131 is used to retrofill PCB and mineral oil filled distribution transformers. MIDEL 7131’s thermal characteristics, cooling performance and electrical properties are equivalent to those of mineral oil. This ensures that there is no need to make any alterations to the design of the transformer or to downrate it when retrofilling with MIDEL 7131.

During retrofill, contamination with other insulating fluids has to be minimized to avoid lowering the fire point or endangering the “non-water hazardous” classification.


For disposal, it is recommended that used MIDEL 7131 or remains of the insulating fluid be burnt in a suitable installation. Waste disposal code 130309 applies (readily biodegradable insulating and heat transmission oils).


Midel is not subject to the regulations for land, sea, and air transport. It does not require labeling in accordance with the Dangerous Substances Regulations.

Midel® is a registered trademark of M&I Materials.

In addition to the properties set out in IEC 61099:1992, MIDEL 7131 is characterized by:

Ecological compatibility

  • readily biodegradable (99% after 28 days)
  • classified as “non-water hazardous” according to Umwelt Bundes Amt (UBA) in Germany
  • low vapor pressure under operating conditions

Low acute and chronic toxicity

  • non-toxic to aquatic life
  • will not effect the degradation activity of activated sludge in biological sewage treatment plants
  • not classified as hazardous

Good thermal properties

  • low thermal coefficient of expansion
  • heat transfer properties comparable with mineral oil.

Excellent dielectric properties

  • high breakdown strength
  • low dependence of dielectric properties on the effects of moisture

Excellent fire resistance

  • high fire point (317+5°C)
  • will not ignite in the event of a major electrical fault and transformer rupture

MIDEL 7131 was found to be non-corrosive in independent laboratory tests to ASTM D1275 B, the Standard Test Method for Corrosive Sulphur in Electrical Insulating Oils. MIDEL 7131 is further noted for its very good lubricating properties. MIDEL 7131 is compatible with all insulating materials used in conventional transformer construction.


  • Power transformers up to 238kV/ 135 MVA
  • Pole-type transformers
  • Distribution transformers in urban areas and buildings
  • Traction transformers
  • Rectifier transformers
  • Tapchangers
  • Thyristor cooling in power electronics


  • Offshore and onshore wind energy
  • Rail and marine transportation
  • Distribution
  • Oil and gas
  • Buildings and construction

Physical properties according to IEC 61099
PropertyTest MethodRequirementMIDEL® 7131Unit
ColorISO2211Max. 200125HU
AppearanceIEC 61099 9.2Clear, free from suspended matter and sedimentClear, free from suspended matter and sediment-
Density @ 20ºCISO 3675Max. 1.000.97kg/dm3
Kinematic viscosity @ 40º CASTM D445Max. 35.028mm2/s
Kinematic viscosity @ -20ºCMax. 30001400mm2/s
Flash pointISO 2719Min. 250260ºC
Fire pointISO 2592Min. 300317+5ºC
Refractive index @ 20ºCISO 5661 +/- 0.01 of the value given by  the manufacturer1.4555-
Pour pointISO 3016Max. –45-60ºC
CrystallizationIEC 61099 9.9No crystalsNo crystals-
Velocity of sound in Midel @ 20ºC--1385m/s

Chemical properties according to IEC 61099

PropertyTest methodRequirementMIDEL® 7131Unit
Water contentIEC 60814max. 20050mg/kg
NeutralizationIEC 61099 9.11max. 0.03<0.03mgKOH/g
Oxidation stabilityIEC 61125
-total acid contentmax. 0.30.01mgKOH/g
-total sludge contentmax. 0.010% mass
Net calorific valueASTM D 240-02<3231.6MJ/kg

Dielectrical properties according to IEC 61099

PropertyTest methodRequirementMIDEL® 7131Unit
Breakdown voltageIEC 60156min. 45>75kV
Dielectric dissipation factor
Tan ð at 90ºC and 50 Hz
IEC 60247max. 0.03<0.03-
Volume resistivity DC at 90ºCIEC 60247min. 2>50Gohm-m

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