High Temperature Operation

One of the key advantages of using high fire point fluids is the ability to run a transformer at a higher temperature. This can directly impact the power output of the transformer, since allowing a higher oil temperature gives designers the ability to get more power out, without increasing the size and weight of the unit.

This principle has been used for many years in rolling stock transformers and compact wind turbine transformers operating with MIDEL 7131. In these cases the solid insulation has also been of a high temperature type, using aramid paper or similar technologies. One example of this is the CG Power MIDEL-filled Bio-SliM® transformers, which are up to 30% smaller and lighter than conventional units with the same power ratings.

In these types of transformer MIDEL 7131 has performed exceptionally well, proving that it is a robust solution for the harshest tasks.

Esters and Cellulose

In more recent times the lifetime extension benefits of using ester fluids such as MIDEL 7131 and MIDEL eN in conjunction with cellulose have been realised and the IEC and IEEE have released some information on running ester-cellulose systems at elevated temperatures.

Ageing studies have shown that if ester is used in place of mineral oil cellulose will live longer and based on this information calculations have been made to work out what increase in operating temperature this would allow. The IEC and IEEE have concluded that in the case of kraft paper the insulation class can be increased from 105 to 120 and in the case of thermally upgraded paper it can be increased from 120 to 140 if ester fluid is used. This translates into the operating temperature rise limits given in table 1.

Table 1 – Operating Temperature Rises[i]

Solid Insulation Kraft Paper Kraft Paper TUP
Liquid Insulation Mineral Oil Ester Ester
Solid Insulation Thermal Class 105 120 140
Top Liquid Temperature Rise (K) 60 90 90
Average Winding Temperature Rise (K) 65/70 75 95
Hot-spot Temperature Rise for Solid Insulation (K) 78 90 110

Overall the increased operating temperature allowed by the uprating of the thermal class can translate into a significant saving in size and weight, while retaining power output and all using standard paper insulation. Since having a smaller transformer also reduces the amount of copper, steel and other materials for manufacture using these principles could potentially offset the additional cost of ester fluid.

Stability of MIDEL Fluids at High Temperatures

High temperature laboratory studies and field experience have demonstrated that both MIDEL 7131 and MIDEL eN are suitable for use in high temperature systems. Both fluids can provide a long service life in sealed systems, while MIDEL 7131 can also be used for breathing high temperature transformers.

Advantages of MIDEL Fluids

The use of MIDEL ester based transformer can bring the following advantages for high temperature systems

  • Increased thermal rating for cellulose, compared to mineral oil
  • Suitable for use in hybrid and aramid insulation designs
  • MIDEL 7131 can be used in both breathing and sealed transformers
  • Greater moisture tolerance than mineral oil and silicone liquid
  • Allows for compact transformer design
  • Better lubricant than mineral oil
  • 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

[i] IEC 60076-14 Ed.1.0, Power transformers – Part 14: Liquid-immersed power transformers using high temperature insulation materials, September 2013