ANSWER:

Mineral oil: aging by-products are short chain acids, water soluble can catalyse paper degradation.

MIDEL fluids: aging by-products are longer chain acids, not harmful to paper insulation.

Because of hydrolysis (the consumption of H2O and production of non-water-soluble long-chain acids) MIDEL fluids can have higher in service acid numbers than mineral oil.

However, being longer chain the acids produced will not dissolve in water and will stay dissolved in the ester. The shorter chain acids from mineral oil are water soluble, will transfer to the paper and catalyse the aging of the solid insulation.

Common transformer oil processing techniques

MIDEL fluids can be processed using common equipment used for mineral oil. Materials and processes, including reconditioning (via degassing) and regeneration (using Fuller’s earth) can be applied to MIDEL fluids.

MORE READING:

CIGRE White Paper: Experiences in Service with New Insulating Liquids – Working Group A2.35

More questions on ester fluids?

This post is one of the 15 top questions we get asked on retrofilling transformers with ester fluids. Here is the full list of the frequently asked questions on ester fluid retrofilling:

1. Miscibility: Which transformer oils are compatible with Midel ester fluids for retrofilling?

2. Free-breathing: Can I use Midel fluids in breathing transformers?

3. Insulation Regeneration: Will Midel fluids heal existing problems in a transformer?

4. Fire Safety: Which dielectric fluids classifications are important to understanding fire safety?

5. Cold Weather: How do Midel fluids perform in the cold?

6. Moisture: What happens to the performance of Midel fluids when they absorb moisture?

7. Acid Levels: If Midel fluids have high acid levels, how do I recondition my fluid?

8. DGA: How can I use dissolved gas analysis on Midel fluids?

9. Power Factor: What is the Power Factor of Midel fluids?

10. Materials Compatibility: Are Midel fluids compatible with all transformer materials?

11. HV Applications: Can I use Midel fluids to retrofill transformers at high voltages?

12. Interfacial Tension: Is interfacial tension a meaningful property for Midel fluids?

13. Reusability: How are used Midel ester fluids processed and handled?

14. De-rating: Will there will be any derating of a transformer by changing it from mineral oil to a Midel ester fluid?

15. Retrofill Process: Are there extra requirements or steps when retrofilling transformers with Midel fluids?