ANSWER:

Interfacial Tension (IFT) is routinely used as an indicator of polar contaminants in mineral oil and can be directly related to oil condition. For ester-based fluids IFT is less sensitive to polar contaminants and not so useful for ageing assessment.

The reason is that ester-based fluids have a slightly polar molecular structure, giving them a lower IFT when new than mineral oil. The value of IFT is also less affected by polar contaminants and the difference between a new ester and an aged ester is smaller than it would be for mineral oil.

Currently none of the ester standards for synthetic or natural esters place specification limits on the value of IFT. IEC 62961:2018 provides some guidance for measurement of IFT in both mineral oils and ester-based liquids, but so far acceptance limits for in-use liquids are not established.

M&I Materials recommends using acid value and dissipation factor for the monitoring of MIDEL fluids in service.

MORE READING

For more on the fluid properties for MIDEL transformer oils, refer to the detailed product brochures: MIDEL 7131 and MIDEL eN 1204.

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?