ANSWER:

Ester fluids can absorb large amounts of moisture with no reduction of breakdown voltage.

They have a higher saturation point – and are more tolerant of water – than mineral oil. A comparison of the saturation curves of different fluids is shown below.

So, what happens in the transformer?

Water moves between the solid insulation and fluid trying to reach equilibrium in terms of relative saturation. Paper produces water as a by-product of thermal ageing, which acts as a catalyst for continued degradation.

Because mineral oil has a very low saturation limit the water stays in the paper.

Since ester fluids have much higher saturation limits some water can migrate into the fluid, having a positive effect on paper ageing rates.

What does this mean for transformer life?

Laboratory experiments have demonstrated that cellulose materials immersed in ester-based liquids will age more slowly those immersed in mineral oil.

Information on the enhanced ageing of cellulose can be found in IEEE C57.154 and IEC 60076-14, including an ageing curve which indicates that ester transformers can run hotter than mineral oil transformers for the same lifetime.

Alternatively, when run at standard temperature, a transformer can have a longer lifetime with MIDEL ester fluid.

MORE READING:

Read up on using MIDEL fluids for transformer life extension and more effective asset management, per ISO 55000.

Read about how Pressboard ages differently in different insulating fluids in this white paper.

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?