ANSWER:

Distribution transformers: unlikely
Power transformers: an assessment may be required

Transformer rating after retrofilling with ester fluids

For distribution transformers there is unlikely to be any need to de-rate the unit after retrofilling. Due to the higher viscosity of MIDEL fluids the transformer may run a few degrees hotter, but this is offset by the enhanced paper lifetime provided by an ester fluid.

For transformers in excess of 66kV a more in-depth assessment may be needed to determine the temperature rise change.

For example:

If the transformer specification reads ’55 AWR’ and the mineral oil filled unit was factory tested at 54 AWR, it may be the position of the manufacturer that the transformer would exceed the temperature rating.

However, this is more than offset by the slower paper aging and MIDEL’s enhanced thermal capabilities. As indicated in IEC 60076-14 and IEEE C57.154 ester-based liquids can operate at a higher temperature than mineral oil for a given paper lifetime. Taking into account this slower paper aging, there is no need to de-rate a retrofilled distribution transformer, even if the operating temperature increases.

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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?