Which transformer oils are compatible with MIDEL ester fluids for retrofilling?

ANSWER:

Miscible: Mineral oil, high temperature hydrocarbon oil

Not miscible: Silicone oil

Miscibility is an indication of the compatibility between different liquids. MIDEL 7131 synthetic ester and MIDEL eN natural ester fluids are fully miscible with transformer mineral oil, high molecular weight hydrocarbons and other transformer ester liquids. They are not miscible with silicone oil.

If unsure about the miscibility of a specific insulating liquid with MIDEL fluids, please contact Insulect for recommendations.

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Can silicone oil transformers be retrofilled with MIDEL fluids?

Retrofilling of a silicone filled transformer with an ester-based fluid needs very careful consideration, since the two types of fluid do not mix. Generally retrofilling of silicone filled transformers is not recommended, but if the user must exchange the fluid then please contact the Insulect technical team for further guidelines.

Can residual mineral oil affect the fire point?

Residual mineral oil after retrofill can have an impact on the fire point of ester-based fluids. As a general guideline the following limits should be applied to maintain the fire point above 300°C.

MIDEL 7131 < 3.5% mineral oil

MIDEL eN 1204 < 7% mineral oil

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MORE READING:

Read more on miscibility and transformer oil compatibility in the Storage & Handling Guides:

MIDEL 7131 Storage & Handling Guide

MIDEL eN 1204 Storage & Handling Guide

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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?

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Retrofilling transformers with ester fluids

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