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


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.

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


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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Retrofilling transformers with ester fluids