Energy Blog

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.

Degradation of the solid cellulose insulation in transformers (insulation ageing) is irreversible. Thermal ageing of cellulose materials cannot be avoided – however it can be slowed. Retrofilling transformers with MIDEL fluids, can have a positive impact on the issue of paper ageing.

According to IEC 61936 transformers filled with K-class fluids can have reduced fire protection measures. In addition, certifiers such as FM Global provide a list of approved Less Flammable Transformer Fluids and guides for safe installation.

There are a range of MIDEL fluids to suit various climatic conditions. The performance of each fluid under cold climate conditions can be characterised by the pour point.

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.

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.

Using much the same oil sampling and analysis methods you would use for mineral oil. The gasses esters produce during electrical and thermal faults are the same as for mineral oil, but the quantities and relative amounts are slightly different.

The dielectric dissipation factor of ester-based liquids is higher than that of mineral oil, due to the polar chemical structure of the ester. This in turn leads to higher power factor values for clean, dry insulation than would normally be seen with mineral oil.

Although MIDEL fluids are suitable for higher voltage levels in specifically designed transformers, retrofilling depends upon the voltage and power rating. As a general guideline the table below can be utilised.

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.

Depending on their condition, Midel fluids can be reprocessed and reclaimed using similar equipment and methods to mineral oil.

There are four main steps to follow to retrofill transformers with ester fluids: Set up retrofilling plant; Reduce mineral oil content as much as possible; Fill again using new ester fluid; Test and nameplate.

There are a range of MIDEL fluids to suit all types of transformer preservation systems. For breathing transformers, we recommend the use of MIDEL 7131 which is oxygen stable and suitable for continuous contact with air. For non-breathing transformers either MIDEL 7131 or MIDEL eN 1204 can be used.

As a rule, components and materials used in mineral oil transformers are also compatible with MIDEL fluids. In all cases the user should confirm the suitability of any material to be used in equipment with MIDEL ester-based fluids.

As a rule, components and materials used in mineral oil transformers are also compatible with MIDEL fluids. In all cases the user should confirm the suitability of any material to be used in equipment with MIDEL ester-based fluids.