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NYTRO® RR 900X and NYTRO BIO 300X Sustainable Transformer Fluids…


Why NYTRO® RR 900X and NYTRO BIO 300X are the optimal choices for sustainable transformer fluids.


Feedstock sustainability

NYTRO RR 900X is a circular transformer oil based on highly powerful and effective re-refining technology. It contributes to sustainable development by reusing only end-of-life transformer oil from the grid. With excellent performance and compliance with the most demanding quality standards, over its lifetime NYTRO RR 900X produces a reduction of at least 70% in greenhouse gas emissions compared to a virgin transformer oil.

NYTRO BIO 300X has strong sustainability credentials, as the feedstock comes from bio-based residues and by-products. It carries a USDA bio-preferred certification and contains >99% bio-based carbon.

While it is based on renewable feedstock, the production of natural esters can interfere with the food value chain, which may diminish its sustainability credentials. Synthetic esters can theoretically have a 50% bio-based component, but in most cases they are not bio-based.

Dielectric properties

Naphthenic mineral insulating oils have a good balance of hydrocarbon structures and stable aromatics, which gives them excellent insulating capability. Natural and synthetic esters contain polar structures, which increases their fire point, but at the same time results in inferior dielectric behaviour, which often means a need for adaptions and component de-ratings.

NYTRO RR 900X has naphthenic character and optimised aromatic content, with no dielectric changes required.

NYTRO BIO 300X shares a high degree of similarity with mineral insulating oils and has more favourable dielectric properties than ester-based liquids.

Oxidation stability

According to IEC 61125, the typical oxidation stability of transformer fluids is 48 hours for natural esters, 164 hours for synthetic esters and 500 hours for inhibited mineral oils. Some transformer fluids containing natural and synthetic esters exhibit an abnormal gassing behaviour, which is related to poor oxidation stability.

NYTRO RR 900X is tested to the highest standard in IEC 60296 Type A, including the new IEC stray gassing test. This is a critical aspect to ensure that stray gassing does not interfere during the monitoring of transformer performance. The same optimal performance can be obtained using NYTRO BIO 300X.

NYTRO RR 900X meets and exceeds the 500 hours in the oxidation stability test, with NYTRO BIO 300X offering even higher levels of oxidation resistance.

Heat transfer

Ester-based fluids have lower convective heat transfer performance due to their viscosity, which ranges from 20 to 40 cst. This requires transformer designs to be modified if a load loss footprint similar to mineral oils is to be maintained.

With a viscosity up to 75% lower compared to esters, NYTRO RR 900X provides the best natural convection flow, significantly lower than the viscosity limits for mineral oils.

Unrivalled heat transfer performance can be obtained using NYTRO BIO 300X, thanks to an ultra-low viscosity of 3.7 cst.


NYTRO RR 900X is part of a truly circular economy. By reusing end-of-life transformer oil, Nynas’ refining process recovers valuable molecules that are no longer fit for service in their present form. Re-refining is best suited to achieve a fluid that can be used in new transformers.

NYTRO RR 900X and NYTRO BIO 300X are both suitable for reclamation techniques that can extend the life of a transformer and its fluid when there is still a little time left in the asset.

For natural and synthetic esters, existing recycling and regeneration technology is not efficient and economical enough to offer a true circular solution. The best option for these fluids is to be downcycled into fuels and base components.

Transformer design compatibility

Transformer OEMs have strict design parameter compatibility, aiming to ensure optimal performance and durability.

NYTRO RR 900X fits existing design parameters as mineral oils, with no changes in design and construction required.

NYTRO BIO 300X allows OEMs to design, at parity of thermal rating performance, smaller and lighter transformers. This bio- based and biodegradable fluid can also offer utilities the flexibility to push the transformer to higher limits.

Ester-based fluids require a change in transformer design, such as dielectric design rules and thermal rating, as the cooling performance of esters as substandard compared to those of mineral oils.

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