Transformer bio-based insulating liquids
Environmental considerations play increasingly important role for electric equipment. In electric drive systems they may affect the selection of the circuit breaker (in case it is gas insulated) or the selection of transformer type. In this post we shortly address the latter option and report about transformer bio-based insulating liquids.
Role of transformer insulation liquid
Insulation fluid is the blood of liquid immersed transformers. The liquid inherently provides following important functions for the transformer:
- dielectric insulation
- cooling / transfer of heat
- protection against ambient
- information carrier
Hence the properties of insulation liquid are crucial for the lifetime, performance and maintenance of a transformer. Today’s expectations on a good power or converter transformers are long lifetime, easy installation, low maintenance, focus on total cost of ownership, high safety level and increased environmental awareness.
Properties of insulating liquid
The properties of insulation liquid shape the performance of transformer. As mentioned in the introduction, the transformer fluid provides at least four vital functions: insulation, cooling, protection and information transfer. Therefore, the liquid shall ideally have high dielectric strength, low viscosity and high oxidation stability. With the environmental and safety awareness additional requirements such as fire safety (i.e. flash and fire point) or biodegradability enter the picture.
1. Dielectric properties
Primary function of transformer liquid is the insulation. Hence dielectric properties are crucial for a perspective insulation liquid. Generally, there are two different insulation materials in liquid immersed transformers. First one is the conductor insulation, e.g. cellulose paper, that provides electrical insulation and mechanical protection. Electric breakdown strength is one of the important parameters. It is expressed as a ratio of breakdown voltage and the distance between electrodes.
2. Cooling properties
The transformer liquid also serves as a cooling medium. Viscosity is therefore of significant importance for the heat transfer from windings and core towards outside. Especially for larger transformers the cooling properties are crucial (larger units in terms of MVA have higher power density). High viscosity prevents efficient heat transfer and increases risk of local hot spots or accelerated aging. In contrary, low viscosity enables higher natural convection and thus better heat transfer. Moreover, kinematic viscosity depends on temperature. This is particularly important for transformers designed to operate at very low ambient temperatures. The increase of viscosity at low temperatures can be very dramatic.
3. Protection against ambient
Transformer winding insulation needs to be protected against external ambient factors such as humidity or oxidation. That is another function the transformer liquid shall provide. Proper impregnation process shall ensure that insulation system is protected against humidity and moisture (sometimes called to be the largest enemy of insulation system). It shall also prevent contact of the insulation system with oxygen and therefore minimize the insulation aging.
4. Information carrier
Valuable information about transformer internal condition can be gained when analysing the liquid samples (e.g. part of regular maintenance) or gas samples (especially after a transformer trip initiated e.g. by Buchholz relay or other protection element). The latter method is called DGA (dissolved gas analysis). An experienced specialist can estimate the health condition of a transformer based on the gas composition.
Mineral oil – Many strengths and two limitations
Mineral oil is used for decades as transformer insulation liquid. Its excellent dielectric and cooling properties make it almost perfect fit for the transformer. However, there are two aspects where mineral oil shows a bit of weakness. One of them is safety – particularly the fire risk (fire hazard). The other one is increasing environmental awareness. Alternative liquids were developed to provide better performance in above two areas. A lot of progress has been made in developing new transformer liquids. Nevertheless, there is no liquid that is superior in all monitored properties. Yet, there are transformer liquids that “check almost all the boxes”.
When a conventional mineral oil is an issue due to environmental concerns there are basically two options:
a) Switch to a dry type transformer technology
b) Use liquid immersed transformer with bio-based insulating liquids
The first option only works up to certain power rating and voltage level as dry type transformers do not cover the same range as liquid immersed transformers do. Let’s look at the other option using biodegradable liquids.
Bio based insulating liquids
1. Natural esters
Natural esters as alternative transformer fluids address the environmental aspects. Those liquids are bio-based – mostly kind of vegetable oil from seeds. Therefore, biodegradability is inherently granted. Many natural esters also feature higher fire point than mineral oil (K-class fire point).
Main disadvantage of most natural ester-based liquids is their high viscosity meaning those liquids are not as good coolants as mineral oil. Consequently, transformers immersed in natural ester based liquids have somewhat larger dimensions compared to mineral oil. Another issue might be somewhat lower oxidation stability and corresponding shorter lifetime.
Selected global manufacturers of natural ester based dielectric fluids:
ABB (Hitachi Energy)
- BIOTEMP®
Cargill
- EnvirotempTM FR3TM
2. Bio-based hydrocarbon
If you do not want to compromise the excellent properties of mineral oil but need to use a biodegradable liquid, then a bio-based hydrocarbon may be the preferred choice. Example is the product NYTRO BIO 300X that contains over 99% bio-based carbon. Hence the liquid meets the Ecodesign Tier II targets. Moreover, this liquid even outperforms mineral oil in many aspects. NYTRO BIO 300X has very low viscosity, superior oxidation stability and is readily biodegradable according OECD 301. The improved cooling can be used in two ways:
- Reduction of average winding temperature à increased lifetime and reduced load losses.
- Keeping by the same winding temperature but saving material –> cost optimization.
Described biodegradable hydrocarbon has lot of goodies apart from one: this liquid does not reduce fire hazard as the fire point is approximately the same as for mineral oil.
Summary
Bio based insulating liquids are an alternative to mineral oil. Their main feature is biodegradability. Most of them also provide higher flash and fire point and thus more safety. Besides those strengths they also have some potential drawbacks such as higher viscosity or lower oxidation stability.
References
[1] NYNAS transformer oils, https://www.nynas.com/en/product-areas/transformer-oils/
[2] MIDEL Natural and Synthetic Esters, https://www.midel.com/
