Synthetic Ester Transformer Fluids : Safe & Reliable

Synthetic Ester Transformer Fluids:
Safe & Reliable
 
Synthetic ester fluid has many benefits to users in the shape of increased fire safety, reduced environmental impact, excellent oxidation stability and enhanced moisture tolerance. This article documents key differences between natural and synthetic esters and mineral oil, specifically with regards to oxygen stability and moisture absorption
 
Mounting energy and peak power shortages are amongst the most urgent problems facing the power sector in India. The need for reliable and adequate supply of electricity is key to the meet the demands of a very fast growing economy.
Efficient and reliable transformers are crucial to ensuring that this need is addressed for both existing and new energy supply.
A key contributor to the reliability of a transformer is the dielectric fluid which is used to cool it. Natural esters, or vegetable oils, were used as dielectric coolants in the very early days of transformer history. However they were soon found to be problematic due to their instability when exposed to air. They were replaced by mineral oils which are more stable when exposed to oxygen. From about 1930 onwards PCBs, sometimes referred to as Askarels, were widely used until their manufacture was discontinued worldwide in the 70s due to their toxicity. Searching for a non-harmful replacement to address the need for improved fire safety, synthetic ester transformer fluids were developed in the 1970s.
Synthetic ester fluid has many benefits to users in the shape of increased fire safety, reduced environmental impact, excellent oxidation stability and enhanced moisture tolerance. Furthermore synthetic esters are very robust and can be used at higher temperatures than mineral oil, this allows designers to come up with innovative compact designs.
These benefits are available in new transformers filled with synthetic ester, but can also be gained by replacing the fluid in the transformer, known as retrofilling.
Where retrofilling is particularly useful is in converting unsafe mineral oil transformers into far more fire safe units, quickly and economically.Other alternative fluids to mineral oil are also available, such as natural esters and some comparison between the two types of ester will be made later in this article.
Various IEC standards have been developed for synthetic esters and the benefits of using high fire point fluids are addressed in insurance documents and a new fire safety standard for rolling stock. In the case of natural esters an IEC standard is due to be published in the near future.
Synthetic estersSynthetic esters are manufactured from specifically chosen acids and alcohol to give a very robust fluid. All the acids used in the manufacture of synthetic ester fluids are fully saturated, which leads to excellent oxidation stability in the final product. Despite being very stable in service synthetic esters are readily biodegradable and quickly broken down by natural organisms if they leak or are spilt into the environment. This process does not occur in a transformer tank due to the conditions being too hot and dry to sustain microbiological life. Having a readily biodegradable fluid is a key advantage in environmentally sensitive areas. In addition synthetic esters are very moisture tolerant, retaining high breakdown strength even with up to 600 ppm of moisture.
Natural estersNatural ester dielectric fluids are produced from renewable edible seed oils. Sunflower oils, Rapeseed oils and Soyabean oils have all been used in the manufacture of natural ester based fluids for transformers since the 1990s.
The chemical structure of the natural ester is based on a glycerol backbone, bonded to 3 naturally occurring fatty acids. Many of the acid chains in natural esters are unsaturated (with C==C double bonds), which leads to poor oxidation stability. This means that natural ester fluids are only suitable for sealed transformers. Natural esters also tend to have high pour points when compared to mineral oils and synthetic esters.
Natural esters do share some advantages with synthetic esters, they have a high fire point, are readily biodegradable and have good moisture tolerance. Since they are manufactured from renewable raw materials natural esters also have a lower carbon footprint than mineral oil, making them a good environmental choice.
Applications of synthetic estersSynthetic esters are used in countless applications where fire safety is critical, such as apartment blocks, oil and gas platforms, airports, hospitals, data centres and tunnels.
They are increasingly used to retrofill existing mineral oil transformers to improve fire safety in buildings, sometimes at the request of insurance companies.
Since the 1990s synthetic esters have also been used in traction transformers, aboard rolling stock. This type of transformer calls for high temperature operation, often with breathing designs and is one of the most demanding applications for the fluid. With increasing concerns over fire safety in trains and in a move to unify diverse standards across Europe, these transformers also comply with the newly introduced technical specification CEN/TS 45545-2:2009, requiring the use of high fire point fluids instead of mineral oil in transformers on trains (1).
The third area where synthetic esters are playing a major role is in slim design transformers. These units use high temperature solid insulation such as NOMEX and are optimised around the ability of the synthetic ester insulation to withstand high temperatures. This provides more power in small spaces. These transformers have been successfully introduced for use in very demanding applications to withstand frequent load changes, such as in wind turbines. The reliability of these transformers and their ability to operate under high temperatures has been well documented (1).
Oxygen stabilityIt is well understood that natural esters are affected by oxygen, but there is currently no IEC standard stability test for natural esters. In order to compare the behaviour of different fluids various laboratory tests have been carried out. In one such test the ASTM D2112 Pressurised Vessel Oxidation Test was conducted by Doble Engineering to compare the stability of MIDEL eN natural ester, MIDEL 7131 synthetic ester and a naphthenic mineral oil Nytro 10CX. 
The time taken for the oxygen to be consumed in the vessel, in minutes, gives an indication of stability. The test results concluded that the natural ester fluid was the least oxidation unstable and the synthetic ester is the most oxygen stable transformer fluid, even outperforming mineral oil.
Oxidation test methods specifically for natural esters will be included in the upcoming IEC standard.
Moisture absorbtionEsters, and in particular synthetic esters, are far more polar than mineral oil and have the ability to form hydrogen bonds with polar water molecules, leading to a much higher moisture saturation limit. At ambient temperature the moisture saturation limit for mineral oil is in the region of 55 ppm, in contrast the saturation limit for synthetic ester is 2,700 ppm. Due to this higher saturation limit synthetic esters retain their dielectric strength with much higher levels of moisture,
This attribute can be very useful in climates with high humidity, where the fluid is exposed to the atmosphere.Another way to describe water behaviour is to look at relative moisture content, rather than absolute ppm values.
Relative moisture content is calculated as
Wrel = Wabs / Wsat X 100
The saturation limit is represented by Wsat in the above formula, Wabs represents the ppm moisture content of the fluid.
Graph 3 demonstrates that synthetic esters such as MIDEL 7131 maintain a high breakdown voltage up to very high relative moisture contents (2). This is important to understand when looking at moisture uptake from the atmosphere.Studies have shown that mineral oil will absorb moisture faster and reach critical relative moisture absorption level (i.e. where breakdown voltage becomes critically low) much more rapidly than the synthetic ester MIDEL 7131 (2).
Further research into this field indicates that even with relative air humidity rates of 90 per cent, the relative humidity of synthetic ester will stay safe and well below the critical point, meaning the dielectric strength is still within norm (Graph 4).A further advantage of the interaction between ester fluids and moisture is the extension of paper lifetime. A variety of laboratory studies has demonstrated that cellulose paper will have a much longer lifetime when immersed in ester fluid, rather than mineral oil. For the user of transformers this leads to either a longer overall life for the transformer, or the ability to run the transformer hotter for the same lifetime. Running the transformer at higher temperatures means that more power output can be gained from the same size unit (6).
RetrofillingIEC standards recognise that the use of less flammable fluids will reduce the fire precaution requirements for transformers erected in buildings. One way to benefit from this is by retrofilling and this can be performed in sites where the only other alternative to improve fire safety would have been investing in a new fire safe transformer. Often the transformers sit in inaccessible places, making replacement expensive and difficult, and a simple fluid replacement comes as the easiest and commercially most attractive solution. Due to the high oxygen stability, synthetic ester can be used in free breathing transformers.
Retrofilling case studyIn the German capital Berlin, the utility Vattenfall initiated research in 1990 into retrofilling some of their transformer estate with synthetic ester both in areas of environmental sensitivity and in buildings.
A pilot was conducted between 1992 and 1994, retrofilling some typical transformers. These transformers were monitored for 13 months monthly testing for moisture content and breakdown strength. As expected, moisture levels started to increase in the synthetic ester MIDEL 7131, but breakdown strength remained constantly very high (Graph 5).Between 1992 and 1997 the utility retrofilled most of their high risk transformers with synthetic ester and continues monitoring moisture and breakdown strength on an annual basis to this day. Even after 13 years in operation without any maintenance, the moisture in the oil in all transformers seemed to level out at around 500 ppm without any degradation in breakdown strength (Graph 6).
ConclusionThis article has discussed some key differences between natural and synthetic esters and mineral oil, specifically with regards to oxygen stability and moisture absorption. It was also discussed how these key properties, coupled with the high fire point, makes synthetic ester a good alternative to increase fire safety in buildings and outside – both in new transformers and through retrofilling existing distribution transformers. In studies it was demonstrated that synthetic ester maintains high dielectric strength even with increasing moisture levels, providing safety and reliability even in humid climates. The benefits of retrofilling existing distribution transformers with synthetic esters have been demonstrated in a long term case study on many indoor and outdoor transformers of a major German/Scandinavian utility.
 

  Mark Lashbrook,  M&I Materials Ltd,     Manchester, UK
  Nitin Satija, M&I Materials Ltd,  New Delhi