Silicone transformer fluids: time for a game change?

As a K-class transformer fluid, silicone has proved a popular choice for applications where fire safety is critical. However, silicone also suffers from a number of disadvantages, notably being leak-prone and non-biodegradable. Esters offer a superior alternative, matching silicone for fire-rating and being completely biodegradable along with other technical advantages. Silicone has defended its position through its cheaper price. However, due to disruption to the global supply chain, silicone prices have risen significantly and now exceed those of ester fluids.

In the 1970s, polychlorinated biphenyls were the cooling liquid of choice for power sector transformers in locations where fire risk was a critical consideration. But then PCBs were classified as carcinogenic and persistent organic pollutants. A safer replacement was needed, quickly, and silicone oil seemed to offer a solution: although not biodegradable, it was non- flammable, the price point was good and it was widely used by operators in the USA, Middle East and Asia-Pacific.

But the popularity of silicone has waned. Better performing and environmentally superior alternatives such as ester fluids have come to market. Now, with silicone oil prices nearly tripling to around $7 per litre, even its financial attractiveness has evaporated.

Silicone characteristics

Silicone is a K class transformer fluid, meaning it has a fire point above 300°C. This means, in the event of a transformer failure due to an arc or lightning strike, there would be no pool fire (as would be the case with mineral oil).

Silicone was used for applications where fire safety was of paramount importance, such as for distribution transformers in built up areas, in oil and gas operations, manufacturing facilities handling flammable substances, trains and wind turbines. In countries with higher ambient temperatures, such as in the Middle East and Asia-Pacific, this is especially important.

Esters and silicone fluids are both K class (non-flammable), making them ideal for applications where fire safety is critical. Esters (both synthetic and natural) offer superior environmental protection. Silicone is also prone to leakage and, once in the environment, takes an extremely long time to degrade. Esters are non-toxic and – crucially – fully biodegradable.

Eclipsed by esters

Esters also have other advantages over silicone. For example, they also exhibit a high water-saturation limit. Water ingress can be a real problem, especially in humid environments. Whether used in breathing or sealed transformer designs, esters can tolerate a lot more moisture before their dielectric properties are compromised. Silicone, like mineral oil, is less tolerant of moisture. This shouldn’t matter, as silicone is mostly used in sealed transformers for this very reason. However, this presupposes regular and comprehensive maintenance, which isn’t always the case in reality.

Esters also win from a cooling perspective, especially in regions such as the Middle East, where ambient temperatures of 60°C and above are relatively common. At these temperatures, silicone has a higher viscosity than ester fluids, hampering the power rating of the transformer. This means an operator can get the same power rating for a smaller transformer, in a smaller space.

In fact, that’s broadly the case for esters, which exhibit better dielectric performance. This means that while silicone is restricted to use in transformers up to around 66 kV, esters have no such limit. It also allows higher power-rated transformers of the same size. For applications such as wind farms where turbines are getting bigger and more powerful, this offers significant performance advantages. Esters are also better to work with. Silicone is well known for how easily it can contaminate its surroundings, how leak-prone it is and how it collects dust and detritus. Transformer manufacturers themselves, or commercial and industrial users with on-site transformers, try to avoid silicone where possible – indeed in some environments, silicone is banned owing to the risk of contamination.

If the price is right

The fact that silicone transformer fluids have persisted for so long speaks to their price advantage. Typically it has been available for between $2.5 and $2.8 per litre.

But a combination of rising demand for silicone for other uses and reduced supply has led to a supply imbalance and, therefore, increased prices. When silicone is a core ingredient in items such as computer chips, transformer fluid manufacturers are unlikely to be near the front of the queue for the limited supplies available.

Silicone oil prices have now reached $7 per litre and they’re unlikely to fall back. Though it’s not possible to retrofill silicone transformers with ester fluids, there’s a powerful case for users who might usually specify silicone for new transformers to opt for esters instead. Utilities, oil and gas companies, rail operators – there are many sectors where fire-safe transformers are mission critical and that is exacerbated in the hot climates of the Middle East and Asia-Pacific. As environmental concerns feature more prominently, esters also stand out for their eco-friendly credentials. Looking forward, could it be the end of an era for silicone transformer fluids? 

Barry Menzies is global managing director of MIDEL