Transformers

Silicones vs. Esters: Which are the Best Transformer Fluids?

Esters could unseat silicone as the global choice for the best transformer fluid based on performance and price.

Glancing back to the 1970s, polychlorinated biphenyls (PCBs) were the cooling liquids of choice for transformers in locations where the possibility of fire was high. Then it happened: PCBs were classified as carcinogenic, as well as persistent organic pollutants. A safer replacement liquid was needed, and quickly. 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 U.S., Middle East, and Asia-Pacific.

But times have changed, and 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 quart, even its financial attractiveness has evaporated. Could it be the end of an era for silicon transformer fluids?

Silicone is a K class transformer fluid, meaning it has a fire point above 572oF (300oC). This means that 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 is an overriding concern. Distribution transformers in built-up areas, oil and gas operations, manufacturing facilities handling flammable substances, trains, and wind turbines…these are the type of places where fire safety is a crucial factor. In regions with higher ambient temperatures, such as the Middle East and Asia-Pacific, this is especially important.

However, silicone has now been superseded by more technically advanced options such as ester-based transformer fluids.

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

So, when it comes to selection criteria, esters match or exceed silicone. They also have other advantages. For example, esters boast a high water-saturation limit. Water ingress can be a serious problem for many transformers, especially in humid environments such as the Asia-Pacific region. Whether used in breathing or sealed transformer designs, esters 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 the real world.

Esters also come out ahead in terms of cooling, especially in regions such as the Middle East, where ambient temperatures of 140oF (60oC) and above are relatively common. At these temperatures, silicone has a higher viscosity than ester fluids, lowering the transformer’s true power rating. This means operators can get the same power rating for a smaller transformer, yielding significant space savings.

In fact, that’s more broadly the case for esters, which boast higher dielectric performance. That means, while silicone is restricted to use in transformers up to around 66 kV, esters have no such limit. It also works in higher power-rated transformers of the same size. For applications such as wind farms where turbines are getting bigger and more powerful as time goes on, this offers significant performance advantages.

More anecdotally, esters are also just nicer to work with. Anyone who works in a manufacturing facility where silicone is involved will tell you how easily it can contaminate things, how prone to leaking it is, and how it collects dust and detritus. Transformer manufacturers themselves try to avoid silicone where possible, along with commercial and industrial users with on-site transformers. In fact, at some places such as an automotive paint plant, silicone is actually banned due to the risks of contamination.

The fact that silicone transformer fluids have last so long speaks to a major advantage they have always enjoyed: price. Though prices fluctuate, silicone has typically been available for between $2.50 to $2.80 per quart.

Not any longer: 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 display screens and computer chips, transformer fluid manufacturers are unlikely to at the front of the queue for the limited supplies available.

Silicone oil prices have now reached about $7 per quart and they’re unlikely to go back down to where they were. Though it’s not possible to use esters in transformers built for silicone, there’s a powerful case to made 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 only exacerbated in the hot climates of the Middle East and Asia-Pacific. As environmental concerns feature more prominently on shareholder and board agendas, esters also stand out for their eco-friendly credentials. Glancing forward, perhaps it’s the end of an era for silicone transformer fluids.

Barry Menzies, is the managing director global for MIDEL.

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