The growing threat of the world’s ageing transformer fleets

An underground fire which led to a partial evacuation at Newark Liberty International Airport on 14 January was just one in a series of blazes with a common cause – catastrophic transformer failure. With global energy demand on the rise, the world’s transformer fleets – which play a vital role in ensuring electricity flows efficiently – are starting to groan.

The boom years of industrial growth between the 1950s and 1980s brought with them substantial investment in energy infrastructure. But as the global population has continued to grow, the rate of investment in this critical infrastructure has slowed.

The average cost of a new transformer has risen by 5.5 per cent annually for around 20 years . This has typically resulted in a 100 per cent increase in the price of a transformer every 12-15 years. Deterred by this, owners have been postponing capital investments. Rather than upgrading the transformer fleets, they are relying ever more on existing assets to cope with a job that becomes more demanding with each passing year.

Transformer failure – and transformer fires – can occur at any time, but the threat increases as they move through their life cycle. Under ideal conditions, transformers are expected to operate for 30-40 years, while industrial transformers have a life expectancy of 20-25 years. As the existing fleets installed during the boom years continue to age, and electricity demand continues to rise, it’s clear that without a change in approach to how these vital assets are maintained, the risk of fire is only going to increase.

A serious blaze

Catastrophic transformer failure can lead to catastrophic fires. At their worst, transformer fires have caused loss of life and significant damage to the environment.

For as long as there have been transformers, mineral oils have been the go-to dielectric fluid, serving to provide electrical insulation and cooling. Largely, they have been up to the task, boasting excellent dielectric and thermal properties.

But mineral oils have one main drawback: flammability.

When a fire does occur, it can put a transformer out of action, in addition to causing serious damage to the surrounding equipment. This can be particularly devastating in highly-populated areas, where transformers can feed critical assets such as schools or hospitals. Additionally, this could cause a risk to business continuity, potentially causing major disruption and delay to operations.

Transformer failure

Fire is commonly involved in transformer failure, but the failures themselves are caused by a number of factors. Transformer failures are usually caused by a combination of electrical, mechanical or thermal issues. According to insurer FM Global, mechanical and electrical failure of components are the main cause of unplanned outages in power transmission and distribution in Europe.

The most common factors in transformer failure include lightning/power surges, overloading, inadequate maintenance, loose electrical and mechanical connections, insulation deterioration, criminal damage and moisture ingress.

The condition of the transformer also plays a significant role in failure. For example, an overloaded transformer with a well-maintained insulating fluid is much less likely to fail than the same unit that has had little or no maintenance. Over the past 10-15 years, the second leading cause of failure has been insulation deterioration, which is also linked to moisture ingress, since water accelerates the ageing of cellulose paper.

Fire is a common outcome of these failures – and this is a result of the hydrocarbon-based mineral oil being used as a dielectric and cooling agent.

Failure of a piece of transformer equipment, such as an oil-filled bushing or onload tap changer, can lead to the bulk oil in the ignition tank to ignite. A fire event, caused by this ignition of flammable insulation oil, will cause catastrophic failure of the transformer.

Mitigate the risk

While mineral oil still makes up the vast majority of the transformer fluid market share, there is an alternative that minimises the risk of fire. Ester fluids are a fire-safe and biodegradable alternative to mineral oil, and are increasingly being chosen by power utilities and end users to minimise fire risk.

Compared to mineral oil, the relatively high fire point and low calorific value of esters mean that they will not sustain fire under all transformer fault conditions. No fires have been reported since the use of esters started almost 40 years ago.

The fire point of ester fluids – which represents the lowest temperature at which the vapour of the fuel will burn for at least five seconds after ignition – is greater than 300°C. This compares with a fire point for mineral oils of 170°C.

A regional fit

Different climates bring different challenges for transformers, and extremes of both hot and cold can pose a failure – and fire – risk. In the Middle East, for example, overheating transformers pose a constant threat to utilities.

One such event occurred in July 2017, when a transformer fire at the Saudi Aramco Mobile Refinery at Yanbu Saudi Arabia was caused by hot weather. On this occasion, the fire was quickly contained, but the extreme temperatures in the region mean that the threat of a far more destructive blaze is ever present.

Elsewhere in the Middle East, transformers in Kuwait are designed for an ambient temperature of 58°C – this is the air that is used to cool the transformer fluid down before it is returned to the transformer. Once back inside, the temperature of the liquid can reach 100°C in a large transformer, or even as high as 130°C in a small transformer – only some 40°C below the fire point for mineral oils. This means that a fault or a hot spot in the transformer would only need to cause a modest increase in temperature to ignite.

With a fire point of more than 300°C, ester fluids provide a much more sizeable buffer between ignition and the ambient air temperature – providing vital protection against fire. Even when inside a transformer in a hot climate, the temperature of ester fluids would have to rise by at least 170°C to ignite, a massive 130°C more than mineral oils.

Insuring the risk

Owing to the fire-safety benefits of ester fluids, several transformer manufacturers, insurance companies and end users have concluded that under all possible conditions that could be experienced in a transformer, ester fluids will not burn.

Ester fluids generate around 30 per cent less energy than mineral oil – which, combined with the high fire point of the ester fluids – means the fire will not be sustained.

Even if the transformer ruptures, the fluid still would not burn, and this has been trialled under test conditions with commercial transformers.

The greener option

The benefits of ester fluids extend beyond minimising fire risk. Unlike mineral oils, they are readily biodegradable and non-toxic. This means that if a transformer is damaged and begins to leak, the chemistry of the esters means it poses no harm to the environment.

Extend transformer life

Another vital benefit of esters is that they help to prolong the life of transformers. A high moisture tolerance allows esters to absorb more water than mineral oil. Because esters have a high moisture saturation level, water migrates from the cellulose into the ester thereby drying the paper in the transformer and reducing the rate at which it degrades.

This not only extends the lifetime of the transformer, but allows it to run at a higher temperature, increasing the available power output.

Retrofilling

Installing new transformers with ester fluids rather than mineral oils is one way to prevent fire. However, there is also substantial potential to retrofill existing transformers with ester fluids.

Retrofilling involves removing the mineral oil in the transformer and replacing it with ester fluid. While if a transformer is approaching the end of its life cycle, it won’t undo the damage to the degraded paper within, replacing the mineral oil with ester fluids will delay the replacement of the transformer. Naturally, the sooner the retrofilling is undertaken, the more the life-cycle of the transformer is extended.

So long as the mineral oil is fully emptied from the transformer by hosing down the sump of remnant material, the transformer will be left with around just three per cent mineral oil, substantially reducing the fire risk.

For distribution transformers up to 66kV without an On Load Tap Changer (OLTC), no design changes are necessary, although if fitted with an OLTC or at higher voltages, further assessment of the transformer may be needed.

Change afoot?

While it is difficult to argue against the high-performance, environmental and fire-safety benefits of ester fluids, the fact remains that the dielectric fluid market is still dominated by mineral oils.

Despite posing a potential fire risk, mineral oils continue to benefit from their own ubiquity. It will take a wholesale change in mentality from a traditionally conservative industry to embrace the alternative.

As the world’s electricity demand continues to rise, and existing transformer fleets grow older, the problem of transformer fire risk is not going away. However, an increased uptake of ester fluids by utilities and transformer OEMs could provide a safe and cost-effective solution.

For more information, go to www.midel.com