Final Word: Playing with fire
Barry Menzies, managing director MIDEL Dielectric Fluids Global talks about the growing threat of ageing transformers
The average cost of a new transformer has risen by 5.5% annually for around 20 years. This has typically resulted in a 100% 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 has become more demanding.
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.
Catastrophic transformer failure can lead to 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.
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 5 seconds after ignition, is greater than 300°C compared to a fire point for mineral oils of 170°C.
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 threat.
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 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.
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.
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.