Transformer fluid is vital to providing electrical insulation and stopping the application from overheating. Typically, mineral oil has been used as the cooling agent but Barry Menzies makes the case for a switch to synthetic organic and natural ester fluids
There has been increased innovation in transformer design
The reliability of power transmission and distribution is key to everyday life and is taken for granted in many developed regions.
A consistent supply of electricity depends on the unified operation of all the applications involved. However, unplanned outages or power cuts bring a hard-hitting realization of how fragile systems can be. With the growing demand for power across the world, utilities are continually looking to specify equipment with robust and reliable performance that complies with safety and legislative requirements and protects against the risk of power supply disruption.
Design engineers anticipate transformers will operate under ‘ideal conditions’ for 30 to 40 years, and around 20 to 25 years for industrial transformers. This means that the applications installed during industrial growth between the 1950s and 1980s are now in the final phase of their lifecycle. Replacing or upgrading equipment in the event of failure can result in a number of issues; lead times required from order to installation can take years to complete in more complex power transformer designs.
Along with rising costs in utility applications, these factors put an even greater emphasis on utilities, planning authorities and key stakeholders, such as insurance providers responsible for covering risk management, to focus on the safety and continued operation of the asset.
Transformer applications can fail for a number of reasons, including a combination of electrical, mechanical or thermal issues, which can result in issues within the power network. According to insurer FM Global, mechanical and electrical failure of transformers are the main cause of unplanned power outages in the UK and Europe. The most common factors in transformer failure tend to include:
• Lightning or power surges;
• Inadequate maintenance;
• Loose electrical and mechanical connections;
• Deterioration of insulation;
• Moisture; and
• Criminal damage or sabotage.
As a main component of a transformer, the transformer fluid is vital to providing electrical insulation and stopping the application from overheating. Typically, hydrocarbon-based mineral oil has been used as the cooling agent due to its dielectric properties.
However, mineral oil is a flammable substance; when a transformer suffers catastrophic failure, mineral oil, due to its low fire point and high calorific value, will readily burn if ignited, leading to potential damage to surrounding equipment and the immediate environment – not to mention placing lives at risk. Despite the conservative nature of the power transmission and distribution industry, there is growing pressure to adopt new technologies that can enhance transformer safety and operating performance.
Over the past decade, there has been increased innovation in transformer design to put greater emphasis on how the transformer will perform at a higher load. Driving the need for improved operational applications are asset owners and utilities looking to optimize the performance of their fleet. This is reflected by the emerging practice of increasing the load through an existing transformer, or by extending its intended operational life to minimize the impact of significant capital expenditure.
In order to meet demand for safe and environmentally friendly transformers, synthetic organic and natural ester fluids have become increasingly popular in the power transmission and distribution market. Based on natural compounds, synthetic organic esters are ideal for applications located in demanding or critical areas such as offshore and underground. In addition, natural esters – derived from organic sources such as rapeseed and soybean oil – are developed for use in sealed transformers.
Ester fluids can have a direct impact on the total cost of operation of a transformer unit. Although the initial cost of the oil is marginally higher, the removal of ancillary equipment such as fire suppression systems, or reductions in containment and associated construction costs, can generate significant savings. The removal of this equipment needed with mineral oil can also shorten the installation time and bring the transformer online faster – all of which very quickly offset the extra capital expense.
In addition, insulation deterioration is one of the main leading causes of transformer failures, which is linked to moisture ingress, since water accelerates the aging of cellulose paper. There is evidence to suggest that insulating paper will have a 20-25 per cent longer lifespan if immersed in an ester, when compared to mineral oil. This extra lifetime can help to minimize maintenance requirements and reduce the overall cost of ownership if considered over the asset’s whole lifecycle.
The unique properties of esters mean utility providers are able to mitigate the risk of unplanned power outages across their networks by offering clear benefits in terms of safety, reduced capital expenditure and asset life extension.
Fire safe properties
Transformer fires are a constant threat to power transmission and distribution networks and can result in catastrophic damages to the surrounding environment. Ester fluids are a fire safe alternative to mineral oil and help to reduce or eliminate the need for fire safety equipment. These fire safe properties also make them ideal for use in transformers in high-risk locations, such as inside a building or underground. Compared to mineral oil’s fire point of 170°C, esters have a significantly higher point, with synthetic organic esters recording at 316°C and natural esters in excess of 350°C.
Figure 2. Impact of transformer operating temperature on insulation paper lifetime
Figure 3. Breakdown voltage vs moisture content at 20°C (IEC 60156 2.5 mm)
The use of ester fluids can also significantly reduce the civil construction costs. Esters’ fire safe properties reduce the need for fire suppression systems, which is supported by FM Global’s guidance document for transformer installations. For example, with transformers specifying a volume range of fluid between 19,000 and 38,000 litres, transformers specifying MIDEL fluid could be installed with no fire walls and only 1.5 metres from a building with non-combustible construction. With mineral oil and the addition of fire walls, the spacing would be 7.6 metres and the fire walls would need to be extended to 15.2 metres on either side of the bounded area.
Extended insulation paper lifetime
Ester transformer fluids have a very high moisture tolerance and can absorb far greater amounts of water than mineral oil and silicone liquid, without compromising dielectric strength. Studies have shown that the interaction between water and esters can lead to a slowing degradation rate of the cellulose paper.
According to guidance contained in IEC 60076-14, Annex C, the longer predicted lifetime of the paper, when immersed in esters, can be used in one of two ways: either the operational lifetime of the transformer can be extended, or the transformer can be run at a higher temperature to increase the available power output from given footprint.
Other advantages of using ester-based liquids are reduced risk of bubble formation during overloads, due to drier paper, and the elimination of free water condensation from the fluid during cooldown.
Even a small amount of water in mineral oil causes a rapid deterioration in breakdown voltage. In contrast, esters and, in particular, synthetic organic esters maintain high breakdown voltage in excess of 75 kV even when moisture levels exceed 600 ppm. For synthetic organic esters at 60°C, water content in fluid of 200 ppm would equate to water content in the cellulose of 1.1 per cent. At the same temperature, mineral oil with a water content of 20 ppm would lead to water content in the cellulose of 2.6 per cent. The extra 1.5 per cent of moisture would equate to at least a ten-fold decrease in the life of the cellulose.
Industry pressures are driving the need for more reliable, fire safe and environmentally friendly transformers. In order to achieve this, transformer fluid specifications need to change from mineral oil to esters during the design phase.
Experienced manufacturers not only provide high-quality and proven products, but also the technical expertise and application understanding to support the need to think beyond conventional solutions. Increasingly, risk mitigation is the guiding imperative behind the design and installation of transformers.
Barry Menzies is Commercial Director at MIDEL, a leading manufacturer and supplier of ester based transformer fluids. MIDEL is a product of M&I Materials Limited.