by Tim Martin, EMEA Sales Director, MIDEL. This content was originally published in Transformers Magazine (April 2021)

European distribution grids will need investments of €375-425 billion until 2030, concludes a study by Eurelectric, Monitor Deloitte and smart grid organisation E.DSO. Based on detailed empirical data from ten European countries, Connecting the Dots: distribution grid investment to power the energy transition suggests investments in distribution grids will need to increase by 50-70 %, from 2020 to 2030, compared with the previous de-cade. That is equivalent to €34-39 billion a year.

The single biggest investment driver is modernisation of ageing infrastructure. The study found that a third of the EU’s grids are already over 40 years old, a figure increasing to fifty percent by 2030. Transformers, designed to step power up or down, are a critical component of any power grid. Upgrading or replacing these assets constitutes a key aspect of any grid modernisation investment programme. Liquid filled distribution transformers are used in medium voltage (MV) and high voltage (HV) applications – historically using mineral oil as the dielectric insulating and heating media fluid. Liquid filled transformers are robust assets, with low load losses and long service lifetimes of around 35 years, making them a popular option for outdoor as well as indoor environments, compared with dry type, or air cooled, transformers, which require more maintenance. However, in our experience, there is a growing awareness of two critical flaws in the use of mineral oil as an insulating fluid – its potential flammability and its poor biodegradation characteristics in case of a major failure.

For any distribution network operator (DNO), or utility, transformer fleet up-grades can potentially be a costly under-taking, especially from a capital expenditure (Capex) perspective. Utilities will want to consider solutions that can safely extend asset life and optimise loading performance. Retrofilling transformers with natural or synthetic ester fluid to replace mineral oil is a perfect example of such a solution. This practice of retrofilling provides several key benefits, including:

• Ester fluids have a higher fire point than mineral oil that virtually elimi-nates the possibility of a transformer fire.
• Being readily biodegradable, unlike mineral oil, ester fluids offer a high level of environmental protection.
• Ester liquids can allow higher operational temperature of a transformer due to their higher flash and fire points as well as different chemical structure which allows a longer lifetime of cellulose materials. This phenomenon can be found in IEC 60076-14 and IEEE C57.154. Based on that it is possible to run trans-former at higher power rate and at the same time also run at higher operational temperature. Synthetic ester fluid is more hygroscopic than mineral oils. It is capable of absorbing more moisture from the solid insulation system used within the transformer (without a reduction in its breakdown voltage). Doing so slows down moisture degradation of the winding paper and extends operational life.
• Retrofilling liquid filled transformers using an ester fluid is a straight-forward process, usually being performed in-situ and with minimal transformer downtime.

Certain forward-looking utilities and transformer OEMs have already been pursuing ways to address the challenges echoed in Connecting the Dots report. A recent project undertaken by Scottish Power Energy Networks (SPEN) identified several capital expenditures (Capex) and operational expenditure (Opex) cost savings derived from the use of ester transformer fluids in its transformer fleet for 132 kV grid, 33 kV primary and 11 kV secondary assets [4]. Considerations in SPEN’s cost-benefit analysis included capital cost, operation and maintenance cost, firewall necessity and fire suppression system necessity. They found that ester-filled transformers offer a significant unit cost saving, particularly for indoor applications where most saving originates from the exclusion of a firewall or fire suppression system. SPEN found that average unit cost savings were found to be around £235,000 per indoor installation. In another example, German transmission system operator TransnetBW tasked OEM Siemens with supplying a power transformer with two critical criteria; namely, the transformer was to be filled with natural ester fluid rather than mineral oil. In addition, the trans-former had to demonstrate robust cold start capability. The use of a rapeseed ester fluid satisfied the local authority’s mandate for the transformer materials to be locally sourced, the rapeseed being grown in central Europe. Importantly, the manufacturer independently acknowledged the superior cold weather performance of rapeseed ester fluid, with its pour point of -31 °C, over soya-based ester fluid that has a pour point of around -18 °C.

Dr Beatrix Natter, CEO of Siemens Energy Transmission commented on this project, saying “The innovative cold start technology in this transformer demonstrates Siemens’ expertise in combining industry leading innovations with eco-friendly solutions for the benefits of our customers and society”. Accommodating more capacity based on distributed sources of clean electricity, while ensuring networks can supply demand for recharging electric vehicles and for heating, inevitably requires in-vestment in new physical infrastructure, including transformers. In densely populated cities and urban areas, where utilities may have to install transformers closer to where people live and work, ester transformer fluids, with their high fire point, clearly strengthen mandated fire safety protocols. In addition, the fluids’ biodegradability means they can be used in transformers installed closer to water-ways and other environmentally sensitive areas.

Increases in renewable energy generation, electrification of transport and electrification of heating and industrial activity are identified in the Connecting the Dots report as other key drivers for investment in European distribution grids to 2030. Ester fluids have a long and proven track record in off-shore  /  onshore wind farms where fire safety and environmental credentials are paramount. Ester fluids’ uptake in the world of solar power – in particular the growth we see in floating solar – is increasing at pace. The correlation is clear: ester filled transformers constitute a wise investment for balance of plant (BoP) electrical equipment at so-lar photovoltaic (PV) power plants and wind farms. BoP equipment is the critical link for delivering electricity generated by these renewable energy plants to the grid. Increasingly wind and solar plants are being built without subsidy, earning their income through electricity sales. Meanwhile, owners of these assets are also focused on reducing levelised cost of energy (LCOE) through reducing BoP equipment Capex and Opex. Ester fluid transformers can operate at higher temperatures, maximising the amount of power distributed and ensuring uptime of BoP equipment and helping to maximise the operator’s return on investment. The Connecting the Dots report sets out clearly, and in granular detail, the level of investment and work necessary in the near future for European distribution grid operators.

There are complex challenges ahead, to be sure. However, by embracing proven technology such as ester transformer fluids in parallel with deploying smart grid processes and innovations, forward-looking operators and OEMs will survive, thrive and ultimately materially contribute to a pan-European landscape of greener, safer, more reliable and better performing power networks.