International energy projects with comprehensive service and team spirit: When power converter stations are rebuilt, submarine cables laid and transformers refilled with insulating oil, every detail counts – especially in large-scale projects that span national borders. As part of the Celtic Interconnector project, Electrical Oil Services demonstrates how technical precision, teamwork and international experience work together perfectly.
"We are proud to be part of the Celtic Interconnector project and to contribute to the development of a secure and sustainable energy infrastructure in Europe. Our commitment to this project demonstrates that EOS is a reliable partner for complex large-scale projects."
– Kilian Gerbl, Managing Director of Electrical Oil Services
The Celtic Interconnector power link is a major project by Irish power grid operator EirGrid and its French counterpart Réseau de Transport d’Electricité (RTE). The aim is to establish a stable power link between Ireland and France in order to increase energy security, reduce electricity costs and promote the integration of renewable energies.
Siemens Energy was commissioned to build the converter stations in Ireland and France. French cable manufacturer Nexans was commissioned to lay a cable almost 600 kilometres long.
The team from Electrical Oil Services (EOS®) was also involved. As a subcontractor to Christof Industries from Graz, the EOS® team was responsible for filling the four large transformers in Brittany, France.
The Celtic Interconnector is the first direct power cable between Ireland and mainland Europe. A 575-kilometre-long HVDC (high-voltage direct current) cable – around 500 kilometres of which runs through the Celtic Sea – will enable up to 700 megawatts of electricity to be transmitted in both directions in future.
This will be enough to supply around 450,000 households. The connection is scheduled to go into operation in 2026.
The HVDC technology used enables low-loss transmission over long distances and strengthens European energy security – a key component for the integration of renewable energies.
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Infobox: The Celtic Interconnector at a glance The project is part of a European energy initiative that will enable the exchange of electricity between Ireland and France in the future.
Further information on the large-scale project Celtic Interconnector. |
Electrical Oil Services was on assignment in France from August to September 2025. On behalf of its Austrian partner Christof Industries (Graz, Austria), the EOS® team took on the task of refilling four large Siemens transformers with new transformer oil.
These transformers will be used in the converter stations being built at both ends of the connection – Knockraha (Ireland) and La Martyre (France). There, the alternating current from the national grids will be converted into direct current and vice versa – a prerequisite for the operation of the HVDC line.
Each of the four new transformers contains around 75,000 litres of insulating oil – a volume that places the highest demands on planning, materials and quality. ‘All processes have to be precisely coordinated here,’ says Peter Klausen, who is responsible for the customer project at Electrical Oil Services. He adds: ‘A key technical element of the project was that each transformer underwent three complete oil circulations – combined with a required vacuum time of over 48 hours. These requirements show how important good preparation and reliable mobile equipment are.’
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EOS® project: Four large power transformers are filled
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Before the first litre of transformer oil flows, Electrical Oil Services spends several weeks preparing: safety briefings, technical coordination, document checks and material logistics.
‘Planning plays an even greater role than usual in international assignments,’ explains Klausen. ‘We prepare every single system, every document and every work step in detail – so that everything runs smoothly on site.’
This also applies to safety standards: all team members undergo special training that meets the requirements of Siemens and the local authorities.
Read more in this interview:
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At the beginning of August 2025, the first team set off for France. Three technicians were permanently on site for between five and seven days before the next crew took over. For each transformer, three complete oil circulations ran through the mobile EOS® systems – under strict purity requirements and continuous monitoring.
‘Coordination between our teams, the customer and our partner companies is crucial,’ emphasises Klausen. ‘We work hand in hand to ensure that vacuum times, oil quality and temperature are maintained precisely.’
The vacuum time of over 48 hours per transformer was implemented precisely – further proof of the technical reliability of Electrical Oil Services' mobile units.
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After around six weeks, the project was successfully completed. All four transformers were filled on time and in accordance with IEC standards. The purity values, temperatures and documentation met all of Siemens' specifications – a result that is only possible through experience, preparation and teamwork.
‘Assignments like this show that we are equipped for large-scale projects – even internationally,’ says Peter Klausen. ‘From planning and safety on the construction site to the final oil sample on the transformer: everyone in the team knows exactly what is important. Our customers benefit from this.’
Find out more about our comprehensive range of services, which covers all aspects of a new transformer project.
Do you have any questions? We would be happy to advise you.
With its successful involvement in the Celtic Interconnector project, Electrical Oil Services has once again demonstrated that quality, safety and efficiency are firmly anchored in its corporate culture. The EOS® team combines technical expertise with years of practical experience – and thus makes an important contribution to the sustainable energy supply in Europe. Projects such as the Celtic Interconnector help to better integrate renewable energies, reduce energy losses and strengthen security of supply.
Sustainable energy supply – that's what Electrical Oil Services stands for. That's why EOS® also supports the energy transition with its own solutions: with EOS® Premium, the recycled transformer oil in new product quality, the company actively contributes to the resource-saving use of transformer oils and to CO₂ reduction in the energy sector.
In addition, Electrical Oil Services can not only carry out refilling, but also provide support for the complete installation, commissioning and other services related to transformers as required. This means that energy companies receive a comprehensive service package from a single source.
Electrical Oil Services offers you safe, sustainable and standard-compliant insulating oil and transformer services – wherever energy is in motion.
FAQHow is a large power transformer refilled? After thorough preparation, the transformer is evacuated, conditioned and then filled with new transformer oil. All parameters such as temperature, flow rate and purity are continuously monitored and fully documented. What are the technical challenges involved in filling large transformers? Large volumes of transformer oil, strict purity requirements and precise vacuum times place high demands on equipment and personnel. Especially when working abroad, all processes must be precisely coordinated to ensure quality and safety at all times. What role does Electrical Oil Services play in the Celtic Interconnector project? Electrical Oil Services is responsible for the professional refilling of the four large transformers used in the project's converter stations. These transformers are crucial for the reliable operational readiness of the systems, as they ensure the necessary power supply and insulation safety. With precise vacuum technology, multiple oil circulations and specialised mobile units, EOS® ensures that the transformers are prepared and filled in accordance with IEC standards – an important prerequisite for stable and sustainable energy transmission in the Celtic Interconnector. How do EOS® mobile processing units work in international projects? Electrical Oil Services' mobile vehicles enable treatment and filling directly at the site of operation – regardless of location, infrastructure or project complexity. With high flow rates, precise temperature control and controlled vacuum technology, they achieve purity values in accordance with the IEC standard. This allows even large transformers with up to 75,000 litres of insulating oil to be filled efficiently and safely. How does Electrical Oil Services organise teams for assignments abroad lasting several weeks? For longer projects, EOS® relies on a rotating team model. Experienced technicians take turns at fixed intervals and are on site for several days at a time. Structured handover procedures ensure that vacuum times, oil quality and technical parameters are seamlessly maintained. This keeps processes stable and efficient over weeks. |