Photo credit: Creative Commons

In 2002, Vattenfall AB identified a need for a new transformer for its Letsi Hydroelectric Power Plant, the first to be developed in the river Lilla Luleälv, Sweden, in 1967. Letsi was built as a cavern plant, with turbines and transformers erected underground, so fire safety was critical. In addition, the 400kV transformer was located in an environmentally sensitive area where mineral oil spillage could cause large amounts of damage. As a solution, Siemens Transformers presented M&I Materials Ltd.’s MIDEL 7131, a fire-safe, synthetic ester-based transformer fluid, which has served the global market since the 1970s.

M&I Materials was involved in the early stages of the tendering process during 2003, supporting Siemens by providing technical information related to MIDEL 7131. Once Vattenfall officials were satisfied that the M&I Materials option was viable for the project’s voltage level, the contract for the transformer was awarded to Siemens AG. It set about building a world-first, large generator step-up (GSU) transformer, filled it with MIDEL 7131, and tested at its factory in April 2004.

Today’s Energy Solutions sat down with Alonso Castillo, president, M&I Materials; and Dr. Stefan Pieper, general manager of the Siemens transformer plant in Weiz, Austria, to get a deeper look at the project.

Today’s Energy Solutions: What were some of the important safety steps M&I Materials took for the Vattenfall project?

Alonso Castillo: MIDEL 7131 – a synthetic ester-based transformer fluid – was selected for this project because it offers distinct advantages over mineral oil in two critical areas; fire safety and environmental security. With a fire point of >300°C, MIDEL 7131 has safety already built-in to its chemical structure. So, in terms of the installation at the Letsi Hydroelectric Power Plant, safety was inherently considered in the choice of transformer fluid.

TES: What were some additional factors for choosing MIDEL 7131 for this project?

AC: There were multiple factors to consider when the time came to choose a transformer fluid for the Letsi units. Of critical importance was Vattenfall’s commitment to corporate social responsibility; it was keen to respect the environment and the Sami ethnic group (indigenous inhabitants of the northern parts of Sweden). Partly for this reason the plant was built in a cavern, with the turbines and transformers deployed underground. As a cooling fluid, MIDEL complemented this perspective. It is readily biodegradable and has a high level of oxidation stability. It also delivers improved cold temperature performance. In addition, it is strongly hygroscopic in comparison to mineral oil. This means it can absorb more moisture while retaining its insulation properties. It is these characteristics that enabled the transformer manufacturer, Siemens Transformers, to design and manufacture units expected to have a longer lifetime.

TES: What were some of the challenges with shipping the transformer to the site?

Dr. Stefan Pieper: Transportation and delivery needed to be well planned, as the new Siemens transformers had to replace the old transformers during ongoing operation. There were several stages to the transportation of the new units and coordination proved complex. The units were shipped from a heavy lift port in Austria to a North Sea port. This was followed by transhipment to a truck and further transport on the road before being shipped to Sweden. The units were then driven, once more, by truck to the location in Letsi. 

TES: What were the other challenges in this project?
Transformer ready for shipment at a heavy lift port in Austria

SP: Removing the already-installed transformer required special care, as space in the cavern was limited. There was also the additional challenge of extreme minus degrees temperatures outside the cavern. For safety reasons, additional steel plates and tarpaulins were laid in the cavern and the unit was loaded onto the truck by a skidding system. An additional towing vehicle was needed for removal, as the cavern had a 14% incline. Installation required similar care, using a skidding system and hydraulic jacks to set the transformer onto the foundation.

TES: What opportunities came from this project?

AC: MIDEL was initially recognised as a viable alternative to mineral oil in niche applications where fire safety or environmental impact presented substantial risks. Over time, its use has grown and MIDEL can now be seen in standard specifications. Its development has grown from small distribution units, indoor and underground sites, through wind, traction, and transmission applications to the latest 400kV-range transformers. The locations vary from high altitude to deep sea and extremes of both cold and heat. MIDEL enables transformer OEMs and service companies across the globe to increase their value proposition and competitive advantage.

TES: What other companies made this project possible?
Ester-filled transformer in the test bay of the transformer factory in Weiz, Austria

SP: Rolf Riedl GmbH was awarded the contract by Siemens AG Austria to manage the transportation and exchange of the four transformer units in the hydro power station.

TES: What was the outcome of this project?

AC: The existing transformer units had been in service for 48 years. Siemens replaced them with four GSUs with ratings of 121.33MVA/60.6MVA/60.6MVA and voltage levels of 433v3kV/16.8kV/16.8kV. However, the completion of the project meant more than just new transformers – there were other outcomes, too. The lower risk of fire translated to lower insurance costs. There were also savings in the building works – for example the elimination of explosion protection walls, the partial elimination of oil sumps, and shorter busbar length. These tangible financial savings were made possible by Siemen’s specially designed transformers and its use of MIDEL 7131.

TES: Does M&I Materials have similar projects in the works?
The transformer is transported to Sweden by ship.

AC: We are working with Siemens and National Grid in an installation in Highbury, a densely populated area in London. The project comprises three grid transformers, each rated at 240MVA and 400kV/132kV, complete with a cooling system with heat recovery.

Vattenfall AB

Siemens AG

IMTS Booth #E-4502

M&I Materials


About the author: Arielle Campanalie is the associate editor for TES and can be reached at or 216.393.0240.