High performance even at low temperatures

Siemens is to supply yet another a 300/400 kV natural ester-filled transformer unit this time even with cold start capability to transmission system operator TransnetBW. This transformer features technology which will give the customer the flexibility to perform a cold start even at very low temperatures and still leverage all the benefits of ester insulation fluid.

Ester at very low temperatures

This is not the first power transformer for the 400 kV level that Siemens delivers to TransnetBW: Already in 2014, Siemens had delivered a 420 kV transformer with natural ester insulation for the substation in Bruchsal. The follow-up order now comes up with yet another innovation, as it provides enhanced cold-start capabilities also in case of low temperatures.


There are many advantages of using ester as an insulation fluid in transformers. For example, its high fire and flash point improves the fire safety of the transformer. Ester fluid is also completely biodegradable, making it extremely well suited to applications in environmentally sensitive areas.


However, when exposed to very low temperatures (e.g. -25°C and below) for an extended period, ester can start to become viscous and develop voids. In real terms this means that the ester oil within the transformer becomes slushy and ceases to circulate properly within the insulation system. Heat given off by the transformer during operation usually keeps the insulating fluid well above ambient temperatures even in extremely cold conditions. However, this characteristic could be problematic in the scenario of a cold start after an outage where the transformer has been exposed to very low temperatures while not in operation.

Innovative cold start solution

In order to provide cold start capability, Siemens has studied the cold start performance of ester-filled transformers under extremely cold conditions at different load conditions. The solution that will be present in the power transformer for TransnetBW is the result of these efforts. The innovation overcomes the tendency of ester to become viscous at very low temperatures and thereby provides the customer with the ability to reliably perform a cold start under these conditions.

More flexibility for the customer

This new transformer unit will provide multiple benefits for the customer. Firstly, the increased fire safety and lower environmental risk that come with using ester as an insulation fluid and secondly more flexibility in how they choose to operate the transformer. For example, if this unit needs to be re-started at extremely cold temperatures, the customer will be safe in the knowledge that a cold start is possible.


„Siemens Transmission Products is leading in terms of innovative and sustainable insulation technologies like clean air and ester. 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,“ said Dr. Beatrix Natter, CEO of Siemens Transmission Products.


By producing ester filled transformers with cold start capabilities Siemens contributing to the United Nations Sustainable Development Goals number 7 and number 9: “Ensure access to affordable, reliable, sustainable and modern energy for all” and “Build resilient infrastructure, promote inclusive and sustainable industrialization and foster innovation”.

Siemens Transmission Products

Transmission products play a vital role in the energy value chain. Siemens Transmission Products offers all key elements including power transformers, gas-insulated and / or air-insulated switchgear and components, individually or bundled, with related engineering. With our global factory network and leading innovations we provide the highest level of quality and reliability to support our customers in achieving their objectives.