Fluid-Immersed Distribution Transformers

Distribution transformers can be both the last step in the conversion chain that bring electrical energy to the consumer as well as the first transformation step to feed decentrally-generated power into the power grid. Distribution transformers by Siemens Energy are designed for superior reliability, efficiency and environmental performance. A modular architecture paired with design options ensure cost efficiency and easy minimal maintenance.

Built with highest quality - for optimal efficiency

The transformer’s core-and-coil assembly consists of the core, windings, and connecting cables. High-quality drying and quick filling with insulation oil play a key role in extending the transformer’s service life. Siemens Energy employs the latest techniques. For example, in vacuum low-frequency plants (LFH – low-frequency heating), the process of drying the solid insulation is combined with vacuum drying of the windings through low-frequency heating. The active part is heated by applying low-frequency current (< 1 Hz) to the high-voltage windings with the low-voltage windings short-circuited. While still in the plant, transformers are filled with preheated insulation liquid which is absorbed by the insulation material, thus preventing the intake of oxygen.

The iron core forms the central element of every transformer. High-grade, cold-rolled – and, depending on requirements laser-treated – metal sheets are precision-cut using computer-controlled machines to ensure compliance with even the smallest tolerances. The individual sheets are then assembled into cores using the step-lap technique or, in the case of transformers with a lower power rating, using precise winding technology. In this way, Siemens Energy's fluid-immersed distribution transformers achieve an especially good flux distribution at the joints, resulting in exceptionally low losses and minimal no-load noise.

Transformer windings are subject to extremely high electrical and mechanical stresses and must therefore be protected. The transformer windings of Siemens Energy distribution transformers are made of copper or aluminum. Low-voltage windings are made from strip or flat wire, and high-voltage windings are manufactured from round or profile wire. The use of insulating paper partially coated with epoxy resin (diamond-dotted paper) bonds the winding into a compact block during the drying process and additionally increases the short-circuit capacity.

The layer insulation is adapted to the alternating voltage strain that occurs in daily operation. Large-scale oil channels are provided to ensure sufficient cooling of the windings and to avoid hot spots. All leads are short-circuit-proof and surge-proof, contributing to the high reliability of the Siemens Energy transformer – and to an above-average service life.

Whether during transport or operation, the tank must remain sealed tight under mechanical stress and regardless of wind or weather. Corrosion protection is especially important. The surface is sandblasted and then multicoated. Upon request, hot-dip galvanization can provide even better protection.

The distribution transformer’s tank must dissipate the entire heat loss. In hermetically sealed transformers, the corrugated walls absorb the changes in the insulation liquid’s volume. Alternatively, the transformer can be designed as an open system or fitted with an oil conservator that compensates for the additional volume.

Some regions also use smooth-walled tanks and built-in radiators. Hermetically sealed units sometimes have an air or gas cushion to compensate for oil expansion. The sheet-steel lid is bolted – or, upon request, welded – to the tank. The steel thickness and the bracing are dimensioned to ensure that the lid withstands all necessary stress.

Transformers of Siemens Energy are dispatched to the customer only after successfully passing the final inspection and testing. Each Siemens Energy transformer factory has a high-voltage test bay, where specialists conduct a wide range of tests, from voltage tests and temperature-rise tests, to special tests for insulation resistance, partial discharge measurement, harmonics, and noise level. Short-circuit, vibration and othe special tests are performed in internationally recognized and approved institutes. Siemens Energy tests in accordance with the applicable standards and provides our customers with additional certificates on request.

Areas of application

Distribution transformers by Siemens Energy cover a wide range of applications.

High-efficiency transformers for daily savings

Siemens Energy offers two types of fluid-immersed distribution transformers – FITformers® and voltage regulators. They are used to convert electrical energy of higher voltage; usually up to 36 kV, to a lower voltage, usually 250 or 450 V, with an identical frequency before and after the transformation.

Standard and large fluid-immersed transformers

Standard fluid-immersed transformers are crucial elements of suburban areas, forming the link between the electrical grid and consumers. Ratings for medium-sized fluid-immersed transformers stand between 2,500 kVA and 6,300kVA. They offer a maximum voltage capacity of 36kV.

Large distribution transformers come with higher voltage capacity and power level than medium-sized fluid-immersed transformers.  The technology differs to ensure the transformer will function at maximum capacity. The voltage of a large distribution transformer can go up to 145kV and its power rating from 6.3 up to 30MVA.

Pole- and pad-mounted distribution transformers

Pole-mounted transformers are available as single-phase or 3-phase units. Siemens Energy also offers pad-mounted distribution transformers with the same ratings as pole-mounted units. Pole-mounted distribution transformers are specifically built for applications in North America because the electric grid there differs from those in other parts of the world, such as Europe or Eastern Asia. Their voltage level and power capacity are similar to those of standard medium-sized  fluid-immersed transformer, with a voltage level of up to 36kV and and a rating of up to 2,500kVA.

Resisting every storm

Wind turbine operators need reliable and safe transformers so that the turbine never fails. With their expertise in transformers since 1892, and about 10,000 units built for wind farms in the last 20 years, Siemens Energy has been at the frontline of wind application transformers technology. An extensive research and development budget and senior engineering experts have contributed to making Siemens Energy the leading supplier for this application.

Transformers for wind applications convert MV to LV with identical frequency.
Ratings: ≤ 20MVA and ≤ 145 kV


  • High-efficiency – low losses
  • Environmentally friendly
  • Largely maintenance free
  • Low weight, high efficiency and reliability
  • Global availability
  • Customized design

On-shore and off-shore design

In wind applications, the characteristics of the transformer depend on the nature of the project. Wind farms on-shore and off-shore will require different power levels.

On-shore: 4.0 - 10MVA
Off shore: 10.0 - 20.0MVA for all types, for installation in nacelles and towers, with forced cooling oil/air, high temperature materials; 6.0 - 10.0 MVA with forced cooling oil/water, massive tank for highest mechanical loads, and for installation in the nacelle. 

Tried-and-tested quality

The Fluid-immersed distribution transformers REN has been subjected to numerous tests according to international standards to ensure maximum capacity and longevity. This includes extensive internal and external programs to test its suitability,
Such as successful testing of short-circuit and vibration stability, highest corrosion, heat run tests as well as salt spray tests for components for offshore application.

1 transformer, 3 possible locations

For wind application, Fluid-immersed distribution transformers REN can be installed in three areas within the wind farm: in the transformer substation located beside the wind turbines, inside the wind turbine tower, or they can even be directly installed inside the nacelle of the turbine. Ratings for this particular application can go up to 9,8MVA and come with a rated voltage of up to 66 kV.

Siemens Energy builds resilient and customizable distribution transformers which not only deliver reliable and superior performance in harshest conditions, but also perfectly fit the needs of customers in industry. Siemens Energy's Fluid-immersed distribution transformers IND are designed to supply energy for heavy-duty drives for steel mills, oil rigs, offshore installations, and conveyance facilities, and for the complete range of uses for drives in industry.

Static converter facilities with appropriate valves are required for transforming and controlling electric energy for these applications. Static converter transformers provide the link to the supply network. They must endure heavier dynamic and thermal stress due to rapid changes in the load and allow for frequent start-uploads. Siemens’ special winding design achieves a high short-circuit withstand capability making the transformer insusceptible to the stress caused by frequent drive peak loads. Another positive factor is the great heat capacity and high overload capability of this type of transformer.

Siemens Energy has developed a laboratory-tested painting system and corrosion protection (e.g. salt spray tests) for Siemens Energy Fluid-immersed distribution transformers IND used for application or in chemical environment.   

Installed next to the solar panels in a transformer substation, Fluid-immersed distribution transformers REN for solar application come with a rating of up to 5 MVA and a rated voltage of up to 36kV. Thanks to Siemens Transformers’ touch-proof design, Fluid-immersed distribution transformers REN can be installed both outside and inside without extra housings.

Solar farms face harsh weather conditions and still need operate at all times. Its design options make Fluid-immersed distribution transformers REN a highly reliable solution – resistant to high corrosion, harsh weather conditions and temperature variations.

Fluid-immersed distribution transformers REN converts MV to LV with identical frequency
Ratings: ≤ 15 MVA /  ≤ 72,5 kV

Make use of our services

For your convenience, we have gathered relevant downloads in one spot. If your question is still unanswered, please feel free to contact our Energy Support Team.    

Brochure downloads

Alternative insulation fluids
PDF (1 MB)
High-voltage, low losses, small size
PDF (476 KB)

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Siemens Energy complies with the laws and regulations affecting distribution transformers

Siemens Energy is closely monitoring the development of transformer regulations worldwide e.g. European Eco Design Directive and US DOE rules. Siemens Energy adheres to the rules and regulations communicated e.g. in US and Europe and complies with all relevant requirements for every transformer product delivered. As a trustful and responsible partner Siemens Energy understands the importance of these regulations and will continue to monitor potential future requirements. We will continue to adapt and comply with all relevant regulatory changes.