FITformer® – 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 Transformers are designed for superior reliability, efficiency and environmental performance. A modular architecture paired with design options ensure cost efficiency and easy minimal maintenance.

Transformer Magazine Interview, April 2021

Interview with Tilo Else, Siemens Energy's Vice President for Distribution Transformers

The energy market is rapidly evolving with the energy transition as one of the most fundamental changes: What challenges does this rapidly changing energy landscape pose for customers?

Please click on the link to read the complete interview
Technology & Benefits

Built for sturdiness and 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 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 LiDTs 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 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 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.

Siemens transformers are dispatched to the customer only after successfully passing the final inspection and testing. Each Siemens transformer factory has a high-voltage test bay, where Siemens 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 tests are performed in internationally recognized and approved institutes. Siemens tests in accordance with the applicable standards and provides our customers with additional certificates on request.

Areas of application

Areas of application

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

High-efficiency transformers for daily savings

Siemens Transformers 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.

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 5,000 units built for wind farms in the last 10 years, Siemens Transformers 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 Transformers the leading supplier for this application.


Transformers for wind applications convert MV to LV with identical frequency.

Ratings: ≤15MVA and ≤72,5 kV

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

Installed next to the solar panels in a transformer substation, FITformer® 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, FITformer® 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 FITformer® REN a highly reliable solution – resistant to high corrosion, harsh weather conditions and temperature variations.


FITformer® REN converts MV to LV with identical frequency
Ratings: ≤ 15 MVA /  ≤ 72,5 kV

Siemens 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’ FITformers® 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 has developed a laboratory-tested painting system and corrosion protection (e.g. salt spray tests) for Siemens FITformers® IND used for application or in chemical environment.   

Voltage regulator

Voltage regulators reduce energy loss from long cables and extended grids. They are used for integrating renewables into the grid. Voltage regulators are predominantly used in grids which comply with IEEE standards.



  • Cost-saving alternative to grid expansion
  • Easily integrated into existing grid structure
  • Easy installation
  • Extensive regulating range
  • Flexible regulation models for optimum operation

Siemens invented the voltage regulator in 1932. These tapped autotransformers maintain a desired voltage level, which means that their reliability is of primordial matter. Voltage regulators are a highly reliable solution for stabilizing voltage fluctuation in the customer’s grid. They are a notably more cost-efficient for customers than a medium-voltage grid expansion.


Voltage regulators and come in 1-phase or 3-phase models. Different cooling systems are available. They can be applied to any electrical system to improve voltage quality.    

The standard voltage regulator provides approximately 10% adjustment. However, in some designs, regulations of 15% and 20% are also available.

Available at 50 or 60 Hz, their ratings are calculated according to the voltage level of the transformer they are connected to (i.e., 10%). 

Voltage level ratings for 1-phase voltage regulators vary between 2.5kV and 19.9 kV and between 31.8kVA and 889kVA. 3-phase units are available at 13.2kV, 34.5kV, or from 500kVA up to 4,000kVA.

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Compliance statement

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.