High-voltage direct-current (HVDC) transformers

Highly efficient power transmission meets huge energy demands 

Expertise and experience at Siemens Energy produce superior transformers in different HVDC schemes like HVDC Classic and HVDC Plus. HVDC transformers are indispensable, where highly efficient bulk power transmission for offshore and onshore grid connections is neccessary.

Siemens Energy’s is supporting highly efficient HVDC power transmission to drive Germany’s energy transition

Siemens Energy is contributing to the Ultranet and SuedOstLink projects, two of the planned electricity highways aimed at advancing the energy transition in Germany. Siemens Energy will supply more than 50 HVDC converter transformers from its transformer factory in Nuremberg – key components to enable transmission of renewable energy from northern to southern Germany.

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Benefits & Technology

From design to delivery

The lead center in Nuremberg ensures an extensive know-how and makes sure that the same technology and quality are available to our customers around the world, while two additional HVDC facilities in China and Brazil can keep distances for transport to a minimum. Where transmission power capability of HVDC systems is steadily increasing, our customers can rely on our experienced design teams to help master transmission and grid challenges around the globe.

HVDC makes a difference - attention to every detail

As a world-class leader synonymous with top quality and reliability, Siemens Energy enables the successful implementation of HVDC transmission systems with dedicated research and development programs.


-       DC voltages: 500 kV, 800 kV, 1100 kV

-       Line ratings: 3 GW, 6,4 GW 8 GW, 10 GW and 12 GW

-       AC voltages: 525 kV, 765 kV,1050 kV


Designing and building a HVDC transformer is a complex process. The insulation to ground and between line and valve winding has to withstand AC and DC stresses. Valve windings need to be tested with voltages determined by the protection level of the DC side. Harmonics in currents cause losses in various parts and DC currents influence the magnetization of the core.


Siemens Energy delivers HVDC converter transformers including all main components to facilitate transportation, installation, and safe operation.

Reliability lowers total cost of ownership

For operators, assessing reliability risks is crucial for evaluating the total cost of ownership of a HVDC transformer. HVDC transformers from Siemens Energy have extremely low mean time between failure (MTBF) rates, compared to our competitors. HVDC transformers of Siemens Energy reach a failure rate well below 0.5 %. According to the international standard defined in IEEE C 57.117 this translates into excellent product reliability. It equals an MTBF rate of more than 200 years of operation. Including failure rate (FR) and MTBF of a HVDC transformer in purchase considerations lowers your risk potential and ensures savings over the entire transformer lifecycle.

Leading competence at the local doorstep

Hundreds of HVDC converter transformers are in successful operation all over the globe. Active in the field since the 1970s, we benefit from a huge experience in designing and manufacturing HVDC units. Rising to challenges, from in-time delivery for large order volumes to building record-setting units that meet highest technical specifications.


A strong factory network around the globe creates leading HVDC converter transformer technology and pre-manufactured components. To meet public local content requirements, Siemens Energy enables and supports local transformer manufacturers to produce HVDC converter transformers based on our technology. However, to ensure high HVDC converter transformer reliability, Siemens Energy implements in-house prototype design and manufacturing.

Areas of application

Bringing stability and security to the grid

HVDC transformers are key components of HVDC stations for long-distance DC transmission lines or DC sea cables, off- and onshore. As coupling elements between AC grids and high-power rectifiers, they adapt the voltage, insulate the rectifier itself from the AC grid and generate the necessary phase shift.

HVDC technologies for long distances and low losses

Siemens Energy offers two HVDC technologies: HVDC Classic and HVDC Plus. HVDC transformers of Siemens Energy meet all requirements for power, voltage, mode of operation, low-noise, connection types, types of cooling, transport, and installation. They also comply with specific national design requirements.

HVDC transformers are subject to operating conditions that set them apart from conventional system or power transformers. These conditions include:

  • Combined voltage stresses with both AC and DC
  • Two or three high or ultra-high voltage windings but no low voltage winding inside of the transformer

  • High harmonics content of the operating current
  • DC pre-magnetization of the core
  • Very narrow tolerances in between units, especially in terms of impedance voltage


Siemens Energy delivers line-commutated converters (LCC) for transmission lines up to 12 GW, up to 1,100 kV DC with connections to AC grids up to 1,050 kV.


The high harmonics content of the operating current results from the virtually quadratic current blocks of the power converter. The odd-numbered harmonics with the ordinal numbers of 5, 7, 11, 13, 17 cause additional losses in the windings and other structural parts. Furthermore, additional DC currents on the valve and/or neutral terminals need to be considered and managed. The high bulk power ratings also require a high rating of HVDC classic converter transformers which must share specific performance parameters, e.g. short circuit impedance.

Siemens Energy delivers voltage-sourced converters (VSC) for transmission lines up to 3000 MW, up to 800 kV DC with connections to AC grids up to 525 kV.


In HVDC Plus mono-pole applications, no DC voltage stresses occurs at the transformer valve terminals. In HVDC Plus, bi-pole applications the valve windings which are connected to the HVDC Plus rectifier/ converter circuit are subject to the combined load stress of DC and AC voltage. The harmonics content of the operating current is much lower than in HVDC Classic applications. It usually meets grid code requirements. Additional DC currents on the valve and/or neutral terminals have to be considered. They are much lower than in HVDC Classic applications.    

News & Events

Learn more about latest references of our HVDC converter transformers

Our HVDC converter transformers are manufactured in 3 different factories for the reliable operation all over the world. Built to comply national and international standards according to customer needs, including excellent customer service - wherever you are.
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