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Home / Technical Articles / Hey Mom, look at that big HVDC Transformer!
Hey Mom, look at that big HVDC Transformer!
800,000 volt transformer for high-voltage direct current transmission. The core of the system are ten transformers for voltage of 800 kV. They are each as large as a house and weigh 350 tons. The photo shows the first ready-to-ship 800 kV transformer during electrical testing at the Siemens factory in Nuremberg, Germany.

Key component

HVDC transformers are key components of HVDC stations.

HVDC converter and inverter stations terminate long-distance DC transmission lines or DC sea cables. This type of transformer provides the interface between AC grids and high power rectifiers and are used to control the load flow over the DC transmission lines.

These actors adapt the AC grid voltage to an adequate level which is suitable for feeding the valve system of DC converter and inverter.

Design options

The design concept of HVDC transformers is mainly influenced by the rated voltage, rated power and transportation requirements like dimensions, weight and mode of transportation. Many large power HVDC converter station are located in rural areas of low infrastructure.

Frequently, special geometrical profiles have to be fulfilled in order to move such transformers by railway.

Converter transformer for UHVDC bipolar transmission system
Figure 1 – Converter transformer for UHVDC bipolar transmission system ± 800 kVDC, 6,400 MW; 2,071 km: single phase; 550 kVAC, 816 kVDC; 321 MVA; high pulse wye system feeding

Typically, HVDC transformers are single phase units containing 2 winding limbs. This concept can include either 2 parallel valve windings (two for delta or two for wye system, figure 1) or two different valve windings (one for delta and one for wye, figure 2).

In order to reduce the total transportation height frequently the core assembly includes 2 return limbs.

Converter transformer for HVDC bipolar transmission system ± 500 kVDC
Figure 2 – Converter transformer for HVDC bipolar transmission system ± 500 kVDC; 2,500 MW: single phase; 420 kVAC; 515 kVDC; 397 MVA; wye system (left side of figure) and delta system (right side of figure)

Due to redundancy requirements in HVDC stations 3 phase units are quite uncommon.

The valve windings are exposed to AC and DC dielectric stress and therefore a special insulation assembly is necessary. Furthermore, special lead systems connecting the turrets and windings have to be installed in order to withstand the DC voltage of rectifier.

Additionally, the load current contains harmonic components of considerable energy resulting in higher losses and increased noise.

Above all, special bushings are necessary for the valve side to access upper and lower winding terminals of each system from outside. Conclusively, two identical bushings are installed for star or delta system.

For approving the proper design and quality of manufacturing special applied DC and DC polarity reversal tests have to be carried out. The test bay has to be equipped with DC test apparatus accordingly and needs to provide adequate geometry to withstand the DC test voltage.

Technical items

In addition to the standard parameters of power transformers, special performance requirements have to be known for the design of HVDC transformers.

These parameters are jointly defined by designers of the HVDC station and transformer design engineers in order to reach a cost-effective design for the entire equipment. Special parameters are:

  • Test levels: DC applied, DC polarity reversal and long-time AC defines the insulation assembly of the transformer
  • Harmonic spectrum of the load current and phase relation generate additional losses, which have to compensated by the cooling circuit
  • Voltage impedance impacting the dimensions of windings and the total height of the transformer
  • DC bias in load and current and transformer-neutral have to be considered for no-load noise and no-load losses
  • Derivative of the load current (di/dt) is a key parameter for the on-load tap changer
  • Overload requirements have to be considered for cooling circuit and capacity of coolers
  • Regulation range and number of steps influence the voltage per turn which is a key design parameter
  • Seismic requirements have to be considered for mechanical strength of turrets, outlets and bushings.

HVDC – Connecting remote power generation to the market

Cant see this video? Click here to watch it on Youtube.

Reference: Siemens Energy Sector – Power Engineering Guide Edition 7.0

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More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Michael
    Apr 02, 2015

    Please sign me up for the newsletter…

  2. Gabriela
    Jun 18, 2014

    HI! I’m an Electrical Engineering Student and I’m at my last year. I’m making a work with HVDC Power System Transmission for my Final Project at my career. I would like to contact you for more information :)

  3. Chuks
    Mar 30, 2014

    Am impressed at the level you go to educate and enkindle our passion for electrical engineering freely.
    I wish I can get the Siemens HVDC presentation video.

  4. Konstantin Filyaev
    Mar 20, 2014

    Yes, Edvard, that transformer really impresses!

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