AC Power Transformers (with Tap Changer)

AC Extra High Voltage (EHV) power transformers are an essential component of the power system enabling change in AC voltage and thus allowing operators to interconnect AC networks of different voltage levels to each other. Power transformers must be built to withstand severe electrical stress from fault currents and transients. Their availability and longevity have a major impact on grid reliability and profitability.

Key functions of Power transformers with tap changers are:

  1. Voltage step-up and -down: As increasing voltage will reduce the currents required to distribute the same electrical power, step-up transformers are used to minimise transmission line losses. Step-down power transformers are used to bring down transmission voltages to usable voltage level for end-customer connections.
  2. Slow dynamic regulation to adjust to changing network conditions supporting the voltage stability of the AC-grid.

Integrating a tap changer with the transformer allows for the regulation of the output voltage by adjusting the number of transformer windings (the transformation ratio). Although the effects on the network depend on the network itself, this nonetheless enables more flexibility to the operator compared to a fixed voltage step up or down ratio.

Technology Types

The main technologies of power transformers highlighting some specificities are reminded below:

  • Transformers per cooling types:

    • Oil Direct Air Forced (ODAF)
    • Oil Direct Air Natural (ODAN)
    • Oil Direct Water forced (ODWF)
  • Power transformers: This transformer is used to transfer the energy to the substation or the public electricity supply.

  • Generator transformer: This transformer, located on a power station, is used to connect the generator output to the grid.

Components & enablers

The exact components depend on the specific transformer. Typical components of an AC power transformers are:

  • Laminated core
  • Windings
  • Insulating materials
  • Transformer oil
  • Tap changer
  • Bushings
  • Oil conservator
  • Cooling units

Advantages & field of application

The transformers constitute an integrated cornerstone in the power systems. The acceptance of these devices from a public acceptance and environmental impact is assessed by the audible footprint and the loss levels. The actual physical footprint is essential since the ‘not in my backyard’ discussions are expected to increase in intensity and frequency.

Technology Readiness Level

It is a mature technology – TRL 9 – System ready for full scale deployment (for standard components). However, a lower TRL prevails for some components’ development, which could lead to a change in the TRL once these components are ready to be commercialised (e.g. development aiming at better performance of the transformers in terms of losses and environmental impacts: cooling system, mechanical design, core and magnetics, insulation system, acoustic / noise, use of ester oil).

Research & Development

The coming generations of transformers will be characterised by a shift in environmental focus. The responsibility of reaching a lower impact on the environment will be shared between producers and users. This includes all steps from production to operation over the lifetime of the equipment and decommissioning / recycling. This shift will impact the following areas:

  • Design and manufacturing: the design of the transformers is key to optimise their impact in terms of weights and losses results, more specifically the use of materials (steel, copper) will be carefully considered at that stage. The type of oil will be also part of the discussion, ester oil having a lower risk impact for the surrounding areas. Manufacturers will be required to produce equipment with a minimum impact on the environment during their production.

    A key design feature for the installation of large power transformers (LPTs) is the transportability constraints due to their dimensions and heavy weight that could require specialised railroad freight infrastructure with high costs.

  • Operation: Life-time assessment will become more important.

  • Importance of losses: optimise losses for the operation.
  • An increased utilisation, which improves return on investment for the users (higher focus on the actual hot spot). Incorrect or sub-optimised design can lead to a temperature rise by 6 – 8 degrees on the actual hot spot, which significantly decreases the lifetime of the equipment.

Best practice performance

Large Power transformers (LPT) refer to units with a power rating higher than 200 MVA or with voltage ratings higher than 275 kV for the three-phase units [4].

The United States Department of Energy defines the estimated magnitude of LPTs [ 23 ]. Typical characteristics for one-phase transmission transformers: 765 – 345 kV transformers with 500 MVA capability rating and a weight of 235 tons. The three-phase transmission transformers encompass several classes:

  • 230-115 kV transformers with 300 MVA capability rating and a weight of 170 tons
  • 345-138 kV transformers with 500 MVA capability rating and a weight of 335 tons
  • 765-138 kV transformers with 750 MVA capability rating and a weight of 410 tons.

On high and extra high voltage, commercial catalogue of manufacturers detail LTP equipment with similar performances: three-phase units up to 1,100 MVA and single-phase units up to 500 MVA, or extra high voltage transformers of 1,000 MVA, 500 / 275 / 63 kV 3 phase [25]; transformers well above 1,300 MVA [16], up to 1,200 MVA, and voltages up to 765 kV [24].

Best practice application



420 kV power transformer (rated power of 400 MVA) for a substation for TransnetBW (TSO state of Baden-Württemberg) to link the 380 kV voltage level with the 110 kV grid.

power transformer with ester oil as insulation. All permissible (over)temperatures have been rated according to IEC 60076-2.

420 kV extra-high voltage level using natural ester. Due to the lower flammability, the transformer also has a higher fire protection class (K instead of O), so that the equipment can be used in densely populated areas.



Siberian Generating Company has commissioned a new 125 MVA transformer at Tom-Usinsk Power Plant as part of the project to improve the reliability of the largest power plant in Kuzbass.

Siemens, 125-MVA autotransformer.

The extreme temperatures in Siberia place special demands on transformers. The autotransformer fulfils these requirements, and losses are below the value stipulated in the Russian Governmental standard.


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[2] eHighway2050. Annex to D3.1 - Technology Assessment Report Transmission technologies: Transformers, Phase shifting transformers, transformers with tap changers and AC breakers [Link]

[3] Siemens. Eco-friendly transformers for Moscow metropolitan area. [Link]

[4] ABB Transformers. Power Transformers / The largest installed base worldwide. [Link]

[5] Osbert Joel C. Parts of a Power Transformer. [Link]

[6] CGGC Gezhouba Group. Jingmen 1000 kV Substation (China). [Link]

[7] HSM offshore. Borkum Riffgrund 2 offshore high voltage substation – 450 MW. [Link]

[8] Siemens. Siemens in Russia. [Link]

[9] Siemens. Benefit of Siemens Ester Filled Transformers. [Link]

[10] ABB. Tap-changer for power transformers. Product Guide. [Link]

[11] Circuit Digest, Different Types of Transformers and Their Applications. [Link]

[12] ScienceDirect, Generator Transformer. [Link]

[13] Circuit globe, Types of Transformer. [Link] Cooling of Transformer and Methods of Cooling. [Link]

[14] Elec-13, Refroidissement des transformateurs. [Link]

[15] ABB, Special report Transformers. [Link]

[16] Siemens, Power transformers. [Link]

[17] Siemens, 420 kV transformer with natural ester insulation. [Link]

[18] Transformers Magazine, TransnetBW commissions first vegetable oil 420 kV transformer from Siemens. [Link]

[19] ABB, Development, innovations and transformer solutions. [Link]

[20] CIGRE, Study Committees A2. [Link]

[21] Transformers Magazine, 125 MVA transformer installed at a Siberian power plant. [Link]

[22] Benefits of Using Mobile Transformers and Mobile Substations for Rapidly Restoring Electrical Service, DOE, 2006. [Link]

[23] Large power transformers and the USA electric grid, DOE, 2012. [Link]

[24] Large power transformers, SGB-SMIT Group. [Link]

[25] Mitsubishi Electric Large Power Transformers, 2005.