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Home / Technical Articles / Change output voltage of transformer with taps
Change output voltage of transformer with taps
Regulate output voltage of transformer with tap-off changers (Siemens GEAFOL dry type transformer)

Most power transformers have taps on either primary or secondary windings to vary the number of turns and, thus, the output voltage. The percentage of voltage change, above or below normal, between different tap positions varies in different transformers.

In oil-cooled transformers, tap leads are brought to a tap changer, located beneath the oil inside the tank, or brought to an oil-filled tap changer, externally located.

Taps on dry-type transformers are brought to insulated terminal boards located inside the metal housing, accessible by removing a panel.

Some transformers taps can be changed under load, while other transformers must be de-energized. When it is necessary to change taps frequently to meet changing conditions, taps that can be changed under load are used.

This is accomplished by means of a motor that may be controlled either manually or automatically. Automatic operation is achieved by changing taps to maintain constant voltage as system conditions change. A common range of adjustment is plus or minus 10%. At Reclamation powerplants, de-energized tap changers (DETC) are used and can only be changed with the transformer offline.

A very few load tap changers (LTC) are used at Grand Coulee between the 500-kilovolt (kV) (volts x 1,000) and 220-kV switchyards.

A bypass device is sometimes used across tap changers to ensure power flow in case of contact failure. This prevents failure of the transformer in case excessive voltage appears across faulty contacts.

SOURCE: U.S. Department of the Interior Bureau of Reclamation – Transformers: Basics, Maintenance and Diagnostics

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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. Gabor Szucs
    Jan 16, 2022

    Can you please simply tell me, if I move the connections from tap #3 to tap #5, will it increase or decrease the output voltage? (three phase transformer).
    Thanks, best regards: Gabor

  2. Nelson Calero
    Aug 20, 2018

    Excelente información Edvard, tengo una pregunta:

    Se puede operar de forma remota (desde la sala de control) un cambiador de taps manual, o debe ser LTC para poder realizar una operación remota..???

  3. coumarane
    Jun 29, 2012

    Many useful information from real field personalities and the portal makes a very good and great service. Congratulation….
    Shortly i will also can share my experience in different area.
    Thanks lot.

    • Edvard
      Jun 29, 2012

      Nice words from you! Thanks, you are the most welcome to contribute in any way and share your knowledge and expirience!

  4. Edvard
    Apr 21, 2011

    Thank you Mady! Glad you like it!

  5. Mady
    Apr 21, 2011

    good portal

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