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 - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears. Professional in AutoCAD programming and web-design. Present on Google+.



9 Comments

  1. [...] dependable and secure protection scheme for the transformer.Errors resulting from CT saturation and load-tap-changers are particularly critical for differential protection schemes where the currents from more than one [...]

  2. [...] Electric Co., Ltd.)Temperature & InsulationIn the U.S., transformers are rated based on the power output they are capable of delivering continuously at a specified rated voltage and frequency under [...]

  3. [...] a portion of a winding can be removed or added to change the transformer turns ratio.The simplest tap-changing device operates on a ‘‘break before make’’ principle and changes taps on the primary [...]


  4. 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!

  5. [...] These devices are calibrated at the factory by changing taps either on the CT or on the autotransformer, or by adjusting the calibration resistors in the [...]

  6. [...] plant. Transformers are used for a wide variety of purposes, with the complete range of voltage and power ratings as well as many special features for particular applications.The following covers the main [...]


  7. Edvard
    Apr 21, 2011

    Thank you Mady! Glad you like it!


  8. Mady
    Apr 21, 2011

    good portal

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