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Home / Technical Articles / An Example Of Transformer Tap-Changer Correct Adjustment

Tap-Changer Adjustment

A 13800V/4160 V transformer has five taps on the primary winding giving -5%, -2 1/2 %, nominal, +2 1/2 % and +5 % turns.

An Example of Transformer Tap-changer Correct Adjustment
An Example of Transformer Tap-changer Correct Adjustment

If, on-load, the secondary voltage reduces to 4050 V then, which tap, should be used to maintain 4160 V on-load (assuming the supply voltage remains constant)?

The following answer results:

To keep the secondary voltage at (or as close as possible to) 4160 V, either primary supply voltage or the HV winding tap position must be altered.

Examining the relationship:

V1/V2 = N1/N2 or V1·N2 = V2·N1 indicates that, to keep the equation in balance with primary voltage and secondary winding turns fixed, either V2 or N1 must be adjusted. Since the objective is to raise V2 back to nominal, then N1 must be reduced.

To raise V2 from 4050 to 4160V requires an increase in secondary volts of: 4160/4050 = 1.027 or 102.7 %. N1 must be reduced to 1/1.027 = 0.974

Basic tap-changer
Figure 1 – Basic tap-changer

Therefore N1 must be reduced by (1 – 0.974) = 0.026 or 2.6 %. Reducing N1 by 2.6 % will accomplish the increase in secondary voltage output.

The nearest tap to select is -2 1/2% (see Figure 1).

How tap-changer works (VIDEO)

Reference: Science and Reactor Fundamentals – Electrical  / CNSC Technical Training Group

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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. magesh
    Sep 21, 2023

    Dear sir,

    It’s very useful to us. I have one inquiry, how will the taping percentages +5%,+2.5%,-2.5%, and -5% be calculated?

  2. Salar
    Feb 19, 2023

    The number of turns between each tap is 28 turns , the transformer voltage 11kv/0.416 kv , five taping in total +5 % and -5% , how I calculate the HV total turns ?

  3. Gowtham
    Oct 26, 2022

    Hello Daniel,

    (V1/V2)=(N1/N2) ie., In order to decrease the secondary output voltage, we need to increase the number of turns in the primary winding (Since, N1 is inversely proportional to V2).
    Now, To reduce 443 volts to 415 volts requires a decrease in secondary volts of 415/443 = 0.936 or 93.6%
    The number of turns in the primary winding has to be increased to 1/0.936 = 1.068 or 106.8%

    Hence, N1 must be increased by 6.8% and the nearest tapping to select in this transformer is +5%

  4. sibo-dennis daniel
    Jun 23, 2022

    so is it possible in this tap changing that this nominal voltage must decrease, what if this voltage increases from 415v to 443v how do you tap change this, based on your above calculations given. That is to reduce 443v to 415v how is this done, please I will need the calculation if possible with a diagram illustration as u showed earlier above.

  5. didahirpa
    Jul 22, 2021


  6. Dinesh Mewhoor
    Nov 26, 2018

    How to calculate the value of transition resistance of an OLTC of a 66Kv/22kv transformer. Thank you in advance

    Jun 09, 2018


    Apr 20, 2018

    How are setting of the OLTC output voltage?

  9. Rakesh Patel
    Dec 24, 2017

    i need formula to set value in RTCC like: Lower,Raise, PV value, to fix LT value. Transformer 11000/415

  10. nitin nakhate
    Dec 08, 2016

    how to calculate % impedance,%reactance

  11. Amit
    May 22, 2016

    in 33kV/433V tranformer if secondary voltage reduce from 433V to 405V ,Any consumption reduce .

    • Anil
      Jul 14, 2016

      You need to reduce your tap by 6.46%, nearest tap is at -7.5%.

    • Nasrelden
      Feb 11, 2020

      V1/V2 = N1/N2
      V1*N2 = V2*N1
      TAP%= (Vmeasure -Vrate)/ Vrate
      = (405 – 433)/433 = -6.46, we select the nearest value
      (by means N1 to be reduce by -5)

  12. Subodh Prakash
    Jun 14, 2014

    Taps on HV winding are provided for variation of HV volts. It may not be good practice to change the taps for reduced LV volts. Though , the extent to which the variation is proposed is Ok. The transformer should be designed suitable for variation of LV volts with taps on HV.

  13. kazim
    Jun 13, 2014

    Nice article but I don’t understand the need to take the inverse of the turns ratio (1.027). Pls explain n thanks

    • Jeff D
      Jun 13, 2014

      Kazim, the ratio of secondary windings to primary windings needs to increase by 1.027. If the transformer in question had multiple taps on the output, we would select the tap closest to +2.7%. Since the secondary doesn’t have multiple taps, we’re stuck with 1/1.027 times as many windings as we’d like on the secondary, so the number of primary windings has to be reduced accordingly.

      Put another way, we want N2actual/N1actual = 1.027*N2nominal/N1nominal, and we’re constrained by N2actual=N2nominal. Solving this system of equations for the ratio of actual to nominal primary windings yields N1actual/N1nominal = 1/1.027.

      • Steffen
        Oct 16, 2015

        What would be th off-nominal transformation ratio of the transformer shown in the example?

        • Adil
          Jun 17, 2019

          Its all ok as explained, kindly also tell about the how we would come to know that tap has been set physically ok to achieve the result, and what would be the risk of operating at higher taps on transformer life.

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