An Example of Transformer Tap-changer Correct Adjustment

An Example of Transformer Tap-changer Correct Adjustment
An Example of Transformer Tap-changer Correct 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. 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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About Author //


Edvard Csanyi

Edvard - 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 fascilities. Professional in AutoCAD programming. Present on


  1. nitin nakhate
    Dec 08, 2016

    how to calculate % impedance,%reactance

  2. 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%.

  3. 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.

  4. 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?

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