An Example of Transformer Tap-changer Correct Adjustment

Home / Technical Articles / Transformers / 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

SEARCH: Articles, software & guides

Premium Membership

Premium membership gives you an access to specialized technical articles and extra premium content (electrical guides and software).
Get Premium Now ⚡

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