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:
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
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