# An Example Of Transformer Tap-Changer Correct Adjustment

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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)?

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

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.

### 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 fascilities. Professional in AutoCAD programming. Present on

1. 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

2. nitin nakhate
Dec 08, 2016

how to calculate % impedance,%reactance

3. 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%.

4. 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.

5. 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?