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.
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
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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 ?
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%
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.
Thankyou
How to calculate the value of transition resistance of an OLTC of a 66Kv/22kv transformer. Thank you in advance
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in 33kV/433V tranformer if secondary voltage reduce from 433V to 405V ,Any consumption reduce .
You need to reduce your tap by 6.46%, nearest tap is at -7.5%.
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)
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.
Nice article but I don’t understand the need to take the inverse of the turns ratio (1.027). Pls explain n thanks
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.
What would be th off-nominal transformation ratio of the transformer shown in the example?
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.