Transformer Tap Changing Methods - How it Works?

Electrical Engineering Electrical Machines
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Taps are generally used to vary the voltage of secondary side of the transformer.

The tap changers are usually provided on HV Side as the current capacity on HV side is lesser than that of LV side.

Generally the myth is that to increase the secondary voltage we have to increase the number of turns but this is not always true.

In some transformers we can find the tap changing values as +5%,+2.5% -5% and _2.5% etc. How the voltage will be changed according to the position of tap changer with reference to the above values. To understand this concept the knowledge of tap changing methods is required.

Before understanding tap changers we have to look at 3 portions of tap changers:

  1. Standard Tap: This is the point where we obtain rated secondary voltage when rated primary voltage is applied.
  2. Negative Portion: In this portion, the number of turns is less with respect to the standard tap. Means as the tap changer moved to negative portion the number of turns from which output is taken will decrease but output is not necessary to decrease.
  3. Positive Portion: In this portion, the number of turns is more with respect to the standard tap.Means as the tap changer moved to positive portion the number of turns from which output is taken will increase but output is not necessary to increase.

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Note: In any transformer, the volt/turn ratio is same on both sides always. (Primary side & Secondary side)

i. e Vp/Np= Vs/Ns

Let us consider two cases here:

  1. Step up transformer with tap changing on secondary [H.V SIDE]
  2. Step down transformer with tap changing on primary [H.V SIDE]

Now let’s assume that in the above cases we need to increase the secondary voltage.

So, in case-1 the tap changer should be moved to positive side.

And in case-2 the tap changer should be moved to negative side.

Here’s how it works:

Case 1:

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Step up transformer with taps on secondary

We know that

Vp/Np= Vs/Ns

We cannot change the primary side turns and voltage as the primary (applied) voltage is always constant and there is no tap changer on this side.

Now Vp/Np= constant

And Vs/Ns= constant

Hence, to increase the Vs the value of Ns should be increased meaning that the tap changer should be moved to positive side.

Case 2:

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Step down transformer with taps on primary

Even here

Vp/Np= Vs/Ns

Rearranging, VpNs= VsNp

Here Ns and Vp are constants.

So VsNp= constant

Hence, to increase the Vs the value of Np should be decreased meaning the tap changer should be moved to negative side.