Transformer Name Plate detail contains the information about Positive and Negative tap changers. It is important for an operator to have the knowledge regarding how these tap changers work.
Working of Positive and Negative Tap Changers:
Any electrical equipment is designed to operate satisfactorily at one voltage level. It is, therefore, important to maintain the consumer’s terminals voltage within the prescribed limits. To control the transformer output voltage and hence the consumer’s terminal voltage, taps are providing either on the primary or on the secondary.
The taps which are using to change turns ratio can be kept on the primary or secondary side. On whatever side they are, it is important to maintaining the voltage per turn constant, as far as possible. The tap changers are usually provide on HV Side as the current capacity on HV side is lesser than that of LV side.
The principle of regulating the secondary output voltage is based on changing the number of turns in the primary or secondary. 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.
Portion of Tap Changers:
Before understanding tap changers we have to look at 3 portions of tap changers:
- Standard Tap: This is the point where we obtain rated secondary voltage when rated primary voltage is applied.
- 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.
- 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.
Note: In any transformer, the volt/turn ratio is same on both sides always. (Primary side & Secondary side)
i.e Vp/Np= Vs/Ns
Application of Tap Changers:
The Positive and Negative tap changers are useful in the following cases.
Let us consider two cases here:
- Step up transformer with tap changing on secondary [H.V SIDE]
- 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:
Application of Positive Tap Changers:
Consider Case 1-Step up transformer with tap changing on secondary [H.V SIDE]
In transformers at the generating stations (Shown in Single Line Diagram), the primary voltage can be kept almost constant, consequently the taps should be provided on the secondary side.
We know that
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.
Application of Negative Tap Changers:
consider Case 2-Step down transformer with tap changing on primary [H.V SIDE]
If transformer is energized from a variable voltage source, as at the receiving end of a transmission line, the taps should be provided on the primary side.
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.