Why Transformer Tap Changing is Required?

Fig-Transformer Tap Changers Line Diagram

What is the Principal of Transformer Tap Changers?

We will try to understand the principle of transformer tap changer and try to find the answer to the question- Can a transformer be tapped when it is on a load? Why is a tap changer used in an HV side?

A tap changer is a mechanism in transformers that allows for variable turn ratios to be selected in discrete steps. Transformers with this mechanism obtain this variable turn ratio by connecting to a number of access points known as taps along either the primary or secondary winding.

A tap is a connection point along a transformer winding that allows a certain number of turns to be selected. This means a transformer with a variable turns ratio is produced, enabling voltage regulation of the output. The tap selection is made with a tap changer mechanism.

You may like to read another post about: 

01. How do you know if a transformer is step up or step down?

02. What is the Function of Earthing or Grounding Transformer?

Can a transformer be tapped when it is on a load?

Yes.

Yes, on-load tap changers are in use to adjust the output voltage of power transformers for a long. OLTC is attached to the power transformers. Control cables run from OLTC to panel in the control room and it can be operated auto /manual.

Variable autotransformers like Variacs and Power-stats have a sliding contact that allows them to effectively change the tap point by turning a knob while they are under load.

Many power utility systems have large, high power tap changers that can change the tap point of a distribution transformer under load to compensate for varying demand conditions.

There are two (perhaps three) types of tap changer available, and the difference lies in the question you ask.

Off-load tap changer, sometimes called offline. Which requires load disconnection before the contacts move. There is usually a downstream breaker to interrupt the load during tap changing.

On load tap changer (make before break). The contacts are designed to have a shorting piece and to make contact with the next tap, before relinquishing contact with the first. The shorting piece has to be resistive, to avoid a shorted turn.

On load tap changer (break before make). This is not really on-load, but the break during tap changing is very short indeed. The contacts very nearly bridge across two taps but not quite. No resistance is needed. Interruptions of 0.1 half-cycles can easily be achieved.

The first type is rarely automated, the second two normally are.

The construction of each is very different, so they cannot be used with swopped duty.

 Why is a tap changer used in an HV side?

In this type, the tap changer circuit is placed on the primary side or supply side. As we know;

    Turns ratio        = secondary winding turns (Ns)/ primary winding turns (Np)= Ns/Np.

           Secondary voltage = (supply voltage or primary voltage) / Turns ratio.

It is mainly due to the current magnitude. The HV side has a higher voltage than the LV side but lowers current than the LV side.

You want as low current as possible to be handled by your tap-changer and, hence, the logical choice of installing the tap-changer on the winding side with the lower current (i.e HV side).

Transformer Tap changer on Primary Side or HV side:

1) In normal operation the tap changer will be at the 0% position to provide the required designed secondary voltage.


2) If the supply voltage increases or load current decreases there will be an increase in supply voltage which is not desirable. In this case, the tap position in the primary winding will rise towards a positive direction i.e. +2.5%, and hence decreases the Np. This will increase the turns ratio (Ns/Np) further decreases the secondary voltage.