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Theory, working and operation of Transformer Tap Changer

In this article we will discuss transformer tap changer from all aspects. Components as well as different types of tap changer are also discussed. What is on load tap changer and how it works. Different steps of operation of OLTC along with labelled diagrams.

Types, components, constuction and operation of on load tap changer

Tap changer is a device which can increase or decrease the output secondary voltage by changing the turn ratio or Primary or secondary winding. Tap changer is generally installed on high voltage side of a two winding transformer due to low current in this side.

Basic principle of tap changer is (Step down transformer)

As a transformer ratio

We want to regulate the secondary voltage (V2)

Since V1 is the system voltage so we cannot change it. As tap changer is installed on the primary side of step down transformer hence N2 cannot be changed. So

N2 and V1 = constant (k)

$latex V2=\ \frac{K}{N1}\$

It means that

  • By increasing primary turn ratio, the voltage on secondary side can be decreased.
  • If the primary turns of transformers are decreased, the voltage on secondary side is increased.

Purpose of Tap changer

Frequent changes occur in the load of the power system, due to which the voltage of the system increases or decreases. So the Tap changer is used to keep the output voltage of a Transformer within prescribed limit.

Types of Tap Changer




If the required change in voltage is infrequent, then an offload tap changer is installed on a transformer and taps can be changed after completely isolating a transformer from the circuit. Such kind of a tap changer is usually installed on distribution transformer (with 5 steps).


With the expansion and inter-connection of power system it often becomes necessary to change the transformer taps several times daily to obtain the required voltage on system as per load demand. The demand of continuity of supply does not permit to disconnect the transformer from system for offload tap changing. To meet this requirement, on load tap changers are installed on the majority of power transformers.


Range tapping ranges between +/- 10 % to +/-16 % is used in normal conditions and +/-22% is in special cases. The number of steps in the range is chosen so that the change in voltage between adjacent positions approximately 1 % to 1.5 % of rated voltage.

Parts of On load tap changes

1- Selector switch

  • Fixed contacts
  • Odd moveable selector
  • Even moveable selector
  • Slip rings
  • Gearing mechanism
  • Reversing switch

2- Diverter switch

  • Main contacts – X
  • Switching contacts Y
  • Transition resistor contacts z
  • Resistors
  • Tension springs

3- Drive unit

  • 3 (Φ) induction motor
  • Controlling switch
  • Limit switches
  • Tap position indicator

A physical, arrangement of on load tap changer in power transformer is shown below.

Physical arrangement of tap changer

A typical on load tap changer is sketched in the attached diagram

On load tap changer

Working of On load tap changer

Let suppose a current inter from point ‘A’ to main winding than enter in tap winding through reversing switch contact N—>L , Taps l & 2 both are connected with move able selectors but the current flows only in tap no. I (Because the diverter side-1 is closed) so the current finally end on point B as shown in above fig.

Now if we want to change the tap position from tap-1 to tap-2, operates the drive unit motor through raise control switch .The motor starts operation (at this time only an action starts in diverter switch because tap -2 is already selected ).

Following operations take place for changing tap during on load

Operational sequence of on load taps changer

Diverter Switch                                                                        



Onload load tap changer operation 1 Operation position side 1 The load current flows via side 1 through the X-Contact
Onload load tap changer operation 2 Contact X. Break of side 1 Load current flows via side 1 through the Y contact.
Onload load tap changer operation 3 Contact Y. Breaks of side 1 The load current flows via side 1 through the Z-Contact

(Arching at Y contact due to break the easy path of current)

Onload load tap changer operation 4 Contact Z of side 2 makes


The load current is divided equally between side 1 tap and side 2 tap flowing via two equal transition resistors R and contact Z. In addition a circulating current flows via contacts Z of both sides
Onload load tap changer operation 5 Contact Z of side 1 breaks The circulating current no longer flowing (circulating  current flows 20ms to 40ms )

The load current flows via side 2 tap through contact Z and transition resistance R (Arching at contact Z of side 1)

Onload load tap changer operation 6 Contact Y of side 2 makes


Load current flows via side 2 tap and through the Y contact.
Onload load tap changer operation 7 Operation position side 2 The load current flows via side 2 through the X-Contact


Operation of tap changing is completed

To see it live click here 

For changing the tap-1 to tap-2 the voltage will be increased on secondary side of transformer because the numbers of primary turns have now reduced.

Similarly, if we want to go on tap no.3, the operational sequence will be same as above but in reverse (side -2 to side-1) and now the operation will become at first in selector switch, as the odd moveable selector selects the tap no.3. For doing continuous changing the taps in step by step, we select the tap-10. At this time only a main winding will be in circuited and current will flows from ‘A’ to ‘B’ via the main winding to diverter switch. The tap-10 of this arrangement is called ‘principal’ tap. At this tap the voltage on both sides will be rated. Now if we want further changing in taps, operates the motor through the raise switch placed in drive unit.

Now the following operations will take place

  • Reversing switch position will be automatically changed (N to M)
  • Odd moveable selector (present position on tap-9), returns on tap -1
  • Load shifts from side -2 to side -1 of diverter switch.

The polarity of tap winding will he reversed which means the primary turns decreased , hence the secondary voltage would increased

The voltages increase, so tap-1 is now called step no 11 and the reversing switch will convert the tapping range (2n-1).

Where n = number of taps

If n=14

The number of steps becomes = 27

If you have any question relating this topic please ask in comments. Dont forget to share it with your friends.

About Syed Noman ud din

Syed Noman ud din is an Electrical Engineer and working in Industry from last 3 years. He writes technical articles for electrical and electronic engineers. He has also published several research publications in renowned international journals.

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