On-Load Tap Changers and Transformers

I am confused as to why we use a reactor as a part of on load tap changer in transformer.. The Book says it helps not to get tapped winding get short circuited as the impedance offered by the reactor is high for a current flowing in one direction so local circulating current flowing through reactor and tapped winding is small..Can somebody please explain me how exactly a reactor helps not to get short circuit of tapped winding or can help me understand what book means

Hi,

(1) We will firstly explain why the tapped winding gets short circuited

(2) Secondly we will explain how the reactor prevents this short circuit.

(3) We will then explain what is meant by part of your question "the impedance offered by the reactor is high for a current flowing in one direction".

Firstly, From time to time we need to change the taps on the transformer in order to maintain the output voltage within specification. Say the transformer is on tap 3, and we need to go to tap 4. Since this is an on load tap changer we cannot break the load, so the tap changer must stay on tap 3 and at the same time make the circuit on tap 4.The voltage drop across taps 3 and tap 4 is now short circuited.

The selector in the on load tap changer does the job of changing the taps. Working in conjuction with the selector is the diverter.The diverter switches the line (usually neutral in a Y connection) using "MAKE BEFORE BREAK" switches in such a manner to connect a reactor so that the current will pass through the reactor before tap 4 is energised by the selector and thus Taps 3 & 4 is no longer short circuited,instead the reactor is now in circiut.

Thirdly, The polarity of the reactor is such that it is in opposition to volt drop of taps 3 &4. As we connect the reactor across taps 3&4, the circulating current that is induced in the reactor is in phase opposition to the current in taps 3&4. Hence we have current cancillation taking place,and the resultant circulating current is small.

Regards