This is a very broard question I suggest you try a google search on the subject FIRST. If you have any specific questions then we should be able to help.
I am giving you a small idea regarding this AVR used in transformer.
1)We are setting a bandwidth i.e voltage range between lower side and higher side.If the voltage crosses the bandwidth,then AVR gives signal to OLTC to rise or lower the tap of transformer.
"Almost" Good Answers: