What are your design specifications?

capacity 100kVA balanced type input voltage range 340v-460v output voltage 400V.

kindly guide me how to learn about designing from 30kVA to 1500kVA capacities

The best way is to go back to college. If you are employed, then go for some distance learning course. One does not learn designing on CR4.

Which museum did you drag that out of?

If it's the type I'm thinking of, just use two voltage relays connected to the tap changer control.

Just looked at Brentford's web site. They've hardly changed in 50 years.

Not that you will become a designer of Brentford type automatic Voltage Stablizer by following these LINKS, peruse them, never the less.

thank you sir(Joshi) for the links. can you suggest me any book to follow for designing..

Look & search for the BOOK here.

The article on Brentford regulators sure brought back memories. I designed very similar devices from 1965 thru 1969 at Glenn Pacific in Oakland, Califorinia, USA.

The regulators only used a on/off control with pre sensing. If the output voltage was increasing, there was a signal inserted to remove power to the motor early and let the motor coast to a stop at the correct voltage. If the output voltage was decreasing a similar approach was used.

The motors and mechanical system had a long time constant so there was no requirement for fast response.

The motors used were usually permanent split capacitor, reversible while running.
If you use this type motor, make ABSOLUTELY CERTAIN that the motors are reversible while running.

Three phase motors were occasionally used, but you have to be absolutely certain that the phase rotation is correct or the motor will over run the limit switch and damage something.

So I can blame you for some late nights. The ones I worked on were installed in 66.

TonyS, just curious, what types of problems did you have?