I have to design a tap changer for a three-pahse transformer which has -10%,-5%,...,+15% taps in its primary (6 taps).
The tap changer must monitor mains voltage trends and choose a suitable tap to compensate mains amplitude increase/decrease in its primary. proper tap is chosen with back to back connected thyristors.
The approach I chose: use a peak detector, then a differential amp., and compare the output DC voltage with 6 references, then decode the result and choose the proper tap. Surge problem can make this circuit choose the improper tap.