On Thursday, February 9, 2017 at 6:43:06 PM UTC-5, James Wilkinson Sword wrote:
They tell me that function is only available on the higher voltage transformers. The final stepdowns are manual only.

Correct. Explains why the utility can only regulate to within 10%. And also why larger variations result in that one transformer cutting off power to all those local transformers. If that one higher voltage transformer does not maintain voltage variations, then a circuit that feeds local transformers is modified so that voltage varies by less than 10% at every time.

If voltage gets too high or too low, then power is cut off to avert damage to consumers appliances.