Voltage regulation on very old distribution circuit

Consider in the rural US, a very old section of a distribution circuit, operating at 4800Y/2770 volts delta. It is fed through a wye/delta transformer
bank fed from a 13.8 kV distribution circuit immediately followed by a pair of voltage regulators connected open delta.
About three miles downstream I have been monitoring the outlet voltage using a Raspberry Pi and a "Back-UPS ES 550G" UPS every 10 minutes. A few questions:
1) When the regulator transformer steps one step, how much would I expect the outlet voltage to change from just that?
2) Over a period of a few weeks, I have seen the voltage range between 116 volts and 125 volts. By current standards, what is the expected range one should see on a nominal 120V outlet? What is the largest acceptible deviation of the average voltage from 120 volts? The building is new, 200A main breaker but not using anywhere near that much, and the UPS is currently the only load on its circuit so the building itself contributes minimally to the deviations.
The power here has always been "bad", noticeable light blinking and there are relatively frequent power outages esp. during storms. The lone 116 volt reading was sandwiched between a 122 volt and 123 volt readings. I just set this up for curiosity, and to eventually check remotely during long periods away in the winter, to prevent pipe freezing if the heat is out for an extended time. (the Pi also measures the temperature. This project is not complete yet)
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On 6/5/20 11:05 AM, Michael Moroney wrote:

You might try the sci.electronics.design group if you're looking for comments. They discuss politics and printed circuit board designs but at least there are some inhabitants.
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