Electric utilities in the U.S. try to maintain voltage levels within +/- 5% of the nominal voltage level however the voltage levels may be at a high of 6% to a low of 13% for a short period, ANSI standard C84. Winter storms causing icing on the lines presents additional issues with the voltage due to the attempt to heat up the lines to melt the ice away, many years ago we would simply drop the voltage at the power plant for short periods of time to heat up the lines, especially the larger 230,000-500,000 volt transmission lines. Nowadays they are using a controlled short circuit for short periods to accomplish this especially on the smaller transmission and distribution lines, all of which causes problems with the line voltage.
Electric utilities in the U.S. try to maintain voltage levels within +/- 5% of the nominal voltage level however the voltage levels may be at a high of 6% to a low of 13% for a short period, ANSI standard C84. Winter storms causing icing on the lines presents additional issues with the voltage due to the attempt to heat up the lines to melt the ice away, many years ago we would simply drop the voltage at the power plant for short periods of time to heat up the lines, especially the larger 230,000-500,000 volt transmission lines. Nowadays they are using a controlled short circuit for short periods to accomplish this especially on the smaller transmission and distribution lines, all of which causes problems with the line voltage.