How do utility companies treat voltage deviations in distribution networks?
From this wiki page:


  Electric utilities traditionally maintain distribution system voltage within the acceptable range using transformers with moveable taps that permit voltage adjustments under load. Voltage regulators located in substations and out on the lines and substation transformers with Tap Changing Under Load are commonly used for voltage control purposes (Load Tap Changer or LTC). These transformers are equipped with a voltage regulating controller that determines whether to raise or lower the transformer tap settings or leave the tap setting unchanged based on “local” voltage and load measurements. 




See also this website primer from UST Power that explains a lot of different techniques for automatic voltage regulation.