Dealing with sustained low or high volts can be cured with something as simple as an auto-transformer. Although auto-transformers are usually specially wound devices where the winding is a tapped winding, making one can be achieved with a suitable standard transformer.
Above is shown a typical configuration. The Primary winding voltage matches the output voltage required, and the Secondary matches the boost or buck voltage required e.g. should the voltage need to be dropped from 250VAC to 230VAC then the Primary is 230V and the Secondary 20V. Please note the secondary must be able to withstand the full load current at 100% duty.
The above configuration is shown in "buck" (voltage drop) mode where the input voltage is dropped to a more suitable level (this is the more usual method of employment). For boost configuration simply invert the connections (swap the input and output).
One of the most common uses is to lower the voltage on lighting circuits to save incandescent lamps (although there is also the spin-off of energy-saving so the transformer pays for itself not just in longer life of lamps, but in energy bills too). A 200VA 0-24V tapped secondary transformer (e.g. RS Electronics part 211-0676) proves more than adequate for a typical 10A lighting circuit.
Legrand appear to have the perfect device if there is a desire to lower the voltage for the whole house (bringing down the deliberately inflated incoming voltage, which averages around 245V, by just 5% i.e. 12V, should lower your energy bills in excess of 10%!).
Their part 6425-10 is a 230V to 2 x 12V, 1kVA transformer (i.e. the two secondaries, when parallelled, are capable of approx. 80A total - more than the domestic property is likely to draw on a continuous basis!). I'm hoping to trial this device in the near future and will post my findings here.
With electricity suppliers pushing the voltage up a little in order to get a quicker return on their purchases (they don't like me saying this!), large users can 'fight back' by bringing it down again to more realistic levels.
In steps the industrial 3-phase auto-transformer. Shown alongside is one model known as the powerPerfector and ranges from 30kVA to 3MVA with tapped input for user-selectable voltage drop.
Such solutions, if the primary windings are carefully connected, can assist in reducing some harmonics too. Even more of a reason to consider them (but, again, do not expect miracles!).