Working with the local electrical utility this problem can normally be resolved reasonably easily, particularly if you are armed with accurate voltage profile recordings from a power quality investigation.
Traditionally this was normally just a function of the tap-setting at the local utility transformer. Working with the local electrical utility this problem can normally be resolved reasonably easily. With imbedded generation increasing significantly (i.e. rooftop solar) it is however becoming more common that an overvoltage at the installation can be caused by the local generation. Such cases need to be evaluated individually so that the most appropriate solution for each case can be found.
Long duration over-voltages and under-voltages are also normally associated with the tap-setting of the utility’s local distribution transformer (as modified by local generation and load). But it is worth noting that such effects can also be caused by utility equipment higher up in the network malfunctioning, or by the utility implementing an emergency configuration change due to the failure of some piece of equipment. In either case it must be followed up with the utility, but you can rest assured that the utility will normally give your query much more prompt attention if you show them accurate record of exactly what your voltage profile has been (as obtained from a power quality survey). If you present the utility with evidence that something is wrong they are obliged to act.