"voltage at few places seem to be as high as 280-300V"

No so.

Do you realise the current taken by a globe (and lots of other devices) will increase considerably as the voltage increases. When the voltage rises by say 10%, the current will rise by much more than 10% and this will put a very heavy burden on the generator and in most cases the generator will not be able to supply the load. This is why the voltage will not rise as you have stated.
Globes and radiators are not linear devices, but if they were, a 1,000 watt radiator at 250v would take 1250 watts at 280v. This is 25% increase in wattage for 13% increase in voltage.
The figures would be much higher for 230v supply.