Getting 240VAC output from 220-230VAC input
I'd be considering using a decent UPS - it can provide a stable output and a decent sine wave with variable input voltage conditions. You must choose one that always converts the input to DC and then uses an inverter to generate the output voltage. Some UPS designs will pass through the input AC voltage and only "switch in" the backup system when there is a power fail. These types are not sufficient for your needs.

Maybe one of these and the giveaway is that they can output 50 or 60 Hz irrespective of the input frequency - this tells you that they are true inverting converters.