How can I vary / control a Voltage Output based on another voltage source.

hello, Please i need help

I have two voltage sources, one is an programmable AC 3 phase and the other is a DC source fed into the Inverter. The programmable AC source is programmed to work at +/-10 % of its capacity, For example if the programmed AC source drops to 90% capacity how can i make the inverter to produce 10% to make the total capacity 100%? And when the programmed AC source is working at 100% capacity the inverter should not produce any power to be fed to the grid.

NB: A DC source is connected to the inverter which converts DC power to AC and feds it into the grid.

regards Thanks