I am designing a changeover circuit that will switch the power of attached appliances between two electricity providers. The two AC mains are of 220V RMS voltage and 50Hz frequency sine waves. The two supplies are independent and out of sync. I will be using a microcontroller to control dpdt relays for switching. I may need to switch between grids every hour or every half hour.
During one cycle of sine wave, what is the best time to switch the load? I did some research and found out that in the UPS, when it's time to switch over from inverter to mains, the controller first syncs the inverter with the grid by varying the frequency of the inverter before switching. But in my case I can not sync the two supplies.
So my question boils down to: What are the things that I should worry about during switching? I want appliances to remain alive during the switching and I do not want to damage my appliances (due to some weird harmonic) during the switching.
It would be much appreciated if someone points in the right direction.