You could introduce a back to back HVDC link [1] on one of the two incoming legs, allowing you to alter frequency and phase freely. [1] https://en.wikipedia.or

You could introduce a back to back HVDC link [1] on one of the two incoming legs, allowing you to alter frequency and phase freely. [1] https://en.wikipedia.org/wiki/High-voltage_direct_current#Back_to_back https://en.wikipedia.org/wiki/High-voltage_direct_current#Ba...