Effects of out of phase angle ac voltage between two power grids
What happens whenever you connect any two voltage sources in parallel that don't have the same voltage at the same time? A massive current flows, limited only by the parasitic resistance of the wiring and the sources themselves.

In synchronous machines such as the alternators used to produce AC power, this current creates a large torque that effectively forces the phase error to zero. After that, it is the external torque on the shaft that determines whether the machine is a net producer or net consumer of grid power.