In the below link you will see an operator go through the process of synchronizing a hydroelectric generator to the national grid. The important aspect of synchronizing to the grid is to make sure the generator's output frequency matches the national grid's frequency. In North America that frequency is 60 Hz. If you watch the control panel in the below link, you will see a synchroscope match to the national grid's frequency.
https://www.youtube.com/watch?v=xGQxSJmadm0&ab_channel=ChrisBoden
