According to the formula n = 60f/p. N is the rotational speed (rpm), f is the frequency (50HZ here), and p is the number of magnetic poles; Obviously, the rotor of the hydro-generator has more than one pair of magnetic poles, so the speed is not 50 revolutions per second, but a fixed number, depending on the nameplate of the generator; As for how to maintain a constant speed, this is achieved by the governor. The governor compares the frequency of the power grid with the frequency of the unit, and then adjusts the opening of the guide vane (equivalent to the valve) of the hydraulic device to adjust the output of the unit, thus realizing the adjustment of the speed of the unit. As for the adjustment, it is controlled according to the principle of automatic control. You can search for "PID" yourself to learn about it. It used to be a mechanical governor, but now it is.