I was reading this article How do you connect hydro to the grid? and the author states there are two methods. One by a fixed-speed induction generator and another by a grid-tied inverter. I understand the grid-tied inverter, but not the fixed speed where they say:
An induction generator is grid-excited, which means that the magnetic field that must be created by the generator’s stator windings is energised by the grid. This has the advantage that by default the electricity generated must be perfectly grid-synchronised because the grid is providing the excitation.
The frequency output of the stator of a hydroturbine induction generator is determined by how fast it rotates which is determined by the flow through it which can be controlled. But how does the frequency controlled by the flow of water, and the frequency connected to the stator by the grid, interact? Does being connected to the grid force the generator to behave as a motor if the flow of water cannot produce a speed/frequency that is slightly greater than that of the grid (in order for it to be generating)? I guess similar to if the flow of water drops off suddenly, would the induction generate just operate as a motor?