How does a hydroturbine induction generator control its own frequency/water flow if it is already excited/pre-determined by the grid?

Question

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?

Answer 1

I'm not going to address the "hydropower" part of this question. First, because it's not about electronics or electrical engineering, and second, because there's such a tremendous array of different ways to extract rotary mechanical power from flowing water that trying to generalize the answer would be impractical.

I'm just going to talk about rotary electrical machines, torque on their shafts, and electrical power.

Note, too, that I'm going to use the term "electrical machine" here, not "generator" or "motor". That's because most things that can be a generator can be a motor, and visa-versa. In simple generating applications as you're talking about, the electrically simple way of making this all work could, at times, involve your machines inadvertently or deliberately turning themselves -- so, they're machines, and it's only intent and circumstance that makes them generators or motors.

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.

Nope.

The frequency of the output of a synchronous generator is determined by its shaft speed. That's what the "synchronous" means.

Synchronous machines are a bit more complicated than that when there's a grid involved, however. They want to lock their shaft rotation to the grid, not just in frequency, but in phase. Once the shaft angle is locked to the grid phase, they want to stay there -- it takes torque in the direction of rotation to make the shaft lead the grid, and torque against the direction of rotation to make the shaft lag the grid.

In a generator application, if the machine is locked and if the applied torque (either load or supply) isn't too large, then it will stay locked, and it will either consume electrical power as a motor, or deliver electrical power as a generator.

Induction machines are not synchronous machines.

The shaft speed of an induction machine is determined mostly by its synchronous speed. Apply grid power to it with a load, and it turns slower than its synchronous speed. Apply grid power to it, and apply torque to its shaft, it turns faster than its synchronous speed and it supplies current to the grid. At all times, the voltage on its electrical connection is determined by the grid.

Answer 2

"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."
This is a mistake. In the induction generator the 'frequency output of the stator' is imposed by the electricity grid. The (excess) speed at which it (the rotor) rotates is determined by how much power the water is delivering to the turbine.

Indeed if the water flow is slower than corresponds to the rotational speed of the rotor if there were no slip (no power), then the induction machine will act as a motor, i.e. it will pump the water, using power instead of generating it.

As a side note: The same occurs in wind turbines during turbulent winds if the wind is temporarily lower than what corresponds to the speed of the rotor. Then the induction machine acts as a motor.

The solution is to connect an electronic circuit which prevents reverse power flow.