I've completed my intern in hydroelectric power plants. I've seen large generators working to maintain voltage in 50Hz. They were turning at 187,5 rpm with 32 poles. But I wondered what stabilizes them? What if water flow makes turbine turn a little bit faster or slower frequency should change, right? When I asked this to them they said, the pipes which turns the turbine are designed to get just enough flow to make turbine turn at 187,5 rpm. But I wonder what if we maintain greater flow or pressure to the turbines, would it produce 55-60-70+ Hz? Are there any electromagnetic force that is holding-back from speeding up?
Are there any electromagnetic force that is holding-back from speeding up?
Yes, the grid is controlled to be 50 or 60 Hz (country dependent) and although a particular hydro plant may produce many megawatts it pales into insignificance compared to the grid. The grid has many power contributors and these are all regulated to push out power at 50/60 Hz. A particular Hydroelectric plant won't be able to fight against this and hope to push-up the operating frequency. This is why grid connected plants have a sophisticated level of control and will shut down or de-pressurize water feeds should anything be going wrong.
If the hysro plant was stand-alone and there were no safeguards against maintianing the correct flow to the turbine then clearly both output frequency and voltage would rise.
The turbine always runs at the speed of the grid.
Power from the water sends power into the grid. Increasing the water flow increases the power into the grid. Turn the water flow off, and the generator becomes a motor, spinning the turbine at grid speed.
If the flow is increased, then the power output must be increased to keep the speed constant - the power output is increased by increasing the excitation current.
Once the maximum power has been achieved then the flow cannot be increased any more as then the frequency starts to increase. There are some hydro turbines that take flows of 45m^3/s which can't be changed very quickly...
This is more of a problem with wind turbines when the wind speed exceeds about 25m/s (depends on the manufacturer of the turbine) and if the wind goes over that limit then the wind turbine is stopped ie brakes on.
I was half-way up a turbine tower when the brake was put on... full speed to 0 rpm bang - a brown-trouser moment - almost...
It helps that gravity and water are two well known and constant factors.
There are two ways to change the speed of the turbine:
- Changing flow. Limiting the intake using valves.
- Changing power output. If you take more energy from the generator, it takes more energy to spin the turbine. Making it run slower.
Maybe there is a third way, by changing the angle of the blades. But I do not know if this is common for hydro generators.
A hydro turbine without these two regulations cannot create a stable AC grid on its own. It will overspeed when you don't consume enough power. And it will stall when you take too much.
Your permanent magnet wind turbine on your yacht is like this. It needs batteries and a controller to create a stable grid with fluctuating winds and loads.
There is a different situation when it is connected to mains. This has a very low impedance due to being many times larger in capacity than one turbine, thus it can only feed power to the grid. To change the frequency of the grid it needs to takeover the load from all other power plants. But it will reach current limits first.