Why Do Some Hydroelectric Control Systems Use Both Speed and Power Feedback?

[vampslayer]

Hey!
It's about control systems for hydro power plants, where one of the main role has turbine-governor which should be able to control speed(frequency) of the generator.

But can someone explain me why on the some block diagrams there are speed and power feedback while some block diagrams use only speed as feedback to control frequency constant. What's deal if we have power too as feedback?
Look the following picture and you will know what I am talking about.

http://oi48.tinypic.com/iqdc08.jpg

 

[jim hardy]

When a synchronous machine is connected to the grid it cannot depart from synchronous speed - it is locked as if by gears.
So one controls power output by controlling admission of water.

Before synchronizing, one of course controls speed by admission of water in order to match the grid so he can synchronize.

The block diagrams you presented are for understanding the control theory and may not resemble the actual hardware block diagram used to implement speed and load control ..

Typical steam turbine governor has a gain of about 20 to 30 so that a demand for ~3% to 5% overspeed translates to a demand for 100% power.
So terms speed and power become interchangeable in some control systems.
Different textbook authors might use them differently.

I assume hydro governors are similar - but was never around one.

Any help?

old jim

 

[vampslayer]

Hi jim.
CAn you just tell me am I right?
Let's say hydro power plant work in off-grid mode, and power plant use synchronous generator. We use turbine-governor to bring speed to the same level as speed of synchronous generator(until that moment generator and load are cut-off). In a moment turbine(generator too has this speed) reach that speed, load can be connected to the generator, and generator's excitation system should start working.
Then we use speed as a feedback to control the frequency because of the load disturbance.
But if we control frequency on this way, we control power too, why do we need power as a feedback too.
And also power depends on the voltage and voltage is controlled by the excitation system of generator(by the exciting current).

So generally we control power with excitation system of generator but in a case of load disturbance we use turbine-governor. Am I right?

And I don't understand what are the differences between controlling the hydro power-plant which works in "off-grid" and "on-grid" mode

 

[jim hardy]

Hi jim.
CAn you just tell me am I right?

I will try.

Let's say hydro power plant work in off-grid mode, and power plant use synchronous generator. We use turbine-governor to bring speed to the same level as speed of synchronous generator(until that moment generator and load are cut-off).

Okay, we are NOT connecting to grid. We call that "islanded" .
Power produced by our generator will be used by nearby loads.
Do I understand the question correctly ?

In a moment turbine(generator too has this speed) reach that speed, load can be connected to the generator, and generator's excitation system should start working.
Then we use speed as a feedback to control the frequency because of the load disturbance.

Yes, the governor will attempt to hold speed wherever we set it, frequency is same as speed presumably near 60 (or 50) hz.

Load disturbance?
Is this what you mean?
A change in load will change the speed of the machine, the governor will sense that and try to restore speed by adjusting valves.
As I mentioned, governor does not have infinite gain so a change in load will result in a speed & frequency offset of $\frac{\Delta Load}{Governor Gain}$ , after the system settles . Remember it's a closed loop.

But if we control frequency on this way, we control power too, why do we need power as a feedback too.

In your islanded situation there is no need for power as automatic feedback.
Just control frequency and voltage. Let the load take whatever power it needs and the governor must follow . That's what governors do.

And also power depends on the voltage and voltage is controlled by the excitation system of generator(by the exciting current).

I don't think you have that relationship quite right.

To a small extent power drawn by a load is affected by voltage, yes. That's why we have an automatic voltage regulator. We hold voltage constant so the customer's lights stay same brightness.
What kind of loads are you powering? For resistive heating like water heaters power is linear with voltage. For induction motors like household appliances power is in proportion to frequency and is comparatively independent of voltage. Think about it - induction motors are just a few % slip, so have fairly constant speed.

Power is a strong function of frequency and a comparatively weak function of voltage.
When islanded we have no control over power - it will be whatever the customer demands.
It is our duty to provide him with normal voltage and frequency.

So generally we control power with excitation system of generator but in a case of load disturbance we use turbine-governor. Am I right?

I don't think you have that right.

OFF GRID the power is determined by what load is connected to the generator. That is not under control of the plant operator. Plant operator can only set frequency and voltage, resulting power will be in accordance with Ohm's law. We do not depart from normal voltage to control power (except in very unusual circumstances )

And I don't understand what are the differences between controlling the hydro power-plant which works in "off-grid" and "on-grid" mode

OFF GRID: as above.

ON GRID: Speed is set by the grid. One plant is not large enough to change grid frequency.
So when unit is ON GRID the governor controls the exact same valves, but because speed is fixed the valves have a different effect.
The valves cannot change the speed, so what do they control instead?
If they move in the open direction they admit more water(or steam) . That produces more torque.
Power = torque X speed X (conversion constant).
So more power flows out into the grid. It only shows on your power meter.

So the same exact knob that controlled speed when OFF GRID
controls power when ON GRID.

-------------------------------------------------------------------------------------------That's the basics of the machine and grid.

Your original question was:

But can someone explain me why on the some block diagrams there are speed and power feedback while some block diagrams use only speed as feedback to control frequency constant. What's deal if we have power too as feedback?

There's a couple reasons to add a power input to the governor.

My plant has a simple circuit that compares generator's electrical power output against mechanical power coming into generator from the turbine.
If it ever sees high turbine power simultaneous with low generator power it reaches into the governor and snaps the valves shut.
The reason for that is to prevent a rapid acceleration and overspeed. That can happen if for example you lose the lines leaving the plant.

Controls also exist to detect and mitigate power system oscillations but most plants don't have them (or didn't in my day).

Bear in mind that the whole grid is an interconnected system having both energy input and inertia. That's an invitation for harmonic motion.
Indeed a large steam turbogenerator when connected to the grid has a natural frequency of about 1 hz, that is it can hunt like a rotating pendulum at 1 hz. So control systems must not excite that frequency in fact they must damp it.
I don't know what a hydro plant's natural frequency is.
But that effect is local.

An entire section of the country can go into oscillation against another part of the country.
I've seen them at $\frac{2}{3}$ hz and divergent. Systems exist to detect and snub those as well, search on "power system stabilizer".

These add-ons generally are not simple linear control blocks as you'd think from looking at your diagrams.

I hope you become interested in power systems. The field needs specialists who love it.

dlgoff has experience in central power system control - I was just at a steam plant so have only a view through a very small window..

Good luck in your studies. old jim

 

[vampslayer]

I will try.


Okay, we are NOT connecting to grid. We call that "islanded" .
Power produced by our generator will be used by nearby loads.
Do I understand the question correctly ?

Yes

Load disturbance?
Is this what you mean?
A change in load will change the speed of the machine, the governor will sense that and try to restore speed by adjusting valves.
As I mentioned, governor does not have infinite gain so a change in load will result in a speed & frequency offset of $\frac{\Delta Load}{Governor Gain}$ , after the system settles . Remember it's a closed loop.

Yes that's what I thought.

And Jim thank you very much. You make me understand theory, and you resolved my puzzles. Now this thing is a lot more clear. This was extremely helpful.
Thank you one more time.

 

[jim hardy]

I thank you for those kind words. Helps an old guy feel perhaps still a little bit useful.