Controlling the Speed of Hydro Turbine

L

Thread Starter

lakmalp

As far as I know, hydro turbine's speed is proportional to the head available to turbine. Please do correct me if I am wrong.

When the power plant is about to sync with the grid, how the RPM of the generator is adjusted to match the frequency of the grid? Since head is fixed, I need to know how the frequency is adjusted.
 
Is the generator a synchronous generator or an asynchronous (induction) generator?

Does the hydro turbine have a governor (control system) to operate a variable control valve (wicket)?
 
R
Over the weekend I did a tour of BC Hydro's museum. Each turbine had it's own Woodward governor, I imagine these are set for slightly over 60 Hz and the generator pulls into sync when it's switched on-line.

Bear in mind, I know zilch about generating stations, I will be watching this thread with interest.
 
Head determines the maximum shaft power a turbine can produce. The power produced when the generator is synchronized is controlled by adjusting the amount of water flowing through the turbine. Power is RPM multiplied by torque and since a synchronous machine runs at constant speed, you get more or less power by producing more or less torque.

The way the flow is adjusted depends on the turbine type; for a Francis turbine, a series of what are called wicket gates are opened/closed to throttle the amount of water.

The offline generator is brought up to slightly above synchronous speed by adjusting the water flow before a sync check and breaker close.
 
1. Generator is a synchronous generator

2. Prime mover is a Turgo turbine w/o control valve. In deed it has a valve, but it is used to control flow manually.
 
Is the manual valve used to control the power produced by the turbine-generator after the generator breaker is synchronized to the grid?
 
>:) Yes exactly

Then I believe that same manual "control valve" should be used to control speed during synchronization, as it's probably also used to accelerate the turbine to synchronous speed. If that's the only means of controlling speed when the generator breaker is open and the only means of controlling load when the generator breaker is closed it is--even if it's not automatically controlled by a governor or synchronizer--the unit control valve.

Are you assuming that because speed isn't changing when the manual valve position is changed when the generator is synchronized to the grid that the speed won't change when the generator breaker is open?

As was said, when synchronized the speed of the turbine-generator is strictly a function of grid frequency. The speed won't change when the manual valve is opened or closed <I>when the generator breaker is closed</i> (I.e., when the unit is synchronized to the (with the) grid) because speed is fixed by frequency.

But when the generator breaker is open varying that valve position will vary speed (unless there's something we don't know about the turbine) because there's no current flowing in the generator stator windings to produce a magnetic field to "grab" and "lock" the generator rotor into synchronous speed (a speed that is proportional to the frequency of the grid and the number of generator rotor poles, the latter which is fixed by the construction of the generator).

The relationship between speed and frequency is defined by the formula:<pre> F =(P * N)/120

where F = Frequency (in Hertz)
P = Number of poles of generator rotor (always a fixed, even number)
N = Generator rotor speed (in RPM)</pre>
Hope this helps!
 
Thanks CSA!

I got the point and learned a lot. Here is what I think would be the process of synchronizing:

1. Frequency is matched by adjusting flow control valve

2. Voltage is matched by adjusting excitation

3. Phase angle is matched by momentarily applying a load to get the signals in-phase

4. Close the breaker

I read a post in another thread in this forum that we should keep the generator voltage little higher than the bus voltage to avoid reverse power situation. Is this applicable to frequency and phase angle as well?
 
laklmap,

1. Yes.

2. Yes.

> 3. Phase angle is matched by momentarily applying a load to get the signals in-phase

No. The only way to "apply a load" is to close the generator breaker.

When there's no current flowing in the generator stator windings there is no magnetic field--but by knowing how the magnetic fields will form from the current which will flow in the generator stator when the generator breaker is closed we can make sure that the magnetic fields will properly aligned when the generator breaker is closed.

Synchronization is done to make sure that when the generator breaker is closed that the magnetic fields of the generator rotor and the generator stator will be very nearly aligned--because if, let's say, they are 180 degrees out of phase (such that the North pole of the generator rotor was directly above where the North pole of the generator stator would be when the generator breaker closed) then the magnetic forces of repulsion (like poles repel; unlike poles attract) would try to spin the rotor very fast in one direction or the other UNTIL the North pole of the generator rotor was aligned with the South pole of the generator stator and then the magnetic forces of attraction would try to STOP the rotor very quickly. And, these forces usually cause mechanical damage to the generator rotor, the load coupling between the generator rotor and the prime mover (the hydro turbine in your case) and the prime mover. They can be VERY destructive.

So, during synchronization even though there's no current flowing in the generator stator windings the magnetic fields that will result when current does flow are known based on the relative waveforms of generator terminal voltage and running (bus) voltage. And, they must be "in phase" (or very nearly in phase) during synchronization so that the magnetic fields of the stator will be properly aligned with the magnetic fields of the generator stator when the generator breaker closes.

Phase angle is checked by looking at the synchroscope/synchronizing lights, and also usually by a synchronizing check relay--to ensure phasing is correct before the generator breaker is closed. So, the two wave forms (from the generator and the bus (grid)) are compared and this is how phasing is verified during synchronization.

> I read a post in another thread in this forum that we should keep the generator
> voltage little higher than the bus voltage to avoid reverse power situation.

Generator terminal voltage needs to equal to or slightly higher than running (bus) voltage so that when the generator breaker closes the reactive current (VArs) will be zero or slightly lagging (considered to be "positive" VArs). Generator terminal voltage--which is a function of excitation when the generator rotor speed is stable--affects reactive current (VArs), not power (watts; KW; MW), when the generator breaker is closed. So, again, when synchronizing the reactive current flow can be "anticipated" and by making the generator terminal voltage equal to or slightly greater than running (bus) voltage so that the reactive current will be low or slightly positive when the generator breaker closes.

Reverse power (negative watts) occurs when the water flow-rate through the hydro turbine is less than what's required to make the generator rotor maintain synchronous speed--and power flows from other generators and their prime movers on the grid to the generator to keep the generator rotor spinning at synchronous speed. This is said to be "reverse power" since power is flowing into the generator, instead of out of the generator. This is also called "motorizing the generator." This is why during synchronization the speed of the generator rotor is made to be equal to or slightly higher than the speed proportional to the frequency of the bus--so that when the generator breaker closes there will be zero watts or slightly positive watts flowing out of the generator. (If the speed of the hydro turbine-generator is slightly higher than the speed proportional to the grid frequency, when the generator breaker closes the speed will slow down to match the grid frequency--and the water flow-rate which was making the speed slightly higher than grid frequency will become positive real power (watts).

> Is this applicable to frequency and phase angle as well?

No.

So, during synchronization hydro turbine-generator speed is made to be equal to or slightly greater than the speed proportional to grid frequency for two reasons:

1) to ensure that when the generator breaker closes zero or positive watts will flow out of the generator,

and,

2) the magnetic field which will result when current starts flowing in the generator stator will be properly aligned with the magnetic field of the generator rotor when the generator breaker closes to prevent serious mechanical damage to the generator, the load coupling and they hydro turbine.

In the same way, generator terminal voltage is made to be equal to or slightly higher than running (bus) voltage so that when the generator breaker closes there will no or slightly positive reactive current (VArs) flowing in the generator stator windings. (There's also a secondary reason for "matching" generator and bus voltage--and that's because when they are nearly equal the potential between them is near zero, and it takes less force to close the generator breaker contacts and there is less arcing across the generator breaker contacts during closure).

The other really important part of synchronization is that the phases of the generator must be matched to the phases of the bus--meaning that "A" phase of the generator must be the phase that's going to be connected "A" phase of the bus, and the "B" generator phase will be connected to the bus "B" phase, and "C" generator phase will be connected to the "C" bus phase. This is done during initial synchronization of the generator with the bus--and since the high-voltage connections are usually rigid and not disturbed after construction and verification it's assumed that once verified they will not change. This is a very important check during initial synchronization of any synchronous generator, and, again, once verified the high voltage links (bus bars; conductors; etc.) are never disturbed. But, if they ever are disturbed/changed/modified, then this verification must be redone to ensure the phasing is correct before the generator breaker is closed.

Hope this helps!
 
> The only way to "apply a load" is to close the generator breaker.

This is a 130kW generator (it is so small) and it has a ballast (set of resistive loads). I noticed, the load is switched ON/OFF (sometimes full set and sometimes partially) when it is being synchronized.

That is why I thought phase angle is synced by using this load. I assumed, since it is not connected to the grid, applying a load suddenly will affect the frequency and phase angle so that they can match these parameters with the bus.

But now I can understand that this cannot be the reason since if it is a much larger generator, then having a massive load to adjust these parameters will not be possible.

Now I have no idea why it is being used anyway.

During the rainy season, they can produce more electricity and they manually adjust the flow depending the flow rate. They wanted me to automate this. That is how I got into this. :)

<b>CSA, Thank you a lot! for your valuable knowledge you shared.</b> If you can think of why they are using the ballast while synchronization, please share your knowledge.
 
CSA,

That's a very interesting explanation, I have a couple of questions.

Suppose we have a 100 MW turbine, roughly what percentage of normal water flow would it take to get the unit up to speed?
If the governor fails is it possible to over-speed the turbine to the point it flys apart or is it naturally limited by the water velocity?

Your explanation of the need to verify the phasing reminds me of a story I was told about a steam fired station in NZ, when they bought the second 120MW unit on line for the first time it was out of phase due to a wiring error in the instrumentation, I was told it just about jumped off the foundation.
 
As I can remember, water jet speed is sqrt(2*g*h). If it is connected to grid, speed won't change. If the generator is not connected to grid, it attain the speed of water jet (mechanical losses will have to be taken into account) and if other mechanical linkages are not designed to operate in this condition, outcome would be destructive.

> If the governor fails is it possible to over-speed the
> turbine to the point it flys apart or is it naturally
> limited by the water velocity?
 
Roy Matson,

Good questions--both. I'm not extremely knowledgeable in hydro turbine operation and application.

I think the answer to the first question is a function of the design of both the turbine and the generator and the type and size (diameter and length) of the inlet "[email protected] (which could be actual pipe or a hole drilled through rock.

And, I think the answer to the second is similar to the first--it depends on the design of the unit and the application (size; volume; velocity; design).

Wish I could provide better answers, but synchronization is more my area of experience and knowledge. As I presumed, the application in question has some previously unknown particulars (the "ballast" resistors), which might be used for some kind of coarse speed control since the "control" valve is manual--and the unit is so small (150 KW).

The mechanical forces of magnetic attraction and repulsion in a synchronous generator are truly great (powerful) and can be very destructive. If they weren't, synchronization wouldn't be so critical. And, people forget that for large generators operating at thousands of volts the synchronization (and metering and protection) circuits use transformers which must be checked for proper polarity before being put into service--or really bad things can happen. I preferred to verify metering PT polarity with high-voltage "hot sticks". But most sites don't have that equipment these days.
 
Roy,

1. It doesn't take a lot of water to keep a turbine on-line at synchronous speed - look up "spinning reserve" where a relatively low flow will suffice to keep a machine running and able to synchronise in a couple of seconds. Limiting factor is the length of the water column that has to be acelerated. If at the end (or the beginning) of a long tunnel or canal system, loading times can be quite long - unloading is possibly worse as a sudden load shed can cause some amazing surge waves on inlet canals. If you've wondered why canals are only half-full, that's the reason.

This has an interesting side-effect on hydro governors - they build in "temporary droop" to deliberately slug the response of the governor to sudden load changes. A typical setting could be up to 40%. The effect decays exponentially over the "temporary droop time constant" which could be 5-10 seconds for a run-of-river station, or up to 15 or 20 minutes for a station using tunnels or canals.

2. With a Pelton Wheel, the speed of the turbine is limited by the speed of the water - which is dependent on the head driving it. 1/2 v^2 = gh. I'm pretty sure that the same applies to the Francis turbine design as well - but at lower head, these need a lot more water flow than Pelton Wheels for the same power.

3. Not sure if the 1st synch of Marsden Unit 2 was 180 deg out of phase or not - I was there about 6 months after commissioning and there wasn't any comments about that then. I did hear the rumour a few years later. It didn't seem to have any long-lasting effects anyway - nothing as serious as the blade fatigue failures we got into with both units. (No 1 machine had been operating for several months with one of the 2nd row HP rotor blades off and lodged behind the stator blades - they were lucky not to "cotton-reel" it.)
 
I would like to thank you all for the knowledge you shared. What a value this forum has in terms of shaping the future/knowledge.

Thank you again!!!
Lakmal
 
Top