Effect of generator load increment or reduction on constant speed turbine??

[baasi]

As load on generator increase it will try to reduce speed of turbine rotor to cope this we will increase steam amount to maintain speed of turbine constant ,so I want to ask that what will happen in actual because speed of turbine is constant in both cases whether load is increases or decreases and if speed is constant it means torque is constant than how power out put of generator will change i.e sometime it is 600MW but on another time it is 250MW
Elaborate it please I am bit confuse about it

 

[lilliput1]

Refer to the unit sequence of operation concerning speed regulation. Usual speed governor controls use round metal weights spinning on a vertical axis and geared to the rotation of the turbine shaft. Linkages on the weight operate a slide valve that is use to control steam flow to the turbine to increase or reduce steam flow to maintain turbine speed at varying load.

 

[gruntguru]

Torque is NOT constant when steam flow varies.

je suis charlie

 

[cranky108]

If the generator is connected to the grid, increasing or decreasing the torque will not change the speed (or frequency) of the grid by very much (depending on the size of the grid). Increasing torque to the generator will only increase the power output, as well as change the angle of the magnetic field between the rotor and stator.

Is this what you are asking?

 

[lilliput1]

It is like a adding more weight to be carried by a car or train. At higher load more power would be required to maintain same speed.The load on the generator imposes magnetic force repelling the rotation of the rotor.

 

[cranky108]

I look at it as increasing the torque is trying to increase the speed of the whole grid. At current grid sizes of around 1 TW, the total mass you are trying to accelerate is a few magnitudes greater than the generator you have. Not to mention that if you do manage to increase to speed, some other generator will drop in output by about the same amount you increased yours.

It's called frequency control.

 

[crshears]

Hello baasi, is this generator connected to an isolated system, a small grid, or a large grid?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[baasi]

crshears
yes generator is connected with large grid

 

[baasi]

Thank you every one for your reply
Actually I have read somewhere that when generator is connected with grid its speed cannot be increase further (Not more than synchronous speed) but its torque will increase, as a result of increasing torque generator amperage will increase which increases output of generator and if we decrease torque below synchronous speed generator output will decrease.
My confusion is that how ampere of generator will increase with increasing torque and what will happened if we want to decrease power output from 600MW to 50MW and we decrease amount of steam once its synchronous speed is achieved.
Once synchronous speed of generator is achieved, is it possible that we can decrease torque to very low value for example from 600MW to 50MW?? Please explain me that is there any relation of synchronous speed and amount of steam that can justify that once synchronous speed is achieved at certain amount of steam but to decrease torque (For example to achieve 50MW at output) amount of steam will decreases. Is it possible that we can decrease amount of steam to such a low limit that we can achieve desired output but its synchronous remain unaffected???

 

[cranky108]

When you increase the torque on the generator you do change the angle between the stater and the rotor. In effect you are attempting to pull the frequency faster.

When you decrease the torque on the generator the reverse happens. In fact if you decrease the torque enough at some point the generator will start turning the turbine to keep the speed constant. This causes some heating in the generator, but more importantly, the turbine also heats up from lack of steam flow to keep it cool. This is what we call motoring.

So the lower limit is normally a turbine limit for heating, and not a generator limit.

This is not to say the generator does not have limits, it is to say those limits are different.

 

[byrdj]

the Turbine's Governor controls the % steam flow to the turbine.
To accelerate to rated speed, the required steam flow is about 2%
Once at rated speed, the Generator is Synchronized to the Grid. the Generators excitiation now controls the generators output VOLTAGE.
The Turbine's Governor now is operator demanded to open CVs and admit more steam. the add steam produces mechanical torque which the Generator produces AMPs

In the MICRO control of the Grid/System, IF you produce ONE WATT more than the what is being used by the Grid, the speed will increase. If you observe your Grid's frequency you will see that it is constantly changing, mind you it is a small devation from rated say +/-0.05 Hertz, but it is having to respond to every time someone turns off/on a single light bulb.

Your Grid frequency is being Monitored by some Agency. they have experiance that they anticipate there will be a need for more MWs early in the AM and will instruct your Turbine's Operators to increase steam to the turbine and make MWs. just prior to people waking up and turning on thier coffee pots, the grid frequency will be greater than rated after you increase to your maximun load by our operators opening the CVs and steam flow going to 100%, but as the people start comsuming power, the frequency will return to rated and possibly go below rated. the opposite occures at night, factories shut down, grid goes high, Agency instructs your turbine to go to minimun load for the night (or maybe even shut down if demand is low enough)

To take the generator off line, the steam flow is reduced to less than 2% that was needed to achieve rated speed. at that instant, the generator becomes a motor and MWs are then used to keep your generator at speed. "Motoring" the generator is not a problem for it, but it will quickly damage the turbine and thus thier will be a reverse power event to seperate the genenrator from the grid

 

[cranky108]

I think for every boiler there is a magic lowest production number, which is likely based on feed pump minimum pumping, feedwater heater capacity, valve closing minimum, etc. So every plant has a minimum production capacity, without blowing off steam. Blowing off steam requires the addition of makeup water, which comes from a water processing plant, or process.

There is a typical electrical demand curve which includes time of year and expected temps which is used to dispatch the several plants that most utilities own, or dispatch over.

 

[baasi]

byrdj (Mechanical)
Is 2% flow is enough to get rated speed??
Is it thumb rule?
If it is like this then 98$ steam is only to increase torque??
Plz expalin

 

[crshears]

Hello baasi,

Once a generating unit is synchronized to a grid, the speed at which that unit rotates follows in lock-step with the frequency of that grid. The rotational speed of the unit therefore effectively does not change with changes in unit loading; instead, as cranky has stated in different words, the torque applied to the shaft and the angle of advance of the rotor ahead of the stator both vary directly and elastically as the output of the unit when generating, and 'stretch' behind the stator when the unit is motoring.

The minimum loading of the units in the steam plant I worked in was due to the minimum stability of one coal pulverizer of six; if the load was reduced too far, the air flow through the mill would become erratic and unstable, causing a boiler flame-out, which then required a furnace purge and re-ignition on oil.

As to motoring of steam turbines, ours generators were equipped with a steam-valve-supervised reverse power relay to trip the unit off line if the unit motored for more than a certain length of time; in such situations the operators would be scrambling in an effort to get "ventilating" steam flowing through the turbine again and beat the trip timer, sometimes successfully, sometimes not.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[byrdj]

2% with steam at rated conditions is an expected "rule of thumb"

the % "corrected" Control Valve (CV) opening should be near % rated steam flow, so at Full Speed No Load (FSNL), the CVs will be about 2% open. If the steam conditions are less than rated (which normally are at startup) the CVs will be open more for FSNL. whether you are just holding speed with the governor or you just close breaker with no load, the required steam flow is the same (~2%).

the controls algorytm for different turbines vary, but once synchronized, the Operator (person or other automation) must demand the CVs to open to increase steam flow and thus load. with the correction for FSNL steam flow, the % MW load and the % steam flow will be nearly linear.

Steam flow through the CVs is not linear with there physical % opening, thus most control systems will determine a "corrected" CV opening which is calculated from % steam flow.

I am mechanical and not knowledgable about the generators design of field angles, but familaer with the concept. I just don't consider them in my understanding.

Something to think about. If 2% steam flow will achieve rated speed, If you were to open the generator breaker with 100% steam flow at 100% load....What speed would the turbine go to.

 

[gruntguru]

Burst speed?

je suis charlie

 

[crshears]

It depends! How many of the following are installed and operate correctly: auxiliary mechanical/electronic governor &/or emergency overspeed governor, main and reheat steam emergency stop valves, turbine trip from inadvertent opening of unit breaker/ "diameter" breakers, release valves, etc.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[cranky108]

Sort of depends on how fast the magnetic field drops, which if it does not would still produce some breaking impedance.

I suggest you don't try this, at least at one of the plants I'm involved with.

But you are correct that on the steam side this can happen for several conditions, that might not be related to steam.

 

[byrdj]

for my last comment...I didn't expect an answer but to emphise to the OP the turbine design is to produce torque and the amount of steam needed for any speed is nothing in comparison. I don't plan to side track the OPs original inquire with a discussion of steam turbines response to full load rejectons

 

[cranky108]

I would like to expand the discussion, just to compare a few things.

If instead of steam, we were to use water, we would have many of the same issues. Except we would have much fewer issues with low fluid flow. Once online the speed is locked and no matter how much water is pushed through that won't change.

If instead of steam, we were to use air, it would be like water or steam. Except low fluid flow would become a large drain of power on the grid, and would be very noticeable. Again once online the speed is locked and no matter how much the turbine pushes out in won't change the speed.

See the theme here? The grid is an energy transfer system that has a huge capacity to keep all the generators in lockstep.
Not that there are no exceptions, but the commonality of the generators sets the conditions.