generator speed control

8/8/2019 Generator Speed Control

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rom the Automation List department...

Steam turbine generator speed control - clarificationPosted by Mikas on 19 June, 2007 - 11:48 am

Hello to all,

I need clarification about a couple of things regarding turbogenerator's speed control.Can you confirm the following:

1. When generator is not connected to the grig, when openning steam regulating valves and allowing more steamto turbine, turbine will rotate faster, right?

2. Generator is mechanicaly coupled with the turbine and it always rotates same speed as turbine.

3. When generator is in parallel with the grid, it cannot rotate faster or slower then 50Hz (suppose for Europe) or3000 rpm (2 pole machine), or in another way, it will rotate the same speed as grid's frequency (for example 49.98Hz).

4. When generator is synchronized with the grid, adjusting steam control valves cannot change the speed of theturbine/generator and it will change only output power.

I'm looking forward to your comments.Thank you very much.

Posted by CSA on 20 June, 2007 - 12:02 am

Mikas,

You have it all correct.

1) When the unit is not connected to a grid (particularly a large, or infinite, grid) increasing the energy beingadmitted to the prime mover will cause the prime mover to increase speed.

When the unit is connected to a grid, increasing the energy being admitted to the prime mover will NOT resultin a speed increase. It WILL result in an increase in the amperage of the generator. In other words, the poweroutput of the generator will increase. The extra torque which would cause the unit to increase its speed whennot connected to the grid gets converted by the generator into additional power output (amps).

2) If the prime mover and generator are directly coupled, in other words, there is no reduction gear or speedincreaser between the prime mover and the generator, the prime mover and the generator will turn at the samespeed as the generator.

Even if there is some gear box (reduction or speed increaser), the speed of the prime mover is still directlyproportional to the speed of the generator rotor. And the speed of the generator rotor is directly proportional tothe frequency of the grid to which the generator is connected.

As has been noted elsewhere on control.com, the frequency of a generator is directly proportional to theproduct of the number of poles of the generator times the speed of the rotor (in RPM), divided by 120: F =(P*N)/120. If a grid is operating at 50 Hz and the generator connected to the grid has two poles, the speed ofthe generator rotor will be 3000 RPM (N = (120*50)/2).

3) AC generators are usually synchronous generators. Synchronous means they are locked in synchronism withthe frequency of the grid to which they are connected, especially if the grid is very large, or, infinite. Suppose a60 MW steam turbine is connected in parallel with other generators on a grid with a total output of 6,000 MW.The little 60 MW steam turbine isn't going to make all the other turbines speed up or slow down detectably asthe prime mover's energy is increased or decreased--there's just too much inertia to overcome.

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Also, there should be operators and control systems somewhere on the grid which would decrease the load ofone or more units to maintain the grid frequency at rated. As units are loaded by their operators they actuallyaccept some of the load from the grid. If enough units are loaded without some units being unloaded equally,the grid frequency will begin to increase.

4) See 1, 2, and 3 above.

But, you've got it!

Posted by CTTech on 22 June, 2007 - 2:20 am

Control systems have a controlled variable and a manipulated variable. If a control system detects no changein the controlled variable, the system has no reason to change the manipulated variable.

If a synchronized generator is asked to adjust load, torque is removed or applied to the prime mover. If thesechanges in speed are "undetectable", the control system has no reason to increase or decrease power input.The changes are infinitely small, yet must exist for the control system to respond. I will concede that multiplethings are happening in one instant of time, but a detectable change is occuring.

Although the hugh power sink in a large grid seem to minimize or absorb changes, measureable changes areoccuring. If not the manipulated variables would not change.

The large "infinite bus", whether controlled by human or non-human controls systems must be able to"detect" these incredibly small changes or no reason would exist for the control systems to change theirmanipulated variables (i.e. other generator sets on the grid.) and maintain this dynamic and delicate balanceof frequency and voltage.

I may be totally wrong. I have found that on the generator sets that I have encountered; Speed is thecontrolled variable and steam or fuel input to the turbine is the manipulated variable. Therefore speed mustchange to induce a change in steam/fuel imput.

I welcome all feedback on why I am wrong.

CTTech

Posted by Bruce Durdle on 23 June, 2007 - 2:02 pm

Lets go back to basics ... A turbine-generator set obeys an energy balance law. Energy in = energy out +change in stored energy. Or power in = power out + rate of change of stored energy.

Power in is found by multiplying the steam flow by the steam enthalpy (- the cooling water flow * increasein CW energy) (- a few other terms that are more or less constant). Power out is the electrical load on thegenerator. Stored energy in the machine is kinetic energy of rotation and proportional to the square of RPM.If the load is increased above the power in, the machine will slow down.

A power system is an energy balance on a very large scale. The energy in is the mechanical energy appliedto all the turbines. The energy out is the sum of the demand of all the light bulbs, wall warts, TV sets, andelectric motors connected to the system. When you turn on a light bulb, all the connected rotating machineswill slow down. Somewhere on the system is a generator that will sense the drop in speed and increasegeneration.

On a large interconnected system such as that in North America, this speed drop is infinitesimal - but it stillhappens. On a smaller system such as we have here in NZ, frequency excursions due to sudden loadchanges are a fact of life - a hiccup on the DC link connecting the 2 islands can cause a 1 or 2 % frequencydrop in 1 or 2 seconds.




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On a steam turbine connected to a grid, a shortfall between power in and power out could occur if there is asmall drop in steam pressure or temperature, as well as a load change. This will cause the rotating parts toslow down. As a result, the internal angle between the rotor poles and the magnetic field set up by the statorcurrents will fall, and the exported electrical power will drop. This restores the balance. The speedexcursion will be very short-lived and will probably not affect a governor.

In older systems, the set-point for a governor was referred to as the "speeder gear". Prior to synchronising, achange in the setting for the speeder gear resulted in a speed change. After synchronising, the same changewould give an increase in power with no obvious change in speed. With electronic governors, the controlstrategy can be a lot more complicated and perhaps needs to be.

On one plant I have worked on we had a small 2.5 MW gas turbine. The governor system was capable ofreacting very quickly to the above-mentioned frequency dips, and would increase generation in response -sometimes to well above the nameplate rating (I have seen the analogue power indication at more than 3.5MW on occasion). An electronic power limit would have been quite useful in that case.

So CTTech is right - speed is the major controlled variable, and changes in speed act to change the governorvalves. The system is also self-regulating in that the electrical power out changes with machine angle.

Bruce.

Posted by Phil Corso on 24 June, 2007 - 10:47 am

I disagree with both CCTech's 23-Jun (02:29) and Bruce Durdle's 23-Jun (14:02) comments stating thatthe major controlled variable is speed.

I believe we can all agree that power-demand (kW) is provided by the prime mover. Furthermore, we canagree that any power demand change will cause the turbine's Speed Regulator, the Turbine GovernorControl (TGC), to intervene, thus correcting deviations. But, the response is relatively slow. So slow infact, that its impact on system stability is ignored! (Just think of the original TGC, the Watt rotating-ballgovernor)

But, now consider the case when load power-factor changes, i.e., power-demand remains constant but theload's power-factor, or as is said, reactive-power (kVAr) changes. Does the speed change? No! Why not?Because reactive-power is not real-power! So what happens? Of course, the generator's current outputchanges! That change, then results in a change of the generator's terminal voltage. What detects thatchange: the AVR!

Thus, the voltage regulator, or in today's jargon, the AVR, changes the generator's field-excitation tocorrect the terminal voltage. (The 'A' in AVR, of course eliminated the need for an operator to keep an eyeon the volt-meter!) The AVR, while it can't supersede the TGC, certainly complements it. It allowsquicker response to output requirements. There shouldn't be any doubt about the improvement in dynamicresponse that today's computer-generated transfer-function models have made to system stability andtransient recovery! But, the real key is the AVR's ability to instantly detect electrical parameter change,not the TGC's ability to control turbine speed!

Phil Corso, PE ([email protected])

Posted by CTTech on 24 June, 2007 - 5:13 pm

I concur with Mr. Corso to a degree. A GE EX2000 Excitation drive is so fast in response time that anadditional feedback signal had to be added to stabilize the drive. The feedback is speed.

I merely wanted to point out that to fully understand this delicate and dynamic balance; one must firstlearn the basics. Generator speed and generator frequency are directly related and this cannot be ignored.




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To infere that generator speed is NOT changing while synchronized is ignoring this relationship.

Once the basics are learned, one can investigate the many other things that are happening in the sameinstant of time to stabilize yet increase response time of the delicate energy balance.

Best regards,CTTech

Posted by Mikas on 9 August, 2007 - 11:33 pm

Thank you all for your replies. I've learned a lot by reading your posts.

However, I'd like to ask you to help me understand what is primary and what is secondary control ofturbine speed.

If I understood correctly, primary control is spontaneous reaction of turbine's controller, but don'tknow what secondary control would be.Please, can you give a detailed explanation?Thanks.

Posted by CSA on 24 June, 2007 - 5:10 pm

Thanks to Mr. Durdle for sparking a return to basics. There's nothing more enlightening than beginning atthe beginning. Unfortunately, we can't discuss the theory of magnetism and amperes and voltage andcurrent, and specifically, of alternating current and voltage; it's presumed we all understand those basics orcan look them up. But, we'll venture again into the breach.

Synchronism: to occur at the same interval or frequency. Synchronous generator: a device, usually with arotating electrical field that, when operated properly (as per Mr. Corso) in parallel with other electricalgenerators, will spin at a speed that is directly proportional to the frequency of the alternating current onthe grid.

From other posts, that speed is N = (120 * F)/P. N is the speed of the rotor, in RPM; F is the frequency of

the AC system to which the synchronous generator is connected, in Hz; P is the number of poles of theelectrical field; 120 is a number which allows for conversion between Hz (cycles per second) and RPM(revolutions per minute).

When a synchronous generator is connected in parallel with other synchronous generators, an electricalmagnetic field is created on the stator, actually three electrical magnetic fields since most synchronousgenerators are three-phase machines. Because of the alternating nature of an AC electrical system, themagnetic fields created on the stator appear to rotate around the stator.

The rotating electrical field of the synchronous generator is locked in to synchronism with the rotatingelectrical fields of the stator and can not spin any faster or slower than the rotating electrical fields of thestator. And that speed is defined by the formula above.

When a synchronous generator is started and accelerated to synchronous speed in preparation forconnecting the generator to a grid in parallel with other generators, any change in energy to the primemover results in a change in speed of the rotor of the generator. That change in speed will result in achange of the frequency of the synchronous generator by solving the formula above for frequency: F =(P*N)/120, which is the same formula above, just solved for frequency.

Once the synchronous generator is synchronized (there's that word again!) to the grid with othergenerators, it's speed is fixed by the frequency of the grid. Any change in energy to the prime mover willresult in a change in the amount of amperes flowing in the stator of the synchronous generator because.And the power produced by a synchronous generator is a function of the number of amps flowing in the




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stator of the generator.

This relationship between frequency and speed is one of the reasons why a synchronous generator must besynchronized with an electrical grid when it is being connected to an electrical grid. The frequency of thesynchronous generator being connected to the grid must be made nearly equal to the grid's frequencybefore the generator breaker is closed for a smooth and stable breaker closure. The speed of the rotatingmagnetic field is directly proportional to the frequency of the synchronous generator, so the prime mover'sspeed is adjusted to make the frequency of the synchronous generator nearly equal to the grid's frequency.

Once the generator breaker is closed, any change in energy being admitted to the prime mover will resultin a change in the torque being applied to the synchronous generator. Because the synchronous generatoris now controlling the speed of the unit (the prime mover and the synchronous generator), the change intorque does not result in a change in speed of the unit, it results in a change in the amperes flowing in thestator of the generator.

So, there's a certain amount of energy that's required to make the synchronous generator rotor spin at aspeed that makes the frequency of the generator equal to the frequency of the grid to which is isconnected. This is the energy required to make the synchronous generator spin at synchronous speed.

Once the synchronous generator is connected to a grid with other electrical generators, an increase in

energy being admitted to the prime mover which would tend to increase the speed of the unit results in anincrease of the electrical power of the generator causing more amperes to flow in the stator of thegenerator, but not an increase in speed of the unit.

If the energy being admitted to the prime mover driving a synchronous generator connected to a grid withother electrical generators is less than the energy required to keep the generator rotor spinning at a speedsufficient to keep the generator frequency equal to the grid frequency the generator will become a motorand spin the prime mover at a speed which is directly proportional to the grid frequency. If the energybeing admitted to the prime mover were shut off and the breaker remained closed, the unit would continueto spin at synchronous speed, as long as the excitation being applied to the synchronous generator rotorremained operational (in deference to Mr. Corso, which is why we must keep making reference tosynchronous generators being operated as synchronous generators, even though he has provided no detailsabout how long the synchronous generator which was operated asynchronously lasted when being

operated without excitation or for how long it operated asynchronously).

To understand AC, alternating current, electrical power generation one must understand the machines usedto generate electrical power and those are usually synchronous machines. When operated as designedwhen connected to an electrical grid with other generators, synchronous generators and the prime moverswhich are usually directly coupled to the generator rotors can spin no faster nor any slower than the speeddefined by the formula above.

Any change in the torque being applied to the generator by the prime mover will result in a change in theamperes flowing in the generator stator. Increasing the torque above that required to maintain synchronousspeed and frequency will result in amperes which can be used to power loads connected to the grid. These

amperes are generally considered to "flow" out of the generator.

Decreasing the torque below that required to maintain synchronous speed and frequency will result inamperes flowing in the generator stator which will cause the generator to become a motor and keep therotor and the prime mover turning at synchronous speed and frequency. In this case, the amperes areconsidered to "flow" into the generator, "motorizing" the generator.

The only difference between a synchronous motor and a synchronous generator is the direction of currentflow, or, from a different point of reference, the direction of torque flow. Torque exceeding that requiredto maintain synchronous speed will cause the electrical machine to become a generator. Torque less thanthat required to maintain synchronous speed will cause the machine to become a motor. Amps will flow




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out of a synchronous electrical machine which has an excess of torque being applied to it, excess meaningmore than required to maintain synchronous speed. Amps will flow into a synchronous electrical machinewhich has a deficiency of torque being applied to it, meaning less torque than required to maintainsynchronous speed.

But in no case will the speed of a synchronous electrical machine be more or less than the synchronousspeed, which is directly proportional to the frequency of the AC grid to which it is connected regardless ofwhether it is a motor or a generator.

The power, watts, produced by a synchronous machine are a function of the torque applied to the machineby the prime mover.

Amazingly enough, reactive power, or VArs, is very similar to watts. Increasing excitation above thatrequired to maintain the generator terminal voltage equal to the voltage of the grid to which thesynchronous generator is connected will cause VArs to "flow" out of the generator. Decreasing theexcitation below that required to maintain the generator terminal voltage equal to the voltage of the grid towhich the synchronous generator is connected will result in VArs "flowing" into the generator.

(This ought to get a response from Mr. Corso!)


From previous posts about droop speed control, which is how most prime movers are operated when thesynchronous generators they are driving are connected in parallel with other generators on a grid, thefrequency of a synchronous generator is defined by the formula F = (P * N) / 120. F is frequency, in Hz; p isthe number of poles of the synchronous generator; N is the speed of the synchronous generator rotor towhich the prime mover is typically coupled; and 120 is a constant which is used to convert all sorts ofthings (radians, and RPM to seconds (Hz), etc.).

The formula can be solved for speed: N = (120 * F) / P. For a two-pole synchronous generator connected toa 60 Hz grid, the rotor will spin at 3600 RPM. For a two-pole synchronous generator connected to a 50 Hzgrid, the rotor will spin at 3000 RPM.

Most prime movers are connected directly to the synchronous generator rotor either though a single loadcoupling or through a reduction gear; very few couplings are variable speed couplings.

Consider a 40 MW turbine-generator connecting to a 6,000 MW grid. It's impossible for that 40 MWgenerator's prime mover to increase the speed of all the other generators on the grid by any appreciableamount. The 40 MW unit's synchronous generator is locked into the same frequency as all the othergenerators on the grid, and because the prime mover is directly coupled to the synchronous generator itsspeed is directly proportional to the generator rotor's speed which is a function of the frequency.

It's a pretty straightforward formula, and when a synchronous generator is connected in parallel with othersynchronous generators, no single synchronous generator can run faster or slower than any othersynchronous generator. (That's kind of the definition of synchronism: everything is occuring at the sameinterval or "frequency", no pun intended.)

If the prime mover could be disconnected from the synchronous generator rotor while the generator was stillconnected to the grid, the synchronous generator rotor would continue to spin at synchronous speed, nofaster and no slower. In fact, if the energy being admitted to the prime mover is cut off and the synchronousgenerator remained connected to the grid, the generator and the prime mover will remain at synchronousspeed.

The next time the units at your site are connected to the grid, check the speed of the prime movers--if theprime movers are directly coupled to the synchronous generators, the speed of the prime mover is fixed bythe generator frequency when connected to the grid.




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No matter how much torque is applied to the generator, as long as the torque doesn't exceed the rating of thegenerator, the speed of the synchronous generator rotor will be fixed by the frequency of the grid to which itis connected. The torque--which would increase the speed of the rotor if the generator were not connected tothe grid--gets converted to amps by the generator. Amps that power devices connected to the grid.

Droop speed control changes the turbine speed reference--it doesn't actually change the turbine speed whenthe synchronous generator being driven by the prime mover is connected to a grid. Droop speed control isstraight proportional speed control. If there is an error between the prime mover speed reference and theactual speed there is NOTHING in the control system which drives the error to zero.

So, when a prime mover is being commanded to operate at 102.4% of rated speed, it can only operate at thespeed which is directly proportional to the frequency of the synchronous generator to which it is directlycoupled. And if the generator is connected to a grid in parallel with other generators, its frequency is fixedby the frequency of the grid. The frequency of the grid is the one thing in a power system that's supposed tobe fixed and constant. That is, unless you're in India where the grid frequency is ANYTHING but constant.Voltage can vary a little, but frequency is supposed to be constant.

If the frequency can't change, the speed of the rotor can't change. If the speed of the rotor can't change, thespeed of the prime mover can't change since the prime mover is usually directly coupled to the rotor,

sometimes through a reduction gear, but that's not variable.

Droop speed control uses the error between the speed reference and the actual speed to increase or decreasethe amount of energy being admitted to the prime mover. As the prime mover speed reference is increased,but the actual speed is constant, fixed by the frequency of the generator which is connected to the grid, theerror between the reference and the actual increases--and the energy admitted to the prime mover increases.The opposite happens with the speed reference is decreased.

It is the fact that the actual speed of the prime mover is constant and the only variable which is changing isthe speed reference that allows prime movers to share load with other prime movers and their generators onan electrical grid when prime movers are operated in droop speed control.

This is a pretty common misconception. Most operators and many technicians all see the speed of the prime

mover increase and decrease during startup and shutdown and just assume that the speed changes when thesynchronous generator is connected to the grid. But it can't.

So, you, too, CTTech, are correct when you say "speed is the controlled variable and fuel is the manipulatedvariable". But, it's the error between actual speed which should be constant and speed reference. It's themagnitude of the error which manipulates the fuel. The thing is: the actual speed is controlled by the gridvia the frequency of the generator and the variable is the speed reference.

And now for the disclaimer: The above applies to synchronous generators which are not being operatedasynchronously. (Even though asynchronous operation of a synchronous generator will usually result in anoverheated rotor, and probably a pretty severe generator failure.)

Posted by Bruce Durdle on 25 June, 2007 - 12:30 pm

Since we are getting into the maths... Power transfer through a reactive transmission line depends on theproducts of the two terminal voltages, is inversely proportional to the reactance, and is proportional to thesine of the power angle or phase angle difference (delta) between the voltages. This is how power systemsfall apart if a line is overloaded - when delta approaches 90 deg, sine delta reaches a maximum. If agenerator is on the end of a line with significant reactance, and power is increased too far, delta exceeds90 degrees and the transmitted power falls - the machine loses synchronism and trips - power available tothe rest of the system falls - more lines are overloaded - and the lights go out.

While the machines on a system are all rotating at the same electrical speed they are not all aligned - the




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rotors of lightly-loaded machines will be more or less in step with each other, but the rotors of heavily-loaded machines will lag by somewhere about 60 deg. Sudden load changes will change this angle whichwill cause a very short-lived apparent speed change. In CSA's example of a 40 MW generator on a 6000MW system, the frequency change needed to accommodate the loss of the 40 MW set is .013 Hz on a 50Hz system, .016 Hz on a 60 Hz one, if all machines have a droop setting of 4%. This may or may not beappreciable. A good time to watch for frequency changes on a power system is between 6 am and 9 am -when loads are increasing and generation is increasing to match.

Bruce

Posted by Prince on 7 August, 2007 - 1:14 am

THANKS A LOT for the useful, easy to understand data! I wish you were my Power Systems professor.This is one VERY informative posting!

Posted by Mikas on 23 June, 2007 - 1:56 pm

Thank you very much for your answer.There are more I'd like to learn.

1. What will happen with grid's frequency if one large transmission tower fail? In that case generators stays

without load and I'm very interesting how frequency is changed!?!

2. Since increasing in steam flow in turbine will cause output power to increase and increasing in power isbecause increasing of generator current, I'd like to know what is happening with the voltage.

3. I know that reactive power is somehow related to generator's voltage and excitation, but that is very blur tome. If possible can you explain this?

Thank you very much!


1. Hopefully enough reliability is built into the system to prevent power loss to customers. If so, powersystem operators will only record a short lived frequency excursion and power will be rerouted. Crews torepair the damage to the tower will be dispatched.

If this is not the case, someone/something/somewhere will be without power until repairs can be made.

2/3. Most large loads on the power system are inductive (induction motors). An area that produces powermust be able to react to the inductive load. They do this by producing VARs(Volt Amp Reactive power).Additional information on VARs is available on the internet.

Capacitance can also be added near a large inductive load reducing the need for VARs. VARs and MWoutput are connected.

VARs reduce the amount of megawatts a generator can produce, therefore a cost is incurred for theproduction of VARs. The power producer for an area must monitor loads through the different seasons andfind a balance. The addition of capacitance in certain areas versus the cost of producings VARs and theresultant loss of system production capacity.

Posted by CSA on 26 June, 2007 - 12:42 am

1. The amount of generation must always equal the load in order for the frequency to be stable. If atransmission tower falls and some generation is removed from the grid unless there is sufficient capacity ofthe remaining generators, the grid frequency is going to decrease if the load exceeds the generation.Sufficient generation capacity means enough of the remaining generators and their prime movers which are




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still connected to the grid are running at part load and not at full rated power output and can be loaded tomake up the difference of what was lost when the tower fell.

If a large block of load is suddenly removed from the grid because of the failure of a circuit breaker in aswitchyard the grid frequency will increase unless one or more units have the energy admitted to the primemovers reduced. If the generation exceeds the load, the grid frequency will increase.

2. When torque input to the synchronous generator increases, the amperage flowing in the stator windingsof the generator increases. This causes the strength of the magnetic fields of the stator to increase, whichcauses the field of the generator rotor to shrink or collapse. If nothing is done and the torque input to thegenerator continues to increase, the generator terminal voltage would tend to decrease. This is commonlyreferred to armature reaction.

In order to maintain VAr "flow" or power factor at a desired setpoint when manually controlling excitationwhile loading a unit (increasing the energy into the prime mover), it is necessary to increase excitation tocounter the armature reaction.

Conversely, when unloading a unit (reducing the energy admitted to the prime mover) and controllingexcitation manually it is necessary to reduce excitation as the unit is unloaded to maintain the desired VAror power factor setpoint.

Some machines have VAr and/or power factor control features to automatically adjust excitation to controla VAr or power factor setpoint.

3. When a synchronous generator is not connected to the grid but is running at rated speed any increase inexcitation will result in an increase in generator terminal voltage. Conversely, any decrease in excitationwill cause the generator terminal voltage to decrease. Synchronous generator terminal voltage is directlyproportional to the speed of the generator rotor (which is held fixed when connected to the grid) andexcitation.

If a synchronous generator is connected to a grid with its terminal voltage equal to the grid voltage, thepower factor will be unity, 1.0, and there will be zero VArs leading or lagging. Once connected to the grid,if the excitation is increased the power factor will shift to less than 1.0 lagging, and VArs will "flow out" of

the synchronous generator on to the grid. So, that in the same way an increase in torque would tend toincrease speed, an increase in excitation would tend to increase generator terminal voltage but the powerfactor and the VAr flow changes.

Usually, an increase in excitation will cause the synchronous generator terminal voltage to increase slightlywhen connected to the grid depending on grid conditions and other system factors, but it would not increaseby the same amount as if the generator were not synchronized to the grid. Increasing excitation above thatrequired to maintain generator terminal voltage equal to system grid voltage is sometimes referred to asover-excitation and results in the generator trying to boost the system voltage.

Decreasing excitation below that required to maintain synchronous generator terminal voltage equal to gridvoltage is sometimes referred to as under-excitation and results in the generator trying to buck the system.

When excitation is reduced below that required to maintain synchronous generator terminal voltage equal togrid voltage, the power factor shifts to leading and less than 1.0 and VArs "flow into" the generator.

Whether they actually flow into or out of the generator seems to have been contended before on this site.Convention talks about VArs flowing into and out of the generator. Some people dispute whether or notamperes flow in a generator stator on an AC system, since it is an alternating current. Others say currentflows from positive to negative in a DC circuit, while still others say it flows from negative to positive.

It all depends on one's point of reference. And if VArs are considered as being consumed and produced asWatts are (and they are--it's just that most people never see a VAr-hour meter, but they do exist!) thensynchronous machines produce VArs when they are over-excited and consume VArs when they are under-




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excited. It's not possible to control VAr consumption of machines like induction motors and transformerslike Watt consumption is controlled; the VAr consumption is a function of how equipment and machinesare built. But if someone doesn't produce VArs to at least partially offset the consumption of VArs, thelights are going to dim and maybe even go out.

And the maths are going to start coming now. Hopefully not, because we're just talking principles in generalterms for operators and technicians, not scientists and engineers. The maths can be found in any text orreference book, but they rarely discuss principles and unless one takes a long time to understand the mathsthen it just confuses things for beginners--and we were all beginners once.

The maths are just proofs of principles, and we need to understand the principles to be good operators andtechnicians. To predict or model we need to understand the maths. But I've run into more than one personwho can cite the maths, but can't explain what's really happening. Vectors and trigonometry and calculus areall wonderful things to engineers and scientists but just add to the confusion of operators and technicians. Ifthey want to look up the maths and the formulae, they can. This seems to be a site where people can askbasic questions and learn and get more information if they desire.


Mr. Tesla and Mr. Westinghouse started out small. They had no idea how far this would go. I am sure that

frequency was "something to watch" on their first outing.

I had no desire to overwelm anyone. For the "newbies": Study Tesla and the induction motor and AC.Learn the basics.

CTTech

Posted by Anonymous on 27 June, 2007 - 11:19 pm

1) Why do we want the VArs to flow out of the generator always?

2) If every generator is forcing VArs out, trying to maintain a lagging pf, then is there someone (agenerator) out there who's allowing those forced out VArs, into them?

3) When you specifically say Synchronous Generator, does it also mean that there are AsynchronousGenerators too? Or it only means that a generator automatically becomes synchronous (or can we say,synchronized?) when it's connected to an infinite bus because it's too small to effect a change on a largesystem and hence has to behave like the grid?

4) Is terminal voltage only due to AVR excitation? Will the torque (and not the speed) of the prime moverhave no role to play in determining the terminal voltage?

5) When the generator and prime-mover are spinning in synch with the grid, does an extra fuel into theprime mover also increase the mass flow of air through the prime-mover (a single shaft turbine), if theaxial compressor air inlet vanes are not modulating with load?

thanks.


1) Because the majority of synchronous generators are not made to run in an underexcited (leadingpower factor) mode. Refer to the reactive capability curve of the generator for specifics of how agenerator may be operated.

Excessively reducing excitation to put the generator in a leading power factor reduces the synchronousgenerator field strength, increasing the possiblity of allowing the torque being input to the rotor toovercome the magnetic attraction between the rotor and the stator, "slipping a pole" which is very




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catastrophic. There are also problems with generator heating when operating in an underexcitedcondition.

2) Lagging VArs feed a lagging load. The majority of reactive loads on most grids are inductive:induction (asynchronous) motors and transformers (yes, transformers are a inductive load on thesystem). The effect of a lagging load is to shift the voltage and current sine waves out of phase with eachother. By providing lagging VArs, the voltage and current sine waves are shifted back towards eachother.

3) Yes, there are induction (asynchronous) generators, though they are usually small machines (smallhydro turbine-generator or small wind turbine-generators).

4) Synchronous generator terminal voltage is a function of two variables: speed and excitation. Since thespeed of a synchronous generator is usually constant, the way to change terminal voltage is to changeexcitation.

As was said previously, armature reaction affects terminal voltage. Increasing armature current reducesgenerator terminal voltage due to armature reaction.

5) When fuel is burned in the combustor of a gas turbine, the pressure in the combustor increases. Axial

compressors don't behave as people expect them to; when the "back pressure" in the combustor increasesdue to the addition of more fuel, the axial compressor discharge pressure increases. So, even though theair flow is not changing because the speed is not changing (for a single-shaft gas turbine) and thevariable inlet guide vanes are stationary, the axial compressor discharge pressure will increase as fuel isincreased.

Extra fuel does increase the total mass flow--but not the mass flow of air, just the axial compressordischarge pressure.

Posted by pv on 13 July, 2007 - 12:16 am

Extra fuel does increase the total mass flow--but not the mass flow of air, just the axial compressordischarge pressure.

1) What happens to the exhaust temperature?

2) If the exhaust temperature increases then the guide vanes will open to maintain the exhausttemperature and otherwards TTXM will go higher. Is it right? Can you please explain?

Posted by CTTech on 13 July, 2007 - 12:46 pm

Please start a new topic. We beat this one to death.

CTTech

Posted by CSA on 15 July, 2007 - 6:46 pmFrom many other posts on this site, as a GE-design heavy duty gas turbine is loaded the exhausttemperature will increase. As the unit is loaded to Base Load, the IGVs of more recent units willmodulate open at various points during the loading depending on the mode of IGV control.

The early versions of IGV control for most simple cycle applications kept the IGVs at the minimummodulating position, usually 57 degrees, until the exhaust temperature reached approximately 900 F.Then as load (fuel flow) was increased the IGVs were opened to maintain 900 F until they were fullyopen, usually 84 degrees. At that point, any increase in load (fuel flow) would cause the exhausttemperature to increase until the unit reached Base Load exhaust temperature control.




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Combined-cycle applications can improve the over-all plant efficiency by maximizing gas turbineexhaust temperature during low load operation. So, the IGVs were usually held at the minimummodulating position until the exhaust temperature was equal to or slightly less than the exhausttemperature control reference as the unit was loaded.

When the exhaust temperature at part load reached the exhaust temperature control reference, theIGVs were opened to keep the exhaust temperature at or near the exhaust temperature controlreference until they were fully opened. At that point the unit was usually at or near Base Loadanyway.

However, when the unit is operating on Base Load, an increase in load results when air flowincreases (usually due to a decrease in compressor inlet temperature) which cause CPD to increase.The increased CPD would tend to cause both the firing temperature and the exhaust temperature todecrease if the fuel were held constant, but the Exhaust Temp Control curve allows a little extra fuelto be burned because it is really trying to maintain a constant firing temperature.

The confusing part of this for most people is that even though fuel flow and load increase slightly,exhaust temperature decreases. The exhaust temperature decreases because the net effect of theincreased air flow and CPD causes the exhaust temperature to decrease because the majority of theincreased air flow is not used in the combustion of the increased gas fuel flow.

The exhaust temperature control curve has a negative slope, so for an increase in CPD, the resultantexhaust temperature reference will decrease. CPD increases as load increases while operating onexhaust temperature control. This just drives some people crazy because it seems to be opposite ofwhat would be expected.

The exhaust temp control curve represents a constant firing temperature--which is not beingmonitored. It's being predicted by the exhaust temperature control curve based on two parameters,CPD and exhaust temperature. If we could measure the firing temperature while operating on BaseLoad, it would be constant at any point on the sloped portion of the curve--regardless of CPD orexhaust temperature and fuel flow.

That's what the sloped line represents: constant firing temperature. Exhaust temperature and CPD

will vary while operating on Base Load, but the firing temperature will not. And that's what BaseLoad is all about: maintaining constant firing temperature and maximizing power output underchanging ambient conditions while optimizing the parts life of the gas turbine.

So, the answers to your questions depend on what type of IGV control is being used, and whether ornot the unit is operating at Base Load. It's not a simple answer, but in general as units are loaded theexhaust temperature will increase as CPD increases until the unit reaches Base Load. At that pointthe unit cannot be loaded any further by the operator. Changes in ambient temperature will causeload to increase or decrease slightly while on Base Load, but the exhaust temperature will respondopposite to what is expected while on exhaust temp control. It drives most people crazy, but that's theway it works.

And, to CTTech's point, we are a little off-topic here. But, a question is a question, and it deserves ananswer. One will find all kinds of drift on topics on control.com.

Posted by CSA on 17 July, 2007 - 9:19 pm

By the way, there is a pretty cool little real-time "ticker" application on http://www.ucte.org of theEuropean grid frequency.

These are the little disturbances that the system operators have to respond to during the day. Checkit out in the middle of the evening, and in the middle of the morning, and in the middle of the day,and on weekends at various times during the day--but check it out!




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Note the resolution of the graph. It's pretty "fine", like thousandths of a Hz. That's why it cansometimes look pretty ragged.

Posted by Anonymous on 1 August, 2007 - 11:17 am

I understand that synchronous generators and the prime movers which are usually directlycoupled to the generator rotors can spin no faster nor any slower than the frequency (speed) of the

main power grid they are connected to. However, how does the amperage in the statorincrease/decrease based on torque?

"Any change in the torque being applied to the generator by the prime mover will result in achange in the amperes flowing in the generator stator. Increasing the torque above that required tomaintain synchronous speed and frequency will result in amperes which can be used to powerloads connected to the grid. These amperes are generally considered to "flow" out of thegenerator.

Decreasing the torque below that required to maintain synchronous speed and frequency willresult in amperes flowing in the generator stator which will cause the generator to become amotor and keep the rotor and the prime mover turning at synchronous speed and frequency. In

this case, the amperes are considered to "flow" into the generator, "motorizing" the generator."

Posted by DB on 3 August, 2007 - 12:05 am

We appear to be making a complex topic out of a simple situation. First generators are not smartmachines and only obey the laws of physics. When we have a generator running under load andwe appy more load the resistance of the system goes down as resitance goes down current goesup. If the generator current goes up and we maintain the same torque the voltage has a tendancyto fall. Exciters (AVRs) correct this but frequency still goes down unless we apply more speed(fuel/torque). If we have have no magnetic field we have no voltage, if we over excite (morevoltage) we produce extra VARs if we under excite (less voltage)we accept VARs. Thechanging of the strength of the magnetic field (excitation) creates a stronger magnetic fieldwhich keeps trying to force the two different magnetic poles to stop in one position creating a

speed drop (load on the prime mover). We must thefore increase the fuel into the system tomaintain the same frequency. If we add more fuel it creates heat and is restricted in its flow outof the system, so we in turn create higher temperatures (the gases need to escape faster). Ourmodern controllers do all of the adjustments automatically not so complex.

Posted by CSA on 3 August, 2007 - 12:09 am

That's EXACTLY what a generator does: convert torque into amperes.

This is exactly what electricity is used for: Transmitting torque long distances via thinconductors.

Of course, these days, a lot of electricity is used for lighting, and computers (virtual torque??),

but in the early days it was for factories and machines (the Industrial Revolution!).

So, one burns a hydrocarbon-based fuel to produce heat which is converted to torque which isconverted to amps which is transmitted via wires to areas where it is reconverted to torque(pumps, air conditioners, elevators, virtual torque (computers), and light and heat).

Actually, when you think about it, the turbine-generator is really doing the work that the pumpsand air conditioners and elevators and computers are doing, by providing the torque which isbeing produced by the motors (and virtual torque motors--microprocessors) driving the pumpsand air conditioners and elevators and computers and lights and heaters.




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Pretty ingenious, huh?

There's a formula which is hard to reproduce on this forum:

T = K(t) * phi(f) * I(A)

where K(T) is a Torque Constant ("K sub T"), phi(F) is Field Flux ("phi sub F"), and I(A) isArmature Current ("I sub A"). In the formula, K(T) is the physical construction of thesynchronous generator--which is fixed and doesn't change as the synchronous generator isoperation (diameter, length, windings, etc.). Field Flux is the strength of the magnetic field ofthe synchronous generator, which is held reasonably constant as the synchronous generator isoperated. Which leaves only two variables: torque and armature current.

Solving the above equation for I(A):

I(A) = T / (K(T) * phi(F))

So, it can be seen that if the denominator on the right side of the equation remains relativelyconstant as the synchronous generator is operated, varying the torque (the numerator) applied tothe generator directly varies the armature current flowing in the synchronous generator stator.

More torque equals more current.

Power in a three-phase electrical system is: P = V(T) * I(A) * 3^2 * pf,

where P is Power, in watts; V(T) is generator terminal voltage ("V sub T"); I(A) is ArmatureCurrent ("I sub A"); 3^2 is the square root of 3; and pf is the power factor of the load. Generatorterminal voltage stays fairly constant during synchronous generator operation; the square root of3 doesn't change as the synchronous generator is loaded/unloaded; and we presume the powerfactor of a load is stable. So, if the only real variable in the power equation is I(A), which is thesame I(A) as in the torque equation, then increasing torque increases armature current whichincreases power (out of the generator).

It don't get no simpler than this.

Posted by S.Hines on 4 August, 2007 - 10:57 am

Thanks. The above information has been extremely helpful and I am greatly appreciative. I amcurrently participating in a hydropower program and I undergo oral examinations thatsometimes exceed four hours in length. My background as a machinist doesn't help much in thearea of electrical power generation. Your comments, however, have helped shed light on whatwould (for me) otherwise be an odious task.

Once again, Thanks!

S.Hines

Posted by CSA on 5 August, 2007 - 2:51 pm

We aim to please!

Glad to be of help! This seems to be a great site for asking basic questions and getting somedecent answers.

But the thing that really makes this site useful is when people write back to say they've learnedsomething or been helped by the information provided. That way, we can all benefit when weknow something has been helpful or informative!




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Although this topic is titled "Steam turbine generator speed control", a steam turbine is nodifferent from a hydro turbine or a gas turbine or a reciprocating engine or a wind turbine or anykind of torque-producing prime mover driving a synchronous generator: they provide torquethat the synchronous generator converts into amperes.

So the principles being discussed here apply to hydro turbine-generators also. Any kind ofprime mover, actually, since a prime mover produces torque which is transmitted to thesynchronous generator rotor through the load coupling and which the synchronous generatorconverts into amperes (when connected to a load).

Posted by GMS on 15 August, 2007 - 10:57 pm

Ok, you've all discussed the connection of a generator to an infinite bus. What about a generatorwhich is feeding its own small island of load (e.g. a wind turbine supplying a remote farm) orfeeding into a weak bus? What is the result here when the wind blows stronger and the prime-mover speeds up? Surely now the frequency will change? What happens to the voltage andcurrent magnitudes? I assume the current becomes purely load dependent?

Thanks,

GMS


Wind turbines operating a small load are a whole 'nuther topic, really, just for the reason youcited: varying wind speed. There are lots of different systems on the market for such anapplication, many involve using a DC generator to charge a battery. Some require theconversion of the loads to run on DC; some use inverters to convert the DC to a relativelyconstant frequency AC at a typical voltage (usually 120- or 220 VAC depending on thegeographical location). In this way, some energy can be stored for use (in the battery) when thewind speed isn't very high, and frequency can be controlled by the inverter.

There's a lot of different systems, and a lot of different opinions as to which is better--but, as

you suggest, it wouldn't be very practical to hook up an alternator (AC generator) directlydriven by a wind turbine to your house/farm which was wired for 60 Hz, 220/120 VAC, and justrelease the blade. If the wind speed was high and the load low, the frequency would beexcessive. If the wind speed was low and the load "high", the frequency would be less thannominal.

Posted by GMS on 19 August, 2007 - 7:50 pm

Thanks for your reply.

I'm very interested in the technical aspects of generation on a small (250kW and less) scale;wind, solar, small hydro, diesel etc etc. Does anyone know of a good source of information onthis stuff?


Actually, I think 3^2 is three squared, and 3^1/2 is the square root of three. I hope this didn'tcause too much confusion.

Posted by Rahul P Sharma on 19 August, 2007 - 7:25 pm

If there are two GTGs connected in parallel (but not to the grid) to supply the load with one ofthem in Preselect Mode and other in floating mode, which generator will control the frequency,with none being in Isoc mode?




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au revoirRahul

Posted by CTTech on 7 August, 2007 - 12:25 am

Outstanding explanation of the the difference between a diffusion flame liquid fuel oil fired 7B anda gas fired IGV controlled DLN 7EA gas turbine.

Regards,

CTTech

Posted by Mikas on 20 August, 2007 - 11:59 am

Hello,

I'd like to ask for further explanation about primary and secondary control (regulation). If Iunderstood correctly, primary control is spontaneous reaction of turbine's controller, but don'tknow what secondary control would be. Please, can you give a detailed explanation?

Thanks.


It seems you're asking a question that's probably related to a particular turbine manufacturer'scontrol scheme or turbine control manufacturer's control scheme. That may be why no one'sresponded; they're not familiar with it because it's not a "generic" term applied to prime movercontrol systems.

Was primary and secondary control ever mentioned in the post prior to the first time you askedabout it?

Where did you read or hear this term; can you provide information/details which we couldreview for comment?

Posted by Mikas on 21 August, 2007 - 4:59 pm

I think that problem is in fact due to terminology. I'm not speaking about any particular turbine.I've tried google terms "primary and secondary regulation" and I failed to find something thatmatches what I need. If I literally translate, then primary and secondary control would be theterms. I'll try to describe what I'm referring to and hopefully someone will recognize what Iwant.

The article I'm reading and which actually has triggered previous questions mentions primaryand secondary control. It's about controlling frequency and active power of one power system.The article says that there is a tight connection between grid's frequency and produced activepower on one side and between voltage and reactive power on the other side. Primary regulationmeans a spontaneous action of primary machine's controllers (turbine controllers) wheneverthere is grid's frequency to change. But because it is related to turbine's controllers primaryregulation is slow and transients disappearing with time constants of cca 10s. Primary regulationhas static steady state error and therefore it is needed that secondary regulation be included.Secondary regulation is added to primary regulation in order to eliminate this error. Productionunits that participate in secondary regulation are often called regulation units....

This is roughly what is stated in the article. If this sounds familiar to someone please offer moreappropriate terminology...




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Thanks.

Posted by Phil Corso, PE on 22 August, 2007 - 11:56 pm

Responding to Mikas' 21-Aug (16:59)query... perhaps Charles Concordia's IEEE paper canhelp:

"Effect of Prime-Mover Speed Control Characteristics on Electric Power System Performance"IEEE Transactions on Power Apparatus and SystemsVol PAS-88; Issue 5 Part-I; May '69; pgs; 752-756.

Regards, Phil Corso ([email protected])


I think you might be reading the wonderful UCTE document regarding requirements forconnecting to the grid. It seems to refer to primary- and secondary frequency control, and myinterpretation is that primary frequency control is most likely regular droop speed control andsecondary frequency control is remote adjustment (by some regulatory agency) of turbine speedreference to try to assist with grid frequency disturbances. It's not too clear without reading the

entire 92-page document (at least that's the size of the English translation one that was given tome was) and a lot of things seemed to have been "lost in translation."

Without being able to speak directly to people who know exactly what was written and/or whounderstand exactly what is meant in that document it's difficult to say. There have been a lot of"interpretations" by many different people from many parts of the world that have read someportion of that document (I don't think most of them have read it all; some of them have beensimple but most have been obtuse. Some extremely obtuse.

Posted by Mikas on 25 August, 2007 - 12:06 am

Probably you're right, but I cannot tall that, because I'm reading article in my langauge which isnot translation of the document you mention (beacause it is not stated that way). If you have thatdocument in electronic form can you send it to me by email to "brobigi at yahoo.com"?Thanks.


The document can be downloaded from http://www.ucte.org.

Posted by Phil Corso, PE on 23 August, 2007 - 12:32 am

Responding to CSA's 28-Jun-07 (23:15) mis-statement, "yes, transformers are an inductive load on thesystem." I would be remiss if I let this error pass without comment.

A transformer is not an inductive load! The only "load" a power source would be "charged" with

(excuse the pun) are the transformer's losses and magnetizing current. Combined, they're aninsignificant "load!"



Oh, come on, Phil. You could be remiss once in your life.

I often wonder what the "1" key looks like on your computer keyboard(s).




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Posted by sud on 13 July, 2010 - 2:00 am

hello,i have a doubt and was hoping if any of you might clear it. what happens when the load of an isolatedalternator is increased or decreased. is there :

1) only change in terminal voltage.2) change in speed and frequency.

3)change in excitation voltage.

according to one source only the terminal voltage changes. but the important point is that there is a so calleddrooping effect. so definitely there should be a change in frequency. if so, why does the speed get altered ?

Posted by CSA on 16 July, 2010 - 1:08 pm

sud,

The clarity you seek can be found in the term 'isochronous control.' There should be no droop control on anisolated system.

If you are reading texts and manuals, skepticism is a good thing, unfortunately. A lot of these things seem to

be produced by people who don't have any real-world experience with small, "islanded" power systems andwho don't properly state all the conditions under which they are making their statements.

An alternating current system is "defined" by it's frequency, usually either 50 Hz or 60 Hz. Maintaining thefrequency relatively constant is very important to an AC system (in most parts of the world, anyway; acertain Asian sub-continent seems to have a different view about this concept).

It's also important to know that the frequency of an AC generator is directly proportional to the speed of thegenerator rotor, which is driven by the prime mover (turbine, reciprocating engine, etc.) of the generator.

Voltage stability is also critical on an electrical system, so maintaining the voltage is very important in mostparts of the world, as well as for an isolated system.

The governor of the prime mover which is producing the torque that the AC generator (more correctlycalled an alternator) is converting to amperes should be configured to maintain rated speed and frequencyregardless of the load. That is the function of isochronous speed control: to maintain rated frequency duringload changes.

The voltage of an AC generator (alternator) is a function of the excitation applied to the rotating magneticfield, which is controlled by the exciter regulator, commonly referred to as the AVR (Automatic VoltageRegulator). The purpose of the AVR is to vary the excitation as required to maintain the generator terminalvoltage setpoint.

So, working together the prime mover governor and the AVR (exciter regulator) should be able to maintainrated frequency and voltage for an isolated system, provided the load does not exceed the rating of the

prime mover and the rating of the exciter.

The authors of many of these texts and references don't properly state the conditions of operation whentrying to describe the effects of loading. They should be saying that if the prime mover governor doesnothing to maintain the rated speed (and hence, frequency) of the AC generator and the exciter regulator(AVR) does nothing to maintain the rated generator terminal voltage, that when load is increased the speedwill decrease and the generator terminal voltage will decrease.

But, in the real world, we don't want those things to happen so the prime mover governors and the exciterregulators are designed to maintain speed (frequency) and terminal voltage as load changes.




19/26

Skepticism is good, and I applaud you for doubting the references you have found.

Remember: An electrical system is just a means for transmitting torque from one place to another, or tomany other places. The prime mover driving the generator is really driving all the loads connected to thegenerator by the wires of the transmission and distribution system. The generator converts the torque fromthe prime mover into amps, and the loads convert the amps back into torque (in various forms, including"virtual torque" of computers).

Posted by Philipe on 13 February, 2009 - 6:35 pm

This is a very interesting discussion. I am wondering if anyone has had experience operating an inductiongenerator at varying speeds above synchronous speeds, say 1810 to 1850 rpm.

Posted by Phil Corso on 15 February, 2009 - 7:21 am

Philipe, I suggest you submit your post as a new topic... "induction generator."

BTW, component substitution is one of the better ways to resolve problems.


Posted by surya on 8 August, 2010 - 6:58 pm

Dear CSA,

you said that increasing torque will increase the current in the generator. my question is torque meansmechanical torque or electromagnetic torque. if it is a prime mover torque could you please explain mebriefly about this subject


A generator is a device for converting mechanical torque into amps.

A motor is a device for converting amps into mechanical torque.

Motors and generators are joined together using wires.

So, in effect, one is just transmitting torque over wires using electricity as the medium.

Same as with a hydraulic system. One uses a pump (driven by an electric motor, usually!) to convertmechanical torque into pressure and flow. And then at the other end of the hose or pipe that pressure andflow is converted back into mechanical torque or work (power).

In a hydraulic system, one is sending mechanical torque from one place to another using hydraulic mediaand means (fluid and pipes).

In an electrical system, one is sending mechanical torque from one place to another using wires.

Hope this helps!

Posted by surya on 12 August, 2010 - 10:42 am

dear csa,

thank u for giving me information but my doubt is if the frequency of grid remains constant and if we




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want to increase the load. what we are doing is we are increasing the mass flow of turbine, increasing themassflow will increase the torque on the turbine. how this prime mover torque is related with increase inthe current. could you please explain me this briefly


surya,

If you're riding your bicycle on a relatively flat and smooth road and you want to maintain a constantspeed then you will apply a relatively constant torque to the pedals. If you increase the torque, then thespeed of the bicycle will increase. If you decrease the torque, then the speed of the bicycle willincrease. But, if you want to maintain a constant speed you will maintain a constant application oftorque to the pedals.

Now, let's say you are riding a tandem bicycle with another person who is also pedaling. And you areon the same relatively flat and smooth road and you are to maintain a constant speed. The two of youwill work together to apply sufficient torque to maintain the constant speed. Now, if you suddenlyincrease the torque you are applying to the pedals and the other rider does nothing, he maintains historque constant, then the speed of the bicycle will increase. In this case the load (the weight of the tworiders and the bicycle and any wind resistance) hasn't changed, but the amount of torque being applied

to the pedals has changed, and that will result in a change in speed. But, that change in speed isundesirable (you're supposed to be traveling at a constant speed, remember?), so the other rider willhave to decrease the torque he's applying to the pedals to maintain the constant speed because you haveincreased your torque.

Now, let's say the two of you are riding on the same relatively flat and smooth road and are workingtogether very well to maintain the constant speed. Suddenly, your young cousin who's runningalongside jumps on the handlebars of the bicycle, increasing the load and decreasing the speed. Eitheryou, or the other rider, or the two of you together, will have to increase the amount of torque beingapplied to the pedals to get back to and maintain that constant speed. Until the two of you can reach aproper equilibrium the speed may vary above and below the desired speed, but eventually everythingsmooths out and you all three are traveling at the desired rate of speed.

An electrical grid is no different. The load on an electrical grid is the sum of all the motors and lightsand devices that are converting amps into power and the amount of generation must exactly match theload in order for the grid frequency to remain constant. On an AC grid, it's very important (in mostparts of the world, except, it seems, for a certain Asian sub-continent) to maintain a relatively stablegrid frequency.

In reality, as motors and lights and other loads are switched on and off and loaded and unloaded, thegrid frequency varies somewhat from 50.00 Hz or 60.00 Hz (which is the typical frequency in mostparts of the world). The variance is usually on the order of hundredths of a Hz (0.0n Hz). It's neverexactly 50.000000 Hz or 60.000000 Hz all the time, because loads are continually being switched onand off. And at certain times of the day and evening and night, the grid operators have to be verycareful to add more generation (increase the amount of torque being produced and/or increase thenumber of generators and prime movers) or decrease generation in order to be able to maintain arelatively stable frequency, not exactly 50.000000 or 60.000000 Hz, but as close as possible. Thevariance from nominal is a reflection of how well the generation is matched to the load. The closer tonominal, the better; the further from nominal, the less better.

Just like the two riders have to do on the bicycle when the load suddenly increases, or decreases.Control systems can be programmed to do lots of this responding to changes in load, but people stillhave to assist these control systems.

It's important to understand that when you increase the "load" on a generator, by increasing the amountof torque being produced by the prime mover driving the generator, that if the load on the grid is not




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changing appreciably, then some other generator and it's prime mover must reduce the load it isproviding, or else the grid frequency will increase. That's what governors and grid operators do: Theycontrol the amount of generation to provide only enough power to supply the load that is currentlyconnected to the grid. If the governor and/or the grid operators don't increase generation when it'srequired, then the grid frequency will decrease. If the they don't decrease power when the loaddecreases, then the grid frequency will increase.

Exactly like what happens on the tandem bicycle. Only, a grid is like a bicycle with many cranks andpeople applying torque to the cranks. And the load is the weight being carried by the bicycle(presuming it's on a relatively flat and smooth road). If the weight (load) is variable, then the amountof torque will have to vary also--to maintain a constant speed!

If there are tens or hundreds of people pedaling this bicycle to carry the load at a constant speed, thenif one person increases the amount of torque he's applying to the pedals and the load is constant at thatpoint then the speed of the bicycle will increase very slightly, almost imperceptibly. But, it's likely thatsomeone is watching the speed of the bicycle and they will either reduce the amount of torque they areproviding or they will tell someone to reduce their torque--in order to maintain a constant speed whilecarrying the load.

Multiple generators on a grid are like multiple people pedaling a bicycle to carry a load at a constant

speed. They are supplying torque to move a load that is likely bigger than any single person couldmove independently. And, their pedals are all linked together by a chain that prevents any one person'spedal speed to be more or less than any other person's. And, the speed of the bicycle dictates how fastthe pedals are turning.

Let's say that the load being transported by the bicycle is on multiple trailers hitched to the bicycle.Further, let's say that several of the last trailers become disconnected from the bicycle; this wouldrepresent a decrease in load. If everyone pedaling the bicycle continued providing the same amount oftorque the speed would increase. So, someone or something will tell some of the people to reduce theamount of torque they are providing, or even to stop pedaling altogether, in order to get the speed toremain as close as possible to the desired speed.

On an AC grid, when the load increases but the generation (the amount of torque being provided to the

generator(s)) does not increase, then grid frequency goes down. Or, when the load decreases but thegeneration (the amount of torque being provided to the generator(s)) does not decrease, then the gridfrequency goes up. So, that's how prime mover governors and grid operators know when to increase ordecrease generation (the amount of torque being provided to the generators): when the grid frequencyis changing. And, good grid operators can anticipate load changes, such as when people wake up in themorning and turn on their lights and stoves and tea kettles and their damned television sets (now there'sa waste of torque if there ever was one!). And when people generally turn everything off at night andgo to sleep.

If you want physics and maths, use your preferred Internet search engine and search for variouselectrical generation articles. There is www.wikipedia.org, www.howstuffworks.com,candu.canteach.org, and any number of other similar sites for the basics. Wikipedia usually has links to

references, which can be very detailed. Use different search terms, as you learn new words and termsand concepts, and you will find no shortage of detailed search results, some better than others.

A generator is a device for converting torque into amps. A motor is a device for converting amps intotorque. Torque is the form of power that is mostly needed by various factories and loads (elevators;water pumps--the largest consumer of electric power (fresh-, grey- and black water); refrigerators; airconditioners; etc.). Lights are converting amps into heat, and that heat is producing light. And mostconsumers of power are not located near large sources of energy (rivers; natural gas pipelines; fuel oilpipelines/storage tanks; coal piles; etc.). So, energy is converted into torque by prime movers whichare coupled to generators which convert the torque into amps which is transmitted by wires to variousloads which are some distance away from the prime mover and its energy source. That's what




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electricity is: Converting power into amps to convert it back into power.

When the amount of torque being applied to a synchronous generator being operated in parallel withother synchronous generators is increased, the speed of the generator rotor cannot be increased. It'slocked into synchronous speed, which is governed by the frequency of the grid with which it isconnected.

So, because the speed cannot be increased, some 'magic stuff' happens inside the generator and the"extra" torque is converted into amps, which can be transmitted over wires to motors and other deviceswhich can convert the amps into power (usually mechanical power) at the other end (of the wire that'sconnected to the generator that's being driven by the torque coming from the prime mover that'scoupled to the generator).

Now, if you want to understand emf and counter emf and radians and armature reaction to satisfy your"doubt" (and that is a mis-use of the word; please see your Oxford's English Dictionary, or any onlinedictionary, for the proper definition and usage of the word 'doubt') then hopefully someone else cancontribute to this thread, or you can use your preferred Internet search engine on any of the site listedabove, and others which have been listed in many related threads on control.com, to answer yourquestion(s) and satisfy your curiosity.

But, that's what generators do: They are devices for converting torque into amps so that the amps canbe transmitted to remote locations and then reconverted into power to be used at the remote locations.Electricity is all about transmitting power from one location to another. There has to be a load for agenerator to produce power. Energy is converted into power in the form of torque by the prime mover,and that energy is applied to the generator rotor, and the generator converts the torque into amps, andwires carry those amps to remote locations, where devices at the other end convert the amps into power(motors, lights, etc.).

Now, it's best to add this disclaimer: This applies to either relatively large grids or to smaller grids withgood frequency control.

Now, surya, if you have observed other physical phenomena with respect to synchronous generators(alternators) being operated in parallel with other alternators and these observations are causing you to

have questions about something you've read or been told please tell us what you have experienced andwhy it causes you to question something you have been told.

Or, if this is just curiosity about electricity and how it's generated, it's okay to say that, too.

But, we digress.

Generators are for converting torque into amps. If there was no electricity to allow torque (power) tobe transmitted by wires to many remote locations, then everyone of those remote locations would haveto have their own sources of torque (power) for their needs. And those sources of power would allrequire energy to be widely distributed. But, electricity makes that mostly unnecessary.

Exactly how those generators work and all the physics and maths is more than I need to know to beable to operate them properly and maintain them. Maybe you have a different need; we don't know,you haven't told us!

But every time someone has used physics and maths to try to explain electrical generation to me, Ihave gotten very confused, and when I have tried to use physics and maths to explain it to people theyhave gotten very confused.

Electricity is not rocket science. There are no rocket scientists working at power plants. (There aresome who liken themselves to rocket scientists, but, ... well, ... I digress. Again.)




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If you consider a bicycle as a means for carrying a variable load or loads (packages, goods, people,vegetables), and if you think of how to carry a variable load at a constant speed on a relatively flat andsmooth road, then it should all become a little clearer. Because it's all about providing torque to a loadat a constant speed, the same as on an AC grid.

Best of luck!


surya,

Here's a link I had been looking for for some time. The frequency graph used to be "real-time" but itseems to be static now.

https://www.entsoe.eu/index.php?id=108

There is yet another explanation of grid frequency, that may be of some help.

You might try looking at this page at different times during the day to see if the graph changes.

Enjoy!

Posted by Shahvir on 13 August, 2010 - 12:34 pm

Dear Surya,

I think the CANDU/CANTEACH link is really great as it even covers generators on finite grid.Also,different operating conditions (like AVR on manual, etc.) are also covers in the articles by Cowling. Ifeel you should really go thru it!

Regards,Shahvir


To Shahvir's point, grids are generally classified into two different types: finite and infinite. Thedescription provided previously is most applicable to what's termed an infinite grid, a very largeelectrical transmission and distribution system with many generators and prime movers connectedin parallel supplying many loads, and the total load is infinitely larger than even the most powerfulprime mover and generator connected to the system could ever supply on its own.

A finite grid is usually much smaller, and composed of a few (one, two, three, seven) prime moversand generators operating in parallel to supply a much smaller total load. Sometimes, the load issmall enough that the most powerful prime mover and generator could supply the load by itself, butfor reliability purposes it's decided to have multiple generators for redundancy. The prime moversrunning these generators would not be operating at maximum output, but would be operating at

"part load", assisting with supplying the load at the desired frequency.

Some of these finite grids have the governor of one large prime mover and generator that isoperated in Isochronous speed control mode, which means that if the load changes (motors andlights switched on or off; motors loaded and unloaded; etc.) which would tend to cause a change inthe grid frequency that the Isochronous governor will adjust the energy being admitted to the primemover to keep the frequency constant. The other generators and their prime movers are typicallyoperated in Droop speed control mode, and they continue producing power at a relativelyunchanged level (presuming the Isochronous governor is well-tuned and fast-acting).

If the Isochronous governor is not well-tuned and/or is not fast-acting then it's possible that the grid




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frequency will vary until the Isochronous governor can stabilize the grid frequency. In this case, thespeed of all the generators and prime movers will vary as the frequency varies.

One more important thing to note is that we are discussing prime movers that are mechanicallycoupled to the generators, either directly or through reduction gears. There are some generatorswhich are driven by "free turbines" which are uncoupled from the prime movers producing theenergy admitted to the "free turbine." In such a case, it's very common for the "power turbines" tovary their speed with load, but the "free turbine" which is mechanically coupled to the generator (totransmit torque) is still held to a speed that is directly proportional to generator frequency.

A finite grid can be considered as a simple tandem bicycle trying to maintain a constant speed on arelatively flat and smooth road. If one rider changes the amount of torque being provided and theother rider does not (presuming the load is constant) then the speed will change.

If the load being carried by the bicycle is variable and the load increases and neither rider increasesthe amount of torque being provided then speed of the bicycle will decrease. It would make sensefor the two riders to agree that one of them would attempt to vary his torque output to try tomaintain a constant speed as load changed, because if they both do so and there is nocommunication or coordination between them then the speed will be unstable until they can bothadjust their output to respond to the change in load. The rider who agreed to adjust his output to

control speed as load changed would be analogous to the Isochronous governor of a generator'sprime mover, automatically responding to changes in load which would tend to cause changes infrequency.

So, it would also be helpful, surya, if you would tell us a little more about your "situation", and ifyou're working on a smaller, finite grid (sometimes called an "island grid"), or if you're working ona larger, infinite grid, and if either of the grids are unstable.

And, as Shahvir has suggested, please review the information on candu.canteach.org, because itreally is some very good and useful material; some of the best I've found and I've looked for a lot ofinformation on the World Wide Web on governor control (which is what this topic is primarilyabout).

Posted by Shahvir on 15 August, 2010 - 2:55 am

Surya, in passing I suggest you also go thru the 'Woodward Governor' website where there's somegreat info on alternator Governor control & behavior of alternators on finite & infinite gridconditions, in Isochronous & Droop control.

Dear CSA, I applaud your patience in writing a detailed explanation for benefit of the posters... inspite of it being extremely exhausting! I thank you on behalf of all the electrical engineers foryour service to the Engineering community. Do keep up the good work & God Bless!

Regards,Shahvir


Shahvir,

Thanks for the help--and the kind words. I just try to remember how difficult it was for me tograsp some of these concepts back when I was reading the available literature (texts andreference material).

However, I don't think we've helped surya. I keep re-reading his posts and I think he's not clearon how generators convert torque into amps.




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I'd wager he has no problem with how motors convert amps into torque. So, I'm just trying tofind a way to help him understand that generators (really the prime movers driving thegenerators) are just converting the torque from the prime movers into something that can beeasily transmitted to electrical machines on the other end that convert it back into power. Powerbeing a time-rate of doing work, it isn't stored like energy can be. The amount of power beingsupplied by all the generators and their prime movers must be exactly equal to the amount ofpower being consumed by the total load (motors, lights, etc.) on the grid. If the load exceeds thepower being provided, then the frequency decreases. If the power being provided exceeds theload then the frequency goes up. It's a balancing act that some grid operators and regulators arevery good at, while others aren't for a variety of reasons.

Power out equals power in minus losses and inefficiencies. And it all has to be done at arelatively constant frequency, which directly translates into speed.

Most people don't seem to have a problem with the fact that AC motors operate at a relativelyconstant speed (those that are directly connected to the mains). But, when it comes to generatorsthey seem to think that because the speed varies during start-up and shutdown that the speedmust vary during loading and unloading, because the fuel is changing during loading andun

generator speed control

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