Steam turbine generator speed control - clarification

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).
 
This is a very interesting discussion. I am wondering if anyone has had experience operating an induction generator at varying speeds above synchronous speeds, say 1810 to 1850 rpm.
 
hello,
i have a doubt and was hoping if any of you might clear it. what happens when the load of an isolated alternator 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 called drooping effect. so definitely there should be a change in frequency. if so, why does the speed get altered ?
 
sud,

The clarity you seek can be found in the term 'isochronous control.' There should be no droop control on an isolated 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 and who 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 the frequency relatively constant is very important to an AC system (in most parts of the world, anyway; a certain 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 the generator 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 most parts 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 correctly called an alternator) is converting to amperes should be configured to maintain rated speed and frequency regardless of the load. That is the function of isochronous speed control: to maintain rated frequency during load changes.

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

So, working together the prime mover governor and the AVR (exciter regulator) <b>should</b> be able to maintain rated 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 when trying to describe the effects of loading. They should be saying that if the prime mover governor does nothing to maintain the rated speed (and hence, frequency) of the AC generator <b>and</b> the exciter regulator (AVR) does nothing to maintain the rated generator terminal voltage, that when load is increased the speed will 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 exciter regulators are designed to maintain speed (frequency) and terminal voltage as load changes.

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 to many other places. The prime mover driving the generator is really driving all the loads connected to the generator by the wires of the transmission and distribution system. The generator converts the torque from the prime mover into amps, and the loads convert the amps back into torque (in various forms, including "virtual torque" of computers).
 
Dear CSA,

you said that increasing torque will increase the current in the generator. my question is torque means mechanical torque or electromagnetic torque. if it is a prime mover torque could you please explain me briefly 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 convert mechanical torque into pressure and flow. And then at the other end of the hose or pipe that pressure and flow 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 media and means (fluid and pipes).

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

Hope this helps!
 
dear csa,

thank u for giving me information but my doubt is if the frequency of grid remains constant and if we want to increase the load. what we are doing is we are increasing the mass flow of turbine, increasing the massflow will increase the torque on the turbine. how this prime mover torque is related with increase in the 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 constant speed then you will apply a relatively constant torque to the pedals. If you increase the torque, then the speed of the bicycle will increase. If you decrease the torque, then the speed of the bicycle will increase. But, if you want to maintain a constant speed you will maintain a constant application of torque to the pedals.

Now, let's say you are riding a tandem bicycle with another person who is also pedaling. And you are on the same relatively flat and smooth road and you are to maintain a constant speed. The two of you will work together to apply sufficient torque to maintain the constant speed. Now, if you suddenly increase the torque you are applying to the pedals and the other rider does nothing, he maintains his torque constant, then the speed of the bicycle will increase. In this case the load (the weight of the two riders 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 is undesirable (you're supposed to be traveling at a constant speed, remember?), so the other rider will have to decrease the torque he's applying to the pedals to maintain the constant speed because you have increased your torque.

Now, let's say the two of you are riding on the same relatively flat and smooth road and are working together very well to maintain the constant speed. Suddenly, your young cousin who's running alongside jumps on the handlebars of the bicycle, increasing the load and decreasing the speed. Either you, or the other rider, or the two of you together, will have to increase the amount of torque being applied to the pedals to get back to and maintain that constant speed. Until the two of you can reach a proper equilibrium the speed may vary above and below the desired speed, but eventually everything smooths 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 lights and devices that are converting amps into power and the amount of generation must <b>exactly</b> match the load in order for the grid frequency to remain constant. On an AC grid, it's very important (in most parts of the world, except, it seems, for a certain Asian sub-continent) to maintain a relatively stable grid frequency.

In reality, as motors and lights and other loads are switched on and off and loaded and unloaded, the grid frequency varies somewhat from 50.00 Hz or 60.00 Hz (which is the typical frequency in most parts of the world). The variance is usually on the order of hundredths of a Hz (0.0n Hz). It's never exactly 50.000000 Hz or 60.000000 Hz all the time, because loads are continually being switched on and off. And at certain times of the day and evening and night, the grid operators have to be very careful to add more generation (increase the amount of torque being produced and/or increase the number of generators and prime movers) or decrease generation in order to be able to maintain a relatively stable frequency, not exactly 50.000000 or 60.000000 Hz, but as close as possible. The variance from nominal is a reflection of how well the generation is matched to the load. The closer to nominal, 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 still have to assist these control systems.

It's important to understand that when you increase the "load" on a generator, by increasing the amount of torque being produced by the prime mover driving the generator, that if the load on the grid is not changing appreciably, then some other generator and it's prime mover must reduce the load it is providing, or else the grid frequency will increase. That's what governors and grid operators do: They control the amount of generation to provide only enough power to supply the load that is currently connected to the grid. If the governor and/or the grid operators don't increase generation when it's required, then the grid frequency will decrease. If the they don't decrease power when the load decreases, then the grid frequency will increase.

Exactly like what happens on the tandem bicycle. Only, a grid is like a bicycle with many cranks and people 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 amount of 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, then if one person increases the amount of torque he's applying to the pedals and the load is constant at that point then the speed of the bicycle will increase very slightly, almost imperceptibly. But, it's likely that someone is watching the speed of the bicycle and they will either reduce the amount of torque they are providing or they will tell someone to reduce their torque--in order to maintain a constant speed while carrying 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 could move independently. And, their pedals are all linked together by a chain that prevents any one person's pedal speed to be more or less than any other person's. And, the speed of the bicycle dictates how fast the 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 would represent a decrease in load. If everyone pedaling the bicycle continued providing the same amount of torque the speed would increase. So, someone or something will tell some of the people to reduce the amount of torque they are providing, or even to stop pedaling altogether, in order to get the speed to remain 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 the generation (the amount of torque being provided to the generator(s)) does not decrease, then the grid frequency goes up. So, that's how prime mover governors and grid operators know when to increase or decrease generation (the amount of torque being provided to the generators): when the grid frequency is changing. And, good grid operators can anticipate load changes, such as when people wake up in the morning and turn on their lights and stoves and tea kettles and their damned television sets (now there's a waste of torque if there ever was one!). And when people generally turn everything off at night and go to sleep.

If you want physics and maths, use your preferred Internet search engine and search for various electrical 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 terms and 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 into torque. 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; air conditioners; etc.). Lights are converting amps into heat, and that heat is producing light. And most consumers of power are not located near large sources of energy (rivers; natural gas pipelines; fuel oil pipelines/storage tanks; coal piles; etc.). So, energy is converted into torque by prime movers which are coupled to generators which convert the torque into amps which is transmitted by wires to various loads which are some distance away from the prime mover and its energy source. That's what 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 with other synchronous generators is increased, the speed of the generator rotor cannot be increased. It's locked into synchronous speed, which is governed by the frequency of the grid with which it is connected.

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 devices which can convert the amps into power (usually mechanical power) at the other end (of the wire that's connected to the generator that's being driven by the torque coming from the prime mover that's coupled 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 online dictionary, for the proper definition and usage of the word 'doubt') then hopefully someone else can contribute to this thread, or you can use your preferred Internet search engine on any of the site listed above, and others which have been listed in many related threads on control.com, to answer your <b>question(s)</b> and satisfy your <b>curiosity</b>.

But, that's what generators do: They are devices for converting torque into amps so that the amps can be 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 a generator 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, and wires 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 with good 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 and why 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) to be transmitted by wires to many remote locations, then everyone of those remote locations would have to have their own sources of torque (power) for their needs. And those sources of power would all require 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 be able 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, I have gotten very confused, and when I have tried to use physics and maths to explain it to people they have gotten very confused.

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

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 and smooth road, then it should all become a little clearer. Because it's all about providing torque to a load at 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 it seems 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!
 
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. I feel you should really go thru it!

Regards,
Shahvir
 
To Shahvir's point, grids are generally classified into two different types: finite and infinite. The description provided previously is most applicable to what's termed an infinite grid, a very large electrical transmission and distribution system with many generators and prime movers connected in parallel supplying many loads, and the total load is infinitely larger than even the most powerful prime 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 movers and generators operating in parallel to supply a much smaller total load. Sometimes, the load is small enough that the most powerful prime mover and generator could supply the load by itself, but for reliability purposes it's decided to have multiple generators for redundancy. The prime movers running these generators would not be operating at maximum output, but would be operating at "part load", assisting with supplying the load <b>at the desired frequency</b>.

Some of these finite grids have the governor of one large prime mover and generator that is operated in Isochronous speed control mode, which means that if the load changes (motors and lights switched on or off; motors loaded and unloaded; etc.) which would tend to cause a change in the grid frequency that the Isochronous governor will adjust the energy being admitted to the prime mover to keep the frequency constant. The other generators and their prime movers are typically operated in Droop speed control mode, and they continue producing power at a relatively unchanged 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 frequency will vary until the Isochronous governor can stabilize the grid frequency. In this case, the speed 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 mechanically coupled to the generators, either directly or through reduction gears. There are some generators which are driven by "free turbines" which are uncoupled from the prime movers producing the energy admitted to the "free turbine." In such a case, it's very common for the "power turbines" to vary their speed with load, but the "free turbine" which is mechanically coupled to the generator (to transmit 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 a relatively flat and smooth road. If one rider changes the amount of torque being provided and the other 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 increases the amount of torque being provided then speed of the bicycle will decrease. It would make sense for the two riders to agree that one of them would attempt to vary his torque output to try to maintain a constant speed as load changed, because if they both do so and there is no communication or coordination between them then the speed will be unstable until they can both adjust 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's prime mover, automatically responding to changes in load which would tend to cause changes in frequency.

So, it would also be helpful, surya, if you would tell us a little more about your "situation", and if you're working on a smaller, finite grid (sometimes called an "island grid"), or if you're working on a 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 it really is some very good and useful material; some of the best I've found and I've looked for a lot of information on the World Wide Web on governor control (which is what this topic is primarily about).
 
Surya, in passing I suggest you also go thru the 'Woodward Governor' website where there's some great info on alternator Governor control & behavior of alternators on finite & infinite grid conditions, in Isochronous & Droop control.

Dear CSA, I applaud your patience in writing a detailed explanation for benefit of the posters... in spite of it being extremely exhausting! I thank you on behalf of all the electrical engineers for your 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 to grasp some of these concepts back when I was reading the available literature (texts and reference material).

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

I'd wager he has no problem with how motors convert amps into torque. So, I'm just trying to find a way to help him understand that generators (really the prime movers driving the generators) are just converting the torque from the prime movers into something that can be easily transmitted to electrical machines on the other end that convert it back into power. Power being a time-rate of doing work, it isn't stored like energy can be. The amount of power being supplied by all the generators and their prime movers must be exactly equal to the amount of power being consumed by the total load (motors, lights, etc.) on the grid. If the load exceeds the power being provided, then the frequency decreases. If the power being provided exceeds the load then the frequency goes up. It's a balancing act that some grid operators and regulators are very 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 a relatively 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 relatively constant speed (those that are directly connected to the mains). But, when it comes to generators they seem to think that because the speed varies during start-up and shutdown that the speed must vary during loading and unloading, because the fuel is changing during loading and unloading just like it does during starting and shutdown.

And, I believe most people don't really understand the whole synchronous part of synchronous motors and generators. That the speed is "fixed" by the frequency of the grid with which the machine is connected, that there are great magnetic forces at work inside the synchronous machine that keep the speed directly proportional to the frequency, regardless of the torque being applied. (The caveat here is that this explanation applies to very large, infinite grids, or to smaller finite grids with good load and frequency control.)

Anyway, thanks for the help with references, and please feel free to offer your own analogies or explanations or clarifications to anything that is written here!
 
Dear CSA,

You are most welcome!... coming to our thread, I think the problem with many posters in grasping these theories is because it looks convincing in print but is hard to visualize! If you do remember, I was stuck with the same problem in the past in which Mr. Phil Corso's help was involved.

The reason being, most of the time, alternator operation is always attempted to be understood when operating on finite grid , wherein the capacity of the alternator in question is more than 5% of total grid capacity. On a finite grid, the terminal voltage too changes with changes in driving torque (considering AVR on manual). I must admit the CANTEACH/CANDU article was of great help to me in understanding the same.

In due course of time, I came to understand how important a role was that of an 'automatic Governal control mechanism' (it is but obvious in practice, but in theory, the automatic Governor speed control is not emphasized upon)...and then there was this 'Woodward Governor' website & everything fell into place.

I think an Engineering concept is best understood if it could be visualized....your beautiful analogy with 2 riders on a bicycle falls into this!

Many a times, posters try to compare theoretical concepts with the practical... the problem arises when they try to visualize these concepts. The conversion of torque into Amperes cannot be easily visualized in AC machines as a lot of electromagnetic physics is involved and there are too many physical events (mechanical to electromagnetic) happening at the same time. We always tend to relate voltage with Amperes (Ohm's Laws) but it's a bit difficult to relate (visualize) Torque with Amperes. This is due to our inherently strict belief in Ohm's Laws taught throughout grad school.

In passing, I feel a concept is best understood if it could be easily visualized. Maybe, the poster is finding difficulty in same.

Regards,
Shahvir
 
Shavir,

Can you share the link to the Woodward Governor Control document which was so helpful? I must have missed that one!

Thanks!
 
Dear CSA,

Following is the link you asked for;

www.canadiancontrols.com (then go to the 'resources' tab for pdf articles)

One could also Google 'Woodward Governors' and get an array of articles on the topic.

Regards,
Shahvir

P.S. - How does one upload attachments on Control.com?
 
Dear CSA,
thank u for giving me the valuable information on the frequency vs torque. My doubt is somewhat cleared after reading the load angle theory.
 
N
My name is NITIN

1.How does steam turbine comes in speed controller from load controller?

2.what is pressure controller and what are its types and how does steam turbine comes in pressure controller from load controller?
 
N

Namatimangan08

>I'd like to ask for further explanation
>about primary and secondary control
>(regulation). If I understood correctly,
>primary control is spontaneous reaction
>of turbine's controller, but don't know
>what secondary control would be.

The function of primary control is to bring frequency rate of change to zero in less than 10 seconds. It can be achieved by means of one way, namely governor speed drop. How does it can do it?

Assuming you are operating 21 turbine generators in parallel to load 6000MW demands including losses. To comply with (N-1) redundancy you need to load each turbine generator at 6000/21 =285.7MW. The biggest per unit capacity of your turbine generator is 300MW. In our case here let us assume you have uniform per unit capacity turbine generator of 300MW but overrated capacity of 305MW.

By doing so you have a sufficient active power spinning reserve in case one of your turbine generators trips off.

Having the spinning reserve is one thing. To activate your spinning reserve to support generation shortfall when it comes to time you need it most is another thing.

Now your system frequency is at 50Hz. All of sudden one of your turbine generators trips off. Just after the tripping you have generation shortfall by 285.7MW. The total demand and losses remain at 6000MW. Obviously you have the scenario where there is net deceleration torque to slow down the system frequency equals to 285.7MW in power equivalent. The system free fall frequency rate (FFR) becomes

FFR = [H(MJ)/285.7MJ/s] seconds (for 50hz)

Where H is inertial energy of rotating masses of you turbine generators and loads.

Let us take a figure for H= 60,000MJ at 50Hz.

FFR= 50 Hz in 210second

FFR above means that such shortfall in generation will consume all inertial energy of rotating masses in 210 seconds. In the other words all your turbine generators' shafts become stand still after 210 seconds! 210seconds? It seems you have ample of times right? Wrong. Lower frequency limit of your system could be around 49.8Hz. Thus you have approximately 1 second to increase generation by 285.7MW in order to match supply and demand before the frequency falls below 49.8Hz. To date only one way it could be done, i.e. via proper setting of the governor speed droops for all your turbine generators.

I don't want to talk about how to set your droops since it has been much talked about.

What actually you want your droops to do? At system frequency => 49.80Hz you want all droops to command the remaining 20 turbine generators to increase output 285.7MW/20 each. If you set it well, each turbine generator will increase its output by 14.25MW at system frequency reaches 49.8Hz. Since you have 20 of them, you will get back 285.7MW generation you have lost. At this moment you should be able to arrest system frequency decay at 49.80Hz. The frequency rate of change (dF/dt) shall become zero. But you have already consumed dH by approximately

dH =(50-49.8)* 6000 =240MJ

Note that supply-demand are already matched. But you still have to accelerate the shaft by providing net accelerating torque. Otherwise the system frequency will stay there at 49.80Hz. This is called secondary control or supplementary controls. There are many methods to achieve this objective. One of the methods is operator's intervention. E.g. all plant operators may change turbine generators set points to 300MW at system frequency 49.80Hz. Note that we have additional unit capacity of 5X20MW to be used to accelerate the frequency. We need roughly 10MW accelerating load that we apply for 24 second to reset system inertial energy of rotating masses to 60000MJ (at 50 Hz). In reality the process it is slightly complex than this since, except the two units that you use to accelerate the frequency, all your turbine generators will withdraw the loads as system frequency is rising from 49.80 to 50Hz due to the nature of speed droops operation. To mitigate this problem plant operators shall change the load set points as system frequency is rising.
 
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