Hello every one.

I'm just a beginner working in a power plant as technician. lately I came to know about this and I appreciate the efforts made by all, and some of my questions about synch generator got cleared. I would like to know more about armature reaction, speed droop characteristics, and pole slip (exactly when is the condition that forces to happen). I read about armature reaction in some of the threads, but it is not clear. kindly give in a lucid way.

 

As a beginner, you are trying to take a bite out of a very big apple.

I have worked with some of the best in the field of excitation. If you want a easy and clear response to a complex issue. I am afraid just a view paragraphs on this forum will leave you wondering.

I am going to follow this thread as well.

 

sudhamadav,

Everyone has to start somewhere, so let's get started.

All of these topics have been covered before on control.com, and it seems you have read some of the previous threads. It would be most helpful if you detailed the areas you would like to have clarified. For the issue of armature reaction, have you searched elsewhere on the World Wide Web? I performed a quick search and unfortunately found a lot about armature reaction of DC motors but not a lot of relevant information about synchronous generator armature reaction. Let's table that topic for a while and start with a topic that's near and dear to my heart: droop speed control.

It's important to know that generators do not have droop--the governors of the prime movers that drive generators have droop (turbines; reciprocating engines; etc.). Droop speed control is the governor mode that allows multiple synchronous generators to be synchronized together and produce stable power output. It is also a method of predicting how much the load of the generator will change for a given change in the energy flow-rate into the prime mover.

The thing to know about droop speed control is that it is how synchronous generator prime mover governors are to be controlled when multiple generators are synchronized together to provide power to a load (motors, lights, computers and computer monitors, etc.) that is larger than any single generator could supply by itself. This is because droop speed control is the prime mover governor mode that allows them all to be operating together as one and to do so in a stable and smooth manner.

It's also important to know that when multiple synchronous generators are synchronized together that no single generator can run at a speed that is not directly proportional to (a function of) the frequency of the grid they are synchronized to. That means that once synchronized the speed of every generator and its prime mover is controlled by the frequency of the grid and changes only when the grid frequency changes. Droop speed control relies in the fact, and in fact, also uses this relationship to help support the grid when frequency is off normal.

The people who work on generator excitation systems (sometimes called "AVRs") don't really know too much about droop speed control. The people who work on the prime mover governors (the turbine or engine control systems) are the ones who (should) know or understand droop speed control. It's actually a very simple--but powerful--concept that is universally misunderstood and poorly documented in texts and reference books and on the World Wide Web. Be very careful when reading about droop speed control because many authors do not properly state the conditions when they describe droop speed control.

I'm going to leave it at that for the time being. If you would tell us a little about the plant you are working at ( type of equipment) that would be helpful.

And, again, these topics have been covered before here on control.com. There is a frequent contributor to this forum who has written a paper about armature reaction; you can send your name, return email address and your affiliation (company; school; etc.) and request a copy from Phil Corso, at [email protected]. (Many of us would be extremely interested to know how informative you found the document should you read it.)

As CuriousOne has said, this is a huge topic--actually two, as we've learned--synchronous generators and droop speed control governor mode. I'd prefer to keep this thread limited to your questions about synchronous generators, and request you to start another thread about droop speed control if, after reading some of the MANY previous threads on control.com, you still have questions.

Welcome to control.com--and don't forget to use the Thumbs-up/Thumbs-down to indicate if you find responses helpful or not--but if not, could you please write back to say why you weren't helped so we can improve our responses (if possible). Also, if you find a particular response very helpful it's always nice--and much appreciated--if you'll take a minute or so and write a line or two to express your satisfaction.

Thanks!!!

 

Hello CSA,

Yes I was really expecting your reply on the first. thanks for that and we have different machines in our plant (13E2, Fr 5, Fr 9 Fr 9b Fr 9e and ABB 13D). its a total 2500 MW power generating station.

and yes regarding the armature reaction I read it on

1. http://control.com/thread/1026244991 (I think the same one you mentioned in your reply) (I felt he left the topic in the middle)

2. http://www.electrical4u.com/armature-reaction-in-synchronous-machine-alternator/
with this it seemed like I understood but then again the same confusion. I feel knowing the armature reaction of synch generator is very important as it deals with the magnetic properties (armature flux, air gap flux and main field flux) which is the source of producing induced emf.

and further I want to understand like when a short circuit (3- phase), which is connected to grid on generator terminals what will be the scenario of the rotor dc? ok the protection systems will act accordingly in tripping, but what would be condition of rotor at that time?

 

sudamadhav,

I find myself waiting for a few hours today, and would like to ask why you are interested in armature reaction as a technician at a power plant. This is something which synchronous generator designers must understand when designing and building synchronous generators.

As for slipping a pole, there are very great magnetic forces at work inside a synchronous generator which is being driven (turned) by the torque transmitted from the prime mover driving the generator. As was said earlier, when synchronized to a grid with other generators and their prime movers each generator is locked into its synchronous speed and can't run any faster or any slower than dictated by the grid frequency.

There are at least magnetic fields interacting in a synchronous generator--the one produced by the rotating field and the one produced when current flows in the generator stator windings (when the generator breaker is closed when synchronized to a grid with other generators). The magnetic field produced by the current flowing in the generator stator windings appears to rotate around the generator stator--and the magnetic field of the generator rotor is LOCKED into rotating at the speed dictated by the apparently rotating magnetic field of the stator windings.

When the strength of the rotating magnetic field is kept within a certain range no matter how much torque is applied from the prime mover to the generator rotor the prime mover cannot increase the speed of rotation of the generator rotor--because it is locked into synchronism with the rotating magnetic field of the generator stator. In other words, the prime mover is always trying to increase the speed of the generator rotor, but the magnetic forces at work inside the generator are preventing the rotor from spinning any faster or slower than dictated by the frequency of the grid the generator is synchronized to.

By the same token, if the energy flow-rate into the prime mover is reduced below that required to keep the generator rotor spinning at synchronous speed as dictated by the grid frequency, the generator will actually become a motor--drawing amperes FROM the grid to keep it spinning at synchronous speed. This is called 'reverse power' and is not a very desirable thing, and is actually extremely damaging to some prime movers while not so problematic for others. The concept is that as long as the generator is synchronized to the grid and the amount of excitation being provided to the generator rotor field is sufficient to keep it locked into synchronism with the stator field the speed of the generator rotor is fixed by the frequency of the grid it is synchronized to. Synchronism means everything is doing the same thing at the same time, and at a speed proportional to the frequency of the grid. Again, one generator can't run at 50.9 Hz, another at 47.6 Hz, another at 51.3 Hz, another at 49.9 Hz, and another at 50.1 Hz--when synchronized they are all running at the SAME frequency and at speeds dictated by the frequency (called the generator's synchronous speed). If any generator could run at any frequency (speed) then it wouldn't be so necessary to go through the process of synchronization--which is done to make sure that when the generator breaker closes the magnetic field of the rotor is "matched" to the rotating field of the generator (approximately) so that when the generator breaker closes the two fields will be very nearly already in lock-step with each other (the opposite fields will be attracting each other). And, from there they remain in synchronism--locked together at the speed dictated by the frequency.

However, should something happen to cause the magnetic strength of the generator rotor to decrease below that required to keep the field of the rotor locked into rotation with the stator field then the generator is said to "slip a pole"--turn at a VERY HIGHER RATE of speed than the 'rotating' magnetic field of the stator. When this happens as like magnetic poles approach each other they are repelled with very great magnetic forces to try to stop the rotor--or if the speed of the generator rotor is so high that it like poles accelerate past each then the magnetic forces will repel them in the direction of rotation even faster and as opposite fields approach each other they are drawn together by the magnetic forces of attraction which can also even accelerate the rotor even faster.

These magnetic forces, once "un-synchronized" result in tremendous forces of repulsion and attraction, at once trying to stop the generator rotor and then a minuscule fraction of a second later trying to accelerate it. The forces can be extremely destructive, especially if some small amount of excitation remains applied to the generator rotor. Actually, this doesn't last very long--probably less than one rotation--before mechanical damage occurs to the prime mover, the coupling between the prime mover and the generator, and probably the generator. It's obviously an extremely undesirable condition, and there are protective relays and excitation system settings designed to prevent this from occurring, or if sensed to open the generator breaker as quickly as possible and shut off the energy flow-rate into the prime mover.

Armature reaction also refers to the interaction between the two magnetic fields--but when they are in synchronism (when the generator rotor field is locked into synchronism with the stator field). That's as much as I'm going to say about armature reaction; again, it's not something that a power plant operator or technician can measure or sense or change. It is a function of the design of the generator. In my view, it can help operators understand some things about why they have to operate the generator the way they do (adjusting excitation as load changes), but that seems to only be my view. But it works for me, and we'll leave it at that for now. Again, it would be very interesting to know why you're interested in armature reaction as a technician.

And, hopefully you'll never be witness to the after-effects of a generator slipping a pole--it's not pretty. And, if you're too near the load coupling when it happens, it could possibly be deadly, as well.

Hope this helps!

 

sudamadhav,

I've re-read Mr. Corso's summary of his armature reaction paper, and I actually think he and I agree--just that my explanations are much more simplistic and don't involve air gaps and the various and sundry EMFs and such--none of which can be measured (they're all mathematical calculations used to predict and model and don't really explain the effects of armature reaction).

Here's a link to a website with the full paper:

http://www.automationwiki.net/images/a/a8/The_Physics_of_Armature-Reaction.pdf

Again, I would be most interested in someone's (ANYone's) review of the paper, and in particular, sudhamadhav, if it helps you to understand what armature reaction does and how it helps you operate your unit(s) better. It certainly has a lot of good maths and explanations of the physics, but have people found it to be useful at power plants? Does it, or has it, improved the understanding of synchronous generator operation, and has it been useful in explaining previously unexplained occurrences (such as, why does an operator need to manually increase the AVR setpoint when loading a machine, and why does an operator need to manually decrease the AVR setpoint when unloading a machine)?

I do share your desire to know about armature reaction--but, if there are values I can't physically measure and quantify (like EMFs and fluxes), how does that help me use my knowledge of the maths to explain to people why it's necessary to monitor VArs/power factor when loading and unloading a synchronous generator? We don't need to know how the internals of a watch works in order to be able to use one to know more precisely what time it is. I helps to know how the internals of a watch work if it's not changing time (the hands aren't moving, or the display isn't working), but it's not helpful to know how to calculate gear ratios or circuit impedances to repair a broken mechanism or replace a dead battery. I only need to know that gears are used to move hands, and charged batteries are necessary for the LCD display to work.

Because, everyone (myself included) believes that an AVR (Automatic Voltage Regulator) operating independently of VAr- or Power Factor Control uses a generator terminal voltage setpoint when operating in Automatic (AC) mode, and if the generator terminal voltage setpoint isn't changing and the excitation is remaining constant as the machine is loaded and unloaded mode why should it be necessary to change the terminal voltage setpoint when the generator armature current changes? Isn't that what we're really trying to understand--not how to calculate EMFs and fluxes? Why does an operator have to manually change the AVR automatic voltage mode setpoint as a machine is loaded and unloaded.

Sure, if there's a grid voltage disturbance and my machine didn't behave as expected it would be helpful to be able to know how to determine what it should have done (provided I can obtain the necessary values to perform the calculations)--but is that an operator's responsibility (other than to possibly retrieve the archived operating data for the engineers to use in troubleshooting and understanding the issue)?

It's really about the interaction between the two magnetic fields inside a synchronous generator when alternating current (and reactive current) is flowing in the generator's stator--armature--windings and what an increase in stator--armature--current flow(s) causes. I can't explain it with maths and vectors, and I may not exactly correctly state what's happening, but I know what operators and I can see--and have seen--on the various meters and displays typically provided with most power generation equipment (none of which includes a synchronous impedance meter, or a synchronous reactance meter, or a flux meter (for the field or the stator), etc.). I see stator amps and volts; rotor volts and/or amps; VArs; Watts; and power factor. And the question I get most often is, "Why do I [as an operator] have to change VArs [using the AVR RAISE/LOWER "switches"] as I load and unload the generator?"

I'm not speaking about a unit operating with VAr- and/or Power Factor Control active, which will automatically <i><b>adjust the AVR setpoint</b></i> to maintain a particular VAr- or power factor setpoint. I'm just talking about the AVR and why it doesn't (can't seem to) control VAr/power factor by automatically adjusting excitation during loading and unloading when generator stator (armature) current changes--specifically amperes, not Volt-Amperes Reactive, or Volt-Amperes--plain old amperes. Because, <i>I believe</i> armature reaction is the reason for this phenomenon--which causes an operator to manually have to adjust the AVR setpoint during loading/unloading of a unit which is not being operated in VAR- or Power Factor Control. I may be wrong--I am quite frequently--and if so, I would gladly welcome being constructively corrected (without maths and vectors)--just the "after-effects" of the maths and vectors on VAr/power factor during loading and unloading.

If you can be more specific about what aspect of armature reaction (the predictive, calculated effects, or, the operational effects) perhaps someone on control.com can help with clarification. Otherwise, the link above will provide all the necessary information for any generator designer or grid engineer (as well as the missing information from the summary in the previous link you provided).

Hope this helps!

 

CSA... you're welcome!

Phil Corso

 

Phil Corso,

If you had answered my question from above (the one operators always ask me), But, you haven't.

And I'll bet dollars to donuts that it's the same question that sudhamadav is trying to get to, as well.

So, you might get a thank you from him if you could answer the question (the one operators always ask me) without any equations or questions. Just the effects: How does armature reaction affect generator terminal voltage and reactive current when loading and unloading the generator? (Or, why is it necessary for an operator to change excitation when the exciter is in automatic (generator terminal voltage mode) during loading and unloading to maintain a certain VAr or power factor setpoint (presuming neither VAR nor Power Factor Control is active).)

 

I have read the article which you have mentioned. but as you said it is all equations. i'm not criticizing but every one have their way of understanding. as you said in your posts it will be if proper to explain it rather than in equations.

but one thing I have noticed is in the article where pictorial representation of flux distribution for the cylindrical rotor is given in the same way, it would be more clear if for different power factors how the resultant air gap flux changes can be shown.

further up to some part I understood but still in the confusion.

but a nice contribution by Phil sir.

 

Sudamadhav... the flux change you noted are shown in my white-paper, "The Physics of... Armature Reaction."

Regards,
Phil Corso