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Paralleling Generators
Field excitation and fuel consumption variations causes different behaviours in power generation when generators are in parallel or in single

Hello

Why when generators are in parallel increasing the fuel will produce more active power without affecting the speed of the prime mover and consequently the frequency? A "single" generator instead will increase the speed/frequency with more fuel injection.

The same for the field excitation that, if increased, will increase, in parallel, the reactive power instead of the voltage when in single?

Regards

Since you are looking at generator fundamentals you would be better thinking of generators in parallel as 'the grid'. Once a single generator is synchronised to the grid, it cannot slow down or speed up - it is synchronised until such time it is electrically disconnected from the grid.

Thus increasing the fuel will produce more active power without affecting the speed.

Ok, but what will keep the frequency of one generator "linked" to that of the other one or as you say of the grid?

As an example, we know that reverse power can occur. I have seen that in order to test the reverse power somebody forces one generator to slowdown until reverse power happen. But the forcing action is performed by increasing or decreasing the speed with the usual button on the main switchboards.

This operate on the fuel injection and so on the speed of the prime mover.

2 out of 3 members thought this post was helpful...

JeffersonAirplane,

>Ok, but what will keep the frequency of one generator
>"linked" to that of the other one or as you say of the grid?

Magnetism. The forces of magnetism keep the generator rotor spinning at the speed that is proportional to the frequency of the AC current flowing in the generator stator windings when the generator breaker is closed. And, the magnetic field associated with the flow of current in the generator stator windings "appears" to rotate around the stator, at a speed that is proportional to the frequency of the grid.

I would suggest you read some references on basic AC synchronous generator fundamentals. And, remember F=(P*N)/120, where "F" is frequency in Hertz, "P" is the number of poles of the generator rotor (which is always fixed), and "N" is the speed of the generator rotor (in RPM). Synchronism and synchronous speed are VERY important words, hence the importance attached to the act of synchronizing a generator to a grid with other generators.

And, it's all really about magnetism. Unlike poles attract each other--with great force, as we all know. And like poles repel each other--with great force, as we all know. And because there are two magnetic fields at work inside a generator (the one associated with the rotor and the one associated with the stator), magnetic forces of attraction are responsible for keeping ALL generators SYNCHRONIZED together on a grid running at the same frequency--which is proportional to speed, based on the above formula (which can be solved for speed or frequency if the other variable is known, along with the number of poles of the generator (which doesn't ever change); N=(120*F)/P is the same as F=(P*N/120)).

As for reverse power, if the torque being applied to the generator rotor (which is a function of the energy flow-rate into the prime mover) is exactly equal to that required to keep the generator rotor spinning at synchronous speed (100% speed) while the generator breaker is closed, the power "output" of the generator will be zero watts (or zero kW, or zero MW). If the energy flow-rate into the prime mover is increased when the generator breaker is closed, the generator speed DOES NOT change (because the generator rotor is magnetically locked into a speed that is proportional to the frequency of the grid the generator is synchronized to (which is 100% speed when the grid frequency is at rated!)), but the additional torque which is trying to accelerate the rotor to a higher speed is converted by the synchronous generator to amperes--and the power output of the generator increases above 0 watts/kW/MW. (Generators convert torque to amperes. Electric motors convert amperes to torque. There is no real difference between generators and motors, except the direction of current flowing in the windings, which determines whether the electric machine is a generator or a motor. The traction motors or most electric vehicles actually becomes generators during braking, to charge the batteries by using the traction motors as generators.)

Conversely, if the energy flow-rate into the synchronous generator prime mover is reduced below that required to maintain 100% speed when the generator breaker is closed then the power "output" of the generator will be negative. Actually, current from other generators and their prime movers on the grid will keep the generator rotor--and the prime mover--spinning at synchronous speed (again, because the generator is always magnetically locked into synchronous speed proportional to the grid frequency). When this happens, the synchronous generator actually becomes a synchronous motor, and a load on the grid (because real power, amperes, are flowing into the generator to keep it spinning at synchronous speed.

As you know, it takes a certain amount of energy flowing into the prime mover of a generator just to get it to and to maintain rated speed (synchronous speed) when synchronizing a generator to the grid--or to produce rated frequency if a single generator is going to power a load. That power is always required to keep the generator running at rated speed, even if it is connected to a grid with other generators. But, if that amount of energy flowing into the prime mover is not sufficient to keep the generator (and the prime mover) running at synchronous speed when the generator breaker is closed and the unit is synchronized to a grid then the generator will become a motor because it MUST spin at synchronous speed due to the magnetic forces at work between the generator rotor and the generator stator in the generator when the generator breaker is closed.

When there is one single generator supplying a load, the amount of energy flowing into the prime mover must be equal to the amount required to spin the generator at the proper speed to achieve the rated frequency (based on the formula above) AND to supply the electrical load(s) the generator is powering (motors, lights, televisions, computers, computer monitors, tea kettles, coffee makers, etc.). If the energy flow-rate into the prime mover is not sufficient to do BOTH (maintain rated speed/frequency) AND power the load(s), then the frequency of the generator (and the loads connected to it) will decrease. There is no other source of current to keep the generator rotor locked into synchronous speed--only the amount of torque being provided by the prime mover. So, if that torque is insufficient to do BOTH (maintain rated speed/frequency AND power the load(s)) then the frequency will decrease.

If the energy flow-rate into the prime mover is greater than that required for a single generator to maintain rated speed/frequency AND power the electrical load(s) connected to the generator then the frequency of the generator (and the loads connected to it) will increase above rated.

Read up on basic AC electrical generator (and motor) fundamentals, and always remember that there are two magnetic fields at work inside a generator when the generator is supplying power to a grid (or load(s)). There are also many excellent (and some not-so-excellent) videos on YouTube and other websites about basic AC electrical generating fundamentals. When the prime mover and generator is synchronized to a grid with other generators and their prime movers, it MUST run at the speed that is proportional to the frequency of the grid (from the formula above) because of those magnetic forces at work inside the generator. The second magnetic field--the one associated with the generator stator) is only present when the generator breaker is closed and current is flowing in the generator stator windings.

For a single generator supplying a load(s), there is still that second magnetic field when current is flowing in the generator stator windings, there's just no other source of current (than the torque being provided by the generator's prime mover). The generator can't become a motor because of reverse power--because there's no other source of current from other generators and their prime movers. So, if the energy flowing into the prime mover is more or less than that required to maintain rated speed/frequency AND power the electrical load(s) connected to the generator, then the frequency of the generator and the load(s) connected to the generator will be higher or lower, respectively, than rated.

Hope this helps!(By the way, do you work in the oil patch, possibly on a supply boat or an oil rig?)

1 out of 2 members thought this post was helpful...

How could this reply have been more helpful (to the person who gave it a thumbs-down)? I have struggled with trying to explain this before (more than once) and I'm open to any suggestions or constructive criticisms.

If I've made any errors with physics, I'm open to corrections. But, it's about as simple as I can make it for someone who's asking a very basic question with, seemingly, little knowledge of AC power fundamentals (which aren't that hard, really).

If there's a better way to explain why machines all spin at their synchronous speeds when synchronized to a grid with other machines and their prime movers, help me find it. 'Cause I've tried. And I don't seem to know how to explain it without emf's and counter-emf's and load angles and vectors--none of which can be measured on site with the instrumentation which is typically supplied with most power generating equipment. And a lot of maths--none of which most people are interested in or care to learn about. One simple formula (the basic formula for frequency and speed)--that's all.

Yes, there's a lot more that can be done--if we could use diagrams and pictures on control.com. But, we can't. And the information that gets posted to web-sharing sites usually disappears after a few months, so that's not a long-term option, either. And, maths can often just serve to add to the confusion, if the basic fundamentals are better understood. (I remember my Electrical 101, 102, 201 and 202 courses in university--they were brutal with all the maths, and no real clear explanations in the texts or from the lectures. The books and the instructor just seem to think the basic "operation" is intuitive and can be derived from the formulae and maths. And, for many people (I speak for many of my fellow students at the time) it just wasn't. We were just memorizing formulae and diagrams--but what was really happening, what was trying to be described by the formulae and diagrams--just wasn't clear. Not until many years later, with lots of hands-on experience, re-reading the texts (some of which were seriously lacking, like on droop speed control!), and piecing it all together in my mind.

Which might be the problem.?.?.? I don't always tend to grasp things the same way other do--most things, yes. But some things, not. Is this one of those things?

I'm looking for some help with explaining this concept as simply and succinctly as possible.

(And if the thumbs-down is about asking if the OP (Original Poster) worked in the oil patch or on a supply boat or rig, that was because they often use multiple generators in parallel (synchronized to each other), but which sometimes operate singly, and the loads can, and do, change very quickly and can lead to some "unusual" operating scenarios, especially if prime mover governors are not properly set up. I have witnessed some drilling rig operations with local generators, and it's pretty impressive, and scary, at the same time. Those generators and their prime movers take a lot of abuse--mechanically, electrically, and otherwise. Some handle it better than others, I'm told. And I wouldn't be surprised to find some technician or mechanic asking this kind of question on control.com trying to understand what is or might be happening; someone without a lot of technical background or electrical education/knowledge. Hence, the question. No other reason.)

Absolutely great.

I provide control system for equipment in the industrial field and at the moment in the naval. We generically call it automation for on-board equipment and this includes also power management. So i deal with hw, sw, and control principles, but I normally do not need to go in deep with some matters. In the case of the PMS i deal with tresholds of power and control the insertion / disinsertions of generators on the net, monitoring the interlocks and so on. But even if i do not need to get the true principles behind, I need to understand that for myself.

One doubt remain. Reading your explanation it seems to me that if the grid is very powerful (big generators synchronized all together that almost nothing could perturb from their constant revolution) the grid will force the generator newly connected to the speed proportional to the frequency of the grid itself. In the case of only two generators, each one forces the other to turn at the speed proportional of the freequency? Alternatively sometimes one dirves the other and the other drives the one?

Thank you,
Roberto

I am totally agreed with you. A simple answer but very powerful, then the math and formulas can follow. Magnetism. And then all the other detail explained in a very intuitive way.

Again, a great post.

0 out of 1 members thought this post was helpful...

JeffersonAirplane,

Thank you very much for the kind words. I'm working on another post at the moment, but will respond to your latest questions soon.

0 out of 1 members thought this post was helpful...

Roberto,

>One doubt remain. Reading your explanation it seems to me
>that if the grid is very powerful (big generators
>synchronized all together that almost nothing could perturb
>from their constant revolution) the grid will force the
>generator newly connected to the speed proportional to the
>frequency of the grid itself.

Doubt is not my favorite word (look it up). I can provide clarification, but I'm not good at assuaging doubts.

Anyway, you are 100% correct--a large grid (often referred to as an infinite grid) will force a generator being synchronized to its synchronous speed with the grid frequency. As you may know, when synchronizing a generator to the grid it is customary for the speed/frequency of the generator to be just a little higher than the frequency of the grid it is being synchronized to. And, there is usually either a synchroscope which rotates to indicate fastness or slowness and the rate of fastness or slowness, or a set of synchronizing lights which also indicate the rate of fastness or slowness (sometimes there are both).

Basically what the synchroscope indicates when it is approaching 12 o'clock (and what the synch lights indicate when they are going dim (usually!)) is that the North pole(s) of the generator rotor are coming into alignment with what will be the South poles of the generator stator field. And usually just before the synch'scope reaches 12 o'clock (or the synch lights go dim) the generator breaker is told to close. There is a slight time lag for the generator breaker to actually close (hence, why the signal is sent a little "early") but the idea is that when the generator breaker actually closes the magnetic poles will be in alignment--and the rotor is "locked" into the speed which is proportional to the grid frequency with very little mechanical force required to "capture" and "lock" the rotor into synchronism and synchronous speed.

Now, the governor is still supplying slightly more fuel than is required to maintain rated speed, and that did, when the generator breaker was open, keep the generator rotor spinning slightly faster than synchronous speed. But, when the generator breaker closes and the generator rotor gets locked into synchronism that extra torque from the generator prime mover gets converted into a small amount of amperes flowing in the generator stator windings, which means the power output of the generator is positive and above zero watts/kW/MW.

If the generator was not synchronized before the generator breaker was closed when the generator breaker was closed one of two things could happen. Either the magnetic forces of the generator rotor and the generator stator would be out of alignment--and force them apart, possibly in the wrong direction!--or the magnetic forces would cause the generator rotor to spin extremely fast as the forces of unlike poles attract each other. In either case, as soon as the unlike magnetic poles attract each other the generator would effectively stop (for a very brief instant in time)--which would transmit forces trying to stop the prime mover back to the prime mover through the coupling connecting the generator rotor to the prime mover output shaft. This causes very great mechanical forces for very short periods of time--but these forces can be very destructive, even catastrophic.

That's why synchronizing a generator to other generators is so important--and so critical--to protect the generator being synchronized, and also to protect the generator breaker and the grid components. So, the generator breaker is closed when the unlike poles of the generator rotor and stator would be in alignment thereby reducing the forces required to lock the rotor into synchronism with the stator. (There is some force as the rotor is "captured" and "locked" into synchronism, but if the synch-scope is going relatively slowly then the forces arent't too great.)

>In the case of only two
>generators, each one forces the other to turn at the speed
>proportional of the freequency? Alternatively sometimes one
>dirves the other and the other drives the one?

In the case of only two generators, usually the one with the more powerful prime mover controls the frequency--not always, but usually. But, again, you are right; they are still synchronized with each other and neither can go faster or slower than the other. There is only one frequency coming out of the receptacle on the wall, remember! There can't be multiple generators all running independently at different frequencies synchronized together on a grid producing a single frequency for the user. It's just not physically possible.

So, I hope this clarifies things for you.

That's very ok thanks again. Now I go back for a while to my first post where I was speaking about field excitation.

Why I hear speaking of increase (decrease) of reactive power when field excitation increases (decreases) in paralleled generators (while in single that variation modify the voltage output)? Isn't it that the load determines the type of power? If I had only a resistance to drive isn't it a active power?

So many questions I know. Hope this does not disturb.

Regards

1 out of 2 members thought this post was helpful...

Roberto,

Excitation is very much like watts (kW; MW). When a generator and its prime mover are connected to a large ("infinite") grid, there is an unlimited amount of load (watts/kW/MW), AND an unlimited amount of reactive load (VArs; kVArs; MVArs). So, when the generator is at rated speed and the generator breaker is OPEN, increasing excitation will increase generator terminal voltage.

But, when the generator is synchronized to a large ("infinite") grid (the generator breaker is closed) and the excitation is exactly equal to what is required to make the generator terminal voltage equal to the grid voltage the generator will not be "producing" (or "consuming") and VArs. The power factor will be unity, 1.0.

If the excitation is increased, the generator is trying to increase the grid voltage--but it can't increase the grid voltage by any appreciable amount, but what happens is that the generator begins to produce VArs--called lagging VArs at the generator terminal.

So, it's very similar to when the energy flow-rate into the generator prime mover is increased which would make the speed increase if the generator breaker were open, but when the generator is synchronized to the grid and generator breaker is closed the speed can't increase and the watts/kW/MW increase.

If the excitation is decreased below the amount required to make the generator terminal voltage equal to the grid voltage then what happens is that VArs (reactive power) flows "into" the generator and the generator becomes a reactive load on the grid. These VArs are called "leading" VArs at the generator terminal.

When there is just one generator supplying a load, you are correct: the reactive load is what it is and the generator can't do anything about that. BUT, changing the excitation does change the generator terminal voltage and the grid voltage at this point. And when there are two generators synchronized together supplying a load, real and reactive, the amount of reactive load is still fixed but the split of reactive load being "carried" by each of the generators can be affected by changing the excitation of one of them (similar to what happens to the real load when two generators are synchronized together).

Finally, if the load on any system is purely resistive then there won't be any reactive power required by the system and changing excitation will have an appreciable effect on system voltage. But, since the majority of electric motors are induction motors the likelihood of that happening is very low. It does happen at some plants (arc furnaces, for example), but even those, I'm told, have some reactive load.

Hope this helps!

Then, can we say that when two generator are paralleled and load is purely resistive, if excitation field increases in one of the two it could be considered like if the other was decreasing its excitation (this because the reference is the output voltage)?. And so is it becoming a reactive load?. In this way even with a pure resistive load, anyway the generator with the increased field excitation will produce reactive power while the other will absorb it being a reactive load.

But as you say, this case is very peculiar (very low probability) and the system will not be working correctly and generating reverse power it seems to me. So control should avoid this.

Then when a generator is generating alone the power for the load and the load is both active and reactive normally, it seems to me, it will not need to modify the excitation field if the voltage remains stable at the output terminals. And if load change there will be more fuel injected whatever it is the type of load. Am I right with this?

If yes in the case of 2 paralleled generators I do not see very clearly the why for modifying excitation field unless that of transferring the reactive load from one to the other as you were describing (when you were speaking of the split).

Regards

1 out of 2 members thought this post was helpful...

When two generators are synchronized to each other and not to a larger grid, if any part of the load is reactive then the excitation systems of the two generators can be used to make the two generators split the reactive load, or have one generator take the reactive load.

In my experience, if one excitation system continues to increase its excitation then the system voltage will increase AND the other generator will eventually start "consuming" reactive power. Regardless of how much of the load the two are supplying is resistive.

The amount of the resistive load shared by the two generators is controlled by the amount of energy flowing into the prime movers of the two generators. And, if either energy flow-rate causes the total energy flowing into the two generators to exceed the amount required to maintain the load AND maintain rated speed/frequency, then frequency/speed of the units will increase.

When two generators and their prime movers are synchronized together and supplying a load (resistive; resistive-reactive) it's customary for one of the to be operated in Isochronous speed control mode and the other to be operated in Droop speed control mode. Sometimes, a separate PMS is used to control the loads AND the frequency of two such generators and both are operated in Droop speed control mode. It can get VERY complicated VERY fast.

>Then when a generator is generating alone the power for the
>load and the load is both active and reactive normally, it
>seems to me, it will not need to modify the excitation field
>if the voltage remains stable at the output terminals. And
>if load change there will be more fuel injected whatever it
>is the type of load. Am I right with this?

Yes.

>If yes in the case of 2 paralleled generators I do not see
>very clearly the why for modifying excitation field unless
>that of transferring the reactive load from one to the other
>as you were describing (when you were speaking of the
>split).

Yes.

Very well. Thank you very much.

Best regards,
Roberto