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Synchronization of a Captive Power Plant
Questions about the synchronization of a captive power plant.

Hello everyone,

I wish Mr CSA and the highly experienced men here can help me with my queries on synchronization of Captive power plant.

Just as I have posted before, the power plant is still at the engineering stage of which am learning from the engineering team. The plant configuration is 8GTGs and 4STGs cogeneration-combined cycle. I understand it will not be connected to the grid being captive. But I want to understand how all the GTGs and STGs will be synchronized and made to operate at same frequency! Of course I asked my mentors and I was told one GTG+Synch generator will be brought to rated frequency and later the second GTG circuit breaker will close with the initial GTG and operate at same frequency! Likewise others.

But am not really convinced and I believe I can get a detailed explanation here.

Looking forward to responses.

Thanks.
Regards,
Rotimi

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

Rotimi88,

Essentially the people you have been talking to are correct. Once one synchronous generator and its prime mover are providing power to the plant when a second one is synchronized to it they will both operate at the same frequency. That word--synchronized--is a very powerful word, almost as powerful as the principles which it tries to describe. One thing which may help to understand what's happening is that when two or more AC generators are synchronized together and supplying electrical loads (motors and lights and tea kettles and televisions and computers and computer monitors) all the generators and their prime movers are essentially acting as one generator and one prime mover, just as all the motors and lights and tea kettles and televisions and computers and computer monitors all appear to the generator(s) as one single load. No single generator can operate at any other frequency than the one of the sytem with which it is synchronized to. That's what synchonized means in this context--all generators which are connected together (synchronized) are operating as one generator, and no single generator (nor group of generators) can operate at any other frequency than the frequency of the grid (of any size!) with which it is synchronized to. It's not possible for one generator to run at 49.678 Hz, another at 50.341 Hz, another at 48.569 Hz, and another at 60.17 Hz while synchronized together.

To understand AC (Alternating Current) power generation, transmission and distribution systems it's necessary to understand one basic formula: F=(P*N)120 [Frequency equals the (Number of Poles of the generator rotor multiplied by the Speed of the generator rotor (in RPM)) divided by 120], and how AC is produced. In other words, a 2-pole generator rotor must rotate at 3000 RPM in order for the generator to produce AC current and voltage at 50.0 Hz. A 2-pole generator rotor producing 60.0 Hz is rotating at 3600 RPM (or must be rotated at 3600 RPM). This formula should be self-explanatory.

The basic physical principles in AC power generation involve magnetic principles; you can find more on this on any number of YouTube videos and textbooks. Rotate a magnetic field past a conductor and a voltage will be produced, and the frequency of that voltage will be proportional to the speed of rotation of the magnetic field. Every magnetic field has two poles--North and South. Another way to produce magnetic fields is to flow current through a conductor (which is what's happening in the generator rotor). And, when current flows through a generator's stator windings (conductors) a magnetic field is also produced.

In every synchronous generator synchronized to a grid (of any size) with other synchronous generators and their prime movers there are two (2) magnetic fields in every generator: one produced by the application of DC current to the generator rotor windings, and the second produced by the flow of AC (Alternating Current) in the generator stator windings. The magnetic field of the generator rotor is rotated by the prime mover, and the magnetic field of the stator appears to rotate around the periphery of the generator because it is being produced by alternating current (again, refer to YouTube and text/reference books for more information on this).

Both magnetic fields, the generator rotor's magnetic field and the generator stator's magnetic field, have North- and South poles. And we all know how unlike magnetic poles attract each other (North attracts South, and South attracts North). And, now we have two "rotating" magnetic fields inside the generator--one created by flowing DC through the generator rotor (which is "free" to rotate) and the other which "appears" to rotate due to the flow of AC through the generator stator windings. The North Pole of the generator rotor is VERY STRONGLY attracted to the South pole of the generator stator windings--and the speed of rotation of the South pole of the generator stator windings is a function of the frequency of the AC flowing in the generator stator windings. Same goes for the VERY STRONG attraction between the South pole of the generator rotor and the North pole of the generator stator windings, which "rotates" around the generator stator at a speed that is proportional to the frequency of the AC grid.

The interaction (magnetic attractions of unlike poles) at work inside every generator when it's synchronized to a grid (of any size!) with other generators and their prime movers dictates the speed of the generator rotor when it's synchronized to a grid (of any size!) with other generators and their prime movers. The speed of the generator rotor, once synchronized to a grid (of any size!) with other generators and their prime movers, is FIXED by the frequency of the AC grid (of any size!) based on the formula F=(P*N)/120.

It's really very important to understand that when generators and their prime movers are synchronized together on a grid (of any size) supplying an electrical load (composed of many smaller loads) that the generators are all acting as one generator--and they are all rotating at speeds proportional to the frequency of the grid with which they are synchronized. Synchronized means they are all rotating at speeds which are directly proportional to the frequency of the grid they are connected (synchronized) to.

So, why can't any single generator (or group of generators) operate at a speed/frequency different from the grid when it's synchronized to the grid (of any size)? We have all played with magnets, and we all understand that North Poles attract South Poles--very strongly, And that South Poles attract North Poles--very strongly. And that like poles repel each other--very strongly. When we try to force the South (or North) poles of two magnets together with our hands, we might be able to make them touch, but when we let go of them--WHAMMMM!!! they fly apart, and sometimes they join each other (North pole to South pole, or South pole to North pole). But, the like poles DO NOT want to be close to each other--not at all.

Let's think about what would happen if a single generator (or group of generators) tried to run at a speed/frequency different from the grid (of any size) with which it is synchronized. Well, that would mean that the generator rotor magnetic field would have rotate slightly faster or slower than the speed of the generator stator magnetic field--which would mean that at some point two North poles would come into close proximity with each other, and at the same time two South poles would come into close proximity with each other. And, that would cause great physical forces of repulsion inside the generator, which would likely severely damage the generator and coupling between the generator and its prime mover. And, if this were allowed to continue indefinitely, well, the damage would be huge. There would great forces of attraction and repulsion every time the poles came into close proximity with each other. As the North pole of the generator came close to the South pole of the generator stator the two would be very strongly attracted, and then they would be torn apart, and as the North pole of the generator rotor passed by the North pole of the generator stator it would be VERY STRONGLY repelled, This is NOT good--and it can't happen (not for long, anyway).

The upshot of all of this is: All generators synchronized to a grid of any size are all rotating at speeds that are proportional to the frequency of the grid with which they are connected--because of very great magnetic forces inside the generators. This effectively makes them all act as a single generator and prime mover driving what appears to be a single electrical load (but which is many smaller, some large, electrical loads). And, the amount of generation (the amount of energy flowing into all of the generator prime movers) must be exactly equal to the amount of electrical load PLUS the amount of energy required to get up to rated speed (synchronous speed--the speed of the generator required to maintain desired frequency). If something happens to upset this delicate balance, the frequency will change--but so will the frequency of every generator synchronized together and this means the speeds of all the prime movers driving the generators will also change. If a generator trips off, and no other generator prime mover raises its output to compensate for the loss of that generator, then the grid frequency will drop and so will the frequency--and speed--of all generators synchronized to that grid. If a large block of load trips off (because a substation breaker somewhere opens) and no generator prime mover reduces it's power output the grid frequency will increase--and so will the frequency, and speed, of all the generators synchronized to the grid. It's a very delicate balancing act.

Now, for what happens at a captive power plant (also called an islanded power plant, because it's like an island in a sea of other plants which might all be connected to a large grid). When a single generator is connected to the power plant load, its governor is (ideally) operating in Isochronous speed control mode. This is the governor mode which senses any change in speed (frequency!) and very quickly adjusts the energy flow-rate into the generator's prime mover to maintain desired frequency (speed) setpoint. Any change in load (as more motors or tea kettles or lights or computers and computer monitors) are added to or removed from the electrical grid will cause the grid frequency to decrease or increase, respectively. And, it's the Isochronous governor that senses that change in frequency (speed) and very quickly changes the energy flow-rate into the generator's prime mover to maintain the frequency (speed) reference at desired, say 50 Hz.

Then a second generator and its prime mover is added to--synchronized to--the first generator and prime mover. And, both generators are producing the same frequency (as we learned above, that's the definition of AC electrial power generation synchronism). Let's say when the second generator is first connected to--synchronized to--the first generator the power output of the second generator is 0 MW, and the electrical load is stable at 20 MW and all of that load is being provided by the first generator, its governor operating in Isochronous speed control mode. By convention, only one generator and prime mover can operate in Isochronous speed control mode so the second generator and prime mover are operating in Droop speed control mode. The Isochronous machine will control frequency--of BOTH machines, because they are synchronized together, and the Droop machine will just "follow along", producing as much power as the operator decides.

We said the electrical load (motors and lights and tea kettles and computers and computer monitors) was stable at 20MW. The operator then increases the load of the second generator and prime mover to 5 MW. The first effect of this increase in generation is to increase frequency (speed)--but the Isochronous governor, sensing the frequency increase, decreases the energy flow-rate into the prime mover to reduce its load to 15 MW, for a total of 20 MW from the two machines--acting as one!--to supply the 20MW load. If 2 more MWs of load are added to the system (more motors and lights and tea kettles) the instantaneous effect on the system would be to decrease the frequency--but the Isochonous governor immediately increases its load to 17 MW, while the Droop machine remains at 5 MW. The operator doesn't have to do a thing--the Isochronous governor does it all automatically--as long as the load on the Isochronous machine doesn't go below 0 MW or above the rating of the prime mover driving the generator.

Both machines are--they have to be!--operating at the same frequency. That's electrical synchronism. And it's the magnetic forces working inside the two generators are keeping them both at the same frequency. And, the Isochronous speed control mode of the first machine's governor is monitoring grid frequency, and when it senses a change--either because the load changes or the generation of the second machine changes--the governor will quickly change the energy flow-rate into the prime mover to maintain desired frequency (speed).

As more loads are added to the system and the total electrical load of the system begins to exceed the amount of possible generation the operators will have to add more generation, but the Isochronous governor will still control the frequency of the system of generators and loads. The operators will have to balance the loads between the generators to keep the load on the Isochronous generator between 0MW and rated MW in order for the Isochronous machine to be able to maintain frequency of the system--and all of the generators synchronized together, acting as one, to supply the load on the system.

The amazing thing to note here is that all the generator prime mover governors are "communicating" with each other--by virtue of the frequency of the system. They don't need any other connection or control system to properly produce power at a desired frequency--they do it all by means of sensing frequency, which is directly related to speed. Of course, this takes some trained operators to recognize sudden changes in load and make appropriate adjustments to the loads of the various machines to help them maintain rated frequency, but it is all possible without any additional control systems and schemes--with trained operators.

Now, what will likely happen is that an "over-riding", external control system will be used to "automatically" do what trained operators would do. The problem here is that the people programming the external control system which will be sending signals to the various prime movers of the generators need to understand how the plant works and how electrical power generation systems work--and that's not very common these days. So, it usually takes a LOT of tuning and tweaking to make these frequency/load control systems work properly. And, in these cases, the governors of the various generator prime movers may all be operating in Droop speed control mode getting their commands from the "automated" external control system, or they might be operated in Isochronous Load-sharing mode (essentially a de-sensitized Isochronous speed control mode, or really Droop speed control). But, that's what will probably happen--someone or some company will try very hard to program a "power management system" to send signals to the generator prime movers to maintain plant frequency as load changes, with as little human intervention as possible. Which means it will take a lot of tuning and tweaking and adjustments--and several plant black-outs, probably--to even begin to achieve. When proper training for the operators--and their supervisors--would be all that was needed.

The keys here are that all generators and their prime movers synchronized together on a grid of any size are all operating at the same frequency, and at speeds which are directly proportional to that frequency. Any number of synchronous generators and their prime movers synchronized together on a grid of any size are effectively acting as a single generator--supplying electrical power at a single frequency. That's the definition of synchronism in this context. And the definition is as powerful as the magnetic forces at work inside every synchronous generator keeping them running at speeds proportional to the frequency of the grid they are synchronized to.

I'm sure, after you digest this, you will have many more questions. And, we will try to answer them as best we can. Just try to keep them simple. This was NOT really a simple question, but the simple answer you received from your colleagues was basically correct, but the explanation is much more involved. Remember, it all gets down to simple physics--magnetism, and what happens when magnetic lines of force cut a conductor, and what happens when current flows through a conductor. And what happens when there are two magnetic fields present inside a generator--and the physical forces of magnetic attraction keeping the rotor magnetic poles "in synchronism" with the stator magnetic poles which are appearing to rotate around periphery of the generator. The rate of rotation of the stator magnetic fields is directly related to the frequency of the current flowing in the stator windings, and that controls the rate of rotation of the generator rotor--and of the prime mover driving the generator.

When there is only one single generator or a small number of generators supplying an electrical load, one of the generator prime movers can be used to control frequency by responding to load changes (that's the Isochronous machine), and the other generators and their prime movers are "just along for the ride", their speed being controlled by the frequency which is being controlled by the Isochronous machine.

I hope this was helpful. It's not easy to write, and it does rely on a good understanding of AC electrical fundamentals and principles, none of which are really difficult to understand and all of which can be understood in videos and text/reference books. It's when generators and their prime movers are synchronized together that the text/reference book explanations begin to get vague, and are frequently fraught with problems. But, it all has to start with basic electrical physical principles and fundamentals, and one simple formula. Yes; there are load angles and vectors and all kind of maths which are only means of predicting and proving what is really happening. And, that's generally where most text/reference books fall short--they forget to explain what's really happening inside each and every synchronous generator that's synchronized to a grid (of any size!) and what synchronism really means. When they get to the maths and angles and vectors they forget to explain what all of that is telling the student/reader--what happens when multiple machines are synchronized together supplying a load (loads), and how multiple machines are to be operated together.

And, doubts--well, look up the meaning of the word in your Oxford's. We clarify; we correct. But we don't like doubts.

Mr CSA,

I am delighted to see this detailed reply. I am going to read this thoroughly in between the lines over the weekend and will ask further questions if I find any.

Thank you very much I must say.

Rotimi88,

There's a couple more things which might help with your understanding.

First, synchronization. It's a BIG deal!!! By, that I mean, the process is very critical and very important, and it involves getting the magnetic poles basically in line with each other (for all three phases--though only one or two phases are used for most synchronization circuits, all three MUST be verified during initial synchronization and any time the bus-bar or PT (Potential Transformer) circuits are disturbed).

Most prime mover-generator units have two (2) synchronization relays--it's that critical. Both must agree the generator breaker can be closed before the signal is sent to close the breaker. And, it's all to try to limit the physical forces required to close the breaker when the magnetic poles are not aligned, and to limit damage to the generator, the load coupling between the generator and the prime mover, and to the prime mover.

Second, each prime mover requires a certain amount of energy just to accelerate the generator rotor to synchronous speed and to maintain that speed during synchronization and during operation. If the energy flow-rate into the prime mover falls below that required to maintain synchronous speed (the speed proportional to the frequency of the grid it is synchronized to) the grid will supply current (amperes) to the generator to keep the generator rotor--and the prime mover--spinning at synchronous speed. That's typically called "reverse power" and is not a desirable mode of operation for most prime movers (it doesn't hurt the generator--as long as the excitation system is working!).

A LOT of people have no problem with the fact that electric motors convert amperes into torque, and they all understand that generators produce amperes that the motors convert into torque. But MANY people have a problem with the fact that generators convert torque into amperes--but that's what they do. Any additional energy flow into the prime mover above that which is required to maintain synchronous speed is converted by the generator into amperes. The extra energy flow-rate would tend to make the generator rotor speed increase--but the grid frequency is holding the generator rotor speed fixed at synchronous speed the generator takes that extra torque that would otherwise go into additional speed and converts it into amperes--which is the variable that causes the electrical power produced by the prime mover and generator increase.

As was noted in the previous explanation, if an operator increases the energy flow-rate into the prime mover of a unit operating in Droop speed control mode while the electrical load of the grid (of any size) is stable, the frequency of the grid will increase. If there is an Isochronous unit (or some other external load control system) operating on the grid, the energy flow-rate into the Isoch machine's prime mover will be reduced to maintain the desired frequency. NO ONE changes the load directly on the Isoch machine--it is monitoring frequency (speed) and adjusting the energy flow-rate as necessary to maintain desired frequency.

Most people know that a generator must be accelerated to synchronous speed, then synchronized, but then they think that as load is increased speed also increases. They just have real issues when it's pointed out to them that the speed doesn't change (unless the grid frequency changes) once that generator breaker closes. Or, they just never think about why the speed doesn't change, and what happens to the extra energy that flows into the prime mover to increase the electrical load.

Once the generator breaker closes, most people are ONLY focused on electrical load--and nothing else matters. (Well, at a combined cycle plant a LOT else matters--but it's got little to do with the generator speed.)

Hope this helps--a little more!