Hello,

Can anyone help me to find a free handbook talks practically about synch. Genrators, modes of operation,...etc.???

<b>Moderator's Note:</b> These topics have been discussed in the Control.com forum a number of times before. Please use the Search Box on the right hand side of the menu bar and search for the relevant terms. For advanced search options, click the Question mark (?) next to the Search Box. If you don't feel your question is adequately addressed, try responding to one of the related posts with a request for more specific information.

 

Find any book. namely rotating electrical machines.
some authors are
1. MG Say
2. Langsdorf
3. PS Bhimbra

The last one is excellent (part-II for synch machines). i had gone thru this during my college days.

 

Diaa,

As our Friendly Moderator says, in one form or another this topic has been covered many times before on control.com. You can use the Search term:<pre> +"synchronous generator"</pre>as your first attempt at finding information you may be looking for.

Not knowing anything about your background or experience in the industry of power generation, and what the nature of your query is (design; operation/production) it's important to begin with a couple of basics regarding electrical power generation.

First is, in the beginning of electrical power generation the primary use of electricity was to drive electric motors to produce torque and work in factories. Previously, many factories (such as textile mills) were located near rivers and water falls to take advantage of the power of moving water to make water wheels rotate. A shaft connected to a water wheel ran into the factory, and belts were used to transfer the rotating energy to various pieces of equipment.

Then some factories began to use steam turbines to get more power, but as that industry was in its infancy there were a lot of fires and explosions and it was expensive to build boilers, turbines and condensers (which required even more water).

Electric motors are a simple method of producing torque and power to do work--such as drive pumps and fans and textile-making equipment and presses and conveyor belts and such. And, electric generators are needed to drive the electric motors. AC was chosen over DC because many of the first generators were driven by hydraulic turbines (hydro turbines) and the rivers and water falls were located far from large cities and the growing industrial centers. So, the electricity had to be transmitted from remote locations to the factories and mills, and AC is a more efficient means of transmitting electricity.

And, that's a very important concept to understand about electrical power generation: a generator (be it synchronous or induction or DC) must be driven by a prime mover. A prime mover can be a hydro turbine, a reciprocating engine, a steam turbine, a combustion turbine--and device that can produce torque. And wires connect the generator to loads (in the beginning, electric motors) and the electric motors convert the electrical power (amperes) back into torque to perform work. So, it's really the prime movers driving the generators that are doing the work of the loads (motors; lights; radios; etc.) connected to the generators by wires. Electricity was initially a means of transmitting torque from a place where it could be found (rivers and waterfalls, for example, flowing through hydro turbines and driving generators), using wires to transmit the "torque" (as amperes) to loads which were located very far away from the hydro turbines and the water that flowed through them. (Uses for electricity have grown exponentially since then to include various forms of light-producing methods, radios, televisions, computers and computer monitors, etc. But, in the beginning it was primarily motors to produce torque and perform work in factories and mills.)

Generators are really pretty "dumb" devices--they convert torque from the prime movers coupled to them into amperes in the generator stator (armature). As such, they really don't have "modes." The more torque produced by the prime mover and transferred to the generator by the coupling between them, the more stator amperes the generators can produce--which means the more motors and more work can be produced at the other end of the wires connected to the generator.

Generators--and their prime movers--can be connected together to supply loads that are much larger than any single generator could ever hope to supply. In AC power systems, this means that multiple generators--and their prime movers--are <b>synchronized</b> together and all basically operate as one large generator. On a 50 Hz system, for example, every generator synchronized together must produce 50 Hz. One generator cannot run at 49.3 Hz, another at 51.276 Hz, another at 59.7 Hz, another at 49.991 Hz, etc. They all have to be producing power at the same frequency--which is the definition of synchronism, everything operating at the same speed, or rythym, or period. Synchronism of an AC power system consisting of multiple generators(and their prime movers) means that all the generators connected together are all operating at the same frequency--none faster or slower than any other one (except in very unusual circumstances, and usually only for short transient conditions).

Which brings us to the second important fact about synchronous generators: the frequency of the generator is proportional to the speed of the generator rotor. And, to make a constant frequency, that means the generator rotor has to spin (or, more correctly, be spun) at a constant speed. And, since prime movers driving generators are almost always directly coupled to each other that means that the prime movers must be operating at a constant speed. The formula that relates speed and frequency is:<pre>F=(P*N)/120

where, F=Frequency (Hz)
P=Number of magnetic poles of the generator rotor (always an even number, North and South)
N=Speed (RPM)</pre>The number of poles of a synchronous generator rotor is always fixed--it can't be changed. So, for a synchronous generator with two poles producing 50 Hz the generator rotor has to turn (be turned) at a steady 3000 RPM--which means the prime mover turning the generator rotor has to spin at a constant speed, and since the prime mover is almost always directly coupled to the generator then it has to spin at 3000 RPM. (Some prime movers are designed and constructed such that they produce maximum torque output at speeds that are not even close to synchronous speeds (for example, 51034 RPM) so they are connected to synchronous generators using gear boxes to change the prime mover speed to match the speed required by the generator to produce the desired frequency.)

So, synchronous generators are driven by prime movers. And, it's usually the prime movers that have "modes"--such as start-up, shutdown, synchronous speed, droop speed control mode, isochronous speed control mode, base load, inlet pressure control, extraction pressure or -flow control mode, power control mode, and so on. Again, generators--even synchronous generators--are all pretty dumb. They just convert torque into stator amperes (and electric motors convert amperes into torque--in this discussion we're still living in the early stages of electric power generation!). More torque--more amps; less torque, less amps. But, it's the prime movers that have the various modes to control the torque that's produced, that the generators convert into stator amperes.

Synchronous generators also some require electric power--usually DC power--at the generator rotor windings to produce the magnetic fields that, when spun, result in voltage, which is very important in the production of electric power. And that power to produce the rotating magnetic fields can be varied while the generator is running, and it's controlled by the generator exciter, or excitation system, or voltage regulator--sometimes called the AVR (Automatic Voltage Regulator).

Generator terminal voltage is a function of excitation and generator rotor speed. And we know that the speed of a synchronous generator rotor must be fairly constant, so by varying the excitation power applied to the generator rotor windings the generator terminal voltage is varied. Normally, generators operate at a pretty constant voltage--meaning that the "AVR" must be able to regulate (control) the excitation power to maintain a stable generator terminal voltage.
And, normally, generator terminal voltage can't be varied more than +/- (plus-or-minus) 5% of generator nameplate voltage rating, so the generator terminal voltage only varies in a small range, which means the excitation power applied to the generator rotor windings must be relatively constant and be able to keep the generator terminal voltage relatively constant.

When a synchronous generator is synchronized to a grid with other generators if the amount of excitation being supplied is exactly equal to the amount required to keep the generator terminal voltage equal to the grid voltage then the power factor of the generator is said to be at unity, or 1.0. In this condition, no reactive current is flowing in the generator windings (0 VArs).

If the excitation power to the generator rotor is increased, the effect would be to increase the generator terminal voltage. But, when synchronized to a grid with other generators the generator terminal voltage can't increase very much, but the effect is to cause lagging reactive current to flow in the generator's stator (armature) windings, and the power decreases to some value less than 1.0 in the lagging direction. This means the VAr meter will swing in the lagging direction.

The opposite happens if the excitation applied to the generator rotor is decreased below that required to keep the generator terminal voltage equal to grid voltage--the power decreases from 1.0 in the leading direction, and leading reactive current flows in the generator stator windings, or leading VArs.

Generators can be operated in what's called "power factor control mode" or "VAr control" mode--by using power factor feedback and adjusting excitation to make the power factor feedback equal to some power factor reference, or the same for VArs. Generators also have "manual mode" and "automatic mode"--where the amount of excitation is varied to control field voltage- or current (manual mode) or generator terminal voltage (automatic mode). But, in reality, these generator "modes" are all really excitation control system (voltage regulator) modes--not generator modes. These "modes" only control the generator rotor winding voltage/current or the reactive current flowing in the generator stator windings, or the machine's power factor, and not the amperes flowing in the stator windings--which is how the power output of the generator-set is measured. And it's torque that determines how many amperes are flowing in the generator stator windings--and it's the prime mover that supplies the torque. And, the prime mover control system (sometimes called the "governor") controls how much torque is produced.

The physics and maths of how all that happens (torque being converted into amperes in the generator stator windings; excitation current causing reactive current to flow in the generator stator windings) is all pretty complicated, but you will find all of that explained in any reference on rotating electrical machinery.

What you won't find is all of the above, in the same manner as above, and related to actual power plant operating conditions in any handbook or manual or reference book--free or otherwise. You will find a lot of talk and maths about emf's, and counter-emf's, and back-emf's, and load angles and air-gap voltages and flux in handbooks or manuals or reference books. But, none of those things are measured or monitored in an operating power plant--only the amount of generator stator winding amperes and the magnitude of the generator terminal voltage, and the amount and type of reactive current flowing in the generator stator windings (from which power factor can be determined). There aren't any emf meters, or counter-emf meters, or back-emf meters, or load angle meters. There are air-gap flux probes, but no air-gap voltage meters. All of those things are important to the generator designers--but not to operators. And you didn't really tell us what the nature of your query is about--synchronous generator design, or synchronous generator operation.

Generators are really generator-sets--a prime mover coupled to a generator. And, even for generator designers it's still important to know the basics of how generators operate, and then they can design and build generators using maths to predict how much power a particular generator can produce--or, how much torque it can convert into amperes. Because, that's what generators do.

Now, go forth and research--and as the Friendly Moderator also said, after you've done some research if you still have specific questions about some aspect of synchronous generators and electrical power generation, post your questions here. And we'll do our best to help. Realize that we can't provide drawings, and the formulas we can provide are crude (we can't recreate some mathematical symbols).

You can also search control.com for:<pre> +"the physics of"</pre> There is a person who responds to a lot of electrical questions/threads who has written a couple of papers on electrical power generation, mostly maths. You can email him, tell him about your affiliation (student at a particular university, or employee of a particular company, for example) and request that he send you a copy of his paper(s).

 

Thank you for your response, I am electrical engineer work in oil and gas field and responsible for the power plant operation (4 gas turbine generator sets). I was asking for a practical handbook for better understanding of some generation concepts. If there is no handbook to explain it practically, so please can you give me an explanation for the following:

1- what is the difference between the various control modes (i.e. Isochronous speed control, droop speed control, droop volt. control, and reactive compensation? and on what bases i decide to operate the machine on iso or droop mode?

2- what is the cross current and how it is initiated and what is the meaning of the cross current compensation?

3- what is the difference between the island mode and synch. to grid mode?

<b>Moderator's Note:</b> Can one of you regulars point him to the threads where these topics have already been discussed? Thank you.

 

Diaa... the free-ist site is Control.Com Archives... as suggested by the ModSquad.

For example,"Cross-Compensation", alone brings up 22 threads. And Iso-Mode some 55!

Regards,
Phil Corso

 

Diaa,

What area of electrical engineering did you study in university? (Electrical engineering can encompass many aspects of electricity, and I'm wondering if you studied anything about electrical power generation--and to what extent the courses prepared you for the work you currently find yourself doing.)

Is the facility you are presently working at operating in parallel (synchronized) to a larger regional or national grid? Does it ever operate independently of the grid?

If the facility operates independently of the grid, does it supply power for a production facility (refinery; natural gas processing plant; or???)?

Does the facility have any kind of 'power management system' (PMS) that controls the level of generation of one or more of the generator-sets, particularly during periods when the facility is operating independently of the grid?

You can search control.com for:<pre> +isochronous</pre>for information on what Isochronous speed control does.

You can search control.com for:<pre> +"droop speed control"</pre>for information on what Droop speed control does.

In general, Isochronous speed control is used when it's necessary to maintain the frequency of some plant or load which is being supplied and operated independently of a larger grid (with many other generator-sets). In general, only one generator-set (generator and prime mover) is operated in Isochronous speed control mode--unless there is some kind of Isochronous load-sharing scheme in place between multiple machines supplying a load independent of a larger grid--which is what a power management system might permit. A generator-set operating in Isochronous speed control mode changes it load (power output) automatically as the load it is supplying changes; an operator cannot apply a load setpoint to a machine operating in Isochronous speed control mode because it must be "free" to changes its load in order to maintain frequency. If an Isochronous machine were to be operated at a stable power output while the load is changing (pumps are being started and stopped; lights are being turned on and turned off; etc.) the frequency of the plant will vary as the load varies--and for an AC power system one of the most aspects of a stable power system is stable frequency.

Droop speed control allows multiple generator-sets (generators and their prime movers) to be able to operate in parallel with each other (synchronized together) to act as one very large generator supplying a load that is perhaps much larger than any single generator-set could supply. Droop speed control permits stable operation of the generator-set when producing watts/kilowatts/megawatts in parallel with other generator-sets; one can set a machine operating in Droop speed control mode at, say, 20.5 MW, and it will remain at 20.5 MW as long as the grid frequency remains stable.

So, Isochronous speed control is only used when a power generating facility is operating independently of a larger grid and it's necessary to maintain frequency as well as load. There are schemes which employ Isochronous load sharing, and there are schemes which employ Droop speed control to control frequency and load--but they are generally provided by some "external" control system, usually called a power management system which sends signals to the generator-sets to control frequency and load.

So, you see--your question, while seemingly simple, is not. We don't know how your facility is configured or how it is operated. There's too much we don't know. And, all of the above is very, VERY general, and while many plants are similar, there are many plants which are very, VERY different.

Reactive current compensation is a topic I suggest you search the World Wide Web for information about. Some generator excitation system manufacturers, such as Basler, and even some generator-set control system manufacturers, such as Woodward, have published and made available "white papers" to describe the topic and how their equipments can be used to accomplish reactive current compensation. These documents can be very useful in understanding some very difficult topics.

When searching the control.com archives (past threads) and the World Wide Web one always picks up useful information in addition to what was originaly being researched. New terms, new explanations, new insights into related topics always come up. You may not get your specific question answered just exactly as you would like, but sometimes that doesn't even happen when you have the full attention of an expert seated or standing directly in front of you.

Again, these questions you have are somewhat basic, but at the same time, they are also very complicated and have many different aspects depending on how a particular power plant is being operated (such as if it's operating in parallel with (synchronized to) a larger grid; or if it's being operated independently of a grid and how many generator-sets are synchronized together to supply a load (refinery; process plant; etc.). In order for us to be of help to you, you need to be more specific about your background and needs--how the plant operate is configured and operated. Because, you don't want us to just spout generalities when your plant has a completely different configuration and some specific requirements and responsibilities (such as maintaining the frequency and load of a process plant or refinery when operating independently of a larger grid). Help us to help you--and do some research, on control.com, and on the World Wide Web. And if you have questions, we can answer them--but please be more specific with the questions and the circumstances.

When reading search results, do not be discouraged if your question is not answered directly. Use the new words and terms you learn to narrow you search to find the answer you are looking for.

That's all I can add without a lot of input from you, Diaa. I can fully appreciate that your area of knowledge with regard to electrical engineering study did not encompass electrical power generation--most universities don't cover this area very well, or in any detail at all. And, most university texts and references aren't very well written with respect to Droop speed control--which is one of the most important aspects of electrical power generation.

So, have a read of some of the past threads on control.com, refine your question(s), and even try to explain what you have learned to see if your understanding is correct. We're happy to help.