Generator MVARS

Critics w/o Critiques by the numbers!
1. Introduction
Everyone should have goals.

2. Definitions
Karapetoff’s definition in 1911 was: "When carrying loads armature-current being a source of mmf, modifies the flux created by the field coils, thus influencing the performance of the machine." I did not extrapolate your preferred definition from Karapetoff’s definition. The keyword is performance. Performance is an energy balance. If performance changes, one can expect the energy balance to change.

3. Synchronous Generator Armature Reaction
I have learned nothing new is this section other than there is no 4, 5, 6, or 7.

8. Conclusions
I agree with your conclusions as long as we are assuming; except the unity part were excitation is increased.
In reality excitation should remain constant. Oh, there is no 9.

10. Addressing Incorrect A-List Responses Related to Armature Reaction.
Placing a dunce cap on someone will not have the desired effect.

I will now digress into oblivion.

I was once told that an inventor is an engineer that must sometimes question his education. Karapetoff addressed his graduating class with the same point in mind.
Karapetoff was also very big on teaching interpersonal skills to his engineers. He reminded to listen and learn.

I have learned from my arithmetic lessons that when performing calculations on numbers with terms one must carry the terms. Current is measured in amperes and the ampere is defined as a flow. Since the VAr depends on changes in field current then it would seem a VAr could or would flow. It is all about who decided to call an apple an apple isn’t it.

An oxymoron is sometimes used by an author to achieve a vision within the reader. Thunderous silence and sweet sorrow are examples. Perhaps reactive power was used for the same effect.

Assumption is the mother of the screw-up. The conclusion cannot be based on the assumption that terminal voltage is constant therefore we cannot summarize. We cannot assume that grid frequency or voltage is constant. The control system themselves induce some deviation because of the accuracy of the input devices and the algorithm used to respond to the setpoint.

For instance, the speed pickups mounted in close proximity of the 60 tooth wheel of a GE frame 7 gas turbine operating at 3600 rpm would have a sampling rate of 216000 hertz. Although a wheel with more teeth would reveal more accuracy in the speed measurement, the increased accuracy is not needed or necessary. All control systems have to be tuned in regards to the accuracy of the measuring device and the capacity to measure and react accordingly. I believe in measure, compare, compute, and correct. A control system can only react to a deviation from a setpoint therefore deviation must occur constantly and be reacted to constantly. The same applies to a voltage control system. Some GE EX2000 excitation control systems have a speed input feedback signal to prevent the system from overcompensating to voltage changes. I will now move away from the effects of the control system.

The grid must have constant changes occurring by design. I will choose the smallest quantity I can think of. An inductive motor bearing is not a perfect device therefore current flowing to the motor changes in some fashion due to the small changes in friction; however infinitely small. Another value that is infinitely small is the amount of time required for an engineering marvel to become and engineering disaster. In my opinion, the transmission grid is never constant in regard to frequency or voltage. The transmission grid is merely in a constant state of balance. The effects of load and how these effects are countered in the real world is what CSA described.

The concept of an infinite grid applies only to mathematics although it does have a definition. Infinitely finite is oxymora. A finite number of generators supply a finite electrical load connected to finite transmission resources at any given time. The only thing that is infinite is the number of things that can change the delicate state of balance of voltage and frequency that exists on the transmission grid.

I once had a professor that to my amazement was also the author of the textbook for this particular subject. The text within this textbook contained all of the knowledge ever obtained on the subject and all of the knowledge that would ever be obtained. No gray area existed on the subject for discussion. I completed the course with the same information that I entered it with. Perhaps, this is why I have not asked to read your paper.

Still stubborn,
CTTech
 
Wow! As a new power plant worker (operator) I was trying to find an explanation of MVARS since it is one of the readings I have to record several times a day. I was having difficulty finding two similar answers to my questions about MVARS. I had no idea any topic about basic electrical generation could cause so many differing answers.

Thanks to all that answered/contributed to this discussion thread! I feel a little less stupid now.
 
B
Hi CSA,

I think we've been here before...

My mental picture of why we need to keep track of VArs (whatever called): A reactive element (inductor or capacitor) absorbs energy and then re-releases it back to the system. If we apply a direct voltage to a capacitor, there will be a period when a current flows from the positive terminal of the voltage source into the capacitor: this current will decay exponentially in the usual way. However, while the current is flowing, we can calculate an instantaneous power by multiplying the current at each instant by the voltage at the same instant. This power will eventually reach zero (in infinite time theoretically but for practical purposes after about 5 time constants).

If the supply to the capacitor is disconnected from the source and connected to ground, a current will flow out of the capacitor (in the reverse direction to the first case). Since the current polarity has reversed and the voltage polarity remains the same, the direction of power flow is reversed. Eventually, it will reach zero. The average value of power during these two periods will also be zero. However, the peak values of power into and out of the capacitor over this cycle are non-zero. The same argument can be applied to an inductor except that the current must be fixed rather than the voltage.

With an alternating source connected to an inductor or capacitor, the instantaneous value of power will go through 2 complete cycles while the source goes through 1. For a capacitor, while voltage is increasing from zero in either direction, the energy stored in the capacitor is also increasing, and so power must be flowing into it. When the voltage is decreasing towards zero, the energy stored is decreasing so power is flowing out.

While the average value of this power over a cycle is zero, and it can do no useful work, if we don't allow for it to exist in the circuit then we won't get full voltage (for a capacitor) or current(in an inductor). So it needs to be taken into account in some way. In applications such as electronic resonance or power factor correction, we use the fact that the two currents are displaced by 180 deg and so cancel to some extent.

For a non-electrical analogy, take a tidal creek with a highway embankment over it. The average value of flow out of the creek depends on the water flow into the streams feeding it at its head, and could be relatively low. However, the design of any bridges on the embankment must take into account the peak cyclic flows as the tide comes in and out - which could be much greater than the average. (When dealing with mechanical engineers who often get stroppy about power factor, I ask them for a simple explanation of entropy - that usually shuts them up!)

Cheers,

Bruce
 
CTTech,

I have two issues: One is with the "definition" of an infinite bus. I have seen it described several ways, but the one I've seen most often says "a bus with an infinite capacity and a constant voltatge". Now that's an egghead's definition as those of us in the field know that no bus can ever have an unending capacity (either demand or supply) and no bus ever has a constant voltage, though the definition of constant can be some source of consternation, as well. (Is that "constant" +/- 5%, or 2%, or what%?) What we're talking about is something like the North American grid, or the European Grid, or some extremely large geographical region which is effectively served by hundreds, thousands, of generators all effectively connected together on the "same" electrical transmission line. I just love it when a utility says you should buy our power because it's 100% renewable. They can't sell you, specifically you, their power unless they connect wires to your house/business; it all just gets pumped on the same grid and ends up supplying the aggregate load, of which your house/business is an infinitesimally small part.

And that is, what I think, is at the heart of the "definition" of an infinite bus: something that is much, much, much larger (from a supply perspective) than an individual load (household or refinery; doesn't matter).

An infinite bus is just something that someone decided was much bigger than someone ever imagined a bus could get or would be, and could, theroretically and within reason, supply just about any load(s) connected to it. The amount of supply (generation *capacity*) exceeds the demand (load).

Personally, I would call a small island like Oahu or Hawaii a finite bus, or a cruise ship, or an aircraft carrier.

I will agree that the amount of power (torque) being input to all of the generators connected to *any* electrical grid (finite or infinite) must be exactly equal to the amount of load connected to that grid. Because, if it's not, then the frequency will either go down or go up depending on whether the torque is insufficent or excessive relative to the load. Remember: a generator is just a means of converting torque into amps which can be transmitted long distances and the reconverted into torque (in the case of motors) or light (which requires torque to cause the current to flow to heat the filament or light the flourescence (ha!)). Prime movers convert chemical or mechanical energy into torque. Too much torque equals high frequency; too little torque equals low frequency. And on an AC grid, frequency is one measure of whether or not the generation equals the load, or if it's excessive or insufficent.

And, the magnitude of VAr flow adversely affects the ability to produce Watts. An increase VAr flow (production) means for the same amount of torque input to the generator, fewer Watts will be produced.

The other point of disagreement I have is with the revolutions vs. toothed wheel thing. A shaft spinning at 3600 RPM spins at 60 revolutions per second (3600 rev/min / 60 sec/min = 60 rev/sec). A 60-toothed wheel spinning at 60 rev/sec would equal 3600 teeth passing a speed pick-up every second, or, 216,000 teeth every minute (3600 teeth/sec * 60 sec/min = 216,000 teeth per min). Because most frequency counters measure in per-second units, a 60-toothed shaft spinning at 60 rev/sec would be measured as 3600 Hz, which is 1 Hz for every RPM, since 60 rev/sec equals 3600 rev/min (RPM). Which is why 60-toothed wheels are nice to have on a generator; it's easy to convert 1 Hz to 1 RPM, as opposed to an 87-toothed wheel spinning at 60 rev/sec.

I agree completely with you characterization of VAr flow, as that's how I've always been taught and read and undertsood and spoken of the "phenomenon." And the direction and magnitude of VAr flow does affect the ability of the machine to produce real power, Watts. Just look at any reactive capability curve for a synchronous machine (generator or motor). More VArs (Lagging or Leading) means less Watts. Lower power factor means less Watts. Period.

I, too, had a similar college professor, and exactly the same
experience: I completed the courses (I had to suffer through five
classes with the man) that I entered them with. He used texts from a colleague, and only chose to "accept" certain passages--and we're talking electrical and electronics courses!

markvguy recommended a site with a real-time plot of grid frequency for Europe: www.ucte.org. It's quite interesting to watch the graph at various times during the day and night. No grid is exactly what it should be; it a a nominal 50 Hz (or 60 Hz) as the case may be. I've been watching it for about an hour, and it has steadily climbed to 50.055 Hz, from about 49.985 Hz and, it's continuing to increase as I write this. Of course, it's near midnight, GMT, so the load is probably decreasing as people turn off their lights and television sets. It just jumped to 50.074 Hz in the space of a couple of minutes. The system operators need to start lowering the load of some of the generators to start reducing the frequency. I wish the graph showed aggregate load, as well! (It's now 20 minutes since my last look at the site, and the frequency has dropped to 49.997 Hz!)

There's the California Independent System Operator website, as well (http://www.caliso.com), and it's sister site http://oasis.caiso.com, both of which display load but not frequency. Anyone know of a site which shows load and frequency, real-time, for a grid?
 
B
Try modifying the definition to "An infinite bus is one in which the change of frequency and voltage is negligible in the event being investigated". So the North American grid is effectively "infinite" if checking the behaviour
of a single alternator, but not if looking at an event where the system splits into 2 large chunks.
Loss of a 6 MW GT from a 10,000 MW system is an "infinite grid" problem: loss of a 1000 MW station from the same system is not.

Bruce
 
Ronbo,

This specific topic will usually cause more conversation and angst amongst power plant workers (including operators, technicians, engineers, managers and accountants) than any other single topic. EVERYBODY has their own idea of VArs (be careful of your spelling here; the "professor" is always monitoring!), and almost nobody's agrees with anyone else's. That's because with Watts, we have a tangible thing we can associate with them: horsepower, work. With VArs, we can't see or hold or smell or taste them--they're intangible.

And many (too many) discussions of, "What's a VAr?" begin with vector diagrams and angles and trigonometry, and most people can't relate that to something they know or have experienced.

Suffice it to say that the effect of too many induction motors on an AC electrical system will cause the current- and voltage sine waves on an AC electrical to begin to shift out of phase with each other. If left unchecked, this will begin to cause brown-outs and even black-outs. One can use a synchronous generator to help to keep the voltage- and current sine waves in phase with each other and provide some "stability" to the system. To do so, one must increase the excitation being applied to the synchronous generator rotor (the "field"). Increasing excitation requires power (that whole amps flowing through a conductor thing, which generates heat, and which requires power). That increased excitation consumes power, because, it doesn't come from thin air. That power is power which can't be sold.

As the excitation is increased, the power factor (the "efficiency" of the generator at producing real work with amount of energy being provided to it) decreases. When the generator is not trying to assist with maintaining sine waves in phase with each other, the power factor is "1", also called unity. As the generator is used to assist with maintaining sine waves in phase with each other, the power decreases below 1.0.

In closing, the more VArs a generator has flowing, the more heat is generated in one part of the generator or another (depending on the "direction" of VAr flow--AAAGGHH! I said it!) Heat is something which must be removed from the generator to make it last as long as possible. Most producers don't get paid for VAr flow; some do.

Hope this helps!
 
In reply sir I have stumbled across this forum and as a Power Station Operator I take extreme exception to your comments. I can only assume you are talking about Operators from the US. I work at Gladstone Power station-the largest in Queensland in Australia. In the early nineties the control systems at GPS had a major upgrade. From two control rooms to running six 280MW from one control room using screen based systems. Foxboro. It was decided by Operators and agreed to by management of the day that they would be involved. A team of about six operators designed, and formatted the graphics interface systems using techniques that even Foxboro claimed were not possible at the time. They now use those graphics around the world. GPS was Queensland's first and only power station to be sold to private enterprise. It is owned by a consortium of companies including Rio Tinto and NRG (US company). I think perhaps MR CSA needs to get out of the US a bit more and into the real world perhaps?

As for all the kVAr stuff, very exciting, especially the bits about people generally not being able to fully explain (in layman's/operator's) terms even what it is. Thanks for the space.

Garth
 
There is a problem with stereotyping, and that is that not every individual (or, in this case, group at a power station) fits the characterization being applied. Sir, a partial list of the nations I have worked in power plants include Canada, Mexico, Qatar, Oman, Saudi Arabia, Malaysia, UK, Taiwan, Indonesia, Ireland, Japan, Bahrain, UAE, and China, as well as many of the United States. The operators at your site is to be commended as they certainly do not fit the stereotype, but as I learned many years ago: There is always an exception to every rule. In this case, it's a pleasant exception.

I note that you have failed to include your thoughts on the subject of VArs other than to say that the variety of explanations is interesting. If you were to take a sampling of people at GPS, how many different explanations would you estimate you would encounter? How many people would utter somthing similar to, "They're like foam on
beer: They're just there, but they don't do anything"? How many people
would start off with a description of phase angle and power triangles?

Also, how are VArs controlled at your plant? Do you use some kind of automatic VAr or Power Factor control? Or, do the operators just monitor the VArs and/or Power Factor and make adjustments to the excitation when they deem it necessary? Is there some kind of VAr or Power Factor limit (upper and/or lower) in effect at your plant?

Tell us a little bit about your plant and how it's operated in this space. We are all trying to learn.
 
I work in a relatively large electron factory in a relatively large grid (it might even be called infinite) and I concur that there are many smart, wise and experienced technical folks in the power plants. However, some of these folks are often times blinded by myths and legends. I've seen many illogical decisions made and executed in response to these perceived myths and legends. On many occasions, I've seen the facts get side-lined in favor of fixing the myth. In my experience, I've found that I must debunk these false impressions before we can begin the work of looking for the facts.

Back to the subject matter. I tend to look at mVAr through the perspective of vibration analysis. To me, it allows me relate it to something more tangible and physical for we all know that electrons are only theory and no one has ever seen one. If you've ever been exposed to vibration analysis, you'll realize that the vibration folks always look at amplitude and phase angle. Phase angle can be thought of as the time shifting of response relative to the excitation. This is how I look at mVAr. It's a time shifting (or delay in response). It isn't as fun as talking about the foam on the beer but it makes more sense to me this way.

By the way, we sure miss you guys down-under. I hope the new owners are treating you well.

mike
 
M

Michael Griffin

In reply to Marc Sinclair: Actually, electrons are produced as a byproduct of anti-matter production. Electron/positron pairs are created when converting energy to matter. The electrons are discarded and the positrons kept to be used directly, or used as one of the components in assembling anti-hydrogen. At least, that's the process that I'm familiar with.

Of course since the electrons are actually a waste product, he would be more correct in stating that he was manufacturing *positrons*. Perhaps though he works in the waste electron treatment part of their factory and so is most familiar with that process. You may not realise this, but your anti-matter powered flying car indirectly produces a lot of e-waste which must be disposed of.
 
P

phil Corso, PE

Responding to CSA's 18-May-08 (17:04) comment... actually they're not, if you are alluding to an electron's movement in a electrical wire, or the like!

But, now we are moving (excuse the pun) well beyond the original thread subject.

BTW, have you noiced that the term "reactive power" seems to have fallen out of favor amongst List responders!

Regards, Phil Corso ([email protected])
 
It's still in wide use everywhere else.

It's probably not in use here on control.com because no one, including me, wants to incur the wrath of the exclamation point.
 
C

College Student

I do not know if my question is appropriate for this technical forum or not. Still when we are discussing positrons and electrons and energy mass equivalences, I am tempted to post my query, which is partly scientific in nature and relates to college physics.

The background of my question:
Sometime back I read a book where in I came across the following paragraph:

"For an example, consider the power transmission lines that deliver the electricity to our homes at 60Hz. The electrons in the wires do not directly transport the energy from the power plant to our homes. On the contrary, the energy is carried in the electromagnetic field between the wires. This fact is often confusing and hard to accept for circuit designers. The wire electrons are not experiencing any net movement. They just slosh back and forth, and through this movement they propagate the field energy down the wires."

Is it really true that the energy/power is carried between the pair of wires and not 'through' the wires? Does it mean, that if I separated my Phase and Neutral wires physically in space by thousand mile, the energy delivered to my house would decrease and my computer wouldn't start?

Thank you,
Robin
 
D

Dooley, Vince

It means that the energy is in the magnetic field surrounding the conductor. If it was transferred through the wire then transformers wouldn't work. Nor would capacitors or alternators or motors or your computer.

Vince
 
Mandatory NERC Standards as of July 2007 only allow US and Canadian Power plant generation in Voltage control mode. NERC Standard VAR-002
 
S

Samiran Bairagya

WHEN GENERATORS ARE RUNNING IN LEADING VAR HOW END-RINGS OF STATORS ARE AFFECTED?

PLEASE CLEAR THIS PORTION.
 
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