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Generator MVARS
What are VARs?
1 out of 1 members thought this post was helpful...


I would like to know how VARs affect a generator when connected to a grid. Is it possible to have negative VARs or zero VARs when connected to a grid?
Also how can we control VARs? Thanks.

3 out of 4 members thought this post was helpful...

Well, this ought to start another string of posts and only a little bit of contention. From previous posts, the droop speed control thing generated a lot of posts but not too much contention; whenever VArs came up there seemed to be some strong opinions. This should be interesting, and, frankly, I'm completely surprised this hasn't come up before (at least in the three or so years I've been reading and contributing). So, let's get to it!

VArs are typically considered to be positive or negative depending on whether they are Lagging or Leading, respectively, When 0 VArs are "flowing", that is considered to be unity power factor (1.0). (VArs and power factor are related.)

When the excitation of a synchronous generator is is such that the generator terminal voltage is equal to the bus voltage of the grid, then the power factor is 1.0 (unity) and there are 0 VArs "flowing."

When the excitation of a synchronous generator is such that the generator terminal voltage is "higher" than the bus voltage of the grid, then the power factor of the generator will be Lagging (and considered to be positive). This condition is also sometimes referred to as "boosting" the grid voltage, because the synchronous generator is trying to increase the bus voltage of the grid. Depending on the grid, this can be a large or a small effect on grid voltage. To cause the synchronous generator's terminal voltage to be "higher" than the bus voltage of the grid the excitation must be increased. Increasing the excitation causes more DC current to flow in the fiend (rotor) windings; more current means more heat. Heat must be removed; the rotor windings can only tolerate so much heat before the rotor winding insulation is damaged.

Increasing the excitation consumes power; that's power that can't be sold.

When the excitation of a synchronous generator is such that the generator terminal voltage is "lower" than the bus voltage of the grid, then the power factor of the generator will be Leading (and considered to be negative). This condition is also sometimes referred to as "bucking" the grid voltage, because the synchronous generator is trying to decrease the bus voltage of the grid. Depending on the grid, this can be a large or a small effect on grid voltage. To cause the synchronous generator's terminal voltage to be "lower" than the bus voltage of the grid the excitation must be decreased. Decreasing the excitation causes a distortion in the rotating magnetic field of the rotor, which allows uneven stator winding flux (magnetic) distributions which leads to concentrated heating in the stator end-iron. (There are "two" magnetic fields in a synchronous generator: the rotor's and the stator's.) Heat causes problems in many ways, including winding insulation degradation and expansion. Heat must be removed.

(From a generator's perspective, Lagging VArs "feed" a Lagging Load.)

In addition to causing stator end-iron heating, decreasing the rotor magnetic strength can lead to a very catastrophic condition called "slipping a pole" which can cause serious mechanical damage to a generator, the coupling between the generator and its prime move, and the prime mover.

VArs are controlled with excitation.

Lastly, VArs reduce the ability of the generator to produce real power: watts. So, more positive- or more negative VArs (a lower Lagging- or Leading power factor) reduces the ability of the generator to produce watts, which is what most power plants get paid for, not for the VArs that they "produce" or "consume". (There, I said it.)

By Phil Corso, PE on 5 April, 2008 - 4:53 pm
2 out of 2 members thought this post was helpful...

CSA, having opened the door, I would like to make two comments before addressing your reply:

1) I believe that the questioner Mr. Thompson, as well as others wanting additional information on this overwhelming popular subject, should avail themselves of the 50+ topics in the Control List

2) You are to be commended on your use of the correct term "VARS", and not the oxy-moron "Reactive Power!

Now, on to your reply:

A) You must be very specific as to stated conditions when discussing a generator's operating in parallel with: a) another generator; or b) with a grid!

If a) in parallel with a second generator, then the generator's terminal voltage could rise, or lower, as a result of a change of excitation.

However, if b) in parallel with a "grid", then excitation or speed changes have zero impact on the generator's terminal voltage. Instead they can only affect the generator's internal generated emf! Recall that a grid is a network of generators whose aggregate size is infinite compared to the generator. Hence the grid's
voltage and frequency are unaffected by the action of the generators speed or excitation systems!

B) Lastly, what do you mean by "opinions?

Phil Corso (

Exactly; when does 1,2,3,nnnnn generators in parallel become infinite? I believe that a finite number of generators combine to produce a power grid.

I hope I do not upset the balance of things but this type of thinking is why engineers are not given tools.

I apoligize for being so blunt.

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


So, you say that there can only be a pre-determined number of generators connected to any electrical grid. What determines that number? Is it variable?

Are you saying that the load determines the amount of generation which can be connected to the grid? And the number of generators is a function of the output (not the capacity but the total output of all the generators at a given instant in time)? Because, I'll agree with that. Just by adding generation one cannont increase the load on the grid; one will only succeed in increasing the grid frequency. So, if you say the total generation of all the generators connected to a grid is exactly equal to the load of the grid, then connecting another generator in parallel with the existing generators and increasing the output of that generator without taking any other action (reducing the output of any other generator by a similar amount) then the grid frequency will increase. So, one should not be adding generation to a grid without reducing the output of another generator by a similar amount if one wants to maintain grid frequency.

But, I'm not familiar with any arbitrary or finite limit on the number of generators which can be connected to a grid. As long as the total output of the generators connected to the grid doesn't exceed the load of the grid, then the grid frequency will be stable. (I'm not taking into account any transformer or line impedance restrictions or amperage restrictions of any transmission lines or substations; that could definitely have an impact on the amount of generation (and the number of generators taking into account the output and capacity of the total number of generators being connected). I'm just talking about generation versus load, trying to keep it simple for the engineers and the wanna-be-engineers and the think-they're-smarter-than-engineers.

Mr. Corso,

I am remiss in not stating the conditions of my reply: a generator and it's prime mover connected in parallel with other generators on an electrical grid of some size (i.e., not a small 1- or 2 MW grid or even smaller, but a larger grid, like that of a region including several "states" (states being states as in the United States of America or the United States of Mexico or states as in nation states in Europe other geographically- and electrically-connected regions of the world). I'm not talking about generators connected to a common bus with no impedance between them.

I have been places in the world where because of many factors (distance between substations; distance between power plants; line impedance; local loading characteristics) that a change in the generator excitation will have a larger effect on grid voltage on that portion of the grid than will another generator of similar output capability (real and reactive) would have at another location on the same grid. I don't have the data or the mathematical ability to describe it or explain it, but I have seen it with my own eyes on more than one occasion. I've even seen the same plant(s) behave differently at different times of the year (winter versus summer), and when other nearby generation is on- of off-line.

By the way, I am also limiting all of my discussions to synchronous generators (more correctly identified as 'alternators'). None of this induction generator discussion here.

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

I apologize to all for not expanding upon my opinions other than screaming "bull". It is a character flaw. I wish to applaud CSA for presenting exactly what I believe occurs in a major electrical transmission system (grid).

I am a firm believer that the speed of a turbine controls generator frequency otherwise no reason exists for a speed/droop control system. I am a firm believer that generator excitation controls generator terminal voltage otherwise no reason exists for a generator excitation control system. I will stand up and admit that many other variables exist; but many questions submitted to this forum are asking for a basic understanding not the complete explanation of armature reaction. I have found that when assisting others on the understanding of speed control and generator operation it is best to use terms that suit their understanding.

Electricity flows like water. Water cannot flow without a pressure differential (voltage). The flow rate is measured as gallons per minute (USA) (amperes everywhere). The myriad of terms merely assist in the understanding of a concept. Once the concept is understood the student can be subjected to the proper terminology. I do not applaud Mr. Corso in regard to his effort to insist that the majority of operators, technicians, and even the engineers cannot understand unless the jargon is absolutely correct. A VAR is a VAr regardless of the spelling. If another acronym for VAr exists that dispels VAR, I will readily use that. For instance RAM could be Random Access Memory in a computer or Radar Absorbent Material in an F-111 attack bomber.

In response to CSA:
Yes!!! A finite number of generators exist. Only the generators needed to support the current load are operated to produce power (makin' megawatts). The others remain at rest awaiting their opportunity to contribute; especially fast start GE frame 7B combustion turbines that respond to turbine trips in less than 6 minutes. According to Mr. Corso turbine trips cannot affect frequency or voltage on the grid because excitation and speed changes have zero impact on the infinite grid. The slow start DLN enabled GE frame 7EA will get their chance when the summer temperatures rise and electrical loads increase while we bask in the sun and our air conditioners run continuously.

While laughing out loud; We will bask in the sun and subject maximum loads to our portion of this infinite grid, voltage will not drop and our generators will not be asked to support voltage with VAR control because mathematically our infinite grid will support us. We will also not be subjected to peak usage charges on our electric bills because the VAr does not consume any fuel or other transmission resources.

A small anecdote to show that you can't always believe the books... A case where power flows in a 220 kV grid network was directly responsible for voltage disturbances. This happened about 35 years ago now.

A 600 MW hydro plant in a remote part of NZ is connected to the rest of the South Island by about 300 km of 220 kV line. There is a substation feeding a city connected into this line at about the midpoint - at the time probably drawing about 70 MW. There were problems reported with voltage swings at the distribution level - caused frequent failures of TV sets, among other effects.

There were no indications of voltage instability at either the remote site of the point of connection to the grid. However, there were definite cycles in the 220 kV voltage with a period of about 5 seconds.

The ultimate cause was traced to governing instability resulting in swings in power flow along the transmission line. Since power flow depends on the angle between voltages at either end, the angular difference was swinging with the power. Even though the voltages at either end were steady, the voltage at the midpoint was going up and down appreciably. Enough to make us believe in phasor theory. (Draw 2 vectors of the same length about 60 degrees apart, and then join the ends with another vector. A line from the centre to the midpoint of the joining line represents the mid-point voltage.) With a remote site connected to an "infinite bus" the same thing could happen.

A few years ago now I had a computer model of the NZ power system to study frequency fluctuations. This modelled the whole system with the mean frequency determined by an overall energy balance and the power variations on individual machines determined by looking at the individual balances at each station. Our system is not an infinite bus by any means as there are one or two stations with unit sizes which are appreciable in terms of the total grid capacity. Falls in frequency from 50 Hz to 48 or below were not uncommon - one result was that the gas turbine on the petrochemical plant I was at used to try and drive the whole North Island back up to 50 Hz, with interesting results on the shear bolts. (I have seen the 2.5 MW set go from 12 MW to 3,5 MW+ over about 2 seconds.)


By Michael Griffin on 10 April, 2008 - 12:55 am

I won't try to interject any opinions about VARs, but to add to Bruce Durdle's anecdote about power transmission, the province of Quebec in Canada has long been unable to connect its grid directly with adjacent regions. They now have back-to-back DC converters (like DC transmission, but the DC bus never leaves the station), but they used to have to disconnect generators from their grid and then reconnect them to an adjacent grid over dedicated transmission lines (or visa versa). Actually, I think they still do this to some extent.

I was told the problem is related in some complex way to their having to transmit so much of their power over very long transmission lines from very large hydro-electric plants (especially Churchill Falls in Labrador and La Grande near Hudson's Bay). They also by the way get instability problems caused by solar storms affecting these same lines.

I suspect that the "infinite grid" theory is a simplifying assumption that works in most situations. The "real" answer probably involves a lot of very complex math that most people don't want to know about.

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


1) It is quite obvious that the system did not meet the definition of an infinite-bus.

2) It is quite obvious that the "computer" model was wrong!

3) Do you remember how the 200-km transmission-line was "modeled?" A short- or a long-line!

4) Do you think you can sent me a simple one-line of the system?

Regards, Phil (

Hi Phil,

The New Zealand grid system does not meet the conditions for an infinite bus. It is divided into 2 halves (the North and South Islands) with a DC interconnection rated at about 600MW at the time I was talking about. Total system load in the North Island was about 4,000 MW: the load in the South Island about 2,000 MW. Pretty well all generation at that stage was hydro - mostly run-of-river in the North, with the majority being on one river and with a small geographic spread. One or two small generating stations were situated remotely and connected by relatively weak links. The major load is Auckland at the north of the North Island: the DC link is connected in to the NI grip near Wellington at the southern tip. So when the DC link dropped a pole (which was about monthly at the time) there was a short-term loss of about 15% of inflow to the north system, lasting for a couple of seconds till the mercury arc rectifiers re-ignited (or whatever - I had nothing to do with that system.) As a result the whole system slowed down as kinetic energy from the machines was used to make up the difference - hence the frequency dips. The model I wrote did a good job of predicting the magnitude and duration of these effects, including what happened when some additional large thermal stations were added in the 70's and 80's. So there can't have been too much wrong with it.

The effects I described on the 200 km link was modelled in the best way - they were observed on the real system. Unfortunately, I cannot point you to a single-line diagram of the system as the fragmentation of the electrical generation and transmission systems means that the info is spread over a number of organisations.


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

CSA and CCTech, I don't understand your position(s), i.e., Critics w/o Critiques! Therefore, following is an abridged copy of the paper presented in January 2007, but without the items that seem to have you stupified such as illustrations, tables, formulae, phasors, or vectors:

1. Introduction. This paper has four goals that are listed below:

(a) Describe Armature Reaction more clearly than hath been presented heretofore (I love lawyer-speak.)

(b) Dispel doubt, misinformation, and misunderstanding that have cropped up in related A-List topics.

(c) Reduce animosity (just kidding) about questionable or ambiguous jargon.

(d) Eliminate myths, misnomers, and omissions. Adjectives describing Armature Reaction are plentiful, some even inventive, but most miss the point! Here are some pairs that were culled from A-List and Off-List responses: adds-subtracts; additive-subtractive; augments-negates; crowded-expanded; decreases-increases; fights-gives up; overcomes-replaces; overtakes-replenishes; magnetize-demagnetize; support-oppose; strengthen-weaken; and swell-shrink. There have been and certainly will be others! Thus far, no-one has used adjectives such as: encourage; discourage; thwart; or tweak! I hope this paper will curtail (hmm, a synonym I hadn't noticed earlier) the seemingly growing list of adjectives.

2. Definitions: Official; Time-Proven; and Preferred [Ref. A & B]. IEEE Std 100 defines armature reaction as: "The magnetomotive force due to armature winding current." Rather sparse! 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 prefer, "Armature-current mmf modifies the field-current mmf so that the resultant armature-reaction mmf, changes the machine's generated emf."

3. Synchronous Generator Armature Reaction (General Theory) [Ref. C & D]. Let's start with the basics for a generator delivering power to an isolated load. The generator has two magnetic structures, one the stator (fixed in space), and the other the rotor (driven by a prime-mover). They are separated by an annular space called the air-gap (regardless of coolant, for those ready to pounce.) Each structure carries windings that are linked by a mutual flux crossing the air-gap, and as a result a generated-emf is produced in the stator. Current in the rotor field-coil produces a rotating magnetic-field called field-flux. When a generator is loaded, current in the stator-winding produces its own synchronously rotating magnetic-field called armature-flux. Two observations can be made: 1) each mmf has magnitude and direction; and 2) they exist independently of one another. When the two fluxes combine the resultant air-gap flux changes generated-emf. And, because field-current is a constant and stator-current a variable, then armature-flux is said to affe ct field-flux. The interaction is called Armature Reaction!

Digressing for a moment... an analogy to the above is the ocean surf's undertow... while the surface current can be seen moving towards the beach an unseen undersea current is also moving, but in a direction away from the beach! When the two currents combine the resultant current speed and direction determine a swimmer's fate. The effects of undertow and armature reaction are similar, differing only in dimensional units, that is, the former uses physical quantities, the latter magnetic quantities.

The unit-dimension for mmf is ampere-turns. Thus, when the statement "the field... opposes... aides…... shrinks... etc" is made, then, what actually occurs is an increase (or decrease) of the air-gap's total ampere-turns. Thus, its mmf can be treated as a vector, i.e., having magnitude and direction, but strongly influenced b the nature of the load.

For a lagging-current load, armature-mmf subtracts ampere-turns from the field-mmf, thus weakening air-gap flux! Conversely, for a leading-current load, armature-mmf adds ampere-turns to the field-mmf, thus strengthening air-gap flux!

A seemingly complex process? Yes! But not if one thinks of the process as a chain reaction: armature-flux modifies field-flux; resulting in an air-gap flux change; changing generated-emf; culminating in a change of terminal voltage; requiring corrective action by the Automatic Voltage Regulator (AVR)!

In general, for cylindrical-rotor machines the modification appears as shift in pattern, while for salient-pole machines there is pattern distortion. The three magnetic-fluxes can be identified by their associated mmf vectors: f is field-flux; a is armature-flux; and r is resultant airgap-flux.

8. Conclusions. Assuming that the terminal voltage is constant, then the effects of load power-factor on armature reaction can now summarized:

• Unity Power-Factor Load. A unity power-factor load, i.e., line-current in-phase with terminal voltage, causes the armature emf to weaken the air-gap flux produced by the field alone! Field-current (excitation) must be increased to maintain terminal voltage!

• Lagging Power-Factor Load. A lagging power-factor load, i.e., line-current lags terminal voltage, causes the armature emf to weaken the air-gap flux produced by field coils alone! Field-current (excitation) must be increased to maintain terminal voltage! (Note: if connected to an infinite-bus system, the machine is said to be over-excited, and it delivers or exports lagging kVAr!)

• Leading Power-Factor Load. A leading power-factor load, i.e., line-current leads terminal voltage, causes the armature emf to strengthen the airgap-flux produced by the field coil alone! Field-current (excitation) must be decreased to maintain terminal voltage! (Note: if connected to an infinite-bus system, the machine is said to be under-excited, and it delivers or exports leading kVAr!)

10. Addressing Incorrect A-List Responses Related to Armature Reaction. Several A-List posters have advanced theories about armature reaction that are plain wrong or confusing. Some fellow posters introduced myths; others mis-name the process of kVAr exchange between a generator and another power source or system; still others, have used incorrect unit-dimensions of electrical parameters. I also omitted an important observation related to end-connection effects. Following are my comments concerning the most noteworthy discrepancies:

• Armature Reaction causes a change in field-current! I am sure that what the A-List contributor observed was the corrective action of the AVR responding to the terminal voltage change!

• Armature Reaction under-excites or over-excites the field! IEEE Standard 100 does not define the term. Neither are there definitions for over-excitation and under-excitation. However, the research done for this paper indicates that, except for synchronous condensers, the terms under-excite and over-excite are in discussions related to interconnected generators, such as those operating in parallel, or those connected to an infinite-bus. In addition, the terms are used more frequently describing synchronous motor operation than for synchronous generator operation!

• Armature Reaction causes the field-flux to modify armature-flux! Just the reverse is true. The field-flux is fixed, but armature-flux is proportional to armature-current. Therefore, it is armature-flux that modifies field-flux!

• Operation in parallel with other sources! In my opinion a synchronous generator does not absorb, or consume, or import, or receive, or take-in, or produce, reactive power. Then, for consistency, the author suggests that the terms listed above, as well as reactive power, be eliminated. Instead, substitute the expression that the generator delivers or exports lagging or leading kVAr! (NOTE: this recommendation does not preclude anyone from using terms they are familiar with!)

• This one is mea-culpa! My response to the thread "Alternator Running in Leading kVAr" addressed the fact that low-pf leading current operation is more deleterious for armature end-connections than low-pf lagging current operation. I presented (correctly) the fact that increased stator-current results in a dangerous increase in the stator conductors' (in-slot) length. However, I failed to include a crucial point. That is, the heat present in the stator winding end-connection is disproportionately higher than that of the portion embedded in the stator core-slot, because their radiating surfaces are different, as well as the manner in which they are cooled!

• Which unit-dimension is to be used: a) KVAr; b) KVAR; c) kVAR; or d) kVAr? Using SI-Units, and multiples and prefixes for SI-Units, then d) kVAr is the correct term! An-aside, SI-Units were approved by the US Congress in 1866. However, stubberness (sound familiar) on the part of our scientists, engineering, and tech communities has prevented general use in the USA! Admittedly, there is a problem if inductive kVAr and capacitive kVAr are to be differentiated in the same discussion! Then, I suggest the use of kVAr(i) and kVAr(c), respectively! One last point on the subject of terminology; the "s" often used to denote multiples of a prarticular dimensional unit should be dropped.

CSA and CCTech, you both must agree, that now, you have something to critique!


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

Splitting hairs, and very fine ones at that. Producing/consuming vs. delivering or exporting. I have almost no issues with the rest of the abridged version, but I do take exception to this minute hair-splitting.

I'm still interested in hearing other's opinions of the paper and how informative and useful it is.

And, I'm not exactly clear how this explains VArs, or kVArs, which is what this thread was about (I think!). Nobody asked about armature reaction; the question, as I recall it, was what was the effect of VARs (sic) on the generator, and explaining armature reaction doesn't explain why kVArs are (need to be) produced or delivered or exported or what the effects of producing or delivering or exporting them is. There is a little bit about the effects of heating, but what about the effects on generation (power production) of producing/consuming or delivering or exporting lagging or leading kVArs? Where's the discussion of the reactive capability curve which depicts what happens when the unit is producing/consuming or delivering or exporting lagging or leading kVArs?

I'm just constantly looking for a utilitarian explanation of (k)VArs that most anyone can understand and might even be able to be used to explain it to others so that they can understand.

Lastly, I am not stubbern; stubborn, yes. Stubbern, no.

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

CSA and CTTech, I want to thank each of you for your invaluable critiques! They certainly were enlightening.


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


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!)



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

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,

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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: 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 (, and it's sister site, both of which display load but not frequency. Anyone know of a site which shows load and frequency, real-time, for a grid?


Give me a break. I was on roll! And yes I did not carry/observe the terms.


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.


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You raise a very excellent question. It's my personal belief that there is this very prevalent saying about VArs: "They are like foam on beer: They're just there, but they doesn't do anything," that causes many people to have something of a cavalier attitude towards VArs. They know enough to know that VARs should be slightly positive, but that's about all they know. They don't understand the relationship between VArs an power factor, or that power factor is really just a method of expressing the efficiency of the generator with respect to producing "real power".

Most sites I have been to are literally afraid to operate in VAr Control or Power Factor Control. And, if one can convince them to try the modes, they generally have not been tuned properly and any kind of excursion or large deadband just makes the operators extremely nervous. I have tried to tune both control modes when commissioning a new power plant, only to be told it will never be used. Then I've had to return to the plant to tune the functions, and then be told they will never be used.

It's been said by others before here on and I can't agree
more: Power plant operators have the highest inertia of any entity
known to man. They are extremely resistant to change of any kind, and one bad experience means with something they don't understand or which hasn't been properly explained means they will only try something new if forced to. Further, they are creatures of habit like no other creature in the world; they are almost superstitious and fanatical with regard to what they "know" has happened in the past and they way they believe something should or does operate, and no one is going to change their mind even if logic diagrams and piping & instrumentation diagrams say otherwise.

I have been physically threatened by operators who insist their unit never did this or that before, and have been flatly called a liar when I produce documentation that says it couldn't have done this or that. Most power plant operators are capable of steady state operation (heck, my fourteen year old son could probably be a power plant operator in a continuously running, "base load" plant), but when it comes to start-up or shutdown the most common utterance heard in the Control Room is, "It's never done that before!"

Any start-up that gets the unit to desired load is a good start-up and most operators are horrible at being able to detail how their power plant starts up or shuts down, even if there are Standard Operating Procedures in place. It's really, really, really sad and says a lot about the management and ownership of power plants, around the world.

Power plant managers usually have some experience with operations, which makes them subject to some of the same tendencies. Further, power plant operators can be such a difficult bunch to deal with (especially when it comes to "change management" (which is my new favorite term)) that most power plant managers won't even try to suggest doing or trying anything new or different for fear of being killed by the huge whine which they know will follow any such suggestion. And, when faced with upgrading control systems or equipment will generally insist that from an operational standpoint nothing should change, because the operators will complain so frigging much.

So, equipment and controls ain't gonna make any difference, sir. Ignorance and habit will trump common sense and correctness every time, especially in a power plant.

Also, some power plants are required to operate at specific power factors or VAr "levels", though it's the odd plant that will use automation to do so. For all the reasons stated above.

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.


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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.

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

By MJPearson on 16 May, 2008 - 9:25 pm

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.


By Marc Sinclair on 17 May, 2008 - 12:37 pm

Strictly you don't manufacture the electrons, don't you just move them about a


By Michael Griffin on 18 May, 2008 - 5:02 pm

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.

By College Student on 22 May, 2008 - 1:17 am

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,

By Dooley, Vince on 23 May, 2008 - 1:01 am

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.


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How true, but they're moved with great force.

By phil Corso, PE on 18 May, 2008 - 6:05 pm
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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 (

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It's still in wide use everywhere else.

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

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Mr. Corso,
Yes, there are many posts on about VArs and more people should use the very powerful search feature of more often than they do for this or any other topic. But, VArs is one of those subjects that is so misunderstood and so difficult to explain and has been explained so poorly by so many people over time that most power plant operators cannot explain something so fundamental and germain to power production.

Sir, I have read several of your brief and nebulous responses to this subject in some of the posts you cited. I have read the exchanges with markvguy. I have not asked for a copy of your paper on the subject, mostly because from what I have read in your posts we have a fundamental disagreement about this subject and I prefer to 'agree to disagree' than to try to offer proof of my experiences without the benefit of data and hard evidence to support my position.

I have read many a section on this topic in many power plant "fundamental" and "operation" and "description" books, some of which I have purchased myself (most of which are a waste of paper and ink when it comes to this subject, and speed regulation and governor control) trying to find a way to explain what I have come to understand through my experiences over the years.

It's just one of those things that I have come to accept that no matter what one says or how one tries to explain, there are people who are going to nod their heads and say, "Yes, I understand," when they don't, or who are going to say, "That's not how it works at all! The angle of the vector and the VA and the load angle..." and they just go off on their mathematical description of this or that which few people can follow much less hope to understand.

It's one of those relative things, relative to the context in which we can understand it or be able to try to explain it to others. It is what it is for each and every one of us, sir.

That's what I meant by opinions, and you understood it exactly correctly.

By Phil Corso, PE on 6 April, 2008 - 9:24 pm
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CSA, you are correct! The subject of Syn-gen operation can be simply explained.

In the past most contributers to meaningful discussion had, unfortunately, missed the point But, if you want it all in one place, then I suggest you avail yourself of the document I offered to all List members in '06/'07!

Called "The Physics of ... Armature Reaction", it had 5 goals:

a) Describes Armature Reaction more clearly than hath been presented heretofore (I love lawyer-speak!)

b) Dispels the doubt, misinformation, oxy-morons, and misunderstanding that often crops up in A-list dicussion on the subject!

c) Reduces anomosity (just kidding) about ambiquious and questionable jargon!

d) Eliminates myths, misnomers, omissions, errors, and extraneous word-pairs often used to describe operation of Syn-Gens!

e) Explains how Lagging or Leading VARs are "exported" by a Syn-Gen... not the myriad of terms such as produced, consumed, absorbed, intake, outgo, etc, etc!

Because the paper contains sketches, formulas, and tables, its distribution via, was ruled out. Instead, I offered an e-mail version
to anyone that wanted it.

So if you want a copy, or anyone else for that matter, contact me!

Now the Caveat: I no longer accept anonymous requests for information or help... something to do with the fact that anonimity breeds a lack of common courtesy! Therefore, anyone wanting the document, must provide a name, affiliation with a company or school, and a location!

Phil (

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Mr. Corso,

I would be more interested to read Ivan's or THOMPSON's review of your paper. As I said before, I will agree to disagree, and since I cannot find the type of explanation and forumulae that I can use to explain my experiences and I lack any hard physical data, I will stay out of the discussion of the principles with you.

I may even be so horribly mistaken in my understanding of the proper physical concept that it would be embarassing. All I know how to do is to relate the principles to concepts I do know and can understand, and when it comes to operation and troubleshooting of power plants my understanding has served me very well over many years.

Lastly, I will say that yours is the only explanation I have ever run across that disagrees with the concept of "reactive power" and VAr flow, which makes me very curious at one level, and very dubious at another. Sometimes, a single person who seems to differ from the horde can have such a keen understanding of a concept or subject that the person is considered to be outside the norm and almost heretical. I have met and come to befriend two such individuals in my lifetime and so I don't discount any beliefs or explanations that I can't absolutely disprove or explain away. Sometimes I regret that position; mostly I learn a lot from the experience. Trying to understand how some people can come to a firmly held belief or understanding can take a long time and a lot of conversation; I have benefitted greatly from learning how people can come to such beliefs and understanding.

But this forum is not the place for such conversations, and because I can't find the clarity I need to be able to support my experiences I can't commit to any further discussion.

I would, still, be very, very, very interested to read anyone's review of Mr. Corso's "The Physics of ... Armature Reaction".

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CSA, I fully understand your reluctance to unburden yourself of a few questionable habits picked up during your years of experience. I know I still have many!

Reminds me of a story! Three 3 hermits are sitting in a cave; soundlessly meditating life! One day an animal passes the mouth of the cave. A year later, one hermit says, "Didya see the cow passin' the cave?" A year later, the second hermit responds with, "Tweren't a cow, t'were a horse!" A year later, the third hermit says, "If you don't stop this constant bickerin', I'm gonna leave!"

"Really", as is said in New Yawk, I don't understand why we have to "agree to disagree!" My offer to you, and others, stands! I'm one that believes "you're never too old to learn from others", especially if an "other" is one with the passion and fervor you seem to still have for your chosen discipline!

Have a good life!

Sincerely, Phil

If producing or consuming Vars reduces the real power, then why with all the sophiticated controllers and electronics around dont the power plants run at exactly unity power factor. I have often witnessed that our generators run at 0.9 pf. The Exciter has a facility of controlling the pf. Once it is set to pf Control mode, it will try to increase or decrease the excitation to maintain the pf. Should we then keep the pf setpoint to 1 and allow the Exciter to modulate the Generator Excitation and keep the pf unity.


By Samiran Bairagya on 18 September, 2011 - 5:38 pm



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Uneven flux distributions.

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Samiran Bairagya... do you mean stator end-turns, or rotor end-bells?

Regards, Phil Corso

By mohammad saeed on 21 March, 2013 - 7:14 am

as u said we can control the mvar by excitation and u said if mvar is zero that's is advantageous for the supplier. then y don't they always try to make it zero? that is always greater than zero and is positive.

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mohammad saeed,

That is the ideal operating condition for the "supplier" (generator).

VAr flow and power factor will change if the system (grid) voltage changes, and it usually does change throughout the day. Operators will respond to system voltage changes by adjusting excitation to maintain the desired VAr/power factor. Because prolonged leading VArs/power factor operation is not good for most generators most plants will have their operators try to maintain a slight lagging VAr/power factor setpoint to provide some "margin" to prevent a leading VAr/power factor condition should the system voltage increase and the operators are not aware of the change.

A small (percentage-wise) amount of lagging (positive) VArs/power factor does not reduce power output by very much. Only when the amount of VArs increases such that the power factor starts to decease below approximately 0.80 will the ability of the unit to produce watts be adversely affected.

Remember: Power factor is really a measure of the efficiency of the generator at producing watts for the amount of torque being provided (really the total amount of power being provided to the generator, including the excitation). At a power factor of 1.0, the generator is not producing any VArs, so it's 100% efficient at converting the power being applied it into watts. As the power factor decreases (as VArs increase) the efficiency decreases.

Hope this helps!

i am confused about your point that positive and negative vars will reduce the watts capability.

ok capability is some what total of watts and vars.... i am not saying sum....
if positive vars are decreasing watts than negative wars should increase it.. isn't so

i hope i had conveyed what i want to say

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Have a look at any explanation of the "power triangle."

VA, the hypotenuse of the triangle, is the total power, which is the algebraic sum of the squares of both watts and vars. If the VA is held constant, and VArs are increased (positive or negative) the watts will decrease.

The same thing can be seen in the reactive capability curve for any generator--which is effectively the power triangle at multiple loads.

Most generators--but not all of them--are not built to "absorb" VArs from the grid. And, consequently, most are not operated with negative VArs.

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VA is the square root of the sum of the squares of the real (Watts) and reactive (VArs) sides of the power triangle.

Sorry for any confusion.

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And "negative" VArs are a recent phenomenon--brought on by digital meters and HMI displays. A lagging VAr (positive, from the generator perspective) is the same as a leading VAr (negative, from the generator perspective) in the power triangle; sign doesn't mean anything. (Even if it did, the square of a negative value is a positive value.)

Remember, the TOTAL power (real plus reactive) is VA. If the if the VA is held constant but the VArs are increased (lagging or leading), the watts will decrease.

It's like the difference between pedaling a bicycle with properly inflated tires versus pedaling one with improperly inflated tires. To go the same speed on the bicycle with under-inflated tires takes more torque than the bicycle with properly inflated tires, or if the same amount of torque is applied to the crank on both bicycles the speed of the one with the under-inflated tires will not be the same as (it will be slower than) the one with properly inflated tires.

And if the tires are over-inflated the bicycle doesn't have the same traction, so while it may be possible to go slightly faster on the over-inflated tires prudence dictates a decrease in speed for safety, not to mention the increased risk of blowout.

Excessive leading (negative) VArs means an increased risk of stator end-turn overheating--as well as a risk of slipping a pole ("loss of traction"), for the majority of synchronous machines.

TANSTAAFL (translation) There Ain't No Such Thing As A Free Lunch

Even with negative VArs.

I understand how a motor would consume reactive power and that a generator would produce reactive power, but how does a generator consume reactive power?

Is this generally poor practice to operate generators this way?

Is this because the excitation is too low?

I am new to a powerplant here and two wheels are operating in parallel yet I noticed one wheel operates with a negative reactive power and another operates with positive reactive power, with both putting out the same MW. This struck me as odd and would like to know the consequences of operating this way.


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Every generator is supplied with something commonly called a "Reactive Capability Curve", sometimes called a "D-curve" (because it has a shape like a capital D). The Reactive Capability Curve defines the limits of operation for the generator--for real power (watts), lagging VArs ("positive" reactive power), and leading VArs ("negative" reactive power). This document defines whether or not a generator might be damaged if it is being operated with a certain reactive power flow ("positive" or "negative") at a certain real power output.

Yes; a leading power factor (leading VArs at the generator terminals) is generally caused by "under-excitation." And, a lagging power factor (lagging VArs at the generator terminals) is generally caused by "over-excitation." When the generator excitation is at the value that makes the generator terminal voltage exactly equal to the grid voltage, the power factor will be unity (1.0), and there will be zero VArs (lagging or leading).

Many times at a plant, two generators are connected to a common bus ahead of (upstream) of a step-up transformer. When this happens (when there is no reactance between generators) sometimes excitation systems require special tuning and configuration in order to be able to "balance" the reactive power between them.

It is the plant operator's responsibility to know the capability of the equipment being operated and to operate it in accordance with the requirements of management, ownership and any contractual commitments. In general, in the absence of any contractual commitments most of the plants I have been to operate their generators at a very slight positive (lagging) power factor (very few lagging VARs). This to allow the operator(s) time to react to changes in VAr flow and correct back to a slight positive power factor (grid voltage does change throughout the day, usually, which will cause the reactive power/power factor to change even if the real power (watts) is steady throughout the day.

Some power plants have the ability to operate the generator(s) in VAR- or Power Factor control mode, which automatically adjusts excitation as required to maintain a VAr- or Power Factor setpoint. )Some grid operators are now forbidding power plants to use either of these modes.)

It is generally considered a poor practice to operate generators such that they are consuming VArs. The effect of VArs on an AC system is to shift the voltage and current sine waves out of phase with each other. If the shift gets too great then power transmission is negatively impacted. So, it's necessary to produce some VArs to keep the AC system transmitting power efficiently. If generators consume VArs instead of producing them they can add to the negative effects of reactive power on the grid.

In some parts of the world where the load is predominantly capacitive (as opposed to most of the world where the load is predominantly inductive) some generators are built to be operated to consume VArs, and some generators are operated as "synchronous condensers" to consume VArs (and in some parts of the world synchronous condensers also produce VArs; just depends on the design of the machine and the needs of the operator/grid in that area).

Lastly, most power plants I've worked at don't have VAr-hour meters which would measure the VAr production for revenue purposes. So, the power plant doesn't generally get paid for producing VArs (which would require a VAr-hour meter) and since producing lagging VArs consumes power (watts) most power plants want to operate at close to unity power factor (zero VAr flow) to make as much money as possible (to keep as much power flowing through the watt-hour meter as possible!).

Hope this helps!

By Power Hungry on 4 April, 2008 - 11:35 pm

Vars will typically be apparent in the generator as heating but in more extreme cases can create system instability.

Excessive Vars are likely to exceed the thermal capability of the generator and lead to alarm/damage/shutdown on temperature depending on your system arrangement and level of excitation. Google "synchronous capability curve" for more info regarding excitation limits, thermal capacity and stability etc.

A synchronous generator of any size should have excitation limiters to prevent operation outside of machine thermal capability and stability limits.

Vars will also affect the power factor as the overall station/board power factor will be the related to the sum of the watts and the sum of the vars of all units on that bus.

Sharing of Watts with other generators/grid is by controlling generator speed (prime mover) and sharing of Vars is by controlling generator voltage (field excitation) relative to the bus it is connected to.

If you have zero vars you have unity (1.0) power factor. This also means the voltage of the generator exactly matches the voltage of the bus it is conencted to.

Google "Power Triangle" for more info.

If you are importing Vars (as opposed to normal case of exporting) you will have a leading power factor which can be a problem - see possibly useful reference:

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Yes; it is possible to have negative VArs, zero VArs, and positive VArs when connected to the grid.

A reactive curve is included in the documentation from the manufacturer of the generator. If not; seek one.

I have no idea how you can control your VAr output. You have not specified your type of control system, your ability to view VArs, or anything else.

As CSA responded: Interesting?

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.

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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!

Dear all

what a nice discussion of VAr and Watt. In a traditional love story of Adamkhan & Durkhani when a farmer starts telling in evening to another and when the sun rises next day the story ended. Then the listener farmer says, " please tell me which one in Adamkhan & Durkhani was male. (Actually he was sleeping).

We are supplying 12MW @11KV rated, power to 14Km away grid. My gen terminal voltage remains 12.5KV, PF leading 0.9, and VArs negative. To get positive VArs, Lagging PF I need to increase more terminal volts by excitation, thats are already 12.5 instead of 11KV.

Is some body can tell me.

Is negative VArs, Leading power factor harmful for generator?
May I increase terminal voltage more then 12.5KV to get positive VArs this will lead to damage the PTs, cables etc.?
May I stop power generation and bear financial loss?


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Seems you were sleeping while reading the above posts.

The reactive capability curve for your generator defines the limits of operation.

The specification sheets for your PTs define the maximum voltages and the saturation voltages they can withstand.

There's just too much you haven't told about the configuration and operation of your site. How long has the situation existed? If for just a short while, what has changed to cause this new operating condition?

Are there any transformers between the generator and the grid connection point? Do they have adjustable taps which could be used to help change the voltage seen at the generator terminals?

Actually, the best thing you can do is to have a proper power system study performed to take into account all of the equipment, the equipment capabilities, the transmission lines and equipment, and make recommendations for improving the condition of operation of the equipment.

By Shaheed ul Islam on 15 February, 2014 - 1:38 am

Yes you are right but the story was too long, why I have given the example.

I don't know the capability curve of reactive power of generator. PTs maximum voltage are 12KV.

1- The situation exist for past 3 months.

2- The grid distribution is in around 120km area and facing power shortfall. To overcome the problem grid commence load shedding. when a heavy loaded feeder goes to load shedding the grid volts get rises.

3- There is no transformer in between grid and generator.

4- Yes tape at grid is adjustable but if they reduce voltage, the receiving end of consumer drop up to 150V instead of 220V, in case heavy loaded feeder gets on. So they do not alter the tape.
We have recommended a voltage regulator transformer ( auto transformer) but you know the cost of regulator transformer of 12MW would be much high! although the working is in progress.


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Although I dislike the use of maths and vector diagrams here's a link to a pretty good explanation of "generator capability curves" (also known as reactive capability curves):,_Colombo,_Sri_Lanka/Course_ppts/lecture_19.pdf

(Remove any spaces inserted by the forum software when pasting into your preferred search engine.)

The curves should be in the instruction manual provided with the generator. If not, try contacting the manufacturer, or post your generator make and model here to see if anyone has a similar generator who could share their curve with you.

My own personal estimation is that when you are running at full load and high voltage it's probably not very good for the machine or the auxiliaries (there's that whole volts-per-hertz thing, too)--but until someone can resolve the overloading issue in a better manner you are probably stuck with the current situation.

If you want assistance from the grid regulators in protecting your equipment--and in maintaining the reliability of your generator-set for the grid--get a copy of the capability curve and use it to help you make your point. Data don't lie.

And it's much more convincing than whinging and pointing to free advice from an Internet forum when dealing with regulators.

Best of luck!