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Generator Load Sharing
Generator load sharing

Hi all

I've been researching for days about synchronous generators and cannot get my head around how real power output of generators can be adjusted when paralleled to the grid or to each other.

I'm across the reactive power side of things, increasing terminal voltage adjusts amount of reactive power absorbed/produced by the generator (i.e. terminal voltage via excitation increased implies producing VARs and decreased implies absorbing VARs) and I understand how generators operate when connected on their own (i.e. load placed onto generator creates opposing force to slow down prime mover, reducing generator output frequency, more fuel is added to prime mover then generator output frequency restores to nominal).

What I don't understand is how do you control the amount of real power the generator produces, when connected to the grid or in parallel with another generator.

My understanding so far is follows (correct me if I'm wrong):

Let's start with the generator output CB open. Generator is started, rotor is excited by DC voltage. Fuel is added to the prime mover and the prime mover begins to spin. As the prime mover spins, a magnetic field is induced in the stator windings and a generator terminal voltage is produced (the generator is currently unloaded). Synch checks are done to ensure stator output frequency, phase and voltage are the same as the grid.

Generator CB is then closed onto the grid (which is just a whole bunch of other generators). The grid induces a voltage back onto the rotor and hence rotor will slow down. Fuel is added to the prime mover to bring the rotor back up to nominal speed, the same way it would if a resistive load was placed onto it.

Now the prime mover is receiving enough fuel to spin whilst connected to the grid, but as I understand it no power is delivered to the grid yet.

More fuel is added to the prime mover and this is where I am lost. If we add more fuel to the prime mover, does it not begin to accelerate creating a gap between the stator magnetic field (from the grid) and the rotor magnetic field? If this is the case, then the rotor will be phase shifted from the stator magnetic field? Looking at the two wave forms, you would have the grid waveform at nominal frequency and the rotor induced waveform phase shifted by this 'gap'.

If there is a phase shift, let's assume it is 90deg, as I've read that 90deg equates to full load output of the generator. Then won't the connected load see the grid voltage plus the induced voltage from the rotor? Adding two phase shifted waveforms would result in higher RMS voltage across the load? This is bad?

Sorry I'm probably way off here, but thought I could get some help understanding what is going on as it's driving me crazy.

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

The thing about AC power generation is that when two or more (or two hundred or two thousand or more) synchronous generators are synchronized together on a grid no generator can spin faster or slower than any other generator based on the construction of the generator. There's this little formula that relates speed and frequency as follows:

     F = (P * N)/120
where F=Frequency (in Hz)
P=Number of poles of generator
N=Speed of generator rotor (in RPM)
You can solve the formula for speed or frequency by rearranging the terms based on algebraic principals. But, basically, the formula related speed and frequency.

When multiple generators are synchronized together, it's the two magnetic fields inside each generator that keep the rotors locked into synchronous speed per the formula above. ALL generators operate at their synchronous speed. For a 50 Hz grid, a two-pole synchronous generator will operate at 3000 RPM; a four-pole generator will operate at 1500 RPM, and so on based on the number of poles.

One, or six, or sixteen or sixty synchronous generators synchronized to a 50 Hz grid with other generators can run at 51.2 Hz, or 49.6 Hz. They all have to run at 50 Hz.

When additional energy flows into the prime mover driving the generator it would seem logical that the prime mover and generator would increase speed--but it can't. The two magnetic fields inside every synchronous generator prevent the generator rotor from spinning any faster or slower than its synchronous speed (based on the number of poles of the generator rotor). And, the generator converts that additional torque into amperes.

And the formula for 3-phase electrical power is:

     P = Vt * Ia * 3^0.5 * PF
where P=Watts
Vt=Generator Terminal Voltage
Ia=Generator Stator Amperes
3^0.5=square root of 3 (1.732)
PF=Power Factor
Generators run at a fairly constant terminal voltage, so that term can be considered to be fixed. And, the square root of 3 never changes, so that term is fixed. And, if for the purposes of this discussion, we consider the PF of a generator output to be 1.0 (resistive), it is also fixed. That means to produce more power the generator stator amperes have to increase. And, that's what a generator does--it converts torque from the prime mover to amperes. In exactly the same way that an electric motor converts amperes into torque. And generator drive motors.

What happens in an electrical generation and distribution system is that a generator converts torque into amperes, which are then transmitted and distributed to various locations via wires, and then motors convert the amperes back into torque. It's as simple as that.

The additional torque being provided to the generator rotor by the generator's prime mover that would tend to increase the generator's speed is converted into amperes because the generator speed can't increase when it is synchronized to a grid with other generators.

Conversely, when the generator prime mover produces less torque and the generator rotor would tend to slow down--but it can't when it's synchronized to a grid with other generators--causes the generator to produce fewer amperes, which means the electrical power output of the generator decreases.

These are AC power generation fundamentals. When synchronous generators are synchronized to a grid with other synchronous generators, all the generator spin at speeds which are proportional to their construction (number of generator rotor poles). And, when the torque being provided to a generator rotor increases--which would tend to increase the generator rotor speed--the generator, which can't speed up (or slow down) converts the torque to amperes. More torque means more amperes; less torque means less amperes.

If the amount of torque being provided to the generator rotor by the prime mover is not sufficient to keep the generator rotor spinning at its synchronous speed then amperes flow into the generator from the grid to keep it spinning at its synchronous speed. That's called reverse power, and most generator rotor prime movers don't like to be spun by the generator, so protective relays open the generator breaker to protect the prime mover.

Other than the above, most of your understanding is basically okay (except for the part about the generator slowing after initial synchronization and the prime mover having to be sped up). Once the breaker closes the speed of the generator rotor--and the prime mover driving the generator rotor--is fixed by the frequency of the grid with which the generator is synchronized. Period. Full stop. It's can't go faster or slower than its synchronous speed. Period. Full stop.

Generators convert torque to amperes.

Motors convert amperes to torque.

Wires are used to transmit torque from generators to motors.

And the speed of synchronous generator rotors is directly proportional to the frequency of the grid they are synchronized to.

Think about it. On a properly regulated grid with stable frequency, every device connected to the grid sees the same frequency--both loads (motors, etc.) and generators. It has to be. If generators could spin at any speed, why would it be necessary to synchronize them with such sophisticated equipment? Why not just connect the generator to the grid at any speed?

Hope this helps!

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

So, as has been proven and said many times in the past, I'm NOT the best proof-reader of my own writing.

>>CORRECTON<<
One, or six, or sixteen or sixty synchronous generators synchronized to a 50 Hz grid with other generators can run at 51.2 Hz, or 49.6 Hz.

The above sentence SHOULD HAVE BEEN WRITTEN TO SAY:

One, or six, or sixteen or sixty synchronous generators synchronized to a 50 Hz grid with other generators CANNOT run at 51.2 Hz, or 49.6 Hz.

My sincere apologies for any confusion.

All generators run at their synchronous speeds (based on the number of poles of the generator rotor) when synchronized to a grid with other generators. The two magnetic fields inside each generator FORCE them to act as ONE SINGLE LARGE generator, supplying one single large load (the total of all the motors and televisions and tea kettles and lights and computers and computer monitors). There can only be ONE frequency for all the generators, and for all the load(s).

It is patently false for textbooks and references to say that synchronous generators slow down as load increases. It just doesn't happen in the real world. And by load increasing, I'm referring to the amount of power being produced by a generator and its prime mover.

Watch the speed (and frequency) of the synchronous generator(s) at your site or ship as it(they) are loaded or unloaded after they are synchronized to a grid with other generators. Unless the grid is small, you will not see any appreciable change in speed (or frequency) unless you have a highly accurate tachometer and/or frequency meter. And, on a well-regulated grid the frequency should stay relatively constant--because AC power is transmitted best when the frequency is at or near rated. And devices work best when the frequency of the grid they are connected to is at or near rated.

By G.A.abobaker on 20 November, 2018 - 5:03 pm

Hi Mr. CSA,

The statement was a little bit confusing but it was Ok. in fact i have another question. which is, if i need to start a isolated (independent) power plant (say 50 MVA seam turbine) using (temporarily) the national grid (infinite bus), and then to put all the loads of that isolated system on the steam generator. how can i share the load between these two power sources? and how can i increase the loads of the steam turbine to eventually take the all loads and to get rid of the other source (national grid) and finally isolate it?

what would make the loads being withdrew from steam generator while the grid still exist?

many thanks for your detailed explanation.

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

G.A.abobaker,

I can say the same thing; your question is a little bit confusing but I will try to answer as best I can.

Here's what I think you're trying to ask. You have a transmission and distribution system that is capable of powering a load independently of an infinite grid at some point in time. The load is being powered by the national grid when the steam turbine-generator power plant is being started, and the auxiliary loads of the power plant are also being powered by the national grid when the power plant is being started.

When the steam turbine-generator reaches rated speed, it would then need to be synchronized to the national grid. AND, at some point the auxiliary power supply to the steam turbine-generator power plant has to be switched from the national grid to the steam turbine-generator. There would likely be a momentary interruption of power as the switching of the auxiliary power is made from the national grid to the steam turbine-generator output depending on how much time the transfer requires (though it could be done in less than one second with the proper switchgear), and this is going to require some dedicated switchgear and protection to accomplish. (There will have to be a "tap" off the steam turbine-generator output--likely before the generator breaker--that would be used to provide the auxiliary power for the power plant AFTER the auxiliary power supply from the national grid to the power plant was opened.)

There will need to be some kind of method of determining, approximately, the amount of load which is to be supplied by the steam turbine-generator power plant when it is separated from the national grid. This amount will have to include the auxiliary load of the steam turbine-generator power plant. The steam turbine-generator will have to be loaded up to equal this amount of power while still synchronized to the national grid in anticipation of separating from the national grid.

Once the steam turbine-generator is loaded up to the amount of the load to be powered by the steam turbine-generator independent of the national grid PLUS the amount of the auxiliary load of the steam turbine-generator power plant, the power plant AND the load to be powered by the plant independent of the national grid must be separated from the national grid (again through dedicated switchgear).

Now, the steam turbine-generator will be powering the load to be powered independently of the national grid PLUS it's own auxiliary power load, and at approximately the right level (MW) and at approximately the correct frequency. At this point, the steam turbine governor should be switched from Droop Speed Control to Isochronous Speed Control--so that any changes in load (either the load being powered independently of the national grid OR the auxiliary load of the power plant) will be handled automatically by the steam turbine generator WHILE maintaining the desired frequency.

That's about it--if I understand the question correctly.

Now, if the steam turbine-generator power plant needs to be shut down for any reason WITHOUT interrupting the power to the load being powered by the plant, it will first be necessary to re-synchronize the steam turbine-generator and it's load(s) to the national grid. (Usually, the steam turbine governor would be switched back to Droop Speed Control immediately prior to re-synchronizing the unit with the national grid--or it would have to be very quickly switched back to Droop Speed Control immediately after re-synchronizing to the national grid to avoid instability of the steam turbine-generator output (load)).

Once re-synchronized to the national grid the load on the steam turbine-generator can be reduced to approximately zero, and then the generator breaker can be opened. The national grid will be providing the power to the load which was being powered by the steam turbine-generator independently of the national grid, BUT the steam turbine generator will still be supplying the auxiliary load of the steam turbine-generator power plant.

At this point, the steam turbine-generator power plant auxiliary power supply will have to be switched over to the national grid (by opening the steam turbine-generator auxiliary power supply breaker and closing the national grid auxiliary power supply breaker--which will cause a momentary interruption in power to the power plant auxiliaries depending on how long the transfer requires). At that point, the power plant can then be shut down using national grid power.

Now, without understanding exactly how your "plant" is powered and connected to the national grid it's extremely difficult to say any more. It would take some "special," dedicated switchgear to accomplish the above, and probably even more to accomplish something other than the above. And, that switchgear would likely have to include multiple synchronization circuits (one to synchronize the steam turbine generator to national grid, and one to re-synchronize the steam turbine-generator back to the national grid to avoid blacking-out the load being powered independently of the national grid when trying to take the steam turbine-generator and its power plant off-line in a controlled and orderly fashion without causing disruption to the load).

I hope I've understood the question correctly, and if I haven't I hope you will read and re-read the information and apply it to your situation because the circumstances you described were incomplete at best and it would probably take a LOT of back-and-forth to answer your specific question or situation (if there even is a real situation and this isn't just a what-if scenario which hasn't really been properly thought-through). I does the best I can with what I was given to work with--which wasn't very much in the way of detail and seems to have made several assumptions which I've had to try to fill in.

It's also made more difficult because what you should be providing is what's called a one-line diagram of the electrical system at the "plant" which would make it much easier to provide specific details about which breakers to open and close when.

Hope this helps! I'm sure it's not the exact details you were expecting, but then there wasn't enough specific information to go on.

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

Many thanks Mr. CSA about explanation.

>Hope this helps! I'm sure it's not the exact details you
>were expecting, but then there wasn't enough specific
>information to go on.

You are right. i didn't give you enough details.

The idea is to start up a power plant supplying chemical plant. the island power system consists of Gas turbine and steam turbine. gas turbine originally intended for starting up. unfortunately gas turbine under emergency maintenance. to start up the steam turbine separate power source needed to start up steam boiler. solution is to connect national grid to that power island (in the same switch gear). then to start the steam turbine, synchronized it with national grid and then put loads on steam turbine. laterally to isolated the national grid.

Hi CSA,

Your above explanation have cleared most of my doubts.
But I would like to know about the role and function of AVR system. My understanding is that its function is to vary the field current in rotor to produce more or less excitation when ever the terminal voltage increases or decreases. As you explained above, if the terminal voltage of generator remains constant once it is synchronized, then what is the purpose of AVR?

Kindly clarify.

Moderator's Note: There have been multiple threads on AVR and excitation on control.com. I suggest you search are archives for information. For directions on formatting search options, click the Question mark (?) next to the Search Box. Just a note, three letter acronyms like AVR all by themselves don't work when you search. You need to add additional terms such as, +excitation +AVR in the search field.

If you don't feel your question is adequately addressed, try responding to one of the related posts with a request for more specific information.

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

esdauto18,

If you haven't tried the Search feature of control.com, it's fast. The syntax of search terms was well explained by the Nice Moderator, and there is Help available, as well.

So, the generators usually driven by GE-design heavy duty gas turbines are synchronous generators (as opposed to non-synchronous generators). Technically they are really called alternators, but nobody uses that term any more.

Synchronous generators use a rotating magnetic field to produce the generator terminal voltage. Under most conditions, you are correct: The generator terminal voltage is constant. (Most generators can only change the generator terminal voltage by approximately plus- or minus 5% of nameplate rating.) However, during any given day the grid voltage may, and usually does, change as the types of loads on the grid change (the number of induction (non-synchronous) motors, mostly--which are used for refrigerators, air conditioners, fans, pumps (some VERY large water pumps for irrigation districts and water treatment facilities (fresh water and sewage water, etc.). Even the number of fluorescent lights can have an effect grid voltage in some parts of the world. So, even if the watts (real power) being produced by the generator set stays relatively constant during the day, the reactive current (also known as VArs, Volt-Amperes Reactive) can, and usually does, change.

You have asked about AVRs (not VArs, AVRs--isn't this fun?!?!?!) in another thread. The purpose of an AVR is to keep the generator terminal voltage at a setpoint. Most generator set operators don't realize they are changing a generator terminal voltage setpoint when they are making an adjustment using the AVR, but that's what's actually happening as they watch the VAr or pf (power factor) meter when making the AVR adjustment. So, the AVR is trying to keep the generator terminal voltage constant (at the setpoint) as the grid voltage is changing.

To vary synchronous generator terminal voltage (since the synchronous generator is always running at a constant speed to produce a stable, and constant frequency--regardless of the amount of load being carried by the generator) one varies the amount of DC (Direct Current) current applied to the rotating generator field. The rotating generator field is a one or more conductors wrapped multiple times around the length of the rotor--these are called windings. (There are rotor windings, and there are stator, or stationary, windings. Only DC is applied to the rotor windings, and that produces voltage in the stator windings as the rotor is being turned by the prime mover. You can find all kinds of videos and references to how AC is produced by using your preferred World Wide Web search engine.)

Another thing that happens is that as the amount of real power (watts) increases the amount of amperes flowing in the stator (the stationary part of the synchronous generator) increases. And this causes the strength of the magnetic field of the stator to increase (more amperes flowing through a fixed number of turns of a conductor (the stator windings) causes the resultant magnetic field to increase). This stationary magnetic field reacts with the rotating magnetic field of the generator rotor and causes the rotating magnetic field strength to decrease somewhat. If the rotor field strength is reduced, then the generator terminal voltage will also be reduced.

Tying this all together, if the amount of DC current (called "excitation") being applied to a synchronous generator rotor is exactly equal to the amount required to make the generator's terminal voltage exactly equal to the voltage of the grid at that moment in time the amount of reactive power (VArs) will be zero (0). And the power factor will be "1" (also known as "unity power factor"). If the synchronous generator terminal voltage is less than the grid voltage at any moment then leading VArs (reactive current) will start flowing in the synchronous generator's stator windings (along with the real amperes associated with the watts being produced by the synchronous generator). And, if the synchronous generator terminal voltage is greater than the grid voltage at any moment there will be lagging VArs (reactive current) flowing in the synchronous generator's stator windings.

Most power plants that are synchronized to grids want to produce as few VArs as possible (minimal reactive current). That's because they don't usually get paid for producing VArs--only for producing watts. ALSO, too many leading VArs are generally not good for synchronous generators (causes unwanted heat in the windings, which can damage insulation). AND, if the excitation being applied to the synchronous generator rotor by the AVR (the exciter, or the excitation control system) is too small then serious mechanical damage can occur to the generator, the coupling between the generator and the prime mover, and even the prime mover (it's called "slipping a pole"--and it's VERY bad).

Most synchronous generators are capable of producing much more lagging reactive current (usually referred to as "positive VArs") than leading reactive current (usually referred to as "negative VArs"). To do this, the excitation control system (the AVR, or the exciter) has to be capable of producing lots of DC current (and voltage) to be applied to the generator rotor. Too much lagging reactive current though can also produce unwanted heat in a synchronous generator, so this too has to be limited under normal operating conditions.

There are many ways to produce the DC to be applied to the synchronous generator rotor windings. But, the AVR, or the excitation control system, or the exciter, is how that DC current (and voltage) is controlled and limited and held stable--to produce stable generator terminal voltage, whether that generator terminal voltage is equal to the grid voltage, or less than the grid voltage or greater than the grid voltage.

To sum up, the AVR is generally used to control the reactive current of a synchronous generator. If the excitation being supplied by the AVR is exactly equal to the amount required to make the generator terminal voltage equal to the grid voltage, then zero (0) reactive current will be flowing in the generator's stator windings. (That's good.) However, if anything happens that causes the synchronous generator's terminal voltage to be less than the grid voltage (either the operator reduces the AVR setpoint, or the operator increases the amount of watts being produced by the generator, or the grid voltage increases because of load changes on the grid) then there will be leading (negative) reactive current flowing in the generator. Too much leading reactive current is not good (causes unwanted heating in the generator).

OR, if something happens to cause the synchronous generator's terminal voltage to be greater than the grid voltage (either the operator increases the AVR setpoint, or the operator reduces the amount of watts being produced by the generator, or the grid voltage decreases because of load changes on the grid) then there will be lagging (positive) reactive current flowing in the generator. And excessive lagging reactive current is also not good (causes unwanted heating in the generator).

So, the AVR, is used to control the direction and amount of reactive current. It's also used during synchronization to make the synchronous generator's terminal voltage at least equal to or, usually, slightly greater than, the grid voltage (so that either zero reactive current or a small amount of reactive current will be flowing immediately after synchronization). The AVR controls the amount of DC current (and voltage) being applied to the synchronous generator's rotor windings--and that affects the strength of the generator rotor's magnetic field, which affects the magnitude of the generator's terminal voltage, which can affect the reactive current of the generator.

Now, all this talk about generator terminal voltage versus grid voltage can be confusing. In general, when a synchronous generator is synchronized to a grid with many other generators the two voltages are generally considered to be equal. BUT, if you have a voltmeter (or two of them, one for the generator terminal voltage and one for the grid voltage) with enough resolution (which most don't have) you can see changes of tens or hundreds of volts as excitation is varied. But, most displays or voltmeters only show one value of voltage, and that is okay. It's really the relative difference between the two that affects the reactive current flow, and that can be measured in tens or hundreds of volts with a high resolution means. We're not exactly interested in the actual value of generator terminal voltage versus grid voltage--EXCEPT during synchronization, and then the difference IS important!--we're really interested in the amount of reactive current flow (which is relative to the difference in voltages). And, so that's why the operator doesn't usually (almost never in most cases) look at the generator terminal voltage meter when making AVR adjustments--they only look at the VAr meter or the power factor meter and adjust the AVR output to make the VAr meter or the power factor meter equal to the desired value.

As a very important aside, the watts being produced by a synchronous generator are a function of the energy flow-rate into the generator's prime mover. And, the VArs (the reactive current) being "produced" (or "consumed") by a synchronous generator is a function of the amount of excitation applied to the generator rotor windings by the AVR.

Isn't this fun???!!!?!?!

Hope this helps! I know it's a lot--but, it's all intertwined and it's all important. Notice I didn't use any maths; I generally try not to. I prefer to describe what the maths can predict or be used to analyze. But, what's happening is more important than the maths--again, the maths can be used to predict or analyze what's happening. We need to what's supposed to happen when, and why. And how.

Welcome to control.com. The Speedtronic (Mark*) turbine control community has been very active here for almost 15 years (I think)!. We thank the Moderators, the host, and the sponsors for keeping this site going and on track (though sometimes some of us stray--I'm one of them who tends to editorialize once in a while; sorry!).

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Again, welcome. And, don't be afraid to share your experiences when someone posts for help or information and you have a relevant experience or knowledge!!!

thank you for your explanations, we (it lest me) noticing how much care you take to your sentences to make it full of meaning. although the terms reactive power (VAr) still not so clear as much as the active (wattage). I myself still have to feel the difficulty when ever try to imagine the reactive power. I didn't work in generator fields. Although that I worked as an electric construction man in area of generation and high yards, have installed panels and wiring in GE generators control room. but I would be more self confident if i had hold the knob of the control. as you said if a drawing or diagrams could be attached to the explanation would be very helpful.

many thanks again

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

Thanks for the kind words. I'm going to try to keep this as "simple" as possible--and I'm really only offering you a way of thinking about reactive power: VArs. It's my own analogy--a way of describing something that is difficult to describe. A way of trying to understand something that is difficult to understand. My analogy is NOT technically correct--but is just an attempt at trying to understand what a "VAr" is and what it "does" and how it is "produced" and "consumed."

The majority of electric loads on a transmission and distribution system are electric motors--driving things like air conditioner compressors; air conditioner blowers (fans); refrigerator compressors; freezer compressors; water pumps (fresh water, and sewage water); fans; elevator motors; crane motors; you name it. And, there are two basic types of AC (Alternating Current) motors: Synchronous and Induction. And, the most common type of motor used for just about every application above is an induction motor--which doesn't have any slip-rings or brushes.

Induction motors rely on the phenomenon of induction to create a magnetic field on the motor's rotor, and that magnetic field is attracted to a magnetic field which is created when alternating current flows in the stator windings (the stationary windings of the induction motor). And it's that reaction between the two magnetic fields that results in the rotation of the motor's rotor--which is how alternating current (amperes) gets converted into torque, which is what air conditioning compressors and blowers, and water pumps, and crane hoists, and elevator sheaves, etc., need to do work (make us or our beverages cold; make water flow under pressure; lift boxes or people; etc.).

In my analogy, it's the act of inducing that second magnetic field in the rotor of an induction motor that "consumes" VArs (Volt-Amperes Reactive--or, reactive power). If we think of VArs as being produced and consumed, just as Watts are produced (by generators) and consumed (by electric motors, and lights, and computers and computer monitors, and televisions, and tea kettles), then you can sort of understand what a VAr is. Remember: this is just an analogy--however, thinking of VArs as being produced and consumed can be very beneficial. (Many people like to think of electricity as flowing water. The water pressure is like voltage, and the flow-rate of water is like amperes. That's another analogy.)

Now, the whole "real" power versus "reactive" power thing. Real power is something we can "see" and it's generally considered tangible ("real")--it's expressed in horsepower (originally). And reactive power is generally something we can't "see" and is considered intangible ("not real")--except that it is real in the sense that without it induction electric motors wouldn't turn. And, since induction electric motors are the overwhelmingly most common type of motors in use around the world (they are the least expensive and easiest type of electric motor to build and operate!) we would have to use more expensive electric motors that require more maintenance and repair. VArs don't "move mass a certain distance in a specific time period" (the definition of a horsepower!)--but they make it possible for induction electric motors to do that.

So, although it's just an analogy--try to think of VArs as just like Watts: They are "consumed" and "produced." They are "consumed" by induction electric motors to create the necessary second magnetic field on the motor's rotor to make the rotor turn. And, they can be "produced" by alternating current synchronous electric generators.

Many HMIs refer to leading VArs at the generator terminal as "negative" VArs, and generally, leading VArs are considered to be "flowing into" the generator from the grid. Lagging VArs at the generator terminal are "positive" VArs, and generally, lagging VArs are considered to be "flowing out" of the generator on to the grid. (There's a saying: Lagging VArs "feed" a lagging load. (The quotation marks are mine.))

Hope this helps! Again--it's just my analogy. It's technically incorrect (as VArs are really MUCH more than that), but it's a way that help me (at least) understand and think about VArs.

I'm presuming I understood ghidan's question(s)--perhaps I didn't, and he gave the 'Thumbs Down' to my previous response. If that's the case, ghidan, can you please clarify your question(s) so I can try again?

If it was someone other than ghidan who clicked on 'Thumbs Down' can you tell what it was about the analogy that you didn't like? How can one explain VArs without vectors and sine waves and maths? And how does using vectors and sine waves and maths explain what VArs "do"?

Because electric motors (and fluorescent lights) cause shifts between the voltage and current sine waves of an AC (Alternating Current) transmission and distribution, and left unaddressed the shift can be very bad for the customers of and the operators of the transmission and distribution system--not to mention the generators synchronized to the transmission and distribution system.

How does one describe that without drawings and graphs?

Isn't a VAr a way of expressing what's causing the shift between the voltage- and current sine waves? Or of expressing the magnitude of what's causing the shift between the sine waves? Isn't the magnitude of the shift related directly to the magnitude of the number of reactive amperes flowing in the system?

Real Power: Volts multiplied by Amperes (when the voltage- and current sine waves are in phase with each other)--such as in a purely resistive load (an incandescent lamp; the heating element in a tea kettle)

Reactive Power: Volts multiplied by Amperes which are NOT in phase with each other--such as in a load with reactance, which can be either inductive or capacitive in nature (think of an induction electric motor; or a fluorescent light fixture)

Reactive power results when a portion of the current flowing through a reactive component is returned to the system on the alternate portion of the current sine wave flowing through the reactive component. So, reactive power flows back and forth without doing any real ("mechanical") work. (That's real descriptive without drawings and vectors and graphs and maths.)

I read that reactive power is just AC power that flows back and forth in a power system--but doesn't do any real work (it doesn't move any mass any distance in any particular time period). (Also, real descriptive, wouldn't you say?)

Another World Wide Web site says, "Reactive power is a necessary evil for induction machines"--that is, induction electric motors (as well as transformers). With which I whole-heartedly agree--but which also isn't very descriptive.

Here's a youTube video (some parts of the world can't access this site):

https://www.youtube.com/watch?v=DPAbMpIOHsU

But, does any of that really explain what a VAr is? Or does? VArs are calculated by mutiplying the magnitude of the voltage and current sine waves at some point in an AC power system at any instant in time. If the two sine waves are in phase with each other, then the load is purely resistive and only real power (Watts) flows in the circuit. But, if the load has either capacitance or inductance (or a predominance of one or the other), the product of the out of phase sine waves at any instant in time at any point in the AC power system is called VArs (Volt-Amperes Reactive).

But, I still ask: How does that explain what a VAr "is" or "does"? Can't we just agree that VArs are the way to calculate or express the effect of inductive and/or capacitive loads on the system? And, by extension can't we agree that VArs are "consumed" (because they are a necessary evil for induction electric motors--the overwhelming majority of the loads on an AC systm)?

What hasn't been said is that, left unchecked, if too many induction electric motors (air conditioner compressors and fans; refrigerators; etc.) are connected to the grid at any one time then bad things start happening to the grid. And, so, grid operators have to have a way of countering the inductive loads and they typically do that by changing the excitation of synchronous electric generators. They can also use fixed capacitors, but they are variable--they can only be switched in or switched out (and when the switching occurs if you're standing near the capacitor banks, usually, there is a big "thud" (a dull, low bang) that can be felt. Sometimes the high tension (voltage) lines (wires) will actually move ("swing" or "shake") when large capacitor banks are being switched in or out (on or off).

So, can't we agree that VArs can be "produced" to counter the effects of the VArs being consumed. Can't we all agree on those two things?

If not, I'm extremely keen to hear how someone else can explain VArs. Simply, and succinctly. Without being able to use drawings and graphs and maths.

Thanks!

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ghidan,

Let's try a slightly different approach.... I'm not trying to describe what VArs "are"--I'm trying to describe what VArs do. Or, the effect of VArs on the system. The effect is to shift the voltage and current sine waves out of phase with each other (on an AC power system there are BOTH voltage and current sine waves, both at whatever the system frequency is (50 Hz or 60 Hz)). So, what VArs do is to shift the voltage and current sine waves out of phase with each other. And, one kind of VAr can offset the other kind of VAr.

I guess that's not the same as what a VAr is--but, for me at least--if I can understand what something does or how it affects other parameters I can usually begin to understand what something is, especially when it's an intangible thing, like reactive current is. There's no real physical way we visualize VArs, except maybe for the heating they cause; but that's not generally they way they are quantified. And just to say it's current flowing back and forth in a system that doesn't do any work (lifting or pumping or whatever) doesn't really describe it either.

You should be able to use your preferred World Wide Web search engine and the term "Volt Ampere Reactive" to find lots of different explanations, most probably better than mine because many include drawings and such.

Finally, we can't use drawings or charts or graphs in the threads on control.com. It's a security thing, and, we have to respect the owners and Moderators of the website and their desire to protect themselves--and us!!! My wife says engineers can't talk without a paper and pencil (to draw and make graphs and lists), and I'm certainly a good example of that, though I try with my responses on control.com, and that's why they get so wordy sometimes--at least that's my excuse and I'm sticking to it!