Reactive power 2

[mab2008]

What is the advantage of reactive power (VAR)? How it is controlled from the power plants? Is it possible to transmit real power (KW) wihtout KVAR?

 

[UtilityDave]

VARs happen naturally. Inductive loads, like motors, light ballasts and others draw VARs.
Also most power delivery lines are inductive and draw VARs to deliver power.

These VARs are "lagging" VARs.

It is necessary and cost efficient to compensate for those lagging VARS with leading VARs that tend to cancel them out. This is done with capacitor banks, or at the generator by changing the excitation.

 

[ScottyUK]

Under normal circumstances generators export lagging VArs, loads consume them. A leading generator is importing (consuming) VArs just as a lagging load does. The leading / lagging terminology can get confusing when considering both motors and generators.


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If we learn from our mistakes I'm getting a great education!

 

[davidbeach]

Lagging VArs go in the same direction as Watts. Leading VArs go in the opposite direction from Watts. Thought of that way, the generator and motor definitions are the same.

 

[ScottyUK]

That's good David. Never heard it explained that way before. Thanks.


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If we learn from our mistakes I'm getting a great education!

 

[Bronzeado]

Hi folks,

I would like to remind you, coleagues, that the unit of reactive power is "var" (not "VAR" nor "VAr").

Best regards,

Herivelto Bronzeado

 

[davidbeach]

Volt Ampere reactive.

 

[itsmoked]

Volt, a guys name.
Ampere, a guys name.
reactive, just a word.


VAr

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

I have gotten lazy - I was taught VA_r.

However Herivelto is correct, at least according to the IEC:

http://www.iec.ch/zone/si/si_elecmag.htm

(bottom of page)

I am somewhat puzzled that there is a 1kg · m/s³ equivalence shown for a quantity that by definition is wattless. It's also just after 0500 here so I will reconsider this comment once I wake up.





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If we learn from our mistakes I'm getting a great education!

 

[davidbeach]

If Volt Ampere is VA then Volt Ampere reactive is VAr. If Voltage Ampere reactive is var, than Volt Ampere is va. I've never seen anybody suggest that Volt Ampere is va, so Volt Ampere reactive must be VAr.

P, Q, and S (W, VAr, and VA) all have the same units/dimensions. There would be many difficulties mathematically if the units were different.

 

[Bronzeado]

David,

I also have never seen anybody suggest that Volt Ampere is "va". It is "VA". However, reactive power is "var", by convention (SI). This was decided a long time ago.

In reality, S, P and Q have different units, VA, W and var, rspectively. If they had of same unit we could add them but, as you know, S is the square root of the sum of P squared plus Q squared.

For current unit we write "ampère" not "Ampère". The symbol is A. The same occur with voltage: "volt" not "Volt" (symbol is "V") unless we talk about Volt, the person. This is as it is by convention (according to SI).

Also, we should write "kV" and "kvar", not "KV", "Kvar" or even "KVAR".

ScottyUK, thank you for showing the link.

Regards,

Herivelto Bronzeado

 

[Denan]

May I add: the person in question is Volta, not Volt.

I also allways thought that the correct is VAr, but it looks like I am wrong, according to IEC

 

[UtilityDave]

Thanks Herivelto,

It is good to be reminded from time to time. I often misuse it and say VAR or KVAR when working with non-Engineers, so they know it is not a misspelling and it can become a habit.

Dave

 

[jghrist]

However Herivelto is correct, at least according to the IEC

Also according to IEEE.

 

[rbulsara]

IEC/IEEE has "decided" many things forming committees of folks like us, expcet they have more time on hand. Not everythig they decided is logical but they are accepted standards but not the laws. While 'var' is what they decided on, its not necessarily logical. I do not have any grief with that however. It is similar to the definition of "counterpoise" discussed in another thread.

It is possible that it may get revised someday. Standards are always evolving so there is no harm questioning some of those that already exist rather than blindly defending them, specially when it comes to conventions.

I have used all of the above combinations, many times to just emphasize that I am looking for kVAR reading and not kVA.

 

[Hoxton]

My Chief Engineer, when I was an apprentice alternator designer, seconded from Sales dept., told me that the only safe way was that to define the alternator's position as either "under excited" or "over excited".

Leading / lagging depends on which way one is looking, out to the system, or in to the alternator.

David Beach gives an excellent definition, however.

 

[pwrtran]

David

I don't think your statement is correct:

Lagging VArs go in the same direction as Watts. Leading VArs go in the opposite direction from Watts.

Lgging and leading vars are opposite to each other but they are both perpendicular to watts.

 

[davidbeach]

scopidia, Watts and VArs are in quadrature with each other, that is true. But both can be spoken of as flowing from a source to a sink. If the Watt source and the VAr source (or sink) are the same, the VArs are said to be lagging. If the source for one is the sink for the other, the VArs are said to be leading. I believe the statement stands.

 

[rbulsara]

scopidia:

Firstly, davidbeach is correct. Your statement is also correct for the vector relationships but vector direction is not same as direction of "flow" of power.

May be this will help: Normally a generator is supplying var and Watts to a motor which consumes vars and watts. So you can say both quantitiesare "flowing" from the generator to the motor. In this case both the generator and the motor will be said to have lagging power factor.

When a grid connected synchronous generator is used as a condensor (capacitor), it still supplies power (watts) into the grid but it is consuming 'vars' from the grid. Thus vars and watts are in opposing direction for the generator so it is said to have a leading power factor, but grid still have lagging pf as both watts and vars are flowing "out" of the grid to other loads. This of course assumes there other inductive loads on the grid.

 

[davidbeach]

rbulsara, a synchronous condenser draws enough Watts to keep it spinning, but supples VArs to the system. Because Watts are in and VArs are out, it is operating at a leading power factor. If something mechanical can make the shaft turn at synchronous speed then you could have VArs out with no exchange of Watts. If that something drives the shaft enough to push Watts out then the power factor becomes lagging, all with no change in the VArs out.