Reactive power
Reactive power
(OP)
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?
When was the last time you drove down the highway without seeing a commercial truck hauling goods?
Download nowINTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS Come Join Us!Are you an
Engineering professional? Join Eng-Tips Forums!
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail. Posting GuidelinesJobs |
|
RE: Reactive power
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.
RE: Reactive power
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: Reactive power
RE: Reactive power
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: Reactive power
I would like to remind you, coleagues, that the unit of reactive power is "var" (not "VAR" nor "VAr").
Best regards,
Herivelto Bronzeado
RE: Reactive power
RE: Reactive power
Ampere, a guys name.
reactive, just a word.
VAr
Keith Cress
kcress - http://www.flaminsystems.com
RE: Reactive power
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.
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: Reactive power
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.
RE: Reactive power
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
RE: Reactive power
I also allways thought that the correct is VAr, but it looks like I am wrong, according to IEC
RE: Reactive power
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
RE: Reactive power
RE: Reactive power
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.
RE: Reactive power
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.
RE: Reactive power
I don't think your statement is correct:
Lgging and leading vars are opposite to each other but they are both perpendicular to watts.
RE: Reactive power
RE: Reactive power
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.
RE: Reactive power
RE: Reactive power
I am still rereading what I wrote, but since you replied, it is true for a just a condensor.
Can a generator be overexcited to a leading power factor while still supplying a load?
RE: Reactive power
Supplying additional VArs to the system requires the generator to be over excited causing a more lagging power factor.
Generators connected to the grid are generally run very close to unity pf as there is someone buying Watts but nobody buying VArs. If the prime mover W rating is close to the generator VA rating, producing many VArs reduces the number of Watts that can be produced without exceeding the Amp limit of the generator.
Generator (Watts out) VArs out = lagging; VArs in = leading
Not a Generator (Watts in) VArs out = leading; VArs in = lagging.
RE: Reactive power
RE: Reactive power
RE: Reactive power
I would like to remind you that the real and reactive power are components (so, maths) of the resultant instantaneous power wave. These components do not exist as separate entities but they can be conveniently considered for purpose of engineering analysis.
As Hoxton said, leading/lagging depends on the reference.
I think the best way to refer to reactive power should be to label as inductive or capacitive vars. In combining the two in analytical work it is important to consider one positive and the other negative regardless the convention employed.
Regards,
Herivelto
RE: Reactive power
Not a Generator (Watts in) VArs out = leading; VArs in = lagging.
Davidbeach:
is it from IEEE /IEE?
From system point of view, it does not matter for MW generator or synchronous condenser as long as it generates Var to system we call it running at lagging pf otherwise it absorbs Vars then we call it running at leading pf. we've been using it for a long time for system planning point of view. it seems you have different definition of leading /lagging in terms of machine.
RE: Reactive power
RE: Reactive power
RE: Reactive power
The cost of supplying VARs to residential users is averaged over the customer base and factored into the KWHr charges.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Reactive power
But also under loaded transmission lines in the under ground, or EHV catogory also produce vars (When highly loaded the same transmission lines consume vars).
RE: Reactive power
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: Reactive power
--peetey
RE: Reactive power
That's a whole 'nother thread
RE: Reactive power
The way that makes most sense to me is that vars are stored power. It is power that is available to return to the source. Leading and lagging simply define phase. It's more like vars are traded than produced or consumed.
Keith
RE: Reactive power
It's referenced on Wikipedias definition of reactive power - which is worth reading too.
RE: Reactive power
It is the nature of AC systems that the load controls the power factor. If the load demands VARs then VARs will flow. (Actually the current phase angle changes, nothing is "created" but VARs are a mathematical description of current phase angle related to watts that is relatively easy to work with.)
The shifted phase angle may be corrected at the load or at some place on the distribution system with capacitors.
A single, islanded, synchronous generator can not change the phase angle of the connected load. When two or more generators are connected in series, They may be adjusted so that the phase angle of the current through one of them is zero, (unity power factor, no VARs, etc.) The phase angle of all the other generators on the system will then shift to compensate for the machine that is not producing its share of the VARs.
VARs are imaginary, a mathematical fiction.
BUT, express a load in ohms and Henrys, and try to calculate the farads needed to correct the phase angle to 25.8 degrees (PF = 0.9) without ever reducing the numbers to kW, KVA or KVARs. The calculations are so much easier when you use the imaginary VARs to express the phase shift between the current and the voltage, as it relates to the power flowing.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Reactive power
I essentially understand all the above from the vector diagrams to the PF meters on the panels and the excitation settings on the AVR's.
I understand that a synchronous compensator operating near a load that needs vars can keep vars from having to be transmitted over long distances hence saving the I2R losses on the transmission lines.
But what I don't understand is how a synchronous compensator located on the end of a transmission line somewhere (not necessarily near a highly inductive load) will raise the voltage on the line by operating as a synchronous condenser and (is it eating or creating?-excuse the ME terminology) vars.
I could really use some help with that.
rmw
RE: Reactive power
RE: Reactive power
Can you elaborate more on what that means? I understand that capacitors shift the phase, not 'how' but 'that' and that condensers serve that function but don't understand how that relates to the line voltage being raised at that time.
The question comes from encountering a synchronous condenser way out in the boondocks way away from any industrial loads that might have had an inductive influence but the condenser was put there to raise the voltage on the end of the line.
To my warped ME way of thinking, it would have made more sense to produce real power and put 'volts' on the line loosely said.
So I didn't understand the function of the sync condenser was in that occasion.
rmw
RE: Reactive power
I remember some parts of the plant had pf of 0.3, has anyone seen lower?
Interesting Topic
Roy
RE: Reactive power
I think all this confusion is because we, as engineers, have the tendency to model everything and, sometimes, forget to come back to the basics.
Also, we always keep trying to visualize mathematical entities as being physical entities.
In reality, there exist the electric and the magnetic filed, which can be associated to voltage (V) and current (A), respectively.
The other electrical engineering "quantities" (S (VA), P (W) and Q (var)) are derived mathematically from these two physical entities (V and A). These "quantities" are defined with base on alternating current (AC) systems with linear components and sinusoidal wave.
The best way to "see" if the vars is leadding or lagging is looking the angle between the voltage and current.
rmw, regarding to synchronous compensator (SC), it can be "visualize" as a voltage source generating a voltage that can be (a) higher, (b) lower or (c) equal to the voltage at the terminal on which it is connected.
In the case (a), the SC (over-excited) will "delivery" current (90 degree leading from the voltage) to the system as current flows from high to low potential (voltage). In this case the SC works as a capacitor.
Conversely, in the case (b), the SC (under-excited) will be "absorbed" the current (90 degree lagging from the voltage), as its voltage is lower than that on the terminal. In this case the SC works as an inductor.
In the case (c), no current will flow. The SC will be is "sleeping".
In all cases, some energy is consumed to supply the losses (friction, heating, etc), as davidbeach rimind us.
This same "picture" can be used for a generator, but in this case there will be a prime mover connected to the generator shaft.
Best regards,
Herivelto Bronzeado
RE: Reactive power
In school way,way back, I got questioned for a correct figure in one of my answers! The professor didn't understand how I got the answer when my formulas are in a mess! We came to an understanding (i.e. me learning) that I got my conventions in reverse!
Having said that, we all can come to an agreement if our conventions in the flow of power, current, etc are the same.
I was taught any quantity entering the system will be considered "positive" and quantities leaving the system as " negative"; outgoing vars-negative, incoming vars-positive, etc.
RE: Reactive power
- why increasing voltage from generator when power factor at lagging pf = 0.84, reduces PF to 0.82 and increases MVAR and kV from generator.
1. if the generator is leading which absorbs VAR? what to do to have PF to 1, and vice versa.
2. is this correct? power station charge companies based on their real power, not apparent power (total).
- why users with low PF get penalty? is it because VAR (phantom power) is difficult to quantify?
RE: Reactive power
PF is the cosine of the angle between the current and the voltage. when you turn the exciter knob to increase the excitation, the angle between the voltage and the supplied current increases. A 0.84 lagging PF means an angle of 32.86 degrees; A 0.82 lagging PF means an angle of 34.91 degrees! See how the angle increases.
On Q 1., All you have to do is increase the exciter voltage until your generator reads 0.9 or 0.85 (depending on the generator temp and what the grid requires you to operate at).
On Q 2., MWHr is energy that power companies charge most users. But users who cannot maintain higher than the specified lowest power factor limit (usually 0.85 lagging) are penalized because the system voltage will go down if loads are too inductive. These MVAr-Hrs (amount of reactive VA times the hours) are sometimes metered on customers known to have PF difficulty and not hard to quantify. Please review your power triangle math.