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Power-User

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Reactive Power

01/28/2017 10:00 AM

famous statement :"an over-excited synchronous generator is said to supply reactive power to system". but wait? didn't we study in text books that reactive power swings back and forth between load and source? i.e generator is not always supplying reactive power .it is taken back by source after quarter cycle.any clarifications please?

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#1

Re: reactive power

01/28/2017 10:14 AM

Reactive power simply means that voltage and current are out of phase. If you multiply two sine waves that are exactly in phase (current and voltage = power) both will either be positive or both negative, and the product will always be positive.

If the voltage and current are not in phase, when the are opposite signs, the product will be negative and power will be returning to the source.

Voltage (blue) x Current (red) = Power (black)

Reactive power flows back and forth between load and source.

Current (red) and Voltage (blue) in phase. Power (black) always positive.

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Power-User

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#14
In reply to #1

Re: reactive power

01/29/2017 4:29 AM

"If the voltage and current are not in phase, when the are opposite signs, the product will be negative and power will be returning to the source"

i have a very simple question ! in power system, loads are predominantly inductive i.e. voltage and current not in perfect phase, so there will be some reactive power flow back and forth between source and load. now if source i.e. generator is delivering reactive power at some time and then receiving that same amount of reactive power another time, then why do we say that power plant generators must be operated in overexcited mode so that they supply reactive power and not absorb.

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#2

Re: reactive power

01/28/2017 11:00 AM

Haris,

Have you ever considered doing any research on your own?

I know that you prefer to have others do your thinking for you, but you might discover some facts all on your own.

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#3
In reply to #2

Re: reactive power

01/28/2017 11:27 AM

Not possible, already he has proven that fact.

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#4
In reply to #3

Re: reactive power

01/28/2017 11:57 AM

Here, Haris Mahmood (@coolyaar) — 22 answers | ASKfm makes the claim that he is an Electrical Engineer from Lahore, Pakistan.

If Haris has a degree, it must have been obtained illegally and is certainly not a claim substantiated by any of his posts here.

He has already admitted that his supervisor doesn't care if he is incapable of doing his job and comes here for answers to all his technical questions. Certainly unethical and illegal.

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#5
In reply to #4

Re: reactive power

01/28/2017 12:03 PM

Yes, I have followed this. I do not have a degree in electrical engineering but after reading all of his posts, I feel I am more qualified then him to do the job. I feel I know more just from experience in the field.

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#6
In reply to #5

Re: reactive power

01/28/2017 12:17 PM

Linkedin says Haris is Manager Training at Pizza Hut - Pakistan (MCR Pvt. Ltd.)

The photo on Linkedin looks just like Haris' photo on the lonely hearts site.

I'd really like to know where Haris is employed, if not at Pizza Hut.

I can't imagine his actually working with high energy electricity.

This comes to mindwhenever I think of Haris in any position of authority in any electrical facility.

@Haris, what is the name of your employer, if not Pizza Hut?

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#7
In reply to #6

Re: reactive power

01/28/2017 12:26 PM

I can cook a pizza with that, can't you?

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#8
In reply to #7

Re: reactive power

01/28/2017 12:36 PM

Now, that's making the best of a bad situation for sure.

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#15
In reply to #4

Re: reactive power

01/29/2017 4:41 AM

you cannot be as dumb as i thought. any one can keep "Coolyaar" Nick and he can be electrical engineer. But pinpointing some unrelated man to a "coolyaar" nick is fantastic. that is a mind blowing google "Research" you did. now i wonder why you always tell me to do a google search. but tell you what ! i never did. That "Haris Mahmood" can actualy sue you . since you are pinpointing him at things he may not even be aware of. be careful dude

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#25
In reply to #15

Re: reactive power

01/30/2017 4:09 AM

<...sue...>

In order for a suit to be successful, the plaintiff needs to prove on the balance of probabilities a financial loss as a result of the defendant's actions.

In CR4, one cannot identify a defendant, as it is an anonymous international forum.

Therefore the concept is a non-starter.

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#27
In reply to #15

Re: reactive power

01/30/2017 10:36 AM

That isn't you? Can you prove that?

It's very noble of you to protect the innocent, if indeed that is the case.

Can you provide verifiable proof of who you really are then?

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#18
In reply to #4

Re: reactive power

01/29/2017 5:25 AM

OT negated again, for the same reasons as before.

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#17
In reply to #2

Re: reactive power

01/29/2017 5:23 AM

Someone gave you an "OT", I have negated this.

Your comment was to my mind valid...the OP's question demonstrated much more than just poor understanding of reactive power to my mind.....or a homework question!!

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#20
In reply to #17

Re: reactive power

01/29/2017 7:51 AM

Fanfare much?

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#9

Re: Reactive Power

01/28/2017 12:53 PM

You're tripping over the word 'supply.'

I can supply you with a bill, in which case you owe me money, or I can supply you with a check, in which case I owe you money, but in both cases I am 'supplying' you with something irrespective of which direction the money actually flows. As a generator my role is as supplier and your role, a load, is as a consumer. 'Supply' as used in your 'famous' statement is speaking of the generator's role, which does not change even when the generator is absorbing power.

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Power-User

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#12
In reply to #9

Re: Reactive Power

01/29/2017 3:27 AM

thanks for understanding my point.if we see the reactive power curve,then VARS are flowing from generator towards load in one quarter cycle .i take that as supply of VARS by generator.in the next quarter cycle,VARS start flowing from load towards generator which i assume means that generator is absorbing reactive power. so generator even if overexcited does not supply VARS all the time.Most of the stupids do not even known what i am taking about.

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#21
In reply to #12

Re: Reactive Power

01/29/2017 8:52 AM

"Most of the stupids do not even known what i am taking about"

Judging from what I have read of these 'stupids' posts elsewhere, and what I have learnt of their considerable competence in this area, I would have to say that you are in absolutely no position whatsoever to be labelling people as 'stupids'.

It is one thing to be frustrated, quite another to be making blanket pejorative statements about forum members. If you cannot get along with people on this forum, you always have the option of leaving, no? Nobody is pointing a gun at your head, making you stay; but if you persist in calling names, you may find the Mods helping you pack your bags whether you wish to stay or not.

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#22
In reply to #12

Re: Reactive Power

01/29/2017 11:15 AM

Haris, if you have a bad day at Pizza Hut, don't blame people here because you do not understand elementary electricity. If you dropped a pizza on the floor and had to clean it up,it's NOT our fault!

If you really had any education you would know that insulting people will not induce them to help you.

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#16
In reply to #9

Re: Reactive Power

01/29/2017 4:46 AM

Ah! actually you are tripping over. in your example , you mentioned supply of two different things. in my case , we are talking about supply of one thing i.e. "Reactive power ". Now can you please elaborate this "one thing" with your classic example?

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#19
In reply to #16

Re: Reactive Power

01/29/2017 7:04 AM

Do you know what an analogy is?

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#10

Re: Reactive Power

01/28/2017 1:58 PM

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#11

Re: Reactive Power

01/29/2017 3:22 AM

You are confusing instantaneous and average values. In an ideal system, the net value of reactive power over time is zero. However, no real-world system is ideal. A device is said to supply reactive power to the system if, over time, it produces more reactive energy than it consumes.

For additional information, there is a very good explanation in this paper by Fetea and Petroianu at the University of Capetown.

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#24
In reply to #11

Re: Reactive Power

01/29/2017 11:40 AM

Thanks a lot dear.That is a really good paper. I am reading "Reactive power control in electric systems by T.J.E MILLER".Hope it clears the idea even more.

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#26
In reply to #24

Re: Reactive Power

01/30/2017 7:25 AM

<...reading...Reactive power control in electric systems...>

On the basis that the contents will satisfy the curiosity to the point where understanding is complete, does that mean that this thread has come to a natural end, please? <sigh>

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#13

Re: Reactive Power

01/29/2017 4:19 AM

Reactive power flows constantly back and forth between the source and the user. As an example, an induction motor needs to be magnetized and demagnetized constantly in order to work. That is done by the "current out of phase with the voltage."

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#23

Re: Reactive Power

01/29/2017 11:19 AM

Enrollment is going on at Kulas Polytechnic. Just saying, if you like.

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#28

Re: Reactive Power

01/30/2017 10:42 AM

An over-excited synchronous generator will change the power factor in the circuit. One of our local utilities used this to lift free electricity from Bonneville Power several years ago when BPA charged for watts. The local utility idled and overexcited the generators at a local dam to throw off the power factor so they were drawing high VAR and then correcting the phase angle downstream. It worked until BPA started charging for VAR instead of watts.

We also had a local foundry that used excitation on idling synchronous motors to correct their power factor to avoid penalty charges from the local utility.

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#29

Re: Reactive Power

02/02/2017 12:28 PM

Most of the Joules are recycled from a reactance load - but the losses in transformers, cables, load etc due to the higher current are not.

Power is lost in a resistance whichever way the current flows.

Also the cables and transformers and generators have to have a higher current rating and cost to carry the increased current due to reactive power. The typical 0.8 power factor means current is 25% higher than ideal with I2R losses 56% higher.

The generators have to supply extra total current whether theory and operating measurement divides it into active and reactive current/power or not.

Transformers and induction motors draw reactive current and it can only come from generators, except in those cases where compensation capacitors/devices are used.

This why it is desirable to have unity power factor.

To understand the difference between stored energy and lost energy in a system consider a wheel barrow moving soil up a hill, you have to move the weight of the wheelbarrow itself & counter its friction, even with no soil is in it. You may get energy back down hill but you have to push harder to get it uphill if the barrow itself is heavy and the barrow has to be stronger to take the push (which makes it heavier with more friction!).

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