How does reactive power compensation work?

Capacitors are installed to compensate for the reactive power requirements of a plant. It is said that the capacitors supply the reactive power to the plant, thereby eliminating the drawing of reactive power from the service provider.

My question is WHERE does this power come from? Won't the capacitor draw reactive power from the line to have itself charged every time? Won't this amount to a lower power factor across the service line?

 

Ram,

You'll probably have more luck using your preferred Internet search engine with the term:

"power factor correction"

The double quotes should help narrow down the search.

As one responder here on control.com is fond of saying: There really is no such thing as reactive "power." And, while, technically, that's true, it's not how most of the world thinks of the phenomenon.

Another poster here is also often saying that reactive "power" is just "power" that is lost on one-half cycle of the sine wave and gained back in the other half-cycle (I think that's a fair summary--at least I hope it is). So, the net effect is really zero--except....

I think it's better to consider what the effects of a low power factor are (regardless if it's lagging or leading): A shift in the angle between the voltage- and current sine waves of an AC system. And, as the shift (angle) becomes larger, the ability of the system to transmit real power (Watts) decreases so the efficiency is reduced. For single plants with high inductive loads (lots of large, inductive motors, usually) the utility wants to get compensated for the VArs they are supplying to the plant (because it reduces the Watts they can sell to anyone). So, they usually install a VAr-hour meter and start charging also for VArs, as well as Watts--to which the plant usually responds by adding "passive" power factor correction capacitors to reduce the inductive VArs "consumed" by the plant, and in so doing reduces the amount of VAr-hours on that VAr-hour meter, thereby reducing the payments to the utility.

So, since in most parts--but not all--of the world (and in many process plants) the majority of the load on AC power systems is inductive, power factor correction capacitors are used to reduce the inductive effects (with the capacitive effect)--which tends to bring the voltage- and current sine waves back closer into phase with each other which also means more efficient transmission of real power (Watts).

Again, I think the search term that will get you the best result is "power factor correction" and "power factor correction capacitor".

Hope this helps!

 

Hello There,

We are using two 30,000 kVA, 0.8PF, 2 pole, 11KV, 50Hz alternator for captive power generation for a large fertilizer complex (captive power) in a gas based power plant. Both GTs' run in parallel. One is in Iso and one is in droop.

Till recently we were using a capacitor bank to maintain power factor (PF) of generation. Now,a senior officer has objected to it and have removed the capacitor bank. earlier our power factor was maintained around 0.86-0.88 now it has come down to 0.82-0.83. the reason which he gave, is that capacitor bank is maintaining the line load rather than generated load and this may result in generator trip owing to some line fault!

As far as i understand, alternator only reacts to the external load demand and as most of the load in complex is inductive henceforth obviously there has to be a lag in voltage and current.

Now my question is, is it really justifiable to remove capacitor bank entirely?

We were or are wrong?

Regards
fluidflow

 

fluidflow,

I would suggest the same thing: Use your preferred Internet search engine to research "reactive power correction" and develop your own understanding of the subject.

In the power generation world, lagging VArs (the result of an inductive load) must be supplied to keep the voltage- and current sine waves of the AC power system closer to in phase with each other. Supplying VArs consumes energy--energy to increase the DC power applied to the generator rotor. Sometimes the DC comes from a rectifier bridge controlled by the exciter regulator (sometimes called the AVR). Sometimes the DC power comes from a shaft-mounted rectifier bridge which is excited by a small AC-powered exciter regulator. In either case, the power that's applied to the generator rotor has to come from somewhere (the generator terminals through a rectifier bridge; station AC AND torque from the generator rotor which is driven by the turbine)--it's not free. And if it comes from the unit(s) themselves that reduces the watts that can be sold or used to power other devices.

Power factor correction capacitors can help reduce the amount of excitation that's supplied to the generator rotor (the capacitive effect counters the inductive effect, in effect). So, that means more power in the form of watts is available for use in the plant if needed. It also means that the units are more efficient in converting hydrocarbon-based energy into electrical energy since less is used ("lost") to produce VArs.

Were you wrong, or are you wrong--depends on point of view and how many VArs were being countered by the power factor correction capacitor bank. You should be able to look at the plant's heat rate before and after, for comparable loads and machine conditions (ambient; compressor cleanliness; filter condition; etc.) and make a pretty simple determination if the power factor correction capacitors were effective at increasing the plant's efficiency.

Right or wrong? Supervisors and managers get paid for their experience and risk-taking capability. It would seem that the new management has had a bad experience with power factor correction capacitors and seemingly deems the risk too great for their appetite. If only management and supervisors would communicate better it would make understanding their motives and decisions easier, and in the process everyone usually learns quite a lot. But, some people aren't very good at communicating, and some people just have their opinions--which may be based on actionable data and hard-earned experience, or which may just be because they themselves don't understand the equipment they are now in charge of operating. It can be very difficult to tell which sometimes, especially when just getting to know individuals.

Remember: They get paid to manage risk, and in this case in their estimation the "risk" of keeping the power factor correction capacitors in service is greater than they wish to indulge in. It may take a long time for you to understand why they made this decision, and, in fact, you may never. It may even be that after the new management looks at plant heat rate before and after their decision, there will be a new decision--to put the power factor correction capacitors back in service. (And that decision may come from the manager's manager!)

It's not really a right or wrong decision, in my opinion. But that's just my opinion--and as a colleague of mine used to say, "Everyone is entitled to my opinion."

 

CSA...

I never said there is no reactive power! My position from day one is that the term "reactive-power" is a misnomer... even and oxymoron!!!

Phil C.

 

Hello there,

CSA, I discussed the situation at length with senior Electrical Engineers and other colleagues working in other plants as well. Perhaps it is a grey area where some use capacitor banks while some not. Nevertheless, a point which came out loud and clear on behalf of new Manager was that since the Generator is designed to perform at 0.8 PF and most of the time Plant is run in Island mode hence removing the capacitor bank is a pragmatic decision, as after removal of capacitor bank PF readily went down from 0.86-0.87 to 0.83-0.84, but still well above 0.8. I went through the capability curves and found the Vars well within permissible limit and V curves also fall under agreement. But, I have noticed that Generator stator winding RTD temperatures have only marginally increased ( 2-3°C only) but the spread between RTD temperature has increased to 10°C which was earlier well under permissible 5°C. The RTDs’ concerned are on R phase and B phase respectively. The same unit went under a major inspection and stator winding underwent Polarisation Index (PI test) and tan-Δ test as well where any partial discharge or water tree was ruled out. RTD are conventional Pt based.

Recently we have taken few motors in line instead of steam turbines which has resulted in an increase of 3-4 MW power demand. The generator in situation runs in droop mode and runs near base load now, though still under igv exhaust temp. control ( runs in parallel with identical unit under Isochronous mode).

Now I want to ask why such a spread in RTD is seen and is there any parameter left which we should consider?

I hope other MVPs like CSA , Phil Corso, Process value help us and throw some light on this problem.

Eagerly waiting for your reply.

Regards,
fluidflow

 

fluidflow,

Hmmm. The old Generator Nameplate game. Just because a generator is <b>capable</b> of operating at nameplate rating does NOT mean it is intended to be operated at nameplate rating, or that it has to be operated at nameplate rating. In fact, operating it at a power factor closer to unity (1.0) is better for the generator, as the excitation power (which produces heat as it flows through the generator rotor) is less when operated closer to unity power factor.

In general, generator stator temperature differences can be attributed to different RTD insertion depths and generator cleanliness. Not every RTD is exactly like every other RTD in the stator--there is an allowable tolerance range. And, not every RTD is inserted in exactly the same way, with exactly the same amount of expoxy or whatever sealing/mounting adhesive is used to secure the RTD in the windings. And dirt can collect at different rates in the generator, based primarily on air flows and incoming and outgoing air flows, humidity, type of airborne contaminant(s), etc.

So, just because a generator nameplate says it's rated for 08. pf does not mean it has to be operated continually at 0.8 pf. That just means that the generator can be operated continually at 0.8 pf without undue harm being caused by the heat produced in the windings due to current flows (stator and rotor currents) to the insulation of the generator.

If we wanted to carry the Nameplate Game to it's ultimate, are you operating the generator at the rated KW output at all times? I sincerely doubt it (and this is an appropriate use of the word 'doubt' in this context).

Generator power factor is a measure of the efficiency of the generator at converting the energy being input to the generator into watts--a power factor of 1.0 means approximately 100% efficient. A look at the power factor triangle confirms this. A power factor of 0.9 means the generator is approximately 90% efficient at converting the power being input to the generator to watts; a power factor of 0.8 means approximatley 80% efficiency.

If you can increase your efficiency through the use of power factor correction capacitors, why wouldn't you? Just because the Generator Nameplate says the generator <b>can be operated</b> at a lower power factor/efficiency doesn't mean its intended to be operated at that power factor/efficiency, or that it operates best at that power factor/efficiency, or that is has to be operated at that power factor efficiency.

The nameplate just says that it can be safely operated continually at that power factor/efficiency without causing excessive damage to the generator windings (rotor and stator).

Use your preferred Internet search engine and do some research on 'power factor' and what it means. But, on the generator nameplate it does not mean that the generator is intended to be operated at that power factor--just that it can continuously be operated at that power factor without harming the generator windings. That, in effect, the generator cooling system can safely remove the heat would would be produced at the currents (stator and rotor) if the generator were operated at that power factor <b> and KW (KVA).</b> I don't know if it's still up, but here's a useful site:

http://www.the-power-factor-site.com

Nothing more. Nothing less. It's a maximum continuous rating which is possible--not a must-operate-at design value. If it were why aren't you generating rated KW/KVA all the time at the rated power factor?

Most of the generators I have ever worked on were rated for significantly more KW than the turbine driving it was rated for (I'm speaking of gas turbines now). That's because under certain ambient and machine conditions the turbine can produce more than rated power, but under most circumstances and in most parts of the world most gas turbines do not operate at or near those ambient temperatures and with near new inlet filters and near new hot gas path parts with near new clearances with nearly perfect compressor cleanliness. No, they are operated at much higher ambients which means the output is lower than rated, so the generator are never operating at or near their rated KW output regardless of power factor.

"Producing" VArs (or operating at higher lagging power factors) requires energy--to overexcite the generator rotor. That energy also generates more heat in the generator rotor as it passes through the windings--heat which must be removed in order to protect the insulation of the rotor. And, most sites don't get paid for "producing" VArs--they only get paid for producing watts (KW). So, they operate at close to unity power factor which is as close to 100% efficient as they can get--which maximizes revenue as the watt-hour meter makes more turns/hour. To follow the Generator Nameplate Game, they would increase excitation to lower the power factor and reduce KW--or have to increase fuel to maintain KW as VArs were increased. And that costs money.

In a "power island" (captive power plant) the generator excitation has to be increased to keep the plant voltage at or near rated as the reactive load increase (in a lagging direction, that is--from the generator perspective). If power factor correction capacitors can reduce the need to divert fuel from excitation power, reducing the fuel costs, why wouldn't you?

But, you have learned a valuable life lesson here. People are different. Their points of reference and their experience, which forms their judgement, are different. And, again, some people take everything they read and see without questioning it (not using their critical thinking skills). Probably all you're going to do is to get yourself labeled as a malcontent by your New Manager if you persist in this. But, you have your own life experiences, and when the time comes for you to use your judgement to make decisions you will have a different point of reference, and by using your critical thinking skills.

Unfortunately, I fear some of the other respected posters here on control.com are not going to weigh in on this since Mr. Corso has. I have agreed to disagree with Mr. Corso. A search of control.com with the search term:

+"reactive power" +"Phil Corso"

will turn up several threads and responses where argues vociferously, even offering to send people papers to dispute accepted norms. Here's one:

http://www.control.com/thread/1026238188#1026238262

Read up and down from this post and you'll see why no one will likely get involved. And this is just one thread; there are several more with similar sentiments, free for the searching and reading. It's been going on for years. And seems to show no signs of abating any time soon, though we had a brief respite recently. I take full responsibility for kicking the sleeping dragon; sorry, everyone.

Do your own research, fluidflow, just as you have been. It is a grey area, this power factor correction capacitor operational choice. But, I can tell you they're not cheap--and they were purchased or installed to achieve a goal, likely one of reducing the need for overexcitation to counter the effects of a lot of induction motors on the system dragging the system voltage down. Think about that. And continue to develop and use your critical thinking skills.

 

Reactive power compensation is a process of changing the voltage or frequency of electrical current in order to maintain a desired level of power in a system. When there is an overage or shortage of electricity, reactive power is used to adjust the voltage and frequency in order to keep the grid stable. By compensating for reactive power, we can ensure that our electrical systems are always running at their maximum possible capacity.

 

Malyah...
Welcome to Control Automation.
Well you certainly got my very old blood moving again!
You are correct when you use the term "compensating"! But your error is the term "reactive power"! It should have been been "reactive component"!
My dissertations concern the use of the term "power' used in conjunction with "reactive" or "capacitive", is what I abhor! I thought my explanations were so simple that I never understood CSA's rage. (BTW, I always considered him a genius in his field)!
Since I don't know what you do, please forgive me for my teacher-like approach:

1) Ampere. Water flowing in a pipe is measured in Gallons per second, that pass a certain cross section in the pipe. Similarly, current flow in an electrical-conductor is measured as coulombs, or electrons that pass a certain cross section in the conductor (yes, we know today they hardly move at all). But, because the number of coulombs is so large, the term Amperes per second was selected by the scientists of that era. Alternating-Current alternates across a point in the conductor, at a fixed frequency.

2) Potential Difference. Whenever current flows an interchange of energy always takes place. Some of the moving particles representing current have an energy increase, others a decrease. This is expressed as as a voltage-drop or gain. Then, if 2 points, a to b exist, the Potential Difference Vab=W/Q, where Vab=potential difference in Volts, W=energy in Joules lost or gained by Q during transfer, and Q=coulombs transferred from point a to b.

3) Typical Power Triangle. The Horizontal-Line, or Abscissa, always represents Power "magnitude" or Watts! The Vertical-Line, whether positive or negative, always represents Reactive "magnitude" or Ohms! The Hypotenuse-Line, whether above or below the Horizontal-Line, always represents Volt-Ampere "magnitude" or VA! So then, if the Horizontal-Line is dimensioned as "power", the Vertical-Line should never be dimensioned as "Reactive power"!

It's an alphabetical problem, not an engineering one! ! !

Q.E.D.
Happy Holidays to all!
Phil Corso (Resume available, cepsicon@aolcom)

 

thanks for the correction
PhilCorso

 

Reactive power compensation is a process of changing the voltage or frequency of electrical current in order to maintain a desired level of power in a system. When there is an overage or shortage of electricity, reactive power is used to adjust the voltage and frequency in order to keep the grid stable. By compensating for reactive power, we can ensure that our electrical systems are always running at their maximum possible capacity.

Okay, I gotta ask. I just gotta.

The first sentence of malyah's post talks about "...changing the voltage or frequency of electrical current in order to maintain a desired level of power in a system...." The second sentence says, "...reactive power is used to adjust the voltage and frequency in order to keep the grid stable...." I call BS on changing/adjusting the frequency of an AC power system (of any size) in order to maintain a desired power level OR stabilize the grid. Big, fat, fresh, stinky BS.

Any, my BS detector is pegged here folks. I can always learn something new, but this idea of adjusting frequency using reactive "power" compensation does not fit with my education or experience. Who can clear this up for me?

 

@ WTF...
Glad you asked!
Since I didn't know anything about Malyah's electrical knowledge or experience I didn't respond to his frequency-related question!
Nor would I use the disparaging term you did, when the purpose of this forum is to be-educating, not be-littling!
I stuck to the original topic of this forum "Power Factor Compensation" (BTW, started in 2014) because I felt it was time to finally end usage of the incorrect term, Reactive-Power!
So, do you accept the term is a misnomer... even an oxymoron?
Secondly, for a better perspective of my contribution to CONTROL Automation use the Magnifying Glass!
Happy Holidays to all!
Regards, Phil

 

Phil Corso,

I don't know, dude. I searched the World Wide Web for "power triangle explained" and found hundreds of references and YouTube videos (so it MUST be true if there are YouTube videos about it!) ALL saying that the three sides of a power triangle are real power (Watts), apparent power (VA), and reactive power (VAr; many relevant searches referred to it with the mathematical representation Q).

Here's one site's explanation:

"Reactive Power Q = I2X = V*I*sin(Φ) volt-amperes reactive" [where I2 means I squared (I*I)]

Here's a written explanation which matches pretty much what I was taught in physics and the electrical/electronics classes I attended in college and what I have experienced in my life and professional career:

"Reactive power (Q), (sometimes called wattless power) is the power consumed in an AC circuit that does not perform any useful work but has a big effect on the phase shift between the voltage and current waveforms."

[The emphasis in the above quotes is mine.]

If your definition of "power" means/includes real, useful work, then I guess I might agree with you.

But, that's about as far as I can go. You seem to be about the only person arguing against the concept of reactive power. I'm tempted to mention Copernicus, who was only recently "pardoned" by the Holy See.... Perhaps you are just ahead of your time?

As far as I'm presently concerned, the power triangle consists of three sides, real-, reactive- and apparent power--hence, a power triangle. I get that no useful, real work is done by the vertical component of the triangle, but when using a synchronous generator to produce VAr to overcome (at least partially) the phase difference between the voltage and current sine waves (phase difference--NOT frequency difference) that consumes real power which could otherwise be watts supplied by the generator. So, to an extent, reactive power production decreases real power production at the synchronous generator. TANSTAAFL as one of my instructors was fond of saying (There Ain't No Such Thing As A Free Lunch). And by keeping the voltage and current sine waves closer to an in-phase relationship (because they are ALWAYS at the same frequency but not always in phase with each other) the AC power transmission and distribution system can operate more efficiently. But, it comes at the cost of real power, to produce VAr, kVAr, MVAr.

In my experience, if I hold the fuel flow-rate to a combustion turbine constant with the synchronous generator power output at unity power factor (1.0--meaning no VArs, kVArs, MVArs--the lengths of the VA and W sides of the power triangle are equal) and then start increasing the excitation power being applied to the generator rotor the amount of Watts (kW, MW) will start decreasing as the power factor decreases--meaning the W, kW, MW of the generator output (the length of the real power side of the power triangle (the horizontal side of the power triangle)) is decreasing--and the VAr, kVAr, MVAr is increasing (the vertical side of the power triangle is lengthening). Essentially, the VA, kVA, MVA is staying the same (as long a fuel flow-rate remains constant), but VAr, kVAr, MVAr is increasing and W, kW, MW are decreasing. In other words, the length of the VA, kVA, MVA side of the power triangle is remaining the same BUT the length of the W, kW, MW side of the power triangle is decreasing and the length of the VAr, kVAr, MVAr side of the triangle is increasing. Generally, power producers don't get paid for VA, kVA, MVA--only for W, kW, MW, so it's not in their interest to produce VAr, kVAr, MVAr and it's in their interest to keep the power factor as close to unity as possible (unless there are contractual or other requirements not included in this discussion). That's my Q.E.D.

But, at the risk of starting another petty action with you, that's as far as I'm willing to go: the reactive side of the power triangle does no real, useful work.

I'm not one of your students, just an interested bystander--wanting to know how reactive power affects frequency, or vice versa. Can you just answer that question without any admission or concession on my part? Please.

And thank you.

 

There is also the V x I component of VAr, kVAr, MVAr. Real power is also V x I, and apparent power also includes V x I.

 

@WTF....
You still don't get it! The product of VI can only be dimensioned as "Power" if I is at 0 degrees with respect to V!
Regarding Malyah's statement, frequency of electrical current in order to maintain a desired level of power... you are right... frequency has zero, 0, nada... effect on power level!
Regards, Phil
PS: there is a power situation when electrical machines having different speed-ratings produce power at the same frequency! But that's a different topic!

 

PhilCorso,

Easy, there. Easy. (Your rage is showing. You have been on this mission discrediting the idea of reactive power for a long, long time.)

Thank you for clarifying the effect on frequency of adjusting reactive power--zilch, zippo, nada.

It's possible to put hundreds of screen captures/snippets of search results which ALL mention real power, reactive power, and apparent power when talking about the three sides of power triangle. Hundreds. But, they're all copy-righted, or at least most of them.

So, you want to call the reactive side of the triangle reactance and dimension it in ohms. That's your prerogative. It is a resistance to the flow of real power--which means real power is diminished. It can be argued this resistance diminishes the flow of real power

I'll stick with the terms of texts and references. It's an analogy, the power triangle, and many analogies are less than 100% accurate. Discrediting it or proving it's incorrect is your battle, and it doesn't involve me.

Thanks again for the clarification about the (un)relationship between reactive ... compensation.

 

WTF?...
Since we both have categorized the other, I would like to Make a Deal!

I will send you my Resume!

You will provide me with an SLD illustrating the Major Electrical Components in a plant encompassing the Hydrocarbon, Chemical, or Power Industries, that you:
o may have worked in;
o may have an interest in;
o know someone planning to construct one;
o know someone planning to upgrade one !

The plant should have three (3) voltage levels:
o HV for major power distribution, say 15kV to 30kV;
o MV for medium-voltage motors or other loads, say 3kV to 6kV;
o LV for low-voltage motors or other loads, say 200V to 600V !

For the major electrical components, you provide:
o Size, Length, Cost, of HV, and MV interconnecting cables;
o Size, Pri'y V, Sec'y V, Cost, of Interconnecting Transformers !

With the information provided above, I will provide a revised SLD that will reveal a cost savings of at least 10% ! Without sacrificing reliability or operability !

Finally, I am willing to negotiate payment for my service!
Sincerely, Phil Corso

 

This may not be most appropriate place to ask,

but is anyone aware of how the addition of fault current limiters upstream of the step-up 34.5/138kVA utility transformers leaving a generating plant might effect power factor? Is it negligible?

 

Well power factor really means about the quality of the utility i means load consumers behaviour so it clearly depends on the nature of the load (inductive or capacitive)
The Step up transformer deal with things like online/offline tap changer for Voltage regulation combined to the Generator voltage

So what i means is that xwit current protections upstream this transfomer there should not be such incidence on PF quality

It is about selectivity /sensitivity of those protections that are important..

PF has to be regulated as best as it can hence to THE Generator GCP (Generator control panel with AVR)

 

Jeff.3300
I suggest you resubmit your question as a new topic !
Regards Phil Corso