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Power Factor Correction 1

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Cerkit

Electrical
Jan 18, 2016
100
Hi,

I have 50VA at 30o leading. I am trying to calculate the inductance required to bring it to unity power factor.

My calculation is Q = sin(30) * 50 = 25VAR

The voltage is 100V.

So L = Q/(V*V) * 1/(2 * PI * 50) = 7.96 * 10^-6Henry

Is this correct?

Thanks
 
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In the load convention a positive Q is an inductive load. To make Q 0 you will need to add capacitance. Or do you mean the current is 30 degrees leading?
 
Is it really worth it? At 50 VA?

Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.
 
Hi,

Yes sorry the current is leading by 30degrees.

Is the calculation correct. I am calaculating the vars that need compensating.Then working out the inductor required in series.

Thanks
 
Q = V * conj(I_L) = V<a * (V<a)/(-jX_L) = jV^2/X_L

L = V^2/(Q*2*PI*f)

Q = S*sin(-30)

 
Check your cals.

For 25 kVAR you need an impedance of 0.4j ohms.

Assuming a 50 Hz feed 0.4/(2 pi 50) does not equal 7.96 * 10^-6Henry.
 
Is it really worth it? At 50 VA?



Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.
 
Thanks everyone for your feedback.

50VA is an arbitrary value I picked, I am trying to make sure I am getting the calculation right first.

Thanks

 
I think that you should get some insights before you start speculating. There are courses and text-books and there are strict rules. Putting a series reactor is something you do to compensate for capacitive effects in transmission lines. Usually at MVA++ levels. Are you a practising engineer at all? I will probably RF this thread. All it does is confuse.
Can you produce a VALID reason for this question?

Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.
 
Just FYI, power factor correction is almost always done with shunt devices.

The vars put into the system with shunt devices will vary with voltage. Capacitor banks are put in shunt to supply vars for reactive loads to help support voltage and reduce loading and losses for vars needing to come from someplace else. Shunt reactors are used to limit over voltages on the system by consuming vars. Over voltage issues can develop on open end lines and low system loading where the capacitance between the phases is producing too much vars for the voltage to stay in the acceptable range. Vars raises voltage and not having enough vars lowers the voltage.


With series devices, you are basically cancelling series impedence or injecting vars based on current flow. Series capactors are used on long transmission lines where it is beneficial for voltage or stability reason to cancel out some of the transmission line's inductance. Power factor correction is not done with series compensation.

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If you can't explain it to a six year old, you don't understand it yourself.
 
Apologies for my lack of knowledge. I guess by offering what I know I am trying to figure out what I don't know so that I can better understand.

I am a practicing engineer but not in the area of power factor correction. But it is something I am interested in and would like to know more about how to perform these type of calculations. Although books are useful talking about it helps as well.

Thanks for all your time.
 
What area you in? It is not easy to imagine any electrical area where fundamental and basic facts like power factor, correction and how it works can be avoided.

Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.
 
Skogs,

There are fresh grads and people switching fields entering power engineering without a solid foundation. I worked with one protection engineer that only learned what a power triangle was during studying for the PE exam. He passed the PE on his second try and was good enough at putting together drawings to be more than capable as an engineer. My first year, I still remember a senior engineer rolling his eyes at me when I asked what a bushing was during my first year in engineering.

Cerkit,

Go grab the IEEE Red book, Power Analysis by Grainger, or something on power analysis from this list. ( I don't know your situation and maybe you do low voltage design or don't have a mentor. One power analysis class, maybe online like through Michigan Tech, University of Idaho or Arizona State, would get you pretty far along on understanding basic concepts. You can do some self study and try to bootstrap yourself, too. Skogs is busting your chops because some of what you are asking with pf correction and units you use in your example make it very obvious you haven't fully wrapped your head yet around some basic concepts. Go get some books and sign up maybe for a class.

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If you can't explain it to a six year old, you don't understand it yourself.
 
Cerkit - no need to apologize and your question was fair game. I think some of the comments you received are owed to the paradigm here in the US and others where electrical power engineering often does not receive much depth of treatment at the university level, and instead, digital systems, bit-pounding, etc receives the lion's share. There are tons of online tutorials and documents detailing power factor correction that should help answer your questions - simply google PF correction.

Good luck

Jim
 
Virtually all industrial power factor correction is done to correct lagging power factor.
Industrial users are typically charged a penalty for a lagging power factor.
There are two main approaches to penalties:
Old school, the penalty was based on a monthly average of PF based on monthly KWHrs and monthly KVARHrs.
With the arrival of electronic revenue meters, penalties may be based on realtime values rather than averages.
Old school, bulk power factor correction and load switched correction was used.
With penalties based on real time values, real time correction must be used.

When a customer came to us with an issue with power factor penalties, I would follow the following steps:

1. A visit to the customers accounting department. A polite request for copies of the power bills for at least the last year and for the last two years if possible.

2. When penalties are charged for poor power factor the following information will be found on the power bill;
Monthly KWrs and Monthly KVARHRS.
The KWHrs and KVARHrs for each month would be entered into a spread sheet.

3. PF penalties generally started at a PF of 0.9 A column would be added to the spread sheet to calculate the KVARHrs needed to correct the PF to 0.9

4. Evaluation; Look at the values for each month. If one month was extremely low, it may be excluded from future calculations.
If one month was extremely high inquiries would be made as to why it was high.
Another column may be added to the spread sheet; The KVARHrs to correct the worst month to 0.9 would be calculated and then a column would be generated to determine what effect that would have on the average PF of the other months.

5. A target for a corrected average PF would be selected. Possibly right on 0.9, possibly 0.92 or perhaps 0.95.
A spread sheet column would be generated with the data for any extremely low months or extremely high months excluded.
A calculation would be done to determine the penalty that would be charged for the extra KVARHrs in the worst month.
Was this worth spending money to correct or can we live with this once or twice a year?

6. Decision time: What will be our target corrected average PF? How many KVARHrs will that take?
How may we apply those KVARHrs?
a) Bulk correction. During times of light loading, capacitors tend to drive the voltage up. Too much correction during periods of light loads may result in the voltage going so high as to damage equipment and burn out lights.
A value based on experience will be chosen. A value that results in a capacitive current of twice the magnetizing current of all the transformers on the system is safe for unswitched capacitors.
Multiply the KVARs by the hours in a month and compare with the needed KVARHrs.

b) Look for large motors or a group of small motors that work together 24/7 and switch capacitors in with the motors. Refer back to item "a)" and beware of system over correction if you over correct the motors.
Multiply the KVARs by the hours per month that the motor(s) run and compare with the needed KVARHrs.

c) Look for large motors that run continually during working hours. Again use judgement. Many large motors are part of a process with a number of smaller motors that run when the large motor is running. Considering this, it is often feasible to use switch twice the KVARs that would be required to correct the large motor to unity.

d) Continue adding protection to progressively smaller motors.
At each step multiply the KVARS by the hours per month that the motor(s) run and compare with the needed KVARHrs.

Power factor correction when real time KVAR consumption is monitored and penalized.
Look at the figures for the worst month.
Divide the KVARHrs by the number of hours in a month to get a value in KVARs.
Buy that many KVARs of PF correction capacitors and a Power Factor Correction Controller.

I have only once ever had to do a calculation to determine the actual capacity in Farads or mfd.
That story also answers Gunnar's question as to why bother for 50 vars.
I had a customer with a small plant and a small PF penalty.
Only a few cents a month, but he wanted to know why, and how to get rid of it.
I forget the actual VARs involved, but a run capacitor intended for a fractional HP motor hidden in one of the panels and connected to one phase as enough to avoid the penalty and make the customer happy. The payback period was less than one year.
Memory fails but I will guess less than 1000 VARs.
The industrial power factor correction capacitors that I used were always rated in KVARs at a stated voltage and frequency.

Bill
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"Why not the best?"
Jimmy Carter
 
I once worked for an Electronic Engineer who specialized in satellite TV systems.
We were working on a vessel that was due to sale and I was too be left "On The Beach".
Once at sea I would have been redundant anyway, and as an EE he had no need for me. He was qualified to do the remaining circuit corrections.
The original job had mistakes in the schematics, mistakes in the shop wiring,mistakes in the field wiring and mistakes in the interconnection diagrams developed by the contractors engineering dept.
I had corrected all the diagrams and the field wiring.
There was a serious mistake in the MCC wiring and all the starter cells needed to be rewired. I had not finished but no problem, right!
The electronics guy could understand the diagrams and the reason for the changes. He just had to complete the wiring changes.
There was the problem. Not a clue about the parts and components of a magnetic motor starter.
Uh, that's the coil? NO. That's the sealing contacts.
Uh that's the main contacts? No. That's the overload relay. etc.
I don't know if he ever figured it out or not.


Bill
--------------------
"Why not the best?"
Jimmy Carter
 
The only time that I have cared about the actual capacitance of the shunt capacitor banks was in setting the relaying protecting the bank. A capacitor bank is made up of cans that are in series and parallel. You have cans that fail short and cans that are fused protected. As cans short, the capacitance of the bank actually goes up. The rest of the cans are strained more because they are under more current and a higher voltage. The rows of cans in series act like a voltage divider so the more rows you lose the larger the voltage is across each row of cans. You want your relaying to trip the bank when too many cans have failed and it starts putting too much strain voltage and currentwise on the remaining healthy cans that it might cause them to fail too and then you loose the whole string.


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If you can't explain it to a six year old, you don't understand it yourself.
 
Apologies, V<a * (V<a)/(-jX_L) should have been V<a * (V<-a)/(-jX_L)
 
HH Do you actually have to work in Farad units?


Bill
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"Why not the best?"
Jimmy Carter
 
I don't know if you have to but at least in the calculations I used it was done. I suppose it can be done with just the voltage and the capacitor bank mva but I just don't find that as intuitive personally as a bunch of capacitors in series and parallel.

I suppose the only other time I used actual capacitance was in turning a capacitor bank filter for a harmonics study but that is a lot different than pf correction.

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If you can't explain it to a six year old, you don't understand it yourself.
 
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