Power Factor incident

[aelectricalman]

I sized my wiring for a 200kVar Capacitor and accidently used KVA instead of KVAR. In turn, I ended up with 350kcmil instead of 400kcmil. My power factor only rose from .75 to about .86, instead of .94. Is this the plausable reason I am not able to get my PF where I need it? Assuming my plans are to reach .94 PF and my Capacitor is correctly sized, is this the probable reason for the PF only increasing to .86 and not .94, because of the difference between 350kcmil and 400kcmil? Seems hard to believe. Im having no luck with the math trying to prove this. Any help would be greatly appreciated.

 

[mivey]

What data did you have for your original determination of the 200 kvar bank size?

Did you check for harmonics?

 

[charlie b]

I sized my wiring for a 200kVar Capacitor and accidently used KVA instead of KVAR. In turn, I ended up with 350kcmil instead of 400kcmil.

I am confused here. The KVA value is always larger than the KVAR value, so a design that is inappropriately based on KVA should have given you a wire that is too large, not too small.

In answer to your question, I don't think the wire size is making the difference. I think it far more likely that the capacitor is undersized. And that brings us back to Mivey's first question.

 

[weressl]

I sized my wiring for a 200kVar Capacitor and accidently used KVA instead of KVAR. In turn, I ended up with 350kcmil instead of 400kcmil. My power factor only rose from .75 to about .86, instead of .94. Is this the plausable reason I am not able to get my PF where I need it? Assuming my plans are to reach .94 PF and my Capacitor is correctly sized, is this the probable reason for the PF only increasing to .86 and not .94, because of the difference between 350kcmil and 400kcmil? Seems hard to believe. Im having no luck with the math trying to prove this. Any help would be greatly appreciated.

Wire size is based on current not kVA nor kVAr. The wire size difference, could not have been resulting in the lower power factor. High harmonic content can throw traditional or rule-of-thumb capacitor sizing to the wind. (Reputable vendors of automatic pf correction equipment often throw free sizing service in with no-uncertain guarantees.)

 

[aelectricalman]

We based our information on a 600V rated capacitor but only need 480V. My ordering agent, thought it was ok to order a 600V cap for a 480V system because he does it all day long for gear and what not. I wasnt clear with him that it mattered for PF correction. I think that is my problem because based on using 600V, my calculations show the PF should be in the range of .84 (currenty where it is). Had I had a 480V cap, we would have achieved the desired .94. I dont believe harmonics are that much of an issue but I still want to look into it deeper. Does anyone have any thoughts on this statement or see anything im overlooking?

 

[T.M.Haja Sahib]

I sized my wiring for a 200kVar Capacitor and accidently used KVA instead of KVAR. In turn, I ended up with 350kcmil instead of 400kcmil. My power factor only rose from .75 to about .86, instead of .94. Is this the plausable reason I am not able to get my PF where I need it? Assuming my plans are to reach .94 PF and my Capacitor is correctly sized, is this the probable reason for the PF only increasing to .86 and not .94, because of the difference between 350kcmil and 400kcmil? Seems hard to believe. Im having no luck with the math trying to prove this. Any help would be greatly appreciated.

The KVAR of capacitor is directly proportional to the voltage across it.So if it provides 200 KVAR at 6ooV,it can provide only 160KVAR at 480V.So you have to provide additional 40KVAR size of 600V capacitor to obtain your target power factor of 0.94.Also check for harmonics.It can destroy the capacitors.

 

[T.M.Haja Sahib]

The KVAR of capacitor is directly proportional to the voltage across it.So if it provides 200 KVAR at 6ooV,it can provide only 160KVAR at 480V.So you have to provide additional 40KVAR size of 600V capacitor to obtain your target power factor of 0.94.Also check for harmonics.It can destroy the capacitors.

Actually additional 40 KVAR of 480V capacitor or 50 KVAR of 600V capacitor is required because reduction in the capacitor voltage rating in the ratio 0.80 causes a reduction in its capacity in the same ratio 0.80.

 

[jcormack]

Actually related to the SQAURE of the voltage diff

Actually related to the SQAURE of the voltage diff

The actual kVAR is related to the square of the voltage measured / volatge rated. A 600 volt cap used at 480 volt would yield 64% of designed rated kVARs. A 600 volt, 200 kvar bank would effectively be a 128 kVAR bank on a 480 volt buss. Here is a site with examples: http://www.lntebg.com/esp/Service/D...ion_Troubleshooting_Capacitors_APFCSystem.pdf

 

[T.M.Haja Sahib]

The actual kVAR is related to the square of the voltage measured / volatge rated. A 600 volt cap used at 480 volt would yield 64% of designed rated kVARs. A 600 volt, 200 kvar bank would effectively be a 128 kVAR bank on a 480 volt buss. Here is a site with examples: http://www.lntebg.com/esp/Service/D...ion_Troubleshooting_Capacitors_APFCSystem.pdf

Thanks,jcormack,for correction.The KVAR of a capacitor is proportional to the square of the operating voltage.With that,the ratio becomes 0.64 and size of the 600V capacitor bank to give 200 KVAR on 480V is 200/0.64=312.5 KVAR

 

[T.M.Haja Sahib]

I dont believe harmonics are that much of an issue but I still want to look into it deeper. Does anyone have any thoughts on this statement or see anything im overlooking?

You may see to it that the resonant frequency of the capacitor bank is lower than any harmonic frequency in the power system.
As for the sizing of the capacitor bank,it may be sized as if the power supply is purely sinusoidal.You would not be penalized by power supply company for poor total power factor due to harmonics.

 

[mivey]

You would not be penalized by power supply company for poor total power factor due to harmonics.

You may or may not be. It depends on your impact to the system and nearby loads. Just because they normally ignore it does not mean they will always ignore it.

 

[T.M.Haja Sahib]

You may or may not be. It depends on your impact to the system and nearby loads. Just because they normally ignore it does not mean they will always ignore it.

I did not talk about the penalty for the injection of harmonics by a consumer into the power system.

 

[Besoeker]

You may or may not be. It depends on your impact to the system and nearby loads. Just because they normally ignore it does not mean they will always ignore it.

In UK, the basic rule is fairly simple in how it is stated. From memory it is something like:
"No one consumer my interfere with the supply to another"

The usual standard applied is the Electricity Association G5/4.
Harmonic distortion limits are not governed by statute.
The enforcement documents is the agreement between the customer and the network operating company (NOC).
The NOC can permit or refuse connection if the distortion is outside the limits of G5/4.
The wording suggests that it would have to be exceptional circumstances to permit such a connection.

I don't recall any situation where connection of any of our installations has been questioned, far less refused.
But then, we take account of the limits at the bid stage....:angel:
With the usual caveat of existing background levels, of course.

 

[mivey]

I don't recall any situation where connection of any of our installations has been questioned, far less refused.

With your attention to detail, I would be shocked if they were.

 

[Besoeker]

With your attention to detail, I would be shocked if they were.

Kind words and appreciated.

 

[robbietan]

In UK, the basic rule is fairly simple in how it is stated. From memory it is something like:
"No one consumer my interfere with the supply to another"

The usual standard applied is the Electricity Association G5/4.
Harmonic distortion limits are not governed by statute.
The enforcement documents is the agreement between the customer and the network operating company (NOC).
The NOC can permit or refuse connection if the distortion is outside the limits of G5/4.
The wording suggests that it would have to be exceptional circumstances to permit such a connection.

I don't recall any situation where connection of any of our installations has been questioned, far less refused.
But then, we take account of the limits at the bid stage....:angel:
With the usual caveat of existing background levels, of course.

However, in our side of the world, poor power factor (below 0.80) and high harmonics (5% VTHD / 5% ITDD) can get you in trouble with the power company. especially if there are other customers complaining about high harmonics from the power grid

 

[mivey]

However, in our side of the world, poor power factor (below 0.80) and high harmonics (5% VTHD / 5% ITDD) can get you in trouble with the power company. especially if there are other customers complaining about high harmonics from the power grid

I think he is saying it would get you in trouble there as well.

 

[Besoeker]

However, in our side of the world, poor power factor (below 0.80) and high harmonics (5% VTHD / 5% ITDD) can get you in trouble with the power company. especially if there are other customers complaining about high harmonics from the power grid

Mivey is right.
It can get you in trouble.

The NOC can permit or refuse connection if the distortion is outside the limits of G5/4.