KVA/KW Calculations

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tkb

tkb

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I had to measure the KVA and KW of a three phase service with a Fluke 43B.
I have the readings of the KVA/KW for each phase.

I want to calculate the total PF and KVAR.

Do I just add the three KW and KVA readings together to get the total KVA and KW or do I also need to multiple or divide by 1.73?

Is my formula correct for calculating total KW or KVA?

Do I need to divide by the √3?

KW-KVA.jpg
 
120910-2026 EDT

If the three loads are identical, then I would suggest that you could say the PF is
(P1 + P2 + P3 ) / (VA1 + VA2 + VA3), but that is no different than P1/VA1. So what would be the point?

If the loads are not equal and have different PFs, then what would be a useful purpose of calculating a PF number from the above equation?

.
 
My goal it to size a power factor correction capacitor for a three phase service by using the 43B.
I was only able to record one phase at a time of a 480 volt Wye system.

The numbers in my example are not my actual readings. They are imbalanced and higher.
I was only able to measure from phase to ground.

I recorded kw, kva, pf and kvar on each phase to ground.
The Fluke 43B is a single phase meter.
I am trying to come up with a total for the system.
 
bob said:
kw = 150 The PF = kw/kva. The √3 does not enter the calculation.
Click to expand...

I agree, if as the OP says he is connecting L to N with the meter. Wether this is the right meter for the job given the unbalanced and changing load is another subject.
 
tkb said:
I had to measure the KVA and KW of a three phase service with a Fluke 43B.
I have the readings of the KVA/KW for each phase.

I want to calculate the total PF and KVAR.

Do I just add the three KW and KVA readings together to get the total KVA and KW or do I also need to multiple or divide by 1.73?

Is my formula correct for calculating total KW or KVA?
Click to expand...
You can just sum the kvar to get the total. Keep in mind that capacitors correct displacement portion of the non-unity power factor and not the portion due to harmonics. Also, if the kvar load between phases is significantly different, you may want to reduce the bank size.
 
mivey said:
You can just sum the kvar to get the total.
Click to expand...
Yes. Power factor in a three phase power supply system= Three phase active power (balanced or unbalanced)/Three phase apparent power(balanced or unbalanced).
But how to switch on shunt power capacitors to bring the existing power factor to that value as computed above is quite another matter IMO.
 
tkb said:
I had to measure the KVA and KW of a three phase service with a Fluke 43B.
I have the readings of the KVA/KW for each phase.

I want to calculate the total PF and KVAR.
Click to expand...

If you have kW and kVA
Then, as Bob noted in Post #4

PF = kW/kVA

You can calculate kVAr as follows

kVA2 = kW2 + kVAr2...........Pythagoras if you wanted to know.

Thus:

kVAr = √(kVA2 - kW2)
 
T.M.Haja Sahib said:
Yes. Power factor in a three phase power supply system= Three phase active power (balanced or unbalanced)/Three phase apparent power(balanced or unbalanced).
Click to expand...
Your calculation method would yield a single number for the power factor which most likely would be a bit meaningless for the unbalanced case.

If the loads are unbalanced the most likely scenario is a three-phase four wire system and the loading on each of the three phases is unlikely to be at the same power factor. Thus you would get the values for PF. Your calculation method would yield just one


T.M.Haja Sahib said:
But how to switch on shunt power capacitors to bring the existing power factor to that value as computed above is quite another matter IMO.
Click to expand...
A bit of fairly basic trig would get you that.
 
Besoeker said:
If you have kW and kVA
Then, as Bob noted in Post #4

PF = kW/kVA

You can calculate kVAr as follows

kVA2 = kW2 + kVAr2...........Pythagoras if you wanted to know.

Thus:

kVAr = √(kVA2 - kW2)
Click to expand...
The OP noted the loads are unbalanced and kW, KVA are for single phase loads in his case. So the formula PF = kW/kVA does not hold here.
 
Verify that it is KW and/or KVA that was measured.

If simple clamp meter was used it was likely Amps that was measured and this would need multiplied by line to ground voltage to get VA, of each phase or by line to line voltage and by 1.732 to get the amount of balanced VA across all three phases and then add the remaining unbalance to get the total power.

Remember KW and KVA are both power units, volts and amps are components needed to be able to get power.

If it is indeed KW or KVA that is measured then just add all three to get the sum.
 
tkb said:
What I have is a plastics plant in MA that has been penalized by the POCO for a PF of .50.
I went to take some readings and couldn't see a PF this low.

I did find a problem with their PFCC that is installed on the service.

Here are my calculations.

Please let me know where I am going wrong.

http://dl.dropbox.com/u/33217279/Plastics kvar Calculation.xlsx
Click to expand...

I've got no idea where the .5 power factor is coming from with the data you provided.

But in your power factor corrected calculations, shouldn't your kW remain the same and your kVA change based on your lower kVAR? 151kW + 69.76kVAR = 166.34kVA? Corrected power factor = 0.91?
 
tkb said:
What I have is a plastics plant in MA that has been penalized by the POCO for a PF of .50.
I went to take some readings and couldn't see a PF this low.

I did find a problem with their PFCC that is installed on the service.

Here are my calculations.

Please let me know where I am going wrong.

http://dl.dropbox.com/u/33217279/Plastics kvar Calculation.xlsx
Click to expand...

A couple or three points in no particular order.

Your spreadsheet gives just numbers, not the basis for their calculation.
Line 13 gives a kVAr greater than the kVA. That obviously can't be right.
Lines 17 and 20, with and without PFC, show same current. If it's the load current downstream of the PFC that's what you would expect. But not upstream of the PFC.
The same two lines show an increase in power from no PFC to including PFC. An additional 11kW. That, if real, would defeat the object and make its presence known fairly quickly. You can't hide 11kW.
 
Besoeker said:
A couple or three points in no particular order.

Your spreadsheet gives just numbers, not the basis for their calculation.
Line 13 gives a kVAr greater than the kVA. That obviously can't be right.
Lines 17 and 20, with and without PFC, show same current. If it's the load current downstream of the PFC that's what you would expect. But not upstream of the PFC.
The same two lines show an increase in power from no PFC to including PFC. An additional 11kW. That, if real, would defeat the object and make its presence known fairly quickly. You can't hide 11kW.
Click to expand...

I corrected line 20 of my sheet. I saw my problem.
I don't see the problem you describe in line 13 though.

You can see my calulations by clicking on the cells.

Here is my corrected sheet.
http://dl.dropbox.com/u/33217279/Plastics kvar Calculation 2.xlsx

Thanks for your help. :thumbsup:
 
Gentlemen, feel free to continue your bickering via PMs, but it is stopping here.

Please stay directly on the OPs questions and do not go off on your own tangents.

If you have your own subjects you would like to discuss you are as always free to start your own threads to do so.
 
iwire said:
Gentlemen, feel free to continue your bickering via PMs, but it is stopping here.

Please stay directly on the OPs questions and do not go off on your own tangents.

If you have your own subjects you would like to discuss you are as always free to start your own threads to do so.
Click to expand...

In what way this question was bickering?:

"OK, what size capacitors will he need to furnish and will they be single phase or three phase units?"

An issue in the OP's question if he needs to correct the power factor in the individual phases, or is it sufficient - even in the case of unbalanced loads - to correct all three phases with the same 3-phase capacitors and over/undercompensate in some phases, but the sum of the PF is still in the POCO's acceptable range.
 
weressl said:
In what way this question was bickering?:

"OK, what size capacitors will he need to furnish and will they be single phase or three phase units?"

An issue in the OP's question if he needs to correct the power factor in the individual phases, or is it sufficient - even in the case of unbalanced loads - to correct all three phases with the same 3-phase capacitors and over/undercompensate in some phases, but the sum of the PF is still in the POCO's acceptable range.
Click to expand...

Unfortunately you had a post that combined an insult with good info, I considered editing it but said the heck with it and cleaned house.
 
This is the capacitor bank that is currently installed.

It is an ABB C485G22.5-3FI capacitor bank connected to the service.
I didn't install it, but I think a capacitor may have failed in it.
I was trying to determine if it is sized correctly or needs to be upsized.
I am sure it's not the correct style unit for the service.

I had the following readings,

Phase A, 19.5a
Phase B, 12.5a
Phase C, 19.5a

A to B, 66.4?F
A to C, 100?F
B to C, 65.8?F

IMG_0526.jpg
 
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