# Capacitor Banks - calculating series/parallel kVAR



## cos90 (Aug 1, 2017)

This is for CI 1-57, 2-17, 3-72, 4-70.

I know how to work this problem, it is simple division, but I was wondering why. Does anyone have a good reference for it?

I could eventually figure it out, but I'd rather keep reviewing practice tests looking for misconceptions I have, and then study Wildi better.


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## rg1 (Aug 1, 2017)

IMHO,  these all are questions of network/circuit theory of EE. Many out of you mentioned, are wrong presentations of questions or answers. If need be we can discuss one by one. Mostly they all have one mistake. That is - Any equipment is rated at some Voltage and some current or power. The questions are not fixing the ratings of the capacitors properly, so once we get the required KVAR 3phase or one phase we need info on the ratings of the capacitors to decide on how we connect them; this is actually creating confusion. They give only KVAR rating of a capacitor which is not enough. But funnily there is only one answer adding to the total KVAR so we are getting the right answer.


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## TNPE (Aug 2, 2017)

I don't have that material, but a description or simply posting an img would be good.  From there, I can try to help you in understanding ratings and analyses methods.  

I have always worked with KVAR, kW and KVA from the power triangle, unless other methods are necessary to solve the problem.  Generally speaking, if it is a PF correction problem, it is simply a case of holding kW constant and analyzing from there.  If it's a delta or wye situation, remember what happens with impedance and phase/line current.... power remains the same!!


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## rg1 (Aug 2, 2017)

TNPE said:


> I don't have that material, but a description or simply posting an img would be good.  From there, I can try to help you in understanding ratings and analyses methods.
> 
> I have always worked with KVAR, kW and KVA from the power triangle, unless other methods are necessary to solve the problem.  Generally speaking, if it is a PF correction problem, it is simply a case of holding kW constant and analyzing from there.  If it's a delta or wye situation, remember what happens with impedance and phase/line current.... power remains the same!!


I suppose the problem is not in calculations of required KVARs . After we get the reactive KVA needed to compensate for a particular pf improvement, the question asks no of capacitors, given KVA rating of one capacitor. So far so good, but sometimes it gives you options of a figure in which it cgives different combination of connections without even giving Voltage of the Capacitor.  So how do you get, how many to connect in series to satisfy Voltage rating. Like if each Capacitor is rated at 2kV, 10KVAR; to connect them to a Voltage of say 12 kV you need a series connection of minimum 6. If you connect less, the capacitors may fail due to overvoltage, if you connect more, they will not give you rated KVAR. So for such questions we need Voltage and KVAR of the unit.


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## TNPE (Aug 2, 2017)

Please post the problem @cos90


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## justin-hawaii (Aug 3, 2017)

The problems listed by OP are power factor correction problems.  

cos90, I think an application guide of how capacitors are used to correct power factor will be a great reference for you.  This reference will help to complement the more school type references, like Wildi.  I like to look for manufacturer websites that provide and design electrical equipment, in order to get a better idea on application of the equipment.   Manufacturers are probably the best experts on the equipment.

ABB Power Factor Correction Guide:  http://www04.abb.com/global/seitp/seitp202.nsf/0/18aa8879b8cc0186c125761f005035b7/$file/Vol.8.pdf

Eaton Power Factor Correction Guide:  http://www.eaton.com/ecm/groups/public/@pub/@electrical/documents/content/sa02607001e.pdf

Aerovex Guide to Power Factor Correction:  http://www.aerovox.com/Portals/0/PDFs/Technical Notes/AN101312 PFC Guide.pdf

Aerovex Capacitor Webpage:  http://aerovox.thomasnet.com/product/power-factor-correction-capacitors/o-1040-vac-4800-vac-medium-voltage-capacitor-cells


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## knight1fox3 (Aug 3, 2017)

justin-hawaii said:


> The problems listed by OP are power factor correction problems.
> 
> cos90, I think an application guide of how capacitors are used to correct power factor will be a great reference for you.  This reference will help to complement the more school type references, like Wildi.  I like to look for manufacturer websites that provide and design electrical equipment, in order to get a better idea on application of the equipment.   Manufacturers are probably the best experts on the equipment.
> 
> ...


And what would a mechanical engineer know about power factor correction??? 

LOL....just kidding. Good info, thanks for sharing.


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## rg1 (Aug 3, 2017)

Yes it really a good information. Specially for me Loss reduction= 100- 100 (Old PF/New PF)**2 was new, though I have yet to maths it. Eaton page no. 6, Case 1, example is reducing KW with PF improvement. Something to mind??. Eatons harmonic explanation is good.


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## TNPE (Aug 3, 2017)

rg1 said:


> Yes it really a good information. Specially for me Loss reduction= 100- 100 (Old PF/New PF)**2 was new, though I have yet to maths it. Eaton page no. 6, Case 1, example is reducing KW with PF improvement. Something to mind??. Eatons harmonic explanation is good.


Unless otherwise told, you always hold kW constant when performing PF correction.  Did you mean KVA or KVAR, because each changes with kW held constant?  Likewise, all quantities change if kW is changed and you're adjusting PF with added reactance.


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## rg1 (Aug 3, 2017)

TNPE said:


> Unless otherwise told, you always hold kW constant when performing PF correction.  Did you mean KVA or KVAR, because each changes with kW held constant?  Likewise, all quantities change if kW is changed and you're adjusting PF with added reactance.


@TNPE can you refer page 6, Case1, of Eaton document.  http://www.eaton.com/ecm/groups/public/@pub/@electrical/documents/content/sa02607001e.pdf , Some confusion there. He varies KW while correcting PF or have I misunderstood?


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## TNPE (Aug 3, 2017)

Don't really have the energy at the moment to comb through "what" they did, but kW is NOT demand!!!  The only time "demand" is equal to kW is at unity.  This problem is not dealing with unity, so I'll have to come back to this later.

Furthermore, highly unlikely you'd see a scenario this convoluted, with a demand and PF correction problem intricately interwoven in this manner.  Once again, this test is heavy on concepts, not how well can you dissect what I said, and then proceed to solve with some method that is as convoluted as the problem statement.

Dont know that I agree with what they did until I can devote some time and investigate a little further.


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## cos90 (Aug 5, 2017)

TNPE said:


> Please post the problem @cos90


Its a really straightforward question:

You need to connect 3000kVAR to correct the power factor and you have 100kVAR cans How many cans per phase do you need to connect?

3000/3 = 1000 kVar per phase 

1000kVAR/100kVAR = 10 cans per phase

Where the questions get strange he draws out networks with series-parallel combinations of capacitor. Only one of these networks has 10 cans, but it was confusing because I thought there had to be other factors which are mentioned by @rg1. Thanks rg1.


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## cos90 (Aug 5, 2017)

justin-hawaii said:


> The problems listed by OP are power factor correction problems.
> 
> cos90, I think an application guide of how capacitors are used to correct power factor will be a great reference for you.  This reference will help to complement the more school type references, like Wildi.  I like to look for manufacturer websites that provide and design electrical equipment, in order to get a better idea on application of the equipment.   Manufacturers are probably the best experts on the equipment.
> 
> ...


thank you for finding these.


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## Messi (Mar 16, 2019)

Does anyone have an idea why we are adding KVARs with both series./parallel cap bank connections ?


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