# 3 phase Power Factor Capacitors



## Handasee (Dec 18, 2009)

I have been told by some suppliers of power factor correction capacitors that the maximum capacitor kvar size can not be more than 20% of the utility transformer size. The capacitors are 3 phase located near the plant service. They are in a fixed arrangement. (always on). The transformer is owned by the utility located outside the plant. I can not find any technical reference to this 20% rule. Does anyone have any reference to this rule?


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## Zog (Apr 15, 2009)

Handasee said:


> I have been told by some suppliers of power factor correction capacitors that the maximum capacitor kvar size can not be more than 20% of the utility transformer size. The capacitors are 3 phase located near the plant service. They are in a fixed arrangement. (always on). The transformer is owned by the utility located outside the plant. I can not find any technical reference to this 20% rule. Does anyone have any reference to this rule?


There is no such rule. It is recommended that you do not add PF caps if you have >20% non linear load without doing an engineering study first, maybe that is what you are thinking of, but even that is not a "rule", just a common practice.

The real question is why are you adding these? Do you pay a penalty?


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## Handasee (Dec 18, 2009)

Customers pf is at 0.48. Utility does charge if its below 0.9.
Service to factory is only 200A, 600V 3ph with a billing demand of 105Kw. 
Thats about 140KVAR required.


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## Zog (Apr 15, 2009)

Handasee said:


> Customers pf is at 0.48. Utility does charge if its below 0.9.
> Service to factory is only 200A, 600V 3ph with a billing demand of 105Kw.
> Thats about 140KVAR required.


I have never heard of PF penalties for such a small service.


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## Electric_Light (Apr 6, 2010)

Handasee said:


> Customers pf is at 0.48. Utility does charge if its below 0.9.
> Service to factory is only 200A, 600V 3ph with a billing demand of 105Kw.
> Thats about 140KVAR required.


He's in Canadiana. 

If the low power factor is caused mainly by lagging current, then perhaps they could use a synchronous motor set to leading power factor. Switching banks of capacitors cause surges that may damage electronic ballasts and computer controlled machines as well as office equipment. Synchronous motor PFC can adjust kVArs more smoothly. 

One rule is that you should NEVER correct beyond unity. If the threshold is 0.9, then you should aim for mid 90s. 

Check the current THD with a PQA. If it's absurdly high, it's caused by a large amount of office equipment, SCR/VFD drives and the like and as far as I know, there's not much you can do about poor power factor caused by non-sinusoidal loads.


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## Handasee (Dec 18, 2009)

I have found one reference in our codebook. (Ontario Electrical Safety Code)
Quote:
*"26-220 Transformers supplying capacitors*​The volt-ampere rating of a transformer supplying a capacitor shall not be less than 135% of the capacitor voltampere rating."
I don't know why this rule was written. Maybe someone has a technical explanation.
As to why the Power factor is so low, that I already know.
This is a machine shop. Machinery is not running at maximum effiecency as the material being worked dictates the motor loading. 

To Electric_Light, Yes I'm in Canada. try it, you might not go back home.
Thanks to everyone for your info.


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