How to calculate total capacitors required for a transformer for APFCP

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Phil Corso

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How to calculate total capacitors required for a transformer for APFCP

Wadi... your original post asked for the formula! Here it is:

If the original apparent-load, kVAo, real power, Po, and power-factor, PFo, and the required power-factor, PFr, are known, then the formula for corrective capacitive reactive-power, kVAc, neglecting capacitor loss, is:

o kVAc = K x Po, where,
o K = tan [ Acos ( PFo ) ] - tan [ Acos ( PFr ) ], and,
o Po = kVAo x PFo,in kW

Regards, Phil Corso
 
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GoldDigger

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Ingenieur said:
anytime you switch caps you get a surge/transient
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And that voltage transient can have a very low source impedance which puts a lot of stress on surge suppressors.
Is the voltage transient limited to the voltage across one capacitor bank step, or do inductive effects cause a much larger ringing transient voltage?

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wadi

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earth
Jraef said:
Just an unsolicited opinion, but APFC systems are disasters waiting to happen in the modern world where a lot of things have electronics on them, all powered by some form of Switch Mode Power Supply or things like VFDs and Servo amplifiers. The stepping in and out of capacitors will destroy your electronics in short order. One single misstep can wipe out a LOT of expensive equipment in a heartbeat. I was once at a fish processing facility in Alaska where this happened, the APFC unit destroyed over $300,000 worth a VFDs in one second.

It might end up a little more expensive, but FAR safer and more effective, to add what's called "At-Load" power factor correction capacitors. They are sized for each INDIVIDUAL load motor, then wired down stream of the controller so that they are turned on ONLY when that load is turned on, and thereby turned OFF when the load is turned off. I would focus only on the 3 phase machinery for this, single phase induction motors tend to have capacitors already, or are so small as to be insignificant.
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Do you think the APFC panel is not a good choice as a system does not has fully inductive loads?
Because i understand from your reply it is not a good choice if i have a lot of machines and devices equipped with control box and motors controlled with VFD and there is no need for APFC panel. Because nowadays all loads are not fully inductive but the motors controlled with VFDs.
But i have some machines like air compressor only start-up (Star-Delta), do you thing needs individual PF correction ?
 
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Ingenieur

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do you have these numbers?
total facility hp (or motor kw)
hp (or motor kw) on vfds
approximate number of machines
how many machines account for 20% of the total hp?
 
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zbang

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wadi said:
But i have some machines like air compressor only start-up (Star-Delta), do you thing needs individual PF correction ?
Click to expand...

Depends on that compressor, otherwise we're making wild guesses with almost no value. To generally answer your question, if the compressor has a PF of 0.92 I wouldn't bother with correction, if the PF is 0.62 and there was a good reason, I would. ("Just because" is not a good reason for engineering :D, excessive/circulating currents or power company rules are.)

Wadi- unless your only goal is to build a APFCP (as an academic exercise?), it would help if you would go back over this thread and attempt to specifically answer some of the questions posed.
 
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