How to calculate total capacitors required for a transformer for APFCP

[wadi]

Hello
I have a project but i am confused on it. The project i want to make an automatic power factor correction panel for a factory which has a transformer 1440 Amp capacity and 3 phase output volt 380 V and the automatic power factor correction relay with 12 steps.
So, how to calculate the KVAR required and the capacitors needed for the panel ? as know that the automatic power factor correction relay control will control the contactories that connected to capacitors and make automatic correction.

 

[Besoeker]

Hello
I have a project but i am confused on it. The project i want to make an automatic power factor correction panel for a factory which has a transformer 1440 Amp capacity and 3 phase output volt 380 V and the automatic power factor correction relay with 12 steps.
So, how to calculate the KVAR required and the capacitors needed for the panel ? as know that the automatic power factor correction relay control will control the contactories that connected to capacitors and make automatic correction.

You need to know something about the connected loads before you can estimate how much PFC would be appropriate.

 

[wadi]

But i see some factories design APFC panel with some specifications, i want to know how to design automatic power factor correction panel. But the loads are machines and the machines installed at a 80% of the transformer capacity.

 

[GoldDigger]

But i see some factories design APFC panel with some specifications, i want to know how to design automatic power factor correction panel. But the loads are machines and the machines installed at a 80% of the transformer capacity.

You have added some information, but still not enough.
You say that the machines total 80% of transformer capacity, but is that in watts or in VA? And do you have any idea of the power factor of the machines? Lightly loaded could be PF=.6 or lower. Fully loaded motors could be PF=.8.
The closer you can estimate the actual load, the more accurate PFC you can design for using a large base capacitor bank and smaller increments for trimming.

 

[Besoeker]

Hello
I have a project but i am confused on it. The project i want to make an automatic power factor correction panel for a factory which has a transformer 1440 Amp capacity and 3 phase output volt 380 V and the automatic power factor correction relay with 12 steps.
So, how to calculate the KVAR required and the capacitors needed for the panel ? as know that the automatic power factor correction relay control will control the contactories that connected to capacitors and make automatic correction.

Is it on an existing installation? Could you measure and the load power factor now?

 

[Ingenieur]

you can assume a worse case say 0.6 corrected to 0.9
but best to measure the existing pf

assuming 1000 kva xfmr (1440 x 380 x sqrt3), 800 kva load at pf 0.6
base
ang arccos 0.6 = 53 deg
S = P + jQ = 800 kva
P = 480 kw
Q = sin 53 x 800 = 639 kvar

corrected
we need the ang to be arccos 0.9 = 26 deg
P remains the same at 480 kw
new S = 480 / cos 26 = 534 kva
new Q = sin 26 x 534 = 234 kvar

kvar required = 639 - 234 = 405 kvar or 34 per step

34000 var = 380^2 / Xl or Xl = 4.25 Ohm
Xl = 4.35 Ohm = 2 Pi (50 Hz) L or L = 13.5 mH

you can adjust as required for different loads, base and corrected pf's

 

[wadi]

Is it on an existing installation? Could you measure and the load power factor now?

yes is it.
I can not because the loads have not installed yet and the loads are industrial sewing machines and 2 air compressors , air dryers , CNC machine, in addition offices with air conditioners, but by taking the current consumption of every machine we decided they need a transformer at least 1440 current capacity.

 

[wadi]

You have added some information, but still not enough.
You say that the machines total 80% of transformer capacity, but is that in watts or in VA? And do you have any idea of the power factor of the machines? Lightly loaded could be PF=.6 or lower. Fully loaded motors could be PF=.8.
The closer you can estimate the actual load, the more accurate PFC you can design for using a large base capacitor bank and smaller increments for trimming.

80% of current capacity
i mentioned that in last replay
the factory has a lot of industrial sewing machines and 2 air compressors with 2 air dryers in addition offices with air conditioners and CNC machine

 

[wadi]

when i search in google i see some automatic power factor correction panels and totally controlled with APFC controller and i dont the steps of designing it for a load or company, and what the measurements it should be taken and calculated.

 

[wadi]

These calculations for known pf and this is fixed only , but i am talking about APFC panel design
which has controller with 12 steps or 6 steps to correct the power factor automatically at 98%
but i dont know the existing pf because i am making the panel for factory from beginning.

 

[Besoeker]

yes is it.
I can not because the loads have not installed yet and the loads are industrial sewing machines and 2 air compressors , air dryers , CNC machine, in addition offices with air conditioners, but by taking the current consumption of every machine we decided they need a transformer at least 1440 current capacity.

Can you get the PF for each of those machines from the supplier?

 

[GoldDigger]

These calculations for known pf and this is fixed only , but i am talking about APFC panel design
which has controller with 12 steps or 6 steps to correct the power factor automatically at 98%
but i dont know the existing pf because i am making the panel for factory from beginning.

To do the calculation that you ask for requires that you estimate the range over which the load will vary during normal operation (such as shift changes, etc).
The variable capacitor bank total size will be the capacitance required to go from the minimum likely PF and load combinagtion (which will be covered by the fixed capacitor bank) and the maximum possible PF requirement, which will be with all 12 steps turned on.
To some extent the variation in PF from minimum motor load (idling) to maximum motor load (highest PF) will actually be small as long as the motor is still turned on.
So what you really need to estimate is how much of the loads will be actually turned off (motor not running) at various times through the 24 hour period that the factory is operating.

 

[Ingenieur]

These calculations for known pf and this is fixed only , but i am talking about APFC panel design
which has controller with 12 steps or 6 steps to correct the power factor automatically at 98%
but i dont know the existing pf because i am making the panel for factory from beginning.

the same calculations apply
instead of using 0.9 use 0.98
I used 12 steps, easily changed to 6 by doubling the per step value
or find the total kvar required to go from say 0.7 to 0.92 and divide by 6 (or 12)
no need to go that high to 98%, 90-92 is good
too high causes switching and over-voltage issues

I doubt your base will be lower than 70%

 

[Jraef]

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.

 

[kwired]

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.

:thumbsup:

Plus you can start out focusing more on larger motors as well as continuously loaded motors and at some point you may have sufficient overall correction that not every single motor will require correction.


A hoist that only runs say minute or less at a time probably not worth correcting at all, unless maybe it is several hundred HP.

 

[Ingenieur]

without knowing specifics hard to say central vs local
soinds like he may have hundreds of small motors, sewing machines?
not sure how many vfd's
if a few motors are most of the load local is the way to go

it has to be engineered properly
do not over compensate
and if do not know enough about sizing them you may not be the guy

the control system is key, slow is better, a weighed avg over time
use fail open contactors
put some Xl in series to slow down response
he's on the right track with at least 10 discrete steps

we are starting to use svc thyristor switched caps
not practical here, makes sense if >5 mva

 

[Sahib]

without knowing specifics hard to say central vs local
soinds like he may have hundreds of small motors, sewing machines?
not sure how many vfd's
if a few motors are most of the load local is the way to go

it has to be engineered properly
do not over compensate
and if do not know enough about sizing them you may not be the guy

the control system is key, slow is better, a weighed avg over time
use fail open contactors
put some Xl in series to slow down response
he's on the right track with at least 10 discrete steps

we are starting to use svc thyristor switched caps
not practical here, makes sense if >5 mva

Sensible suggestion against a silly apprehension because good quality electronics items are equipped with surge arresters.

 

[zbang]

without knowing specifics hard to say central vs local sounds like he may have hundreds of small motors, sewing machines?

Let's back up a moment- is PF correction needed at all? If the equipment is pretty much unknown, then answer is also unknown. Only the OP can answer that.

Also, what is the low PF penalty? If that doesn't kick in until the plant drops under, say 0.80, then it doesn't make much sense to correct up to 0.95.

(I agree that at-load correction is much easier to deal with, and may be all that's needed.)

None of this answers the original question, of course .

 

[Sahib]

The OP may look for similar working installations for typical power factor values to form a basis for his design.

 

[jumper]

Sensible suggestion against a silly apprehension because good quality electronics items are equipped with surge arresters.

Please explain to me what surge arresters have to do with pf.