Automatic Cap Banks

Status
Not open for further replies.
P

Phil Corso

Senior Member
Location
Boca Raton, Fl, USA
RaiderUM?
Given a load, kW, and an original power-factor, PFo, here is a formula to determine additional capacitive-reactance to raise the power-factor to a new value, PFn:
∆ kVAr(c) = { Tan [ Cos ⁻? (PFo) ] ? Tan [ Cos ⁻? (PFn) ] } x kW
Regards, Phil Corso
 
R

raiderUM

Member
Location
Ohio
bob said:
If your PF is 0.89, you need to add caps. If you add about 5000 kvar and improve the

PF to 0.96 your new demand would 19125 kva and a savings of $220000 per year.

Yes this is very close to the number I got



Not necessarily. You can not assume. The only way to know for sure would be to check the actual monthly demand and PF. You may find that the PF can improve if the load drops off.
Click to expand...

Check out my thread on Chiller Plant Power Factor.

I have left the Fixed Cap Banks on at a couple locations and when I did that I saw the same thing happen that is explained in the "Chiller Plant Power Factor". I am now tracking and recording PF, which I just started doing in March. We have an E-Mon system that allows me to track all the other buildings to see Electric usage and other utilities. This allows me to see what the Caps are doing and I don't have to wait and get a bill from the City Utility. I hope that makes sense?? But this is the reason why I don't leave them on. I can see at night when the buildings are empty and our "BAS" system shuts off or cuts way back on KW usage, the PF falls to almost nothing. THis leads me to believe that when there is no Load we have to much capacitance and it causes a bad PF.
 
R

raiderUM

Member
Location
Ohio
Phil Corso said:
RaiderUM?
Given a load, kW, and an original power-factor, PFo, here is a formula to determine additional capacitive-reactance to raise the power-factor to a new value, PFn:
∆ kVAr(c) = { Tan [ Cos ⁻? (PFo) ] ? Tan [ Cos ⁻? (PFn) ] } x kW
Regards, Phil Corso
Click to expand...

Yes, this is true. This is not how I do it but you still get the same answer. I have to see things, so I draw two small triangles and find the difference, or if I want a quick answer I use this web site http://www.maxenergyindia.com/kvarCalculator.htm
 
R

raiderUM

Member
Location
Ohio
Phil Corso said:
RaiderUM... if you don't disconnect the PFC when demand is low, then you should get a "thankyou" letter from POCO!

Regards, Phil Corso
Click to expand...

This is what I originally thought, but the PF tracking I have been doing shows the PF at a very low number when there is no load and the Caps are on... this is what is confusing me, starting to think I dont know which way is up anymore..:?
 
R

raiderUM

Member
Location
Ohio
raiderUM said:
This is what I originally thought, but the PF tracking I have been doing shows the PF at a very low number when there is no load and the Caps are on... this is what is confusing me, starting to think I dont know which way is up anymore..:?
Click to expand...

So basically looking at the Chiller Plant thread and the Docs I posted, its safe to say the E-Mon system doesnt know what to make of the Leading PF so it shows it as NEG Phase angle and low PF? Would you say this is correct?
 
bob

bob

Senior Member
Location
Alabama
raiderUM said:
So basically looking at the Chiller Plant thread and the Docs I posted, its safe to say the E-Mon system doesn't know what to make of the Leading PF so it shows it as NEG Phase angle and low PF? Would you say this is correct?
Click to expand...

The meter reading should be negative but not low. If the load is very low you should see a PF of -0.98 but not low. The meter may not be designed to make this reading.
You should check with the utility and see if there is a charge when the PF is negative.
In line with what Phil said, you should be charging them. If this utility is like others, you are charged a KVA demand based on the peak KVA demand for the year. This peak will likely occur during the hot months. So as far as the demand cost, there is not advantage of removing the capacitors. Your PF is already too low. In the off peak months you may save some KWH's with the caps off.
 
O

Open Neutral

Senior Member
Location
Inside the Beltway
Occupation
Engineer
Sidenote:

I always hated "lower power factor" from a language standpoint. I always try to talk "better" or "worse" as especially if you are talking to a PHB vice a PhD; he may not grasp which is your goal.
 
R

raiderUM

Member
Location
Ohio
I just found this on Siemens I posted the link below. It puts this thread a little more on track.

It says this: Power factor capacitor should be connected on the load side of the contactor and switched as a unit with the motor. This arrangement minimizes the switching costs as an additional switching device is not needed.

And I call the Utility to have a power quality test done this week, so I can see what is actually happening with the capacitors when there is no load and how that translates to me seeing it on the graphs that I have shown. They do it for free so might as well.



http://www.google.com/url?sa=t&rct=...sg=AFQjCNHvB1Y79WQZdCZcHqidWwdMZiMCNw&cad=rja
 
Besoeker

Besoeker

Senior Member
Location
UK
Open Neutral said:
Sidenote:

I always hated "lower power factor" from a language standpoint. I always try to talk "better" or "worse" as especially if you are talking to a PHB vice a PhD; he may not grasp which is your goal.
Click to expand...

Perhaps PFC covers it nicely?
Power factor correction.

And if the PHB doesn't understand it probably isn't worth the effort in trying to explain anyway.
 
Besoeker

Besoeker

Senior Member
Location
UK
raiderUM said:
I just found this on Siemens I posted the link below. It puts this thread a little more on track.

It says this: Power factor capacitor should be connected on the load side of the contactor and switched as a unit with the motor. This arrangement minimizes the switching costs as an additional switching device is not needed.
Click to expand...

We always switch PFC capacitors separately. This avoids the possibly of self excitation if the PFC is connected directly across the motor terminals.
It can happen (I've seen it) and the article does mention it.
 
R

raiderUM

Member
Location
Ohio
Besoeker said:
We always switch PFC capacitors separately. This avoids the possibly of self excitation if the PFC is connected directly across the motor terminals.
It can happen (I've seen it) and the article does mention it.
Click to expand...

Self excitation only happens when to much Kvar is added "over correction". This is when capacitance is larger then the magnetized reactance of the motor.

There are two locations that come to mind. Both locations have two chillers in each room and putting fixed caps on each Chiller starter would greatly increase PF.

The one location has a Avg Load of 148.27KW since March 31, 2012 and an Avg PF of .7096. So 120Kvar would be perfect for these chillers if this is the correct application for load side caps?
 
R

raiderUM

Member
Location
Ohio
Phil Corso said:
RaiderUM...

what did you say the chiller nameplate rating was?

Phil Corso
Click to expand...

Stamped on the chiller itself: Each Chiller has a Min Cir Amps of 465 and Max overcurrent protection Amps of 649. Two parallel Nema size 5 for each chiller. Is that what you are looking for?
 
Status
Not open for further replies.
Top