Automatic Cap Banks

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raiderUM

Member
Location
Ohio
Ok, I have some threads on here about Capacitor banks and I seem to be learning more about them everyday. I've been an electrician for 10 years and am about to finish up my construction management degree, so I'm trying to apply the things I;ve learned in math and physics to my current job.

We have multiple locations on Campus that have Fixed Cap banks. The banks are turned off right now because we are doing load shedding with our Building Automation System (BAS) system. If I were able to turn these fixed Cap banks into Automatic ones we would be able to control them with our BAS system.

The question is how would I go about doing it? Could I take a 150KVAR fixed bank and turn it into 3 - 50 KVAR steps? By using a 200 amp lighting contactor on each step? So I would have three 200 amp contactors now being individually switched by our BAS system allowing us more flexibility. Would I need a special contactor besides the 24V coil I need to control it with the BAS system.

Let me know what you guys think.

Thanks
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
This is just my personal OPINION, not shared by everyone, but shared by many.

Automatic PF correction (APFC) is better than no PFC and better than what you have now, which is called Bulk PFC, but it is not the best solution. Even if broken down into smaller steps, the tendency to over correct is still there. The goal of PFC is to avoid penalties from the utility right? So if you are penalized below a .95PF, what's the use of correcting to .94? So if adding 50kVAR takes you to .94, but adding 100kVAR takes you to 1.10, the tendency will be to go with the 100kVAR. In the modern day of electronic power supplies on virtually everything, that over excited system leads to equipment failure, sometimes on a wide scale in a facility. People then blame the utility, or lightning, or "surges" that nobody sees anywhere else, but completely overlook the APFC system.

The best approach in my opinion is to use what is called "At-Load" PF correction. Put smaller caps only on the loads that are creating lagging PF, such as induction motors, and have them switch on with the motor. That way you can NEVER over correct.
 

Open Neutral

Senior Member
Location
Inside the Beltway
Occupation
Engineer
This is just my personal OPINION, not shared by everyone, but shared by many.

Automatic PF correction (APFC) is better than no PFC and better than what you have now, which is called Bulk PFC, but it is not the best solution. Even if broken down into smaller steps, the tendency to over correct is still there. The goal of PFC is to avoid penalties from the utility right? So if you are penalized below a .95PF, what's the use of correcting to .94?
......
The best approach in my opinion is to use what is called "At-Load" PF correction. Put smaller caps only on the loads that are creating lagging PF, such as induction motors, and have them switch on with the motor. That way you can NEVER over correct.

A) One goal is avoiding PF penalties, but isn't another reducing currents within the plant grid?

B) The "At-Load" brings its own issues. Can you fit them at each location? Does it cost more to buy/install more smaller caps than fewer large ones? And now I wonder...will the starter contactor object?
 

Besoeker

Senior Member
Location
UK
A) One goal is avoiding PF penalties, but isn't another reducing currents within the plant grid?
Yes, but if you fit them on the supply to the whole plant the current you will reduce with be that supplied to the plant rather than within the plant.
And that's fine if you want to avoid PF penalties.
 

bob

Senior Member
Location
Alabama
Raider
Before you do anything you need to look at the method of the utility billing.
1.Are you billed KW or KVA. If it is KW then the capacitors are moot.
2.If you are being billed for KVA, then determine what you power factor is.
3.If for example you have a 0.90 pf then you can save money by improving it.
4.You can determine how much KVAR is required to increase the PF to 0.96 or what ever.
5.Some of the capacitors will not need to be de-energized. It depends on the load.
6.How will you decide when to energize a capacitor bank? How many to de-energize?
7.Usually the utility has a smart meter that will provide information to do this.
 

raiderUM

Member
Location
Ohio
Raider
Before you do anything you need to look at the method of the utility billing.
1.Are you billed KW or KVA. If it is KW then the capacitors are moot.
2.If you are being billed for KVA, then determine what you power factor is.
3.If for example you have a 0.90 pf then you can save money by improving it.
4.You can determine how much KVAR is required to increase the PF to 0.96 or what ever.
5.Some of the capacitors will not need to be de-energized. It depends on the load.
6.How will you decide when to energize a capacitor bank? How many to de-energize?
7.Usually the utility has a smart meter that will provide information to do this.

We are billed for KWH and KVA. I know our PF and averages around 89% for the year. Last month our KWH was 8456400. KW 18360 KVA 21014, so there is plenty of money to be saved. I wont get into the price we pay for our electric but as you can see we are a big consumer.

I have all the data at each building for the last 6 years and I know how to determine how much KVar is needed a each location

We can use our Building Automation System (BAS) to energize, monitor and control the Caps just like we are use them to control our Load Shedding to keep our peak down.

The reason I am trying to do go this route is because we already have Fixed cap banks at all of our 480V locations. We have a couple quotes on getting Auto Cap Banks put in and the quotes are way to expensive. So I'm trying to fig out a way to make our own Auto Cap Banks by breaking down our Fixed ones. The fixed ones will work but only when all the equipment is running normal, but now that we have this new BAS program it regulates the buildings load at night and shuts it down to almost nothing. This causes our fixed bank to put way to much Capacitance in the circuit causing our PF to be even worse, so we need a way to control them.
 

raiderUM

Member
Location
Ohio
If I am breaking the steps into 50KVAR each that mean I have 60 amps of load per step, but I dont know what the code says about Capacitive switching and what the starters need to be rated for??
 

bob

Senior Member
Location
Alabama
We are billed for KWH and KVA. I know our PF and averages around 89% for the year. Last month our KWH was 8456400. KW 18360 KVA 21014, so there is plenty of money to be saved. I wont get into the price we pay for our electric but as you can see we are a big consumer.

It looks like you need to add 10000 kvar to approach a PF of 1.0. Why do you feel you need to switch the present capacitors off? Are you planning to add capacitors to improve the power factor? Depending on your rate, you might see a saving of over $150000 per year.
 

raiderUM

Member
Location
Ohio
It looks like you need to add 10000 kvar to approach a PF of 1.0. Why do you feel you need to switch the present capacitors off? Are you planning to add capacitors to improve the power factor? Depending on your rate, you might see a saving of over $150000 per year.

This is where it gets a bit confusing. The numbers I gave you were a sum of electric consumption for over 60 buildings. All these buildings are on the same primary meter from our Utility company. So obviously each building is going to be different in KVAR needs. Plus with this being a College campus all the students go home for the summer our load changes greatly, so its important to have flexibility with our KVAR correction. And yes, by my calculations we would be able to save even more the 150000 per year.
 

bob

Senior Member
Location
Alabama
Ok. So what happens, with regard to the billing, if you do not switch the caps. What happens if you do. The point I was making is, if you are going to add the 10000 kvar, why not just monitor those and leave the other as they are. Obviously I do not have the whole story. I know in most projects, it is better to have the caps at the load. As for just billing that is not necessary. You could put the new caps in a room and switch them as needed. What is the cost per KVA from the utility? I used $5/kva.
 
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raiderUM

Member
Location
Ohio
Ok. So what happens, with regard to the billing, if you do not switch the caps. What happens if you do. The point I was making is, if you are going to add the 10000 kvar, why not just monitor those and leave the other as they are. Obviously I do not have the whole story. I know in most projects, it is better to have the caps at the load. As for just billing that is not necessary. You could put the new caps in a room and switch them as needed. What is the cost per KVA from the utility? I used $5/kva.

I don't know what happens to the billing because I have never left them all one for an extended amount of time, but I would assume that our PF would be not so good. To much capacitance will also cause bad PF.

We don't need to add any more Kvar. We have banks all over campus, most of which is on the 480V gear because that's where our PF is very bad. All the 208V seems to be very good. We also have to large units at two distribution center, one is 750 Kvar and the other is 450 Kvar, both on 4160V. I am not really concerned with those at this time, to many questions going on.

I know in a couple of our buildings the Chillers are causing the lower PF. Does anyone know if we could put the Fixed caps "sized correctly" on the load side of the starter? Would the starter for the chiller handle the Caps also?

We pay around $9.7 per KVA... so yeah
 

bob

Senior Member
Location
Alabama
I know our PF and averages around 89% for the year.

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.

I don't know what happens to the billing because I have never left them all one for an extended amount of time, but I would assume that our PF would be not so good. To much capacitance will also cause bad PF.

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.
 
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