Where to put PF correction capacitors?

[MD88]

Hello there,

What are the advantages/disadvantages of putting PF correction capacitors at the main substation of a plant, as opposed to smaller ones in each electrical room?

I'm working on an expansion at a mine site, and while the utility doesn't charge for kVARS (only straight kW), the plant engineer is pushing for capacitors in each electrical room, stating "if we put them at the substation, only the utility gets the benefit. If we put them in each electrical room we'll get substantially less cable and transformer losses on the feeders to each room." From this I have 2 questions..

1) Are the savings in cable losses really that much from say a PF of 0.7 compared to 0.95? and
2) If we put the capacitors at the main sub, will all the downstream feeders still run at a low PF? So only upstream devices get the benefit of power factor correction?

Typically we only put them at the substation, but I think this is mainly to save on utility penalties.

Thanks very much for the help

 

[StephenSDH]

1) Are the savings in cable losses really that much from say a PF of 0.7 compared to 0.95?

It depends on the feeder distances. You are saving on line losses, or the ability to use a smaller feeder. With most plants it will not save you money, but with mining, I bet you have some pretty impressive feeder lengths.

2) If we put the capacitors at the main sub, will all the downstream feeders still run at a low PF? So only upstream devices get the benefit of power factor correction?

Yes, if you place PF correction at the main sub, it will only effect the PF between the main sub and utility. It will have no effect on the PF and line losses downstream.

 

[MD88]

Thanks! I found this really good article through one of the other threads:

http://ecmweb.com/mag/electric_correcting_power_factor/

The main feeder I'm concerned with is a 1000' foot long run from the main sub to a distribution centre closer to where the loads are. It's a 3000A cable bus laid in a precast concrete trench (unventilated) and while the manufacturers of the bus are very confident it will be fine, I'm still nervous about overheating the cables. So if I put capacitors at the "distribution centre closer to where the loads are" I mentioned, it will save some heating on the cable bus.

Thanks!

 

[bob]

The plant engineer is pushing for capacitors in each electrical room, stating "if we put them at the substation, only the utility gets the benefit. If we put them in each electrical room we'll get substantially less cable and transformer losses on the feeders to each room."

Maybe. It depends on the load and the number of hours the load is running.

(1) Are the savings in cable losses really that much from say a PF of 0.7 compared to 0.95?

Calculate the load amps at pf 0.70 and pf 0.95. Determine the difference.

(The difference amps)? x resistance of the cable = watts loss. Determine the hours that the load is running. Watts loss x hours running/1000 = KWH's saved. If the load runs 24 hours the savings could be large

2) If we put the capacitors at the main sub, will all the downstream feeders still run at a low PF? So only upstream devices get the benefit of power factor correction?

Yes.

Typically we only put them at the substation, but I think this is mainly to save on utility penalties.

you said in your post there were no KVAR penalties. The caps are not saving you anything if there is no penalty.

 

[MD88]

Yeah I think it may be useful to put them at each electrical room, here's the data:

Run from Sub to Distribution Centre:

3000A, 1000'

Run from Dist Centre to ER 1:

600A, 400'

Run from Dist Centre to ER 2:

800A, 600'

Run from Dist Centre to ER 3:

400A, 900'

and they are all running pretty much at this loading, 24hrs a day

 

[bob]

Looks like you can save some money. Why don't you do one of the calculations and see what the saving might be and let us know.

 

[LarryFine]

What are the advantages/disadvantages of putting PF correction capacitors at the main substation of a plant, as opposed to smaller ones in each electrical room?

The closer to each offending load its correction device is, the less of your electrical system that must be sized for the reactive current as well as the non-reactive. Ideally, each load gets local correction that gets switched out when that load is deeneergized.

 

[Jraef]

The closer to each offending load its correction device is, the less of your electrical system that must be sized for the reactive current as well as the non-reactive. Ideally, each load gets local correction that gets switched out when that load is deeneergized.

I agree. And not only the savings issue, but this also eliminates the problem of over correcting and helps prevent setting up resonance. If you do "bulk power factor correction" as you propose, every time an inductive load switches on or off it will change the amount of capacitance you need. An APFC controller would have a series of caps and contactors to switch them in and out. I have seen one of those go bad, it gets really bad when that happens, in the plant I was involved it the voltage spike took out every VFD front-end in the plant.

"At-Load" PF correction is what it's called when you put the PFC caps near each load on the downstream side of the contactor. That way you only tun on as much capacitance as that load requires, never more.

http://www.lmphotonics.com/pwrfact.htm

 

[IslandSparky]

I used to always prefer putting the caps at the points of use, but as harmonic loads on the system increase, you need to make sure the capacitors are properly de-tuned so you don't get into any resonance problems.

For OP's stated configuration, filtered caps in each room looks most appropriate. Long enough runs that you should be able to go from (say) 5% voltage drop down to 4%. Caps just at the distribution center would have the fastest payback though.

 

[MD88]

Yeah resonance is a major concern... Because we have a lot of 300HP+ motors on 6-pulse VFDs I was going to put harmonic filters on the biggest ones to get the harmonics down to IEEE 519, with the PCC being the main bus at each electrical room.

But, these motors have a really high PF anyway, so what do you guys think of maybe putting individual PFCs on the small motors with low PF and leaving the big motors alone?

Generally, each electrical room has maybe 6 big 300+HP motors on VFDs and then like 20 small (under 50HP) motors, some on VFDs, most not.

I haven't got around to doing any calcs yet, just wondering if anyone has any comments on that idea.