power factor correction

[alberto]

goog morning every body, i hope you can help me.

a factory has a power factor of 75.21% and ask me to calculate the power factor for correcting above 90% and i did the following calculation:

the energy bill shows a peak demand load of 164 KW and a power factor of 75.21%.
i want to correct the power factor to 97%

KVAR = KW x (TAN(A)-TAN(B))
A = COS-1(0.7521) = 41.22
B = COS-1(0.97) = 14.06

KVAR = 164 x (TAN(41.22) - TAN(14.06))
KVAR = 164 x (0.87605-0.25044)
KVAR = 102.60

i propose to install a 100 KVAR capacitor but the maintenance engineer thinks that a 100 KVAR capacitor is too big, the factory has a 300 KVA, 13200-480/277 volts substation, it doesn't have equipment in 480 volts, has 4 dry transformers in main panelboard (150,75,75,45 KVA).
other person told to the enginner that a 30 KVAR capacitor was enough for improving the power factor, this factory makes toys.
the calculation that i did was correct or there is another method for calculating the correction of power factor.

 

[jtester]

Re: power factor correction

Your approach is a bit simplistic. It would only work if the pf was relatively constant. A better approach would be to get a pf vs time chart or other set of data, and coordinate var demand with time of day.

Jim T

 

[bob]

Re: power factor correction

the energy bill shows a peak demand load of 164 KW and a power factor of 75.21%.i want to correct the power factor to 97%

KVA = kw/pf = 164/0.75 = 219 kva
KVAR=sqrt(KVA? - KW?)= sqrt(219? - 164?)
KVAR = 145 KVAR.

If you assume the PR = .97 then
KVA = 164/0.97 = 169 KVA
KVAR = sqrt(169? - 164?) = 41 kvar.

Reqired KVAR = 145 - 41 = 104 kvar added or 100 kvar as you said.
If the customer is being billed on kw then the additional caps will do nothing to reduce the bill.

[ March 01, 2006, 02:03 PM: Message edited by: bob ]

 

[alberto]

Re: power factor correction

I understand that the calculation that i did is for a constant demand, but the customer doesn't want to spend more money for making a real study of the demand and maybe the capacitor would be lower than 100 KVAR, but what happen, let's supose if making a real study results on a 60 KVAR and a 100 KVAR is installed would have some problem in the installations or what would happen.

 

[bob]

Re: power factor correction

IF you are billed a demand charge, it is usually based on the max. The the correction should be for the max. Yes, there are time of the day when you don't need the total caps but so what.
You keep using the KW demand. How is installing the caps a benefit fore the customer?

 

[coulter]

Re: power factor correction

Originally posted by alberto:
... the energy bill shows a peak demand load of 164 KW and a power factor of 75.21%. ...

Wow, Assuming you mean the pf is lagging, that is as low as I have ever seen. Most loaded motors run .85 or better.

I'd be curious as to what is causing it. Lots of lightly or unloaded motors is one cause. I don't know about HID lighting. Maybe that could be a problem. Could be that someone here knows about HID and pf.

Originally posted by alberto:
... the calculation that i did was correct or there is another method for calculating the correction of power factor.

Yes, however rather than putting in one big bank, another method is to identify the feeders/circuits with low pf and put smaller cap banks on each circuit, wired to come on with the load. This corrects the pf as needed, lightens the load on the xfmrs and switchgear, and prevents overvoltage from excessive VAR correction.

Originally posted by alberto:
... i propose to install a 100 KVAR capacitor but the maintenance engineer thinks that a 100 KVAR capacitor is too big ...other person told to the enginner that a 30 KVAR capacitor was enough for improving the power factor ...

Two things here:

1. An un-substantiated claim made by a partially knowledgeable person that is connected to the management network carries a lot of weight. In fact sufficient weight to upset physical laws, like the Law of Gravity (in the words of my daughter, "eeuwwwww - that is one sick pun")

2. You don't have to correct the pf to 1.0. You just have to get it below the limit where a penalty is charged. Check the operating agreement or tariffs. That will tell you where the penalties kick in. Could be a smaller bank is okay.

Originally posted by alberto:
... the customer doesn't want to spend more money for making a real study of the demand ...

Yeah, that is a bummer. Problems are hard to solve when you don't get to know what they are. In my mind, the .75pf is not the problem, it is a symptom of the problem. That low would really freak me out - I'd be looking hard for why os low. Rarely does patching the symptom fix the problem.

Originally posted by alberto:
... let's supose if making a real study results on a 60 KVAR and a 100 KVAR is installed would have some problem in the installations or what would happen.

Again, check the tariffs. Could be a leading pf is also penalizied. Another issue is excess VAR correction can (will) lead to high voltage during certain operating conditions.

Maybe you will get lucky and convince the management to rent a meter and do a little sluething to find the culprits. Good Luck

carl

 

[bdarnell]

Re: power factor correction

We've installed lots of capacitor banks on several different utilities with varying PF penalties and clauses. Here's the way we approach it:

1. Go to your customer and ask for the last year's worth of bills (12 months) showing demand, kwh, pf, etc.

2. Turn all of that over to your local cap supplier (ABB, Commonwealth, whomever) and let them tell you what you need. That is their business and they typically are very good at it.

I'd be leery of installing a fixed bank at service entrance if that's what you are planning. We ALWAYS do automatic banks unless they are at the point of use. You want to avoid leading.

Good luck with it.

 

[beanland]

Energy or Demand Billing

Energy or Demand Billing

You need to know if the customer is billed for power factor on an average monthly basis, power factor at the time of the peak monthly demand, or on some other basis. Every utility approaches power factor charges a little differently. If an average monthly power factor is billed, look at average monthly demand and average monthly power factor. Add capacitors running 24x7 to bring you from existing average PF to the point where no penalty is charged. If the utility uses some other power factor penalty system, find out what it is and figure out the best way to eliminate it with a combination of fixed and switched capacitors.

 

[cduranph2006]

I agree with JimT. you have to determine the kW demand & pf in different time frame. Get the average and use your formula. It could be different to what you have obtained.


Charlie D.