power factor question.

[electricalperson]

last night at school we were talking a little about power factor. our school had a power factor of .55 due to the large amount of motors, lighting and other inductive loads. after the power company installed a capacitor bank it brought it up to .99 power factor. the teacher said you dont want to have a 1.00 power factor or have a capacitive problem. he didnt explain what would happen but im curious. what would happen if you had a 1.00 power factor or had a capacitive problem when current leads voltage?

 

[bob]

last night at school we were talking a little about power factor. our school had a power factor of .55 due to the large amount of motors, lighting and other inductive loads. after the power company installed a capacitor bank it brought it up to .99 power factor. the teacher said you dont want to have a 1.00 power factor or have a capacitive problem. he didnt explain what would happen but im curious. what would happen if you had a 1.00 power factor or had a capacitive problem when current leads voltage?

Are you sure the utility installed the capacitors? Some utility customers are
penalized if the PF is too low. If the customer is being penalized, they will hire an electrician to install the caps to correct the problem. There is no problem with having a 1.0 PF. However, if you have equipment that produces harmonics such as, variable frequency drives, computers, electronic ballast,
or other equipment, the harmonics can react with the capacitors to cause problems.

 

[electricalperson]

Are you sure the utility installed the capacitors? Some utility customers are
penalized if the PF is too low. If the customer is being penalized, they will hire an electrician to install the caps to correct the problem. There is no problem with having a 1.0 PF. However, if you have equipment that produces harmonics such as, variable frequency drives, computers, electronic ballast,
or other equipment, the harmonics can react with the capacitors to cause problems.

well the power company done a survey and had the school install capacitors. im sure the electrical shop done the job. maybe an outside electrician at night came in or a weekend to shut down the switchgear. its a 5000 amp service coming in

the school has about 3000 flourecant lights and about 300 motors of some sort

 

[charlie b]

The teacher said you dont want to have a 1.00 power factor or have a capacitive problem. He didn't explain what would happen but I'm curious.

You wind up with an overvoltage situation. I don't think I can easily explain how or why that works, or how bad it can get. So I'll just give you "the short answer,"and let someone else fill in the blanks.

 

[electricalperson]

You wind up with an overvoltage situation. I don't think I can easily explain how or why that works, or how bad it can get. So I'll just give you "the short answer,"and let someone else fill in the blanks.

so you would use inductors to fix that?

 

[charlie b]

so you would use inductors to fix that?

No, you already have plenty of inductance available at this site. You just prevent the problem by not putting in too much power factor correction capacitors.

 

[electricalperson]

No, you already have plenty of inductance available at this site. You just prevent the problem by not putting in too much power factor correction capacitors.

no i was asking how you would fix a capacitive circuit. like if voltage leads current too much would you use inductors to bring it more in phase

 

[mikeames]

How do you get over voltage with unity? Are you saying having a corrected power factor of 1 is bad in case the inductive load is reduced and then the capacitive correction produces the over voltage???

 

[charlie b]

How do you get over voltage with unity?

You don't.

Are you saying having a corrected power factor of 1 is bad in case the inductive load is reduced and then the capacitive correction produces the over voltage???

Precisely.

 

[electricalperson]

You don't.

Precisely.

im starting to understand power quality a little better now

 

[Strahan]

Wow power factor of .55 and I thought our power factor at our plant was bad at .80. Larry is it common to obtain a power factor of 1? I know this is the best possible solution but is it reasonable to obtain?

 

[LarryFine]

Larry is it common to obtain a power factor of 1? I know this is the best possible solution but is it reasonable to obtain?

I don't know why you asked me, but since you did...

As with most things, you attain what's known as diminishing points of return, where the value is no longer worth the cost.

As other's have mentioned, you don't want to over-correct anyway, so no, I wouldn't spend the money to try for a full 1.0 PF.

 

[bob]

Wow power factor of .55 and I thought our power factor at our plant was bad at .80. Larry is it common to obtain a power factor of 1? I know this is the best possible solution but is it reasonable to obtain?

Power factor correction is required when the customer is being billed for a low power factor. This is usually done by billing the KVA demand. If your location has a 0.80 Pf, you might want to check and see what the cost would be to raise the PF to 0.95. Also you would consider what the extra cost would be to go to a PF of 0.99. The reduced billing may be such that the additional cost could be justified. It all depends on the pay back.

 

[Strahan]

I don't know why you asked me, but since you did...

As with most things, you attain what's known as diminishing points of return, where the value is no longer worth the cost.

As other's have mentioned, you don't want to over-correct anyway, so no, I wouldn't spend the money to try for a full 1.0 PF.

I asked you because you're the most censored member:grin: I respect your answers on all previous posts. I'm talking of a very large industrial plant and I would agree increasing our power factor from .85 to upper .90's would greatly benefit us. I also agree you can over compensate with smaller loads I was always under the impression 1 is what you strive for but when dealing with such a big facility it was always satisfactory to get close that 1 was unattainable when talking savings. Thanks didn't mean to pull you in.

 

[don_resqcapt19]

There are automatic power factor correction systems for use with varying loads to avoid over correction. Here is one example.

 

[zog]

No, you already have plenty of inductance available at this site. You just prevent the problem by not putting in too much power factor correction capacitors.

Or better yet, a variable PF correction cap bank.

Edit to add: "Sorry Don, I posted before reading your post"

 

[weressl]

Wow power factor of .55 and I thought our power factor at our plant was bad at .80. Larry is it common to obtain a power factor of 1? I know this is the best possible solution but is it reasonable to obtain?

Usually you correct to 0.85-0.9 maximum. Economically it just would not make sense to go higher. We never point this out but inductive power factor is lagging, eg. should have a '-' sign fornt of it, and capacitive power factor would be leading and it has a positive value. It does not matter because both signifies the reduction usable electrical work delivered.

Now these economics had worked with rates about 5 years ago, so it may be different today.

Capacitors may be a cure to power factor, but they are surely a source of a host of other problems. As resonant circuits with the inductances they are the primary source of harmonics, long before the large amount of ASD's started bringing and awareness to harmonic problems. Harmonics of course create additional heating, therefore reduce insulation life. They will also create overvoltages and the switched currents take longer to extinguish causing extra contact wear.

 

[beanland]

Voltage Rise

Voltage Rise

Capacitors, by the creation of leading reactive current, will interact with the impedance (mostly inductive) of the POCO transformer, leading to a voltage rise. The voltage rise is (esentially) linear with the capacitive current injected. Each kVAr will create roughly the same voltage rise.

If you install fixed (not switched) capacitors for the worst case loading, when all the motors and lights are off, the capacitive current through the transformer can rise dramatically, leading to a significant rise in voltage, and voltages above normal can occur.

Most facilities are safe if they correct to ~90%. They may go leading under light load but not so far as to cause voltage problems.

 

[Besoeker]

Capacitors may be a cure to power factor, but they are surely a source of a host of other problems. As resonant circuits with the inductances they are the primary source of harmonics, long before the large amount of ASD's started bringing and awareness to harmonic problems. Harmonics of course create additional heating.

I usually see it from the other perspective. Harmonics from variable speed drives cause capacitors to get overloaded.
A solution I have seen, and used a number of times, is to fit "detuning" reactors in series with the capacitor. Normally they are tuned to be resonant at a little below fifth harmonic, thus being inductive at harmonic frequencies.

Of course it puts the capacitor volts up some, 5-10% depending on resonant frequency chosen, but as long as the capacitors specified correctly at the design stage, this shouldn't be a problem.
For retrofit, it may not be a solution unless it is known that the capacitors will remain within the tolerance of their voltage rating.

 

[weressl]

I usually see it from the other perspective. Harmonics from variable speed drives cause capacitors to get overloaded.
A solution I have seen, and used a number of times, is to fit "detuning" reactors in series with the capacitor. Normally they are tuned to be resonant at a little below fifth harmonic, thus being inductive at harmonic frequencies.

Ya think...just because the capcitors were there first:grin:

They are like humans, they show little tolerance toward others when they exhibit their own worst shortcomings.:smile: