PF leading....any savings missed....

[aelectricalman]

So, a customer has an existing 1500kVAR fixed bank.

The utility company here in my state does not bill the customer for leading power factor. In the utility's eyes they look at 99% leading the same as they do 99% lagging. The only thing is they don't like it (because it affects their load planning) but they do not penalize you any different for having it.

Also, the utility demand structure is on the basis of coincident PF at peak demand.

With this being said, I have a customer that says, " I don't like the fact that on the weekends, my power factor leads at 45%". Note that they are lightly loaded. Since the utility is not billing them for this and it is not affecting his voltage adversely ( enough to trip drives), I don't see the harm. Its been this way for 10 years with no problems or drive failure.

Enter a new maintenance engineer..... He says, This is not going to work any longer. He wants to know what he can save by closely monitoring his PF and keeping it near or at unity. I told him there is little or no benefit unless he wanted to keep track of carbon emission.

Does anyone see another way this customer could save by turning off the banks on the weekend? Am I missing something?

Before you answer with the answer I think you will, consider that when the customer operates on the weekend they are at 1/10th of their normal load, therefore the PF @ 45% leading does nothing for the billing of demand. It's the PF at peak load that matters (for the utility). Peak PF always occurs during the week under load.

 

[xformer]

IMHO, since this is a fixed bank, there is not much that can be done. It sounds to me like a recalculation needs to occur and a new bank installed.

One could always turn it off when not needed. but i think that defeats the purpose.

 

[kingpb]

Leading or lagging it doesn't matter; the current drawn will be higher.

Just because his load goes down on the weekend doesn't mean he isn't drawing more current than need be, and since current translates to losses, I²R, he is operating less efficiently.

I agree there is not much difference between 99% lagging or leading, but certainly 45% is a big difference.

 

[Pharon]

Unless I'm missing something, it doesn't matter if his current is higher or if he's more inefficient as a result -- as long as he's being billed in kWh and not kVAh, there's no cost savings in changing anything from the way it is now.

 

[aelectricalman]

Unless I'm missing something, it doesn't matter if his current is higher or if he's more inefficient as a result -- as long as he's being billed in kWh and not kVAh, there's no cost savings in changing anything from the way it is now.

Assuming that we are using kWh, a watt is a watt is a watt, when determining a watt-hour. If we increase the voltage we are actually lowering the current. Am I wrong? so, technically wouldn't it be better on the system assuming we are not causing drive failures and other PQ issues.

 

[aelectricalman]

IMHO, since this is a fixed bank, there is not much that can be done. It sounds to me like a recalculation needs to occur and a new bank installed.

One could always turn it off when not needed. but i think that defeats the purpose.

Good suggestion but not very practical from a cost standpoint.

 

[Pharon]

Assuming that we are using kWh, a watt is a watt is a watt, when determining a watt-hour. If we increase the voltage we are actually lowering the current. Am I wrong? so, technically wouldn't it be better on the system assuming we are not causing drive failures and other PQ issues.

Huh? Capacitor banks don't raise the system voltage -- all they do is introduce negative kVARs into the system to cancel out inductive kVARs.

But even raising the system voltage doesn't give cost savings, because as you say -- a watt is a watt.

 

[xformer]

Huh? Capacitor banks don't raise the system voltage -- all they do is introduce negative kVARs into the system to cancel out inductive kVARs.

But even raising the system voltage doesn't give cost savings, because as you say -- a watt is a watt.

Kvars use current.... and any additional current could add to the power bill.

 

[Besoeker]

Huh? Capacitor banks don't raise the system voltage.

They can and do.
That's why going leading is a bad thing.

 

[GoldDigger]

But only through resonant effects, no?

Tapatalk!

 

[xformer]

They can and do.
That's why going leading is a bad thing.

Would a utility transformer change effect the KVars?

 

[GoldDigger]

As the caps correct PF toward 1the line current decreases along with the vector IR, and therefore the observed voltage at the motor terminals can increase. But if we are looking only at linear effects, without resonances, the load voltage should start to decrease as the PF goes back below 1.

Tapatalk!

 

[Besoeker]

As the caps correct PF toward 1the line current decreases along with the vector IR, and therefore the observed voltage at the motor terminals can increase. But if we are looking only at linear effects, without resonances, the load voltage should start to decrease as the PF goes back below 1.

Tapatalk!

Draw out the vectors and you'll see what I mean.

 

[xformer]

Draw out the vectors and you'll see what I mean.

I dont really need to that... we are after all taking about power transfer from one point to another. Since we are talking about power and power is the product of I and E then if I decreases, E must increase to obtain the same quantity of Power.

 

[GoldDigger]

Only for a motor load or a variable resistive load.

Tapatalk!

 

[Besoeker]

I dont really need to that... we are after all taking about power transfer from one point to another. Since we are talking about power and power is the product of I and E then if I decreases, E must increase to obtain the same quantity of Power.

I and E can remain the same and the power change.

 

[xformer]

I and E can remain the same and the power change.

Can you elaborate on that?

 

[Besoeker]

Can you elaborate on that?

You need to include the cos phi.
Same E, same I, different cos phi different power.

 

[junkhound]

Only appreciable savings would be elimination of capacitor ESR (dissipation factor) losses on the weekends, plus could result is slightly longer lifetime of the PF correction capacitors since they would be running cooler on the weekends.

You can calculate the ESR losses easily with the capacitor data; would be surprised if the cost of adding switches could ever be recovered in power savings.

OP: BTW, ya get your harmonics figured out?

 

[aelectricalman]

Only appreciable savings would be elimination of capacitor ESR (dissipation factor) losses on the weekends, plus could result is slightly longer lifetime of the PF correction capacitors since they would be running cooler on the weekends.

You can calculate the ESR losses easily with the capacitor data; would be surprised if the cost of adding switches could ever be recovered in power savings.

OP: BTW, ya get your harmonics figured out?

I agree, the savings are going to be negligible. The capacitors should last longer being left in the fixed state. Once you start introducing switching transients and alternating cooling and heating, seems we are going to be shortening the life a bit.


Regarding the previous harmonics issue, we feel like a particular utility company is not doing this particular customer any justice. When you are providing 8.4% V total harmonic distortion to your customers and you are relying on the customers transformers to bring your distortion levels below IEEE thresholds, there's a problem. At the customers utilization equipment, they have passed through only 2 transformers. There is no way these harmonics are being mitigated to safe level only passing through a 4160V, 2500 kVA 5.7% Z xfmr and an 18 pulse DC Drive system. We are performing a harmonic spectrum analysis next week to see where these harmonics live. That will tell us what to do next I suppose. Thanks for all of your help.