A "Where to" Power Factor correction debate.

[aelectricalman]

I have been in the Power Factor correction industry going on 5 years. I have designed, engineered and installed cap banks and have learned from some of the best in doing so. There are some minor chinks in my armor in that I have a lot left to learn. When dealing with customers that are billed primary meter rates, I have chosen a pole mount (out on the line) approach for several reasons. The reasons are,

1. Your whole objective with Power Factor correction is to please the meter! So, why not correct at the meter (on the pole). Don't forget, Im talking primary industrial.

2. If you correct on the line side of the step down transformers, you do not have to account for the power factor of the step down transformer. Usually they are 90% or so. That means reduced capacitance required.

3. The higher the voltage you correct at, the smaller amount of KVAR needed to do the job. Therefore, you can again reduce the equipment size. the equipment we buy cost roughly the same at 13.80kV as it does at 480V.

4. You cut down on line loss to the meter. If you place the cap banks at the equipment, you are moving far away from the meter. The whole objective is to satisfy the meter.

5. There is no overcurrent protection needed other than cutouts and fuse links. Very cheap compared to adding breakers and building an enclosure to house the breaker, not to mention wiring all of the equipment together.

6. The cost to install the equipment is relatively the same. You need 2 bucket trucks and a medium voltage crew but you reduce the amount of labor to install. You reduce the amount of materials needed to install. So, I give the heads up to Medium Voltage.

Both require brief downtime. If youre licensed around here to do so, you may not even need to bring the building down to add the banks on medium voltage.

What I would like is for someone to challenge me and tell me why it is better to install capacitors on the individual pieces of service or utilization equipment. I am open to learning, I just dont know the pros of individual correction. I am not talking about using banks on the 480V side. I know it makes more sense to go medium voltage in this instance. I'm just not sold on individual cap banks scattered amongst a building. Thanks for you time. I hope this turns out to be a good discussion.

 

[mivey]

1. Your whole objective with Power Factor correction is to please the meter! So, why not correct at the meter (on the pole). Don't forget, Im talking primary industrial.

Because another objective can be to reduce losses. Another is to not over-correct. Another is increased system capacity.

2. If you correct on the line side of the step down transformers, you do not have to account for the power factor of the step down transformer. Usually they are 90% or so. That means reduced capacitance required.

Not sure why you think that is so. For a primary metered account, the transformer bank is seen as another inductive load.

4. You cut down on line loss to the meter. If you place the cap banks at the equipment, you are moving far away from the meter. The whole objective is to satisfy the meter.

That is backwards. The closer the capacitor is the the inductive load, the less the line loss. The var current is causing line loss so if we can let a remote capacitor and inductive load swap energy way down the line, we are not swapping this energy all the way back past the meter (i.e. we are not pushing this energy up and down the main line every 1/2 cycle).

5. There is no overcurrent protection needed other than cutouts and fuse links. Very cheap compared to adding breakers and building an enclosure to house the breaker, not to mention wiring all of the equipment together.

That is because of poor planning. If the var correction would have been installed at the beginning, it could have been incorporated along with the motor control.

What I would like is for someone to challenge me and tell me why it is better to install capacitors on the individual pieces of service or utilization equipment.

One good reason is because when the equipment is off then the correction is also off and you don't over-correct. Another is the reduction in losses from the meter to the equipment. Another is reduced capacity requirements.

 

[petersonra]

I don't know if the utility cares if you overcorrect, but if they do, you may need an automatic bank instead of a fixed bank. You would have the same problem downstream of the xfmr though.

you have to correct downstream of the meter or the utility won't see it. I am not sure what you mean by putting it on the pole.

line losses from PF issues can only be resolved by putting the caps close to the load. this is not usually a significant issue though.

i don't see how being licensed means you can do the work live. it is not part of the equation of determining whether the work can or cannot be done with the power on.

it probably makes the most sense if you are going to put in a single bank over putting it on the LV side.

 

[wirenut1980]

I don't know if the utility cares if you overcorrect, but if they do, you may need an automatic bank instead of a fixed bank. You would have the same problem downstream of the xfmr though.

you have to correct downstream of the meter or the utility won't see it. I am not sure what you mean by putting it on the pole.

line losses from PF issues can only be resolved by putting the caps close to the load. this is not usually a significant issue though.

i don't see how being licensed means you can do the work live. it is not part of the equation of determining whether the work can or cannot be done with the power on.

it probably makes the most sense if you are going to put in a single bank over putting it on the LV side.

Putting it on the pole means installing on the medium voltage side like utilities do. I agree with the comments posted here about locating capacitors at the loads might be better depending on whether the goal is to free up capacity on a certain part of the electrical system.

 

[petersonra]

Putting it on the pole means installing on the medium voltage side like utilities do.

as long as it is downstream of the meter, I guess you can put it on a pole.

 

[Besoeker]

I don't know if the utility cares if you overcorrect, but if they do, you may need an automatic bank instead of a fixed bank.

Overcorrection can cause the voltage to go higher than it ought to be and is often considered to not be a good thing.

 

[robbietan]

Overcorrection can cause the voltage to go higher than it ought to be and is often considered to not be a good thing.

utilities certainly wont like someone putting capacitor banks on the same voltage as the distribution grid - especially without a distribution impact study

 

[aelectricalman]

I don't know if the utility cares if you overcorrect, but if they do, you may need an automatic bank instead of a fixed bank. You would have the same problem downstream of the xfmr though.

you have to correct downstream of the meter or the utility won't see it. I am not sure what you mean by putting it on the pole.

line losses from PF issues can only be resolved by putting the caps close to the load. this is not usually a significant issue though.

i don't see how being licensed means you can do the work live. it is not part of the equation of determining whether the work can or cannot be done with the power on.

it probably makes the most sense if you are going to put in a single bank over putting it on the LV side.

The utility doesnt care so much in most instances.
On Primary billed customer the meter is usually on the customer pole fed just off the primary.
It makes is simple to control from the load side of the meter.
When projecting KVA savings on a primary customer, I have never been wrong in my estimates of kVA saved by placing Caps at the pole. Remember, you are not saving KW by installing capacitance, just KVA off the top. So, you main job is to safisfy the meter.

 

[aelectricalman]

utilities certainly wont like someone putting capacitor banks on the same voltage as the distribution grid - especially without a distribution impact study

Why not. They are constantly adding capacitance to help regulate and add the voltage of their system. I cant think of a situation where they dont appreciate it. It just mean less on their end. Plus, its not adding anything back on the utility side. Its correcting the customer after the meter. We use Automatic Switched banks with controls to regulate to 99.5% lagging.

 

[aelectricalman]

Overcorrection can cause the voltage to go higher than it ought to be and is often considered to not be a good thing.

IEEE say that you can have up to a 5% over voltage. As long as you stay within a couple of percent you will be fine. Leading kVAR translates to increased voltage, so you just have to pay attention to sensitive motors. Realistically, I dont like correcting past unity. We use controllers to regulate it. Most cap banks we use have voltage regulators built into them anyhow, so its not a big concern.

 

[mivey]

Remember, you are not saving KW by installing capacitance, just KVA off the top.

With your method you are not, but with correction at the load you will save kW.

So, you main job is to safisfy the meter.

But doing so while neglecting excess plant losses, excess plant capacity requirments, and excess plant voltage drop may not be the best route.

 

[mivey]

We use Automatic Switched banks with controls to regulate to 99.5% lagging.

And these controls add quite a bit to the cost of the cap bank. Some utilities require the var correction to be switched with the load.

 

[mivey]

IEEE say that you can have up to a 5% over voltage. As long as you stay within a couple of percent you will be fine.

According to who? The utility may already have their taps set accordingly and would not want you to add additional overvoltage to their equipment. They are already accounting for allowable tolerances on their system. If you own everything past the primary meter then you can make your own decisions.

The primary meter point is not always the separation point between the customer's and utility's equipment.

Leading kVAR translates to increased voltage, so you just have to pay attention to sensitive motors. Realistically, I dont like correcting past unity. We use controllers to regulate it. Most cap banks we use have voltage regulators built into them anyhow, so its not a big concern.

He was talking about overvoltage on the utility side. Many utilities do not permit excess var correction from the customer.

 

[mivey]

Why not. They are constantly adding capacitance to help regulate and add the voltage of their system.

But they make studies to make sure they don't create overvoltage problems or create a ferroresonance conditon. I'm assuming you are working with the utility when installing these device on the utlity primary so you probably know this already.

 

[robbietan]

Why not. They are constantly adding capacitance to help regulate and add the voltage of their system. I cant think of a situation where they dont appreciate it. It just mean less on their end. Plus, its not adding anything back on the utility side. Its correcting the customer after the meter. We use Automatic Switched banks with controls to regulate to 99.5% lagging.

the utilities do appreciate (and I mean REALLY appreciate) added capacitance in the lines as long as they are informed before installation. one problem is the effect of additional capacitors on voltage regulation.

 

[BPoindexter]

With your method you are not, but with correction at the load you will save kW.

How does that work?

 

[petersonra]

How does that work?

Because the extra current leads to additional I^2R losses all the way from the service to the load.

The amount of losses and thus the cost (and potential cost saving) is very hard to pin down.

I have seen numbers in the 5% range and numbers in the 0.5% range.

Some plants have opted for central systems on the grounds that even an automatic bank that costs a bit more is less costly up front than putting capacitors at the loads themselves, and whatever energy savings there might be from putting the caps at the loads is not worth the extra up front cost.

Other places have claimed there is a very short payback, especially on larger motors.

Most plants seem to have some kind of standard where they put PFC on any motor of a certain size or larger (like 20HP) or none at all.

I have also seen some places install them after the fact because the plant's electrical system (or part of it) is close to maxed out and it is cheaper to put in PFC than whatever else they might do to reduce the load. You have to remember than in many cases motors are grossly oversized for their true loads so they may be pulling 80-90% of FLC at 50% of full HP. if you put in PFC, you may now run at 60-70% of FLC when at 50% of HP. That can add up.

 

[BPoindexter]

Thanks Bob. I actually knew that... but wasn't clicking for some reason!

The jobs I have done we put them at the motor starters so that they were not in service when the motor was not running. I have done two jobs where we did this. Both times reputable engineering was not employed and we ended up with severe Harmonics issues.... burning up soft starts, overheated transformers, hot running motors, etc. In one case was so bad they removed them rather than get the necessary engineering. :slaphead:

 

[petersonra]

Thanks Bob. I actually knew that... but wasn't clicking for some reason!

The jobs I have done we put them at the motor starters so that they were not in service when the motor was not running. I have done two jobs where we did this. Both times reputable engineering was not employed and we ended up with severe Harmonics issues.... burning up soft starts, overheated transformers, hot running motors, etc. In one case was so bad they removed them rather than get the necessary engineering. :slaphead:

I doubt that adding across the line PFC would make any difference to a transformer. It ought to cut the current so it should run cooler.

PFC should have no effect at all on the motor. the motor sees the same exact current that it saw before the PFC was added.

Every soft start I have used requires the PFC be installed on the line side, sometimes with some minimal in line inductor requirements to feed them. if you experienced soft start failures related to PFC use, it may have been due to improper installation.

 

[BPoindexter]

The problem at least on the one case was that we had a very high harmonics and the way the PFC's were sized and installed were amplifying. 5th harmonic was very high which was causing the overheating of the motors. There was also quite a bit of 3rd harmonic in addition. Certain tranformers were not only loud but very very warm. Our system voltages on some of MCC's were getting high enough to burn up the soft starts. Several of our soft starts went out within the first week. At that point they did hire an engineering company to come out with recorders and determined where we needed filters installed. This was several years ago and I assume the technology has improved since then.