poweringtech
Member
First is lagging powerfactor considered as power quality problems?
Powerfactor and cable
- Thread starter poweringtech
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bphgravity
Senior Member
- Location
- Florida
Re: Powerfactor and cable
Absoulutely. Wouldn't any loss in a system not being applied to perform work be considered a power quality issue?
A highly inductive system requires more VA with less W performing the work.
Absoulutely. Wouldn't any loss in a system not being applied to perform work be considered a power quality issue?
A highly inductive system requires more VA with less W performing the work.
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Re: Powerfactor and cable
It is a power quality issue, it is only a problem if it is out of a typically expected range. As Bryan stated, it is not efficiently using the energy that you pay for. It may be a result of the type of loads you have within the facility.
There are several ways to "correct" (adjust) the power factor if you determine it is a problem. Maybe you experience utility penalties, or the load capacity (kW) is reaching the rating of your equipment and you don't want to upgrade your service, etc.
It is a power quality issue, it is only a problem if it is out of a typically expected range. As Bryan stated, it is not efficiently using the energy that you pay for. It may be a result of the type of loads you have within the facility.
There are several ways to "correct" (adjust) the power factor if you determine it is a problem. Maybe you experience utility penalties, or the load capacity (kW) is reaching the rating of your equipment and you don't want to upgrade your service, etc.
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: Powerfactor and cable
I disagree. But that is only because I am using a different definition of ?power quality? than Bryan and Ron. However, I do agree with the rest of their comments.
My definition tells me that if the voltage and current are pure sine waves, then the power quality is perfect. You lose quality when you introduce harmonics, and cause the voltage or current to be no longer a pure sine wave.
So for us to give you a proper answer to your question, we would need you to tell us what you mean by the term ?power quality.?
By the way, if this is ?First,? then what comes ?Second??
I disagree. But that is only because I am using a different definition of ?power quality? than Bryan and Ron. However, I do agree with the rest of their comments.
My definition tells me that if the voltage and current are pure sine waves, then the power quality is perfect. You lose quality when you introduce harmonics, and cause the voltage or current to be no longer a pure sine wave.
So for us to give you a proper answer to your question, we would need you to tell us what you mean by the term ?power quality.?
By the way, if this is ?First,? then what comes ?Second??
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
Re: Powerfactor and cable
When I hear the term "power quality" I think of things such as over/undervoltage conditions, dropouts, RF noise, etc.
I don't consider a poor PF to be a power quality issue.
When I hear the term "power quality" I think of things such as over/undervoltage conditions, dropouts, RF noise, etc.
I don't consider a poor PF to be a power quality issue.
charlie
Senior Member
- Location
- Indianapolis
Re: Powerfactor and cable
That is interesting Carl. We don't care if the customer corrects their power factor or not. We will do it for him but he will pay for it through the penalties. We also do not care where he corrects his power factor to unless he goes past unity. We charge a penalty until 85% and give a credit past 85%.
On our higher voltage customers, we meter on the low side and charge an adder for the substation transformer losses. It is very expensive to meter on the high side.
It is really interesting how different electric utilities do things.
That is interesting Carl. We don't care if the customer corrects their power factor or not. We will do it for him but he will pay for it through the penalties. We also do not care where he corrects his power factor to unless he goes past unity. We charge a penalty until 85% and give a credit past 85%.
On our higher voltage customers, we meter on the low side and charge an adder for the substation transformer losses. It is very expensive to meter on the high side.
It is really interesting how different electric utilities do things.
- Location
- Illinois
- Occupation
- retired electrician
Re: Powerfactor and cable
Charlie,
Don
Charlie,
How high is "higher"? The plant where I'm at is metered on the 34.5kV side of the transformers.
Don
charlie
Senior Member
- Location
- Indianapolis
Re: Powerfactor and cable
We normally feed an industrial substation with 138 kV and a couple at 345 kV. Metering that power on the high side is prohibitive. We have 345 kV, 138 kV, and 34.5 kV then distribution voltages, 13.2 kV and 4.16 kV. We only have a few industrial substations where we feed them with 34.5 kV and I don't know how they are metered.
We normally feed an industrial substation with 138 kV and a couple at 345 kV. Metering that power on the high side is prohibitive. We have 345 kV, 138 kV, and 34.5 kV then distribution voltages, 13.2 kV and 4.16 kV. We only have a few industrial substations where we feed them with 34.5 kV and I don't know how they are metered.
poweringtech
Member
Re: Powerfactor and cable
Thank you very much for all the reply.
My second question is other than the savings and penalty introduced by the utility company, Is there a computation on the amount of energy savings a company will get out of installing a capacitor?
Saving computation in terms of less I^2 R losses in the cable
computation on Additional life of cable and equipment because of less current that will flow in the cable and equipment
Savings on the maintenance cost, downtime cost etc.
Thank you very much for all the reply.
My second question is other than the savings and penalty introduced by the utility company, Is there a computation on the amount of energy savings a company will get out of installing a capacitor?
Saving computation in terms of less I^2 R losses in the cable
computation on Additional life of cable and equipment because of less current that will flow in the cable and equipment
Savings on the maintenance cost, downtime cost etc.
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Re: Powerfactor and cable
There are calculations you can done to determine payback and eventual savings after the installation of power factor correction capacitors. Depends on whether you install a fixed amount of capacitance or a dynamic amount. The capacitor manufacturers have lots of literature for you to consider.
http://www.esipower.net/_private/GEH-323C2.pdf
http://www.egenergy.com/power_factor.htm
http://bg.ecmweb.com/ar/electric_primer_power_factor/
There are calculations you can done to determine payback and eventual savings after the installation of power factor correction capacitors. Depends on whether you install a fixed amount of capacitance or a dynamic amount. The capacitor manufacturers have lots of literature for you to consider.
http://www.esipower.net/_private/GEH-323C2.pdf
http://www.egenergy.com/power_factor.htm
http://bg.ecmweb.com/ar/electric_primer_power_factor/
Re: Powerfactor and cable
(snip) We don't care if the customer corrects their power factor or not. We will do it for him but he will pay for it through the penalties. ... We charge a penalty until 85% and give a credit past 85%.(snip)
*******************************
Charlie -
You are absolutely right. I didn't phrase my comments very well.
In this case, the customer had paid for the design and installation of the substation, but the utility handled the maintenance on it up to the transformers. Part of the deal with the utility was the caps that were installed on the 13.8kV side of the xfmrs. I suspect that had to do with the adder one pays when the metering is on the xfmr secondary. And, like you said, if the customer doesn't add the vars required, then the utility will, and will meter them and charge for them. They have to, it takes rotating capacity to generate and transmit vars and it has to be paid for.
Question: Occasionally we would shut off most of the motors, and be a little slow about getting the caps shut off. The pf would go leading. One time, the utility noticed and the dispatcher was right on us to get the caps shut off. What does a leading pf do to the utility?
I've worked with a lot of generator regulation, small stuff by utility standards (multiple 800kw, paralleled is the biggest) and there is ususlly words about a leading pf causing instability, but I've never figued out why.
carl
(snip) We don't care if the customer corrects their power factor or not. We will do it for him but he will pay for it through the penalties. ... We charge a penalty until 85% and give a credit past 85%.(snip)
*******************************
Charlie -
You are absolutely right. I didn't phrase my comments very well.
In this case, the customer had paid for the design and installation of the substation, but the utility handled the maintenance on it up to the transformers. Part of the deal with the utility was the caps that were installed on the 13.8kV side of the xfmrs. I suspect that had to do with the adder one pays when the metering is on the xfmr secondary. And, like you said, if the customer doesn't add the vars required, then the utility will, and will meter them and charge for them. They have to, it takes rotating capacity to generate and transmit vars and it has to be paid for.
Question: Occasionally we would shut off most of the motors, and be a little slow about getting the caps shut off. The pf would go leading. One time, the utility noticed and the dispatcher was right on us to get the caps shut off. What does a leading pf do to the utility?
I've worked with a lot of generator regulation, small stuff by utility standards (multiple 800kw, paralleled is the biggest) and there is ususlly words about a leading pf causing instability, but I've never figued out why.
carl
charlie
Senior Member
- Location
- Indianapolis
Re: Powerfactor and cable
Carl, it depends on the electric utility you are dealing with but we attempt to keep the power factor close to unity. If you have a lot of VARs, you would be over correcting the other direction and driving the voltages on the lines nuts. Additionally, you would be swinging the power factor triangle in the other direction and actually increasing the kVA they need to generate.
Carl, it depends on the electric utility you are dealing with but we attempt to keep the power factor close to unity. If you have a lot of VARs, you would be over correcting the other direction and driving the voltages on the lines nuts. Additionally, you would be swinging the power factor triangle in the other direction and actually increasing the kVA they need to generate.
poweringtech
Member
Re: Powerfactor and cable
Hello Charlie,
I have a friend in one of the utility companies here and his explanatin on the discounts and penalties provided by the utility company is utility economics.
Power Factor correction installed at the load side of the utility distribution system corrects and improves the system losses of the distribution company.
To save on cost of correcting the power factor of the system. The utility have decided to give incentive to their customer and penalize them if the power factor is below the base of 85%.
In one of the books that I have read the best way to install power factor correction is to place the capacitor as nearest as cost effective as possible to the load. By doing this system losses inside the plant will also be reduced.
Would you have some computation on how to determine system losses inside the industrial plant?
poweringtech
Hello Charlie,
I have a friend in one of the utility companies here and his explanatin on the discounts and penalties provided by the utility company is utility economics.
Power Factor correction installed at the load side of the utility distribution system corrects and improves the system losses of the distribution company.
To save on cost of correcting the power factor of the system. The utility have decided to give incentive to their customer and penalize them if the power factor is below the base of 85%.
In one of the books that I have read the best way to install power factor correction is to place the capacitor as nearest as cost effective as possible to the load. By doing this system losses inside the plant will also be reduced.
Would you have some computation on how to determine system losses inside the industrial plant?
poweringtech
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
Re: Powerfactor and cable
Its less true for smaller motors, yet you can often get very large loads from having many smaller motors where it is not economical to put PFC at each motor. Thats when it may make sense to have a PFC system that can be switched in and out as needed on a more centralized basis.
I have seen some of the calcs people do to justify PFC, and the economics just are not always there. You need to run the numbers and see just what they really are.
A load that only runs a few hours a day (like a sump pump) may take many decades to break even on. Just like going with higher efficiency motors does not always make that much sense.
Another thing is that VFDs tend to make the motor PF very close to unity. In many cases it is far mor cost effective to put a VFD on a motor than it is to put PFC capacitors on the load, since not only will you get PFC you can also get load matching.
This is one of those sort of true things. Its true that you normally want the PFC as close to the load as possible. This is almost always true for larger motors, which is why most industrial specs require PFC for larger HP motors.
Its less true for smaller motors, yet you can often get very large loads from having many smaller motors where it is not economical to put PFC at each motor. Thats when it may make sense to have a PFC system that can be switched in and out as needed on a more centralized basis.
I have seen some of the calcs people do to justify PFC, and the economics just are not always there. You need to run the numbers and see just what they really are.
A load that only runs a few hours a day (like a sump pump) may take many decades to break even on. Just like going with higher efficiency motors does not always make that much sense.
Another thing is that VFDs tend to make the motor PF very close to unity. In many cases it is far mor cost effective to put a VFD on a motor than it is to put PFC capacitors on the load, since not only will you get PFC you can also get load matching.
Re: Powerfactor and cable
I agree that the replacement of standard starters with VFD's is generally the best solution for power factor correction. VFDs have many other advantages. They allow for soft starts protecting you from peaking to high demands when restarting after a power outage and speed control of course. One consideration: Always check with the motor manufacturer to make sure the bearings installed in your motors are appropriate for speed control and that there are not motor characteristics that make VFDs not appropriate.
As for where to place power factor correcting capacitors use the example of the run capacitor that is found on most large single phase refrigeration compressor motors. That run capacitor is for power facter correction. It is parallel to the run winding.
I'm big on VFDs but please don't ask me how they work. I think its magic or electronics or a comnbination of both.
One thing on a serious note: Don't mix VFDs and capacitors for power factor correction.
Bob
I agree that the replacement of standard starters with VFD's is generally the best solution for power factor correction. VFDs have many other advantages. They allow for soft starts protecting you from peaking to high demands when restarting after a power outage and speed control of course. One consideration: Always check with the motor manufacturer to make sure the bearings installed in your motors are appropriate for speed control and that there are not motor characteristics that make VFDs not appropriate.
As for where to place power factor correcting capacitors use the example of the run capacitor that is found on most large single phase refrigeration compressor motors. That run capacitor is for power facter correction. It is parallel to the run winding.
I'm big on VFDs but please don't ask me how they work. I think its magic or electronics or a comnbination of both.
One thing on a serious note: Don't mix VFDs and capacitors for power factor correction.
Bob
Ed MacLaren
Senior Member
Re: Powerfactor and cable
While some of the better VFDs can have a PF as high as 90%, keep in mind that a unit that does not include an input reactor typically will have a lower PF (66-65%).
The high kVA demand of a VFD that causes the low power factor is due to the harmonic content in the input current waveform.
It is the reduction of the harmonics, either by a line reactor, or by a harmonic filter that leads to the improved power factor.
Some lower cost units don?t have them factory installed, but they can be added.
Another important advantage of some VFDs is the ability to operate a three phase motor from a single phase supply.
Ed
[ December 19, 2004, 01:53 PM: Message edited by: Ed MacLaren ]
While some of the better VFDs can have a PF as high as 90%, keep in mind that a unit that does not include an input reactor typically will have a lower PF (66-65%).
The high kVA demand of a VFD that causes the low power factor is due to the harmonic content in the input current waveform.
It is the reduction of the harmonics, either by a line reactor, or by a harmonic filter that leads to the improved power factor.
Some lower cost units don?t have them factory installed, but they can be added.
Another important advantage of some VFDs is the ability to operate a three phase motor from a single phase supply.
Ed
[ December 19, 2004, 01:53 PM: Message edited by: Ed MacLaren ]
Re: Powerfactor and cable
It's hard to type with my tongue held firmly between my teeth, errr, my fingers held in my teeth.
But I can't let it go. I can't say I agree with some of the statements on VFDs
To paraphrase a conversation with an AB rep:
Rep: "At 60Hz it's 95% efficiency and 95% pf."
My response: "So what you are saying is if I want to run at line frequency rpm I only have to pay for 105% power. Think about this for a minute, why would I care about the eff and pf at 60Hz? If I was going to run at 60 hz, I wouldn't need a VFD. I'm much more interested in the eff and pf at say 45 hz. So what is it at 45 Hz?
Rep: "Uhhhh ..."
It seems to me that if line freq rpm is fine, then soft start is best accomplished by using a ... drum roll ... Soft Start - say an AB SCR soft start. And power factor correction is best accomplished by caps.
Before I get beat to death, please keep in mind I've rarely seen any need for soft starts or pf correction under 50 hp.
All of the cases where I have used VFDs, were rpm/torque application specific. PF correction/VAR correction were not the issue.
I can't imagine a VFD is cost effective for only
VAR/pf - especially since you get to throw away 5% of your power.
Question for bg concerning: "Reduced danger of transformer overheating and equipment damage associated with harmonics."
Reduced from what? A motor without a VFD doesn't have any. Caps don't introduce any. A soft start only does during the few seconds while it starts. This one eludes me.
VFDs are great devices - if you need one - otherwise its just a power and maintenance hog. And yes, generally speaking, a DIT preferably or input reactors are absolutely necessary or the VFD can introduce horrible notching on the power supply.
carl
It's hard to type with my tongue held firmly between my teeth, errr, my fingers held in my teeth.
But I can't let it go. I can't say I agree with some of the statements on VFDs
To paraphrase a conversation with an AB rep:
Rep: "At 60Hz it's 95% efficiency and 95% pf."
My response: "So what you are saying is if I want to run at line frequency rpm I only have to pay for 105% power. Think about this for a minute, why would I care about the eff and pf at 60Hz? If I was going to run at 60 hz, I wouldn't need a VFD. I'm much more interested in the eff and pf at say 45 hz. So what is it at 45 Hz?
Rep: "Uhhhh ..."
It seems to me that if line freq rpm is fine, then soft start is best accomplished by using a ... drum roll ... Soft Start - say an AB SCR soft start.
And power factor correction is best accomplished by caps.Before I get beat to death, please keep in mind I've rarely seen any need for soft starts or pf correction under 50 hp.
All of the cases where I have used VFDs, were rpm/torque application specific. PF correction/VAR correction were not the issue.
I can't imagine a VFD is cost effective for only
VAR/pf - especially since you get to throw away 5% of your power.
Question for bg concerning: "Reduced danger of transformer overheating and equipment damage associated with harmonics."
Reduced from what? A motor without a VFD doesn't have any. Caps don't introduce any. A soft start only does during the few seconds while it starts. This one eludes me.
VFDs are great devices - if you need one - otherwise its just a power and maintenance hog. And yes, generally speaking, a DIT preferably or input reactors are absolutely necessary or the VFD can introduce horrible notching on the power supply.
carl
Re: Powerfactor and cable
Coulter my friend, you are scope bound on the efficiency of the VFD and not looking at the efficiency of the system of which the VFD is a major contributing part. Systems where motors operate equipment under widely varying load conditions are not just 5% more efficient with VFDs they are better than 20% more efficient. Wasteful 3 way valves can be replaced with less costly and more efficient 2 way valves, static control dampers are no longer needed, vane load controls are no longer needed, etc. Now understand I'm talking about systems employing motors well over 100 HP as you would find in any large HVAC installation. Look around you, the use of VFDs is growing at an almost vertical rate at a time when utility companies are giving incentives to increase power efficiency.
Bob
Coulter my friend, you are scope bound on the efficiency of the VFD and not looking at the efficiency of the system of which the VFD is a major contributing part. Systems where motors operate equipment under widely varying load conditions are not just 5% more efficient with VFDs they are better than 20% more efficient. Wasteful 3 way valves can be replaced with less costly and more efficient 2 way valves, static control dampers are no longer needed, vane load controls are no longer needed, etc. Now understand I'm talking about systems employing motors well over 100 HP as you would find in any large HVAC installation. Look around you, the use of VFDs is growing at an almost vertical rate at a time when utility companies are giving incentives to increase power efficiency.
Bob
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