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PFC Capacitors on Primary Side of Transformer
- Thread starter bnsmith2
- Start date
As the voltage goes up, the price per KVAR usually goes down.
A capacitor that supplies 20 KVAR at 480 volts will produce only 5 KVAR at 240 volts.
jtkirb's observation is valid. If you have a transformer that is heavily loaded with a poor power factor load, consider connecting the capacitors on the secondary to reduce the transformer current.
If you have a choice between capacitors on a 208 volt system and a 480 volt system, you get over 5 times as many KVARs per uF at 480 compared to 208 volts. The cost of insulation in a capacitor doesn't go up nearly as fast.
Yours
A capacitor that supplies 20 KVAR at 480 volts will produce only 5 KVAR at 240 volts.
jtkirb's observation is valid. If you have a transformer that is heavily loaded with a poor power factor load, consider connecting the capacitors on the secondary to reduce the transformer current.
If you have a choice between capacitors on a 208 volt system and a 480 volt system, you get over 5 times as many KVARs per uF at 480 compared to 208 volts. The cost of insulation in a capacitor doesn't go up nearly as fast.
Yours
Regarding the harmonic resonance, by adding a large lump of capacitance to the system, I'd have thought you are likely to change the overall system resonance point - to what extent would depend on size of sytem/amount of capacitance being added.
Need to be careful here as if you have any non-linear loads on the system (VSD's, rectifiers etc) which generate harmonics at frequencies which co-incide with the system resonance point then you will have problems.
I'd suggest if you have non-linear loads on the system, what you propose in terms of PFC capacitors needs to be modelled to verify that you won't have any harmonic issues to consider.
On the other hand if all off your load is linear, it may not be an issue!
Need to be careful here as if you have any non-linear loads on the system (VSD's, rectifiers etc) which generate harmonics at frequencies which co-incide with the system resonance point then you will have problems.
I'd suggest if you have non-linear loads on the system, what you propose in terms of PFC capacitors needs to be modelled to verify that you won't have any harmonic issues to consider.
On the other hand if all off your load is linear, it may not be an issue!
- Thread starter
- #5
Thanks for the responses.
The primary side voltage is 13.2 kV and the secondary is 480 V. I was wanting to install on the primary side of the transformers before the meter point to alleviate the power factor demand charge, and hopefully use the transformers as a sort of isolation device.
If capacitors were attached on the secondary, it would cost a lot more because of multiple transformers. If the caps were attached on the primary, the cost per kVAR would be cheaper, and only two cap banks would be required instead of ten.
My main concern is harmonics on the secondary caused by capacitors on the primary. Is there any literature that could help me determine this, or do any of you know how to determine the resonant harmonic on the secondary, when the caps are on the primary?
The primary side voltage is 13.2 kV and the secondary is 480 V. I was wanting to install on the primary side of the transformers before the meter point to alleviate the power factor demand charge, and hopefully use the transformers as a sort of isolation device.
If capacitors were attached on the secondary, it would cost a lot more because of multiple transformers. If the caps were attached on the primary, the cost per kVAR would be cheaper, and only two cap banks would be required instead of ten.
My main concern is harmonics on the secondary caused by capacitors on the primary. Is there any literature that could help me determine this, or do any of you know how to determine the resonant harmonic on the secondary, when the caps are on the primary?
If you connect the capacitors on the supply side of the Metering, it is a nice thing to do for the power company, but the metering will not see the capacitors and you will not aleviate your power factor charges even though you may have corrected the power factor.
The capacitors must be on the load side of the meter.
I don't quite understand this.
Do you have primary metering?
How many meters do you have.
If you have primary metering you may connect primary capacitors on the load side of the metering transformers.
The capacitors must be on the load side of the meter.
I don't quite understand this.
Do you have primary metering?
How many meters do you have.
If you have primary metering you may connect primary capacitors on the load side of the metering transformers.
- Thread starter
- #7
waross, thanks for your post. Let me try to clarify.
Yes, the facility is primary metered, but there are two seperate meters at the facility. These two meters are combined for the monthly bill.
My main question is can I put non-tuned capacitors on the primary side of the transformer with little effects to the harmonics on the secondary side of the transformer?
Yes, the facility is primary metered, but there are two seperate meters at the facility. These two meters are combined for the monthly bill.
My main question is can I put non-tuned capacitors on the primary side of the transformer with little effects to the harmonics on the secondary side of the transformer?
You will be less likely to see problems due to harmonics if the caps are on the primary side. We have done this fairly often in the past.
But it is not a guarantee, so I would still want to evaluate the harmonic distortion on the primary and also how the caps will effect the system resonant frequencies. You might still want to apply some minimal filtering to the caps, even on the primary. Failure of a 15 kV cap can be much more spectacular than a 480V cap. Since you will have one big chunk of caps, you also need to evaluate voltage and pf at low loads to determine if you need automatic switching.
But it is commonly done and can be less expensive and less troublesome than fighting the 480V drives on the low side.
But it is not a guarantee, so I would still want to evaluate the harmonic distortion on the primary and also how the caps will effect the system resonant frequencies. You might still want to apply some minimal filtering to the caps, even on the primary. Failure of a 15 kV cap can be much more spectacular than a 480V cap. Since you will have one big chunk of caps, you also need to evaluate voltage and pf at low loads to determine if you need automatic switching.
But it is commonly done and can be less expensive and less troublesome than fighting the 480V drives on the low side.
If there is a non-linear load on the secondary, the harmonics would see the transformer inductance in parallel with the capacitors if the capacitors are on the secondary. There will be a parallel resonant point and if it is near the harmonic order where the load creates harmonic currents, then there may be excessive harmonic voltages produced.
If the capacitors are on the primary, they will be in series with the transformer inductance, so they will not create a parallel resonant point with the transformer. There will still be a parallel resonant point between the primary system inductance and the capacitors, but it is less likely to be at a low enough harmonic order to cause problems.
If there are other capacitors on the secondary, there may be voltage surge problems on the secondary during primary capacitor switching.
If the capacitors are on the primary, they will be in series with the transformer inductance, so they will not create a parallel resonant point with the transformer. There will still be a parallel resonant point between the primary system inductance and the capacitors, but it is less likely to be at a low enough harmonic order to cause problems.
If there are other capacitors on the secondary, there may be voltage surge problems on the secondary during primary capacitor switching.
Good advice dpc.
I usually try to direct connect as much capacity as I can without overvoltage problems at light loads.
I also start by calculating how many KVARHrs I need to correct the power factor to 90% (Or whatever figure your penalty kicks in at).
I divide this by the number of hours in a month to get my needed KVAR rating. It is often possible to avoid the expense of a power factor controller.
The resulting KVAR rating of the capacitor bank is much smaller than the KVAR capacity required to provide 100% correction under all circumstances.
The power company is penalizing you for using more KVARHrs in a month than you are entitled to based on your KW consumption.
Power factor is an easy way of expressing this relationship, but at the end of the month, when you write the check for the power consumption and any penalties, The bottom line is "You used too many VARHrs."
I realize that there are regional exceptions to this general statement. I understand that some utilities may use either mechanical or software ratchets on the meters to prevent subtracting VARHrs when your power factor goes leading.
My general comments are still applicable.
If, after evaluating the system, it is determined that more capacity is needed than can be safely permanently connected, look for motors that run 24/7. A good example is the fans in a lumber drying kiln at a sawmill. If all the kiln motors are corrected to 100% PF, this often goes a long way to bringing the plant power factor up to the penalty cut-off point. The kiln fans are often started in groups. It may be more economic to use a signal from the kiln control panel to bring in one large bank of capacitors when the motors have started rather than correcting each motor individually.
I realise that every-one doesn't have a sawmill to use for power factor correction, the point is, look for the largest loads you can and see what you can do with them.
In the case in question, I would start by installing enough primary capacitors to correct all the transformers to at least 100% power factor, and see how many KVARHrs I can get and how close to the penalty cut-off I am. If I need more I would consider enough capacitors to over correct the transformers to 90% or 80% leading.
If you need still more capacity, look around for a big motor that typically runs for most of the operating hours of the facility. Step one, correct it to 100% power factor. If you need more capacity look for similar motors to correct or use a contactor to connect a capacitor bank directly to the line that is about twice the capacity required to correct the motor to 100%. Typically, if the large motor is running there will also be a number of small motors running at the same time to absorb the extra KVARs.
Power factor correction can be a science or an art.
The science is to specify a power factor correction unit.
If your budget is less forgiving than Haliburtons, you can save a lot of expense by resorting to the "Art" of power factor correction.
Look around, use ingenuity, and you can often drastically reduce the cost of power factor correction.
Re transformer correction. Measure the no load current of the transformer. Select a capacitor bank that will result in an equal current and you are close enough to 100% correction. Very much closer than the 15% tolerance allowed in the ratings of power factor correction capacitors.
Respectfully.
I usually try to direct connect as much capacity as I can without overvoltage problems at light loads.
I also start by calculating how many KVARHrs I need to correct the power factor to 90% (Or whatever figure your penalty kicks in at).
I divide this by the number of hours in a month to get my needed KVAR rating. It is often possible to avoid the expense of a power factor controller.
The resulting KVAR rating of the capacitor bank is much smaller than the KVAR capacity required to provide 100% correction under all circumstances.
The power company is penalizing you for using more KVARHrs in a month than you are entitled to based on your KW consumption.
Power factor is an easy way of expressing this relationship, but at the end of the month, when you write the check for the power consumption and any penalties, The bottom line is "You used too many VARHrs."
I realize that there are regional exceptions to this general statement. I understand that some utilities may use either mechanical or software ratchets on the meters to prevent subtracting VARHrs when your power factor goes leading.
My general comments are still applicable.
If, after evaluating the system, it is determined that more capacity is needed than can be safely permanently connected, look for motors that run 24/7. A good example is the fans in a lumber drying kiln at a sawmill. If all the kiln motors are corrected to 100% PF, this often goes a long way to bringing the plant power factor up to the penalty cut-off point. The kiln fans are often started in groups. It may be more economic to use a signal from the kiln control panel to bring in one large bank of capacitors when the motors have started rather than correcting each motor individually.
I realise that every-one doesn't have a sawmill to use for power factor correction, the point is, look for the largest loads you can and see what you can do with them.
In the case in question, I would start by installing enough primary capacitors to correct all the transformers to at least 100% power factor, and see how many KVARHrs I can get and how close to the penalty cut-off I am. If I need more I would consider enough capacitors to over correct the transformers to 90% or 80% leading.
If you need still more capacity, look around for a big motor that typically runs for most of the operating hours of the facility. Step one, correct it to 100% power factor. If you need more capacity look for similar motors to correct or use a contactor to connect a capacitor bank directly to the line that is about twice the capacity required to correct the motor to 100%. Typically, if the large motor is running there will also be a number of small motors running at the same time to absorb the extra KVARs.
Power factor correction can be a science or an art.
The science is to specify a power factor correction unit.
If your budget is less forgiving than Haliburtons, you can save a lot of expense by resorting to the "Art" of power factor correction.
Look around, use ingenuity, and you can often drastically reduce the cost of power factor correction.
Re transformer correction. Measure the no load current of the transformer. Select a capacitor bank that will result in an equal current and you are close enough to 100% correction. Very much closer than the 15% tolerance allowed in the ratings of power factor correction capacitors.
Respectfully.
In the olden days, we liked to put capacitors as close to the motor as possible, allowing them to be switched on and off with the motors. There was a lot to be said for this approach when Adj. Freq. Drives were a rarity.
But I believe that this approach has to go by the wayside along with ungrounded 480 V deltas and Askarel transformers.
There are just too many non-linear loads at 480V these days and that trend seems certain to continue. There is a good probability that the 480V caps are going to have problems unless they are properly filtered. I suspect there may be more of these old 480V caps at MCCs and motors with blown fuses than without. In just about every plant I visit, the local caps have either been taken out of service intentionally, or the fuses have taken them out of service unknowingly.
Putting in larger switched caps with filters and perhaps some resistance to limit the inrush, is the future, I'm afraid.
But I believe that this approach has to go by the wayside along with ungrounded 480 V deltas and Askarel transformers.
There are just too many non-linear loads at 480V these days and that trend seems certain to continue. There is a good probability that the 480V caps are going to have problems unless they are properly filtered. I suspect there may be more of these old 480V caps at MCCs and motors with blown fuses than without. In just about every plant I visit, the local caps have either been taken out of service intentionally, or the fuses have taken them out of service unknowingly.
Putting in larger switched caps with filters and perhaps some resistance to limit the inrush, is the future, I'm afraid.
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