PF Correction yields cost savings?

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paulengr

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The plant I work at averages about 5-6 MW at full production at a PF of about 0.75. We do not have any cap banks present. We are not tariffed for power factor, so there's no immediate power savings from reduced tariff charges. We have a new substation going online in November, consisting of 2 transformers sized at 6.25 MVA with the fans, 5 MVA without.

If we install a cap bank, we could easily operate on a single transformer in a pinch. We would also reduce the load on the distribution system upstream of the cap bank, so the best option would probably be to hit all the major motors or at woest, MCC's throughout the plant as opposed to one large varcap bank for the whole plant.

I've been asked whether there's a power reduction. Simple physics says yes (reducing I^2*R), but I'm unsure how much. So if I add X kvar in capacitance to the system, is it valid to divide by the system nominal voltage, square the result, multiply by the system impedance (recent arc flash/coordination study documented this), hours, and $/kwhr?
 
paulengr said:
I've been asked whether there's a power reduction. Simple physics says yes (reducing I^2*R), but I'm unsure how much. So if I add X kvar in capacitance to the system, is it valid to divide by the system nominal voltage, square the result, multiply by the system impedance (recent arc flash/coordination study documented this), hours, and $/kwhr?
Click to expand...
If the utility is providing primary service and you are being billed on a KW basis, your savings will not matter whether the caps are on the sub or in the plant. You will improve VD with the caps on the motors.
The system impedance includes the system resistance and system reactance.
As you said the savings is I?R. You will need each of the motor amps and the total of the conductor resistance feeding the motor. That could be a big undertaking. Don't forget the transformer losses. That could be a major savings.
 
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paulengr said:
The plant I work at averages about 5-6 MW at full production at a PF of about 0.75. We do not have any cap banks present. We are not tariffed for power factor, so there's no immediate power savings from reduced tariff charges. We have a new substation going online in November, consisting of 2 transformers sized at 6.25 MVA with the fans, 5 MVA without.

If we install a cap bank, we could easily operate on a single transformer in a pinch. We would also reduce the load on the distribution system upstream of the cap bank, so the best option would probably be to hit all the major motors or at woest, MCC's throughout the plant as opposed to one large varcap bank for the whole plant.

I've been asked whether there's a power reduction. Simple physics says yes (reducing I^2*R), but I'm unsure how much. So if I add X kvar in (capacitance to the system, is it valid to divide by the system nominal voltage, square the result, multiply by the system impedance (recent arc flash/coordination study documented this), hours, and $/kwhr?
Click to expand...
If you are not penalised for operating at low power factor, I'm not sure that installing PFC wil get you much benefit.
Sure, if installed at local MCCs, it would reduce losses in your distribution system. Correct to 0.95 and you could cut distribution loses by nearly 40%.
But 40% of what? Probably not a lot given that the distribution system (presumably) already exists and is rated for existing currents.
Transformer losses will also be reduced but again, this is not likely to a very great saving. In my experience, copper losses for the size of transformers you mention, is in the region of 1% of rating.

Bob makes a good point. Savings would be for I?R losses. System impedance is usually mostly reactive, so basing calculations on that would be a bit misleading. In the absence of any better information I usually take resistance to be 0.24 times the impedance.

In summary, I think for your particular application, it might be difficult to justify the capital cost of PFC based on energy savings alone.

Just my thoughts.
 
paulengr said:
The plant I work at averages about 5-6 MW at full production at a PF of about 0.75. We do not have any cap banks present. We are not tariffed for power factor, so there's no immediate power savings from reduced tariff charges. We have a new substation going online in November, consisting of 2 transformers sized at 6.25 MVA with the fans, 5 MVA without.

If we install a cap bank, we could easily operate on a single transformer in a pinch. We would also reduce the load on the distribution system upstream of the cap bank, so the best option would probably be to hit all the major motors or at woest, MCC's throughout the plant as opposed to one large varcap bank for the whole plant.

I've been asked whether there's a power reduction. Simple physics says yes (reducing I^2*R), but I'm unsure how much. So if I add X kvar in capacitance to the system, is it valid to divide by the system nominal voltage, square the result, multiply by the system impedance (recent arc flash/coordination study documented this), hours, and $/kwhr?
Click to expand...
Sure,if installed at local MCCs, it would reduce losses in your distribution system. Correct to 0.95 and you could cut distribution loses by nearly 40%.
But 40% of what? Probably not a lot given that the distribution system (presumably) already exists and is rated for existing currents.
Transformer losses will also be reduced but again, this is not likely to a very great saving. In my experience, copper losses for the size of transformers you mention, is in the region of 1% of rating.

Bob makes a good point. Savings would be for I?R losses. System impedance is usually mostly reactive, so basing calculations on that would be a bit misleading. In the absence of any better information I usually take resistance to be 0.24 times the impedance.

In summary, I think for your particular application, it might be difficult to justify the capital cost of PFC based on energy savings alone.

Just my thoughts.
 
paulengr said:
The plant I work at averages about 5-6 MW at full production at a PF of about 0.75. We do not have any cap banks present. We are not tariffed for power factor, so there's no immediate power savings from reduced tariff charges. We have a new substation going online in November, consisting of 2 transformers sized at 6.25 MVA with the fans, 5 MVA without.

If we install a cap bank, we could easily operate on a single transformer in a pinch. We would also reduce the load on the distribution system upstream of the cap bank, so the best option would probably be to hit all the major motors or at woest, MCC's throughout the plant as opposed to one large varcap bank for the whole plant.

I've been asked whether there's a power reduction. Simple physics says yes (reducing I^2*R), but I'm unsure how much. So if I add X kvar in capacitance to the system, is it valid to divide by the system nominal voltage, square the result, multiply by the system impedance (recent arc flash/coordination study documented this), hours, and $/kwhr?
Click to expand...
I have tried to reply to this before, so if this appears to be a duplicate (or triplicate) post, I apologise.

If you are not penalised for poor pf, then I think you won't save much by installing pfc capacitors.
If you fit them at the MCCs and inprove pf from 0.75 to 0.95 you could potentially reducedistribution losses by up to 40%, but 40% of not a lot, I suspect.
Transformer losses woul be reduced too, but given that copper loss is likely to be about 1% or less, there isn't much scope there either.

My guess is that the capital cost could yield more savings elsewhere.
 
paulengr said:
The plant I work at averages about 5-6 MW at full production at a PF of about 0.75. We do not have any cap banks present. We are not tariffed for power factor, so there's no immediate power savings from reduced tariff charges. We have a new substation going online in November, consisting of 2 transformers sized at 6.25 MVA with the fans, 5 MVA without.

If we install a cap bank, we could easily operate on a single transformer in a pinch. We would also reduce the load on the distribution system upstream of the cap bank, so the best option would probably be to hit all the major motors or at woest, MCC's throughout the plant as opposed to one large varcap bank for the whole plant.

I've been asked whether there's a power reduction. Simple physics says yes (reducing I^2*R), but I'm unsure how much. So if I add X kvar in capacitance to the system, is it valid to divide by the system nominal voltage, square the result, multiply by the system impedance (recent arc flash/coordination study documented this), hours, and $/kwhr?
Click to expand...
I have tried to reply to this a few times now, and my post doesn't appear to make it on to the thread.
Sorry.
 
Boseoker, welcome to the forums, your posts should appear immediately now.

Roger
 
paulengr said:
The plant I work at averages about 5-6 MW at full production at a PF of about 0.75. We do not have any cap banks present. We are not tariffed for power factor, so there's no immediate power savings from reduced tariff charges. We have a new substation going online in November, consisting of 2 transformers sized at 6.25 MVA with the fans, 5 MVA without.

If we install a cap bank, we could easily operate on a single transformer in a pinch. We would also reduce the load on the distribution system upstream of the cap bank, so the best option would probably be to hit all the major motors or at woest, MCC's throughout the plant as opposed to one large varcap bank for the whole plant.

I've been asked whether there's a power reduction. Simple physics says yes (reducing I^2*R), but I'm unsure how much. So if I add X kvar in capacitance to the system, is it valid to divide by the system nominal voltage, square the result, multiply by the system impedance (recent arc flash/coordination study documented this), hours, and $/kwhr?
Click to expand...

Simple answer: no.

Capacitors will also add harmonic components that may have undesirable effects.
 
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