kW reductions from capacitors

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petersonra

Senior Member
Location
Northern illinois
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engineer
the OP seemed to be talking about PFC in general and not for a specific size motor.

I have often had this vision of every motor on the grid being on a VFD and the current waveforms causing the utility lines to dance. :)
 

mivey

Senior Member
But that's usually not a lot.
Not for many cases but the net adds up for some cases (large poor pf loads located a long way from the meter that run a lot of the time).

OK, no. First off I have NEVER seen a utility that bills solely on kVA demand. They bill on kWh, then in SOME cases there is a PENALTY for exceeding an allotted kVA demand. But they do not "bill on" kVA demand.
He did not say solely as we know there are other charges. But as for billing on kVA, some do. Here is just one example: http://www.cityofames.org/index.aspx?page=113

Used to see it used a lot with irrigation rates also.

Secondly, "marked savings" is the kind of nebulous fluff terminology you see in a marketing pitch, I differ with what it implies.Hmmm... I'd say that value is debatable. If the conductors are properly sized to code requirements and adjusted for voltage drop under full load, then the losses would be very low to start with. I believe that 1-2% would be a stretch.
I agree they are small but can make the installation worthwhile in some cases (see response to Bes).

That would be your first purely true statement. The reason they are not installed to reduce kW is because... doing so doesn't save enough kWh to make it worth doing!
You would be right for many cases but not all.

Do they exist? Yes. Are they worth the cost of installing PFC capacitors JUST for the purpose of attaining that miniscule reduction in losses? No.
Maybe. Run the numbers then get back to me.

Suppose we have a 480 volt, 150 amp load, 78% pf, 200-300 ft from the meter, running 50%-60% of the time, $0.10-$0.20/kWh incremental cost. Correct to 93-94% pf at using $20/kvar installed. Unless I just fat-fingered the numbers, the payback is favorable.
 

GoldDigger

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Placerville, CA, USA
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Retired PV System Designer
OK. "Billing on KVA demand" is perhaps too loose a term.
1. Billing per KVA for the highest 15 minute KVA recorded during the billing cycle seems pretty close to the concept of a penalty for high KVA, just without the step function at some minimum level.
2 "Billing for KVA" is not to be confused with billing per KVAH, which I do not know of any POCO doing.

Is anybody aware of a utility that bills based on highest KW as well as on cumulative KWH?
 

mivey

Senior Member
1. Billing per KVA for the highest 15 minute KVA recorded during the billing cycle seems pretty close to the concept of a penalty for high KVA, just without the step function at some minimum level.
It is just another way to bill for kW and poor power factor.

2 "Billing for KVA" is not to be confused with billing per KVAH, which I do not know of any POCO doing.
Me neither. I have seen the use of kWh and kvarh to calculate the average kVA for billing purposes.

Is anybody aware of a utility that bills based on highest KW as well as on cumulative KWH?
Most utilities do it that way for demand rates.
 

mivey

Senior Member
I was specifically asking about billing based on highest kWh only rather than folding kWh and KVAR together into one KVA charge.
Your units are mis-matching so your question is not clear.

But separate kW and kvar charges seem to be more common than a combined kVA charge.

Along with base charges and energy charges, some also have demand charges like:
kW charges
separate kW and kvar charges
kVA charges
I seem to recall seeing a separate kW and KVA charge where the generation and transmission costs were kW based and the distribution costs were kVA based.

in addition, some or all of the above demand charges may be included as part of the energy charge by using an hours use of demand rate with increasing rate blocks. The hours use of demand rates are load-factor sensitive and the total charges decrease as the load factor improves because most of the demand charges are collected in the first energy blocks.
 

Sahib

Senior Member
Location
India
The OP wants a criterion to ascertain whether there is any KW saving by installing load side capacitor in an electrical installation. The formula for reduction in voltage drop in post #17 may well serve as the criterion. For example if 10 MVA is the fault level and 0.7 MVA is the PFC capacitor size, then applying the formula, the reduction in voltage drop =(0.7/10)*100=7%. So the power saving may be up to 7%, worth a try to do further calculations to confirm it. But if the fault level is 100 MVA instead of 10 MVA, then the reduction in voltage drop is =0.7% only. So there would be no power saving in this case.
 

mivey

Senior Member
The OP wants a criterion to ascertain whether there is any KW saving by installing load side capacitor in an electrical installation. The formula for reduction in voltage drop in post #17 may well serve as the criterion. For example if 10 MVA is the fault level and 0.7 MVA is the PFC capacitor size, then applying the formula, the reduction in voltage drop =(0.7/10)*100=7%. So the power saving may be up to 7%, worth a try to do further calculations to confirm it. But if the fault level is 100 MVA instead of 10 MVA, then the reduction in voltage drop is =0.7% only. So there would be no power saving in this case.
Not sure about that.

With a 480 volt scenario similar to my prior post we have about a 1.5% volt drop with few tenths of percent change in the volt drop and about a 10-15 year simple payback, depending on parameters.

For a higher volt drop, say using 240 volts, we are in the 3% volt drop range with about a 1/2 percent volt drop change but the payback goes to 5-10 years, depending on parameters.

So, the higher the volt drop, the more the opportunity for a faster payback even if the % change in volt drop is not several percent.
 

Sahib

Senior Member
Location
India
I've seen it done, and done it myself for two main reasons.
It reduces maximum demand in terms of reduced kVA. Exceeding maximum demand is extremely costly in this country.

The other application is reducing the size of conductors and/or supply capacity. We currently have an installation for pumps where the customer decided to change the motor from a synchronous induction motor with unity power factor to cage induction motors with 0.76 p.f. One serious consequence of this was that the unit transformer supplying the original motor was not rated for the increased kVA. PFC fixed this by getting the pf up to about 0.97.

Energy saving? Not really. As is our usual practice, we fitted detuning chokes and the losses in these, though not a lot, was about the same as the reduction cable losses resulting from the reduced current.
Your solution to use the same transformer to power the low power factor induction motor by connecting PFC capacitor after its starter would create problems for the motor and other connected loads if the corrected pf is more than 0.95.See
http://ecmweb.com/power-quality/basics-pf-correction-single-induction-motors
But if the connection of the capacitor is before the starter i.e if the capacitor has a separate ON-OFF control, there is no problem:you could correct the pf up to unity.
 

mivey

Senior Member
Perhaps not, because for cable size up to 35 sq.mm, approximately,70 Kcm, the reactance may be ignored and only resistance of cable may be taken into account.
Voltage drop is a function of magnitudes, not vectors. I've discussed this before in another thread somewhere.
 

Sahib

Senior Member
Location
India
Not sure about that.

With a 480 volt scenario similar to my prior post we have about a 1.5% volt drop with few tenths of percent change in the volt drop and about a 10-15 year simple payback, depending on parameters.

For a higher volt drop, say using 240 volts, we are in the 3% volt drop range with about a 1/2 percent volt drop change but the payback goes to 5-10 years, depending on parameters.

So, the higher the volt drop, the more the opportunity for a faster payback even if the % change in volt drop is not several percent.
The goal for the OP is to ascertain power saving irrespective of system voltage. If the lines are of predominantly resistive in comparison to its inductive component i.e its inductive can be ignored, then it is easy to see that percentage reduction in voltage drop is the same as the percentage reduction in power loss i.e percent KW saving ( it can be derived from the formula in post#17).
If the inductive component of the lines can not be ignored, then the loss power factor of the lines need to be taken into account to work out the reduction in power loss in the lines. This divided by the input power and the result multiplied by 100 gives the percentage reduction in power loss.
 

Besoeker

Senior Member
Location
UK
Your solution to use the same transformer to power the low power factor induction motor by connecting PFC capacitor after its starter would create problems for the motor and other connected loads if the corrected pf is more than 0.95.See
http://ecmweb.com/power-quality/basics-pf-correction-single-induction-motors
But if the connection of the capacitor is before the starter i.e if the capacitor has a separate ON-OFF control, there is no problem:you could correct the pf up to unity.
I don't think it is a very good article. The very notion that the PFC would be connected downstream of the motor overload is not something I would expect any half competent engineer to do.And on making the capacitor smaller if the motor is not running at FLC is a bit odd as well. Draw out the Steinmetz equivalent circuit and you may begin to understand.

And we always fit a contactor for the PFC. No doubt you will be aware of the risk of self-excitation if you don't.
 
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