I'm a newbie, so be gentle. Anyway, Was studing for test and ran across the VA vs Watt (power factor) discussion. Couldn't remember and then couldn't figure out how a Meter works. Is it smart enough to compare lead/lag in phase? How? If I was a power company, think I would want to bill on VA. Don't they have to provide enough to meet my voltage and current demands (like a transformer has to be sized.)?
True Power - how does the utility co know?
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Re: True Power - how does the utility co know?
The meter records energy used in KWH. That is, it integrates V*I*PF.
You may also have to pay a demand charge and/or a PF penalty over and above the charge for energy used.
[ December 13, 2005, 02:03 PM: Message edited by: rattus ]
The meter records energy used in KWH. That is, it integrates V*I*PF.
You may also have to pay a demand charge and/or a PF penalty over and above the charge for energy used.
[ December 13, 2005, 02:03 PM: Message edited by: rattus ]
rick hart
Senior Member
- Location
- Dallas Texas
Re: True Power - how does the utility co know?
And yes, billing on VA takes care of a lot of paperwork and adjustments to bills. Industrial clusters- Houston, TX- does bill on KVA, not KWh. That makes it the consumers problem to have an efficient electrical system
Larger electric consumers do have "smart enough" meters to compare current/voltage lead lag.
And yes, billing on VA takes care of a lot of paperwork and adjustments to bills. Industrial clusters- Houston, TX- does bill on KVA, not KWh. That makes it the consumers problem to have an efficient electrical system
jtester
Senior Member
- Location
- Las Cruces N.M.
Re: True Power - how does the utility co know?
A very common approach is to bill on KW and KWH, and with larger customers include a power factor clause. The pf clause will increase an indvidual bill based on some predetermined penalty for low pf.
I worked for a utility for a while who refused to start a pf clause. Their theory was that the utility costs for covering poor power factor were existing in all rates, and if they implemented a pf penalty where KVARS were charged at the same rate as KVA, some bills would double.
They couldn't stand the political pressure to focus the poor power factors on the customers who caused it, they let everybody pay for it. They still have an insignificant pf penalty, although at least they have something in their rates now.
Jim T
A very common approach is to bill on KW and KWH, and with larger customers include a power factor clause. The pf clause will increase an indvidual bill based on some predetermined penalty for low pf.
I worked for a utility for a while who refused to start a pf clause. Their theory was that the utility costs for covering poor power factor were existing in all rates, and if they implemented a pf penalty where KVARS were charged at the same rate as KVA, some bills would double.
They couldn't stand the political pressure to focus the poor power factors on the customers who caused it, they let everybody pay for it. They still have an insignificant pf penalty, although at least they have something in their rates now.
Jim T
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- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: True Power - how does the utility co know?
No politics, please, or you'll be sent to the penalty box.
No politics, please, or you'll be sent to the penalty box.
Re: True Power - how does the utility co know?
P = VxIxPF
Which is the average power. The gears and dials integrate this power to provide an energy (not power) reading in KWH.
Electronic meters I am sure use DSP to perform the same function with no moving parts.
[ December 14, 2005, 11:31 AM: Message edited by: rattus ]
The induction watt-hour meter consists of a current winding and a voltage winding in an eddy current motor. The speed of the disc is proportional to the product of V, I, and PF,
P = VxIxPF
Which is the average power. The gears and dials integrate this power to provide an energy (not power) reading in KWH.
Electronic meters I am sure use DSP to perform the same function with no moving parts.
[ December 14, 2005, 11:31 AM: Message edited by: rattus ]
rick hart
Senior Member
- Location
- Dallas Texas
Re: True Power - how does the utility co know?
KVA is power and that is what the utility provides; the consumer uses energy and the power needed to deliver the energy at their location.
This way the utility can charge for used and borrowed power, the same as a power factor penalty, but even charge for a 0.99 power factor.
As far as a residential meter goes- I have not seen to many plug in watthour meters that can see power factor. The loads are not largely reactive in a typical residence making them pointless.
KVA is power and that is what the utility provides; the consumer uses energy and the power needed to deliver the energy at their location.
This way the utility can charge for used and borrowed power, the same as a power factor penalty, but even charge for a 0.99 power factor.
As far as a residential meter goes- I have not seen to many plug in watthour meters that can see power factor. The loads are not largely reactive in a typical residence making them pointless.
Re: True Power - how does the utility co know?
Rick, according to my book, the typical watt-hour meter exhibits a PF error which is compensated. If it does not take PF into account, you cannot call it a watt-hour meter. It is true though, that the meters do not display PF directly.
I would argue also that residences with central air conditioners present a significant reactive load, so PF is a consideration.
Can you provide a copy of the commercial rate which charges for KVA? I am especially interested because my first job out of HS was as a clerk computing electric bills.
Rick, according to my book, the typical watt-hour meter exhibits a PF error which is compensated. If it does not take PF into account, you cannot call it a watt-hour meter. It is true though, that the meters do not display PF directly.
I would argue also that residences with central air conditioners present a significant reactive load, so PF is a consideration.
Can you provide a copy of the commercial rate which charges for KVA? I am especially interested because my first job out of HS was as a clerk computing electric bills.
rick hart
Senior Member
- Location
- Dallas Texas
Re: True Power - how does the utility co know?
Yeah, I guess there must be a power factor in there to get watt-hour. But.....
Don't those meters use an assumed .8 power factor? I can't recall for sure but seem to remember seeing that on the label.
AC loads are significant in some residential loads, minimal in some and non-existent in others. Residential loads are much smaller than commercial/industrial, spread out over a larger geographical area and tend to be heavier in off-peak hours. The simple glass jar scaler meter that I have seen on houses don't know what the exact power factor is at any given time or what time of day it is. They do apply a power factor though.
Demand me
For a KVA bill- someone from Houston should be able to supply one of those. I'm in Dallas and have only heard stories...
Yeah, I guess there must be a power factor in there to get watt-hour. But.....
Don't those meters use an assumed .8 power factor? I can't recall for sure but seem to remember seeing that on the label.
AC loads are significant in some residential loads, minimal in some and non-existent in others. Residential loads are much smaller than commercial/industrial, spread out over a larger geographical area and tend to be heavier in off-peak hours. The simple glass jar scaler meter that I have seen on houses don't know what the exact power factor is at any given time or what time of day it is. They do apply a power factor though.
Demand me
For a KVA bill- someone from Houston should be able to supply one of those. I'm in Dallas and have only heard stories...
rick hart
Senior Member
- Location
- Dallas Texas
Re: True Power - how does the utility co know?
How the heck do you edit a post anyway????
DEMAND ME????
That was supposed to be something about DEMAND MEter. I forget now where I was taking that!
Shheeeessss! :roll:
How the heck do you edit a post anyway????
DEMAND ME????
That was supposed to be something about DEMAND MEter. I forget now where I was taking that!
Shheeeessss! :roll:
Re: True Power - how does the utility co know?
Rick,
I suspect that the modern electronic meters integrate KWH and KVARH separately. Then the poco can bill at different rates for real and reactive energy. Such meters can be designed to do just about anything, even figure your bill if they wanted to do that.
The old-fashioned demand meters I recall carried a 30-day chart recorder to record demand. I don't recall how they measured PF.
Rick,
I suspect that the modern electronic meters integrate KWH and KVARH separately. Then the poco can bill at different rates for real and reactive energy. Such meters can be designed to do just about anything, even figure your bill if they wanted to do that.
The old-fashioned demand meters I recall carried a 30-day chart recorder to record demand. I don't recall how they measured PF.
jtester
Senior Member
- Location
- Las Cruces N.M.
Re: True Power - how does the utility co know?
Watthour meters don't assume a power factor. The construction of the meter is such that any variation in power factor is undetectable.
They use voltage coils with hundreds of turns, and current coils with only a few turns. The phase angle between voltage and current in a watt hour meter is very close to 90 degrees regardless of the power factor.
There is a large disk, that you can often see, in the typical kwh meter that senses torque created by the voltage-current interaction. This torque is proportional to watts. The revolutions of the dial provide the hour part of the equation to the dial that you read.
Jim T
Watthour meters don't assume a power factor. The construction of the meter is such that any variation in power factor is undetectable.
They use voltage coils with hundreds of turns, and current coils with only a few turns. The phase angle between voltage and current in a watt hour meter is very close to 90 degrees regardless of the power factor.
There is a large disk, that you can often see, in the typical kwh meter that senses torque created by the voltage-current interaction. This torque is proportional to watts. The revolutions of the dial provide the hour part of the equation to the dial that you read.
Jim T
Re: True Power - how does the utility co know?
Right, Jim T!
Common sense tells us that the tiny amount of power used by the watt-hour meter would be proportional to the product of the in-phase components of voltage and current, but the meter does not measure power factor per se. As you say, the torque and speed of the disk are proportional to this power. Integrate this power over time to obtain energy used.
Now some electric rates assume a standard PF, say 90%. If the actual PF is worse, you are penalized; if it is better, you are rewarded.
Right, Jim T!
Common sense tells us that the tiny amount of power used by the watt-hour meter would be proportional to the product of the in-phase components of voltage and current, but the meter does not measure power factor per se. As you say, the torque and speed of the disk are proportional to this power. Integrate this power over time to obtain energy used.
Now some electric rates assume a standard PF, say 90%. If the actual PF is worse, you are penalized; if it is better, you are rewarded.
rick hart
Senior Member
- Location
- Dallas Texas
Re: True Power - how does the utility co know?
If the scaler meter only detects volts and amps, and does not consider/assume PF, the meter is actually reporting KVAh, not KWh?
If the scaler meter only detects volts and amps, and does not consider/assume PF, the meter is actually reporting KVAh, not KWh?
Re: True Power - how does the utility co know?
The poco charges for energy in KWH, and some charge extra for reactive energy in KVARh. This amounts to a PF penalty.
Rick, your assumption is wrong. The speed of the disk is proportional to VxIxPF. True, PF is not measured directly, but it is part of the equation.
The poco charges for energy in KWH, and some charge extra for reactive energy in KVARh. This amounts to a PF penalty.
ramsy
NoFixNoPay Electric
- Location
- LA basin, CA
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- Service Electrician 2020 NEC
Re: True Power - how does the utility co know?
With only a voltage and current coil, these analog service meters may be designed for another Watts formula (load Volts * Device Amps) or Watts=(Er*Ir), which is the same as Watts=(VA*PF).
That formula doesn't require any assumptions for PF, to get watts.
[ December 23, 2005, 09:15 AM: Message edited by: ramsy ]
With only a voltage and current coil, these analog service meters may be designed for another Watts formula (load Volts * Device Amps) or Watts=(Er*Ir), which is the same as Watts=(VA*PF).
That formula doesn't require any assumptions for PF, to get watts.
[ December 23, 2005, 09:15 AM: Message edited by: ramsy ]
beanland
Senior Member
- Location
- Vancouver, WA
Re: True Power - how does the utility co know?
The phase angle between voltage magnetic field and current magnetic fields is largely removed in a rotating-disk watthour meter. Only that portion of current magnetic field in phase with the voltage magnetic field produces torque in the disk, causing it to rotate. The torque is an indication of real power flow. The rotation of the disk integrates the power over time, giving energy. The number of rotations is counted, giving kWh. Electronic meters do the same thing (multiply in real time the voltage and current) to get real power.
Some utilities use kVA meters. Utility capital costs are driven by connected kVA and fuel costs are driven by kWh. Some combination of billing for real and reactive power or real power and power factor is common to recover capital costs AND energy costs. Billing for power factor in small customers has tradionally been too expensive to be worthwhile.
The phase angle between voltage magnetic field and current magnetic fields is largely removed in a rotating-disk watthour meter. Only that portion of current magnetic field in phase with the voltage magnetic field produces torque in the disk, causing it to rotate. The torque is an indication of real power flow. The rotation of the disk integrates the power over time, giving energy. The number of rotations is counted, giving kWh. Electronic meters do the same thing (multiply in real time the voltage and current) to get real power.
Some utilities use kVA meters. Utility capital costs are driven by connected kVA and fuel costs are driven by kWh. Some combination of billing for real and reactive power or real power and power factor is common to recover capital costs AND energy costs. Billing for power factor in small customers has tradionally been too expensive to be worthwhile.
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