Mismatched Electric Meter
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jtester
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- Las Cruces N.M.
Re: Mismatched Electric Meter
martinm
Can you describe the metering better? Is it instrument transformer metering, 2 stator or 3 stator, etc.? Secondly, is the delta meter for a high leg delta or just a 3 wire? Finally, is the wye likely to be unbalanced, is there any netural current?
In looking thru my old metering books, a high leg delta self contained meter has one coil connected l-l, and the other(s) l-n. It might end up difficult or impossible to deal with converting readings if one leg is l-l.
If you can give a bit more information, I'll search my books further.
Jim T
martinm
Can you describe the metering better? Is it instrument transformer metering, 2 stator or 3 stator, etc.? Secondly, is the delta meter for a high leg delta or just a 3 wire? Finally, is the wye likely to be unbalanced, is there any netural current?
In looking thru my old metering books, a high leg delta self contained meter has one coil connected l-l, and the other(s) l-n. It might end up difficult or impossible to deal with converting readings if one leg is l-l.
If you can give a bit more information, I'll search my books further.
Jim T
Re: Mismatched Electric Meter
Meter is 120/240v 4-prong meter designed for use on delta service installed in a 4-prong base in a 120/208v wye service where there should be 5-prong meter and base. I believe I understand the problem and would like to have a second opinion. This is my line of reasoning: The meter in place is designed for 240v and sees the 208v as low at one-half the square root of three (0.86603) times the 240v on which the meter is calibrated. In addition, the current in the 208v is shifted 30 degrees from the location normal for a 240v system. The meter perceives the 30 degree current shift as causing a power factor of 86.603%. Since 0.86603 times 0.86603 is equal to 0.7500, the meter would give a readout of consumption as 75% of actual usage. In other words, the customer would be getting 1/3 more kWh than he is paying for if the mismatched meter is used for billing. What do you think of this logic?
Meter is 120/240v 4-prong meter designed for use on delta service installed in a 4-prong base in a 120/208v wye service where there should be 5-prong meter and base. I believe I understand the problem and would like to have a second opinion. This is my line of reasoning: The meter in place is designed for 240v and sees the 208v as low at one-half the square root of three (0.86603) times the 240v on which the meter is calibrated. In addition, the current in the 208v is shifted 30 degrees from the location normal for a 240v system. The meter perceives the 30 degree current shift as causing a power factor of 86.603%. Since 0.86603 times 0.86603 is equal to 0.7500, the meter would give a readout of consumption as 75% of actual usage. In other words, the customer would be getting 1/3 more kWh than he is paying for if the mismatched meter is used for billing. What do you think of this logic?
Re: Mismatched Electric Meter
I'm not really familiar with meters, but both a 240V delta and a 208V wye have each phase 120 degrees apart. I don't understand why you are saying a 208V system would have the voltage shifted by 30 degrees? Unless it is a single phase meter, but that couldn't be used on a 3 phase service. Or are you only using 2 lines of a 208V wye?
Don't most power meters actually measure the voltage and the current and use both to calculate true power? If that is the case, wouldn't your meter read correctly?
Steve
I'm not really familiar with meters, but both a 240V delta and a 208V wye have each phase 120 degrees apart. I don't understand why you are saying a 208V system would have the voltage shifted by 30 degrees? Unless it is a single phase meter, but that couldn't be used on a 3 phase service. Or are you only using 2 lines of a 208V wye?
Don't most power meters actually measure the voltage and the current and use both to calculate true power? If that is the case, wouldn't your meter read correctly?
Steve
jtester
Senior Member
- Location
- Las Cruces N.M.
Re: Mismatched Electric Meter
If I understand your meter description correctly, your 4 prong meter is what is I think of as a 5 terminal socket meter. This would measure a 3 wire 3 ph delta, or ungrounded wye. I don't believe that calculations will help you convert readings to a grounded wye service. The meter needs one additional stator to accurately measure 4 wire energy. Blondell's theorem, the basis for polyphase metering, states that with a N-wire system you need N stators, but if one wire is a common, you can get by with N-1.
One thing that confuses me is your description of a 5 prong meter. Typically a 4 wire service would require a 7 terminal meter at a minimum.
What brand is the meter?
Is the service grounded wye with any imbalance? If not, it may look like a delta to the meter, and not matter anyway.
Jim T
If I understand your meter description correctly, your 4 prong meter is what is I think of as a 5 terminal socket meter. This would measure a 3 wire 3 ph delta, or ungrounded wye. I don't believe that calculations will help you convert readings to a grounded wye service. The meter needs one additional stator to accurately measure 4 wire energy. Blondell's theorem, the basis for polyphase metering, states that with a N-wire system you need N stators, but if one wire is a common, you can get by with N-1.
One thing that confuses me is your description of a 5 prong meter. Typically a 4 wire service would require a 7 terminal meter at a minimum.
What brand is the meter?
Is the service grounded wye with any imbalance? If not, it may look like a delta to the meter, and not matter anyway.
Jim T
Re: Mismatched Electric Meter
martinm -
Metering is not my strong suit, but this gave me a chance to look up and see what I could find. My source is ANSI C12.1-2001, Appendix A.
Quote from martinm: If you install a 240v delta meter in a 208v wye service, what is the error between actual and apparent kWh consumption and what is the mathematical equation used to calculate the error?
Near as I can tell, there isn't one. You just aren't measuring part of the 208 Wye, so that part is unknown.
For 240 delta, the only four-jaw connection I could find won't do 120/240 delta. As jt said, any of the metering ckts that will do 120/240 delta are 7 jaws. And some of them are fine for 208 Wye.
Unless, again as jt said, you are not using the 208 neutral. In this case, it wouldn't matter, it would just look like the delta. The change in voltage from 240 to 208 is taken care of in the meter. The change in phase angles wouldn't matter either. Watt meters measure the voltage, measure the current, and do a vector multiplication.
From the data we have, I don't think you can get here from there. Maybe some of our utility metering whizzes will step in.
carl
martinm -
Metering is not my strong suit, but this gave me a chance to look up and see what I could find. My source is ANSI C12.1-2001, Appendix A.
Quote from martinm: If you install a 240v delta meter in a 208v wye service, what is the error between actual and apparent kWh consumption and what is the mathematical equation used to calculate the error?
Near as I can tell, there isn't one. You just aren't measuring part of the 208 Wye, so that part is unknown.
For 240 delta, the only four-jaw connection I could find won't do 120/240 delta. As jt said, any of the metering ckts that will do 120/240 delta are 7 jaws. And some of them are fine for 208 Wye.
Unless, again as jt said, you are not using the 208 neutral. In this case, it wouldn't matter, it would just look like the delta. The change in voltage from 240 to 208 is taken care of in the meter. The change in phase angles wouldn't matter either. Watt meters measure the voltage, measure the current, and do a vector multiplication.
From the data we have, I don't think you can get here from there. Maybe some of our utility metering whizzes will step in.
carl
Re: Mismatched Electric Meter
Let me be more specific. I apologize for the confusion I caused. The situation in question is that a three-wire single-phase delta meter base with meter is installed on a three-wire single-phase wye service. The problem seems to be that the delta meter is using a 208v component of voltage of the wye circuit in the metering equation but the meter is feeding two 120v circuits. This would cause the meter to indicate only 86.6% of the kWh consumption since 208 is 0.866X240. In addition, the actual current vector in the wye service is shifted 30 degrees from where the delta meter expects it to be, so the meter takes this current vector shift as a power factor of 86.6% since cosine of 30 degrees is 0.866. Therefore, since 0.866X0.866=0.75, the meter would read only 75%of actual consumption. All of this assumes that the actual power factor in the wye circuit is near unity. Is this line of reasoning logical?
Let me be more specific. I apologize for the confusion I caused. The situation in question is that a three-wire single-phase delta meter base with meter is installed on a three-wire single-phase wye service. The problem seems to be that the delta meter is using a 208v component of voltage of the wye circuit in the metering equation but the meter is feeding two 120v circuits. This would cause the meter to indicate only 86.6% of the kWh consumption since 208 is 0.866X240. In addition, the actual current vector in the wye service is shifted 30 degrees from where the delta meter expects it to be, so the meter takes this current vector shift as a power factor of 86.6% since cosine of 30 degrees is 0.866. Therefore, since 0.866X0.866=0.75, the meter would read only 75%of actual consumption. All of this assumes that the actual power factor in the wye circuit is near unity. Is this line of reasoning logical?
jtester
Senior Member
- Location
- Las Cruces N.M.
Re: Mismatched Electric Meter
martinm
I think I understand a little better.
First, the torque that turns the watthour disk is generated by a voltage and a current. Because of the large # of turns in the voltage coil, the voltage and current in that particular stator are nearly 90 degrees apart. If you have more than 1 of these stators, you are measuring the energy in more than one circuit. There doesn't need to be any particular phase relationship between additional stators. If you wanted, you could use the same phase and run it thru 2 stators and get twice the torque.
A single phase meter with the chance for unbalance will need 2 stators, one for each phase to recognize that there is some neutral current. A single phase meter with no imbalance will need only 1 stator. Remember that the phase relationship is not a part of the torque generated by 2 stators. You simply have 2 little motors driving that disk.
Secondly the delta meter has voltage coils connected l-l, so the only concern is whether the voltage magnitude difference is sufficient to cause a misreading. I think shouldn't be too much of a problem. 5 terminal meters have the voltage coils connected l-n.
Your delta meter might be a single stator, my old Westinghouse meter guide calls it a 2 wire meter. More likely it would be a 2 stator meter, called a Combination 2-3 wire meter. I say this because you describe it as a 120/240 delta which indicates a neutral.
If it is a 2 stator meter, it probably measured the kwh accurately. None of the phase shifts, 1.732 factor, etc. apply.
I am long winded, but I hope that helps.
Jim T.
martinm
I think I understand a little better.
First, the torque that turns the watthour disk is generated by a voltage and a current. Because of the large # of turns in the voltage coil, the voltage and current in that particular stator are nearly 90 degrees apart. If you have more than 1 of these stators, you are measuring the energy in more than one circuit. There doesn't need to be any particular phase relationship between additional stators. If you wanted, you could use the same phase and run it thru 2 stators and get twice the torque.
A single phase meter with the chance for unbalance will need 2 stators, one for each phase to recognize that there is some neutral current. A single phase meter with no imbalance will need only 1 stator. Remember that the phase relationship is not a part of the torque generated by 2 stators. You simply have 2 little motors driving that disk.
Secondly the delta meter has voltage coils connected l-l, so the only concern is whether the voltage magnitude difference is sufficient to cause a misreading. I think shouldn't be too much of a problem. 5 terminal meters have the voltage coils connected l-n.
Your delta meter might be a single stator, my old Westinghouse meter guide calls it a 2 wire meter. More likely it would be a 2 stator meter, called a Combination 2-3 wire meter. I say this because you describe it as a 120/240 delta which indicates a neutral.
If it is a 2 stator meter, it probably measured the kwh accurately. None of the phase shifts, 1.732 factor, etc. apply.
I am long winded, but I hope that helps.
Jim T.
Re: Mismatched Electric Meter
A delta meter is a single element meter that runs on 240.A wye meter is a two element meter that needs the fith position grounded to get 120volts.If that is not grounded the meter will only get 104 volts per coil reducing torque on disk.208/2=104
A delta meter is a single element meter that runs on 240.A wye meter is a two element meter that needs the fith position grounded to get 120volts.If that is not grounded the meter will only get 104 volts per coil reducing torque on disk.208/2=104
gregoryfast
Member
- Location
- Alaska
Re: Mismatched Electric Meter
There is no calculation that will tell you what the difference in registration would be of a varying load and changing phase angles. However, all of the phase to phase ( 208 volts) appliances such as water heater, dryer and range will measure 100% of connected load. If the power factor is unity (which is not very likely), then the registration on the phase to ground (120 volt) loads will be 75%. Depending on how much 120 volt load is on each leg and the power factor of the connected load, the registration can be anywhere from 44% to 140%. I have all of the calculations in a work book from a class I took that shows it.
It is also covered "Handbook for Electricity Metering".
The problem is that the phases are separated by 120?, instead of 180?. The modern residential meter, Form 2S, meters 1/2 the amperage and double the voltage for the line to ground loads.
There is no calculation that will tell you what the difference in registration would be of a varying load and changing phase angles. However, all of the phase to phase ( 208 volts) appliances such as water heater, dryer and range will measure 100% of connected load. If the power factor is unity (which is not very likely), then the registration on the phase to ground (120 volt) loads will be 75%. Depending on how much 120 volt load is on each leg and the power factor of the connected load, the registration can be anywhere from 44% to 140%. I have all of the calculations in a work book from a class I took that shows it.
It is also covered "Handbook for Electricity Metering".
The problem is that the phases are separated by 120?, instead of 180?. The modern residential meter, Form 2S, meters 1/2 the amperage and double the voltage for the line to ground loads.
jtester
Senior Member
- Location
- Las Cruces N.M.
Re: Mismatched Electric Meter
gregoryfast
I have a copy of the Handbook For Electricity Metering. Could you direct me to that calculation? My other reference books show a 2S as one potential coil connected L-L and 2 current coils. With only one potential coil, I don't see where phase relationship enters into the calculation, it seems like only magnitude will matter. Thanks.
Jim T
gregoryfast
I have a copy of the Handbook For Electricity Metering. Could you direct me to that calculation? My other reference books show a 2S as one potential coil connected L-L and 2 current coils. With only one potential coil, I don't see where phase relationship enters into the calculation, it seems like only magnitude will matter. Thanks.
Jim T
gregoryfast
Member
- Location
- Alaska
Re: Mismatched Electric Meter
The issue is addressed in the 9th edition of the Handbook on page 149, under Three -Wire Network Service.
I have the calculations in a separate workbook. There are 16 sets of calculations. They show untity PF, 75%, 50% and 30% with all of the load on one leg, 25/75, 50/50, 75/25 load. Unless you knew precisely the phase angle and current of the connected load and it was constant for the billing period, you cannot come up with any meaningful data.
If you want to PM me, I can fax what I have to you. It is about 4 pages long. There really isn't any value to the calculations, other than to understand the possible consequences of applying a Form 2S meter on a 3 wire 208Y/120 system.
The issue is addressed in the 9th edition of the Handbook on page 149, under Three -Wire Network Service.
I have the calculations in a separate workbook. There are 16 sets of calculations. They show untity PF, 75%, 50% and 30% with all of the load on one leg, 25/75, 50/50, 75/25 load. Unless you knew precisely the phase angle and current of the connected load and it was constant for the billing period, you cannot come up with any meaningful data.
If you want to PM me, I can fax what I have to you. It is about 4 pages long. There really isn't any value to the calculations, other than to understand the possible consequences of applying a Form 2S meter on a 3 wire 208Y/120 system.
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