Standard vs net meter

[BillK-AZ]

Does anyone know, as a measured fact, how the present generation of bi-directional meters measures and reports the following situation:
-PV inverter, connected L1 to L2 for a nominal 220V service, is producing 5 kW
-The local load is also 5 kW, but is all on L1 to N

An older analog (spinning disk) meter would read zero because the forces driving the disk balance.

Does the modern digital meter do the same?

Or, does the digital meter record both a forward 2.5 kW (L1) and a reverse 2.5 kW (L2)? Makes a difference if there are different rates for buying and selling.

I asked this of a local utility years ago and they said they did not know for sure and had never measured such a situation as a test (meter shop did not have access to a PV system).

 

[electrofelon]

Does anyone know, as a measured fact, how the present generation of bi-directional meters measures and reports the following situation:
-PV inverter, connected L1 to L2 for a nominal 220V service, is producing 5 kW
-The local load is also 5 kW, but is all on L1 to N

An older analog (spinning disk) meter would read zero because the forces driving the disk balance.

Does the modern digital meter do the same?

Or, does the digital meter record both a forward 2.5 kW (L1) and a reverse 2.5 kW (L2)? Makes a difference if there are different rates for buying and selling.

I asked this of a local utility years ago and they said they did not know for sure and had never measured such a situation as a test (meter shop did not have access to a PV system).

interesting scenario. I do not know the answer. I have simple net metering so it wouldnt matter for me, but I have been curious how much data my meter records, even if my utility doesnt need/use the it.

 

[jaggedben]

I don't know for sure, but I think most likely it just reads zero. I've never seen any metering data that differentiated between L1 and L2. Not to say it couldn't exist, but I think it would impose on the hardware design, not just the software.

 

[electrofelon]

@Hv&Lv might know.

 

[Carultch]

Does anyone know, as a measured fact, how the present generation of bi-directional meters measures and reports the following situation:
-PV inverter, connected L1 to L2 for a nominal 220V service, is producing 5 kW
-The local load is also 5 kW, but is all on L1 to N

An older analog (spinning disk) meter would read zero because the forces driving the disk balance.

Does the modern digital meter do the same?

Or, does the digital meter record both a forward 2.5 kW (L1) and a reverse 2.5 kW (L2)? Makes a difference if there are different rates for buying and selling.

I asked this of a local utility years ago and they said they did not know for sure and had never measured such a situation as a test (meter shop did not have access to a PV system).

A single phase meter for 120/240V uses only 4 jaws to connect the globe to the socket. Line 1 in, line 1 out, line 2 in, and line 2 out. The neutral doesn't get connected. It measures the line-to-line voltage, and makes the assumption that the line-to-neutral voltage of each line is equal. To measure current, it passes the conductor from both lines through a shared CT core, and then uses the secondary of that CT as the input of the metering hardware. Line 1 is passed directly through this CT, and line 2 is passed through in reverse.

This means it can only measure (I1 - I2), and cannot tell whether there is any imbalance between the two lines. Because I2 is usually the opposite sign as I1 at any instant in time, the value of (I1 - I2) would ordinarily add up the magnitudes of the two current waveforms (even though it is a subtraction problem).

Considering your example:
A local load of 5 kW all on line 1, would be 41.6A. Generation of 5kW equally on both lines, would be 20.8A. The net line 1 load would be 20.8A consumption, and the net line 2 load would be 20.8A export. On line 1, define positive as consumption. This would mean consumption on line 2 at the same instant is negative, and production on line 2 at the same instant is positive. The value of (I1 - I2) would therefore be zero. The meter would stay in place during this condition of production equaling consumption.

Very few (if any) utility meters would have separate registers for kilowatt-hours by phase. Especially on a residential scale system, where service metering is kept as simple as possible.

 

[electrofelon]

To measure current, it passes the conductor from both lines through a shared CT core, and then uses the secondary of that CT as the input of the metering hardware. Line 1 is passed directly through this CT, and line 2 is passed through in reverse.

This means it can only measure (I1 - I2), and cannot tell whether there is any imbalance between the two lines.

But aren't you making some assumptions here? Could not a net meter be different so as to measure imbalance?

 

[winnie]

Interesting scenario indeed.

As Carultch notes a standard residential meter only measures L-L and assumes voltage balance to the neutral. Thus the unbalanced loading (voltage drop on L1, voltage rise on L2) would cause an error.

I believe that the mechanical spinning disk meters used separate current stators for each line and thus registered the net torque on the disk. Running both lines through the same CT would have a similar effect.

I would expect such meters to use a single CT in the interest of lower component count, however it is plausible that a meter could separately measure the two line currents, perhaps to keep high current paths shorter.

So I think the answer in this scenario is again 'depends on the particular meter's

Jon

 

[JoeNorm]

OP here. I accidentally posted the original on an older account. Thanks for all the responses, definitely beginning to clear up the mystery.

Usually there is not enough time in between commissioning a system and having the utility swap the meter to notice this phenomenon, But in this particular case there was and it appears all the solar export looks to the meter like consumption. Over 250KWH in this case.

As to the question of whether small solar arrays export, of course they do. Especially in climates that do not require AC.