Where does power meter sense current?

[Evil Bunny]

I've been trying to determine how a power meter (like a utility company attaches to your house to determine your bill) works, but I can't find anything that explains exactly where the meter is taking it's readings.

On a center tapped transformer, you have L1, L2, and Neutral, but the loads aren't always balanced. Are they sensing total current on L1 and L2 and then averaging that total? I suspect this is how it works... Anyone know for sure?

 

[berkeman]

I've been trying to determine how a power meter (like a utility company attaches to your house to determine your bill) works, but I can't find anything that explains exactly where the meter is taking it's readings.

On a center tapped transformer, you have L1, L2, and Neutral, but the loads aren't always balanced. Are they sensing total current on L1 and L2 and then averaging that total? I suspect this is how it works... Anyone know for sure?

The current in each phase is sensed (either with a shunt or a current transformer), and the current and voltage of each phase are used for the total power calculation.

 

[Evil Bunny]

Right... the voltage is known, but the current must be the average of the sum of L1 and L2, correct?

 

[berkeman]

Right... the voltage is known, but the current must be the average of the sum of L1 and L2, correct?

No. The power in each phase is calculated separately, and added up to give the whole power. You need to sense both the voltage and current in each phase -- the voltage is not "known". The voltage in each phase can vary quite a bit, and the actual voltage needs to be used for the power calculation. The waveforms are sampled at a fairly high rate, to get accurate power numbers.

 

[Evil Bunny]

The 240 Volts coming into my house varies quite a bit? I did not know that... Interesting.

So they're using neutral as reference and then calculating 120V (or whatever the "actual" voltage is) times the current in L1 and then 120V times the current in L2 and adding them together... I guess that makes more sense than averaging the current off the two hots and sensing the total voltage between them.

Thanks! Could not find that information out there...