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how to measure the direction of a.c. power flow
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soldar:

--- Quote from: CodeDog on January 18, 2017, 03:54:44 am --- But how to measure the direction in which energy is moving in an a.c. line? Phase angle doesn't do it.
--- End quote ---

Huh? Phase angle is exactly what counts. If the current is exactly in phase with the voltage then power is flowing in the direction of the current and there is only real power. If there is some phase difference then you have reactive power flowing back and forth.

Measuring instant voltage and current is trivial and it should not be difficult to design a simple circuit (maybe with op amps) which would continuously multiply both magnitudes and it would read positive when power goes in one direction and negative when it goes in the other.
David Hess:
To do it non-intrusively, I would use a current transformer and electrostatically couple to the lines to measure the voltage.  Between the two, the phase can be found to determine direction.  I suspect the simple way would be to only detect the polarity electrostatically and use that to multiply the current by 1 or -1 so a positive or negative current is returned indicating direction and magnitude.

Why not get a Kill-O-Watt or whatever and permanently wire it into the circuit?
Circlotron:
Assuming a unity PF load, if at any moment your house is pulling the instantaneous supply voltage down (incoming power) then we would say the current is in phase with the voltage. If things change and then your house is pulling the instantaneous voltage up (outgoing power) then the current would appear to be 180 deg out of phase with the voltage. The current trace would invert on your scope and the voltage trace would stay the same. Think in terms of DC and it becomes clear. That's how I see it.
soldar:

--- Quote from: David Hess on May 16, 2019, 02:47:22 am ---I suspect the simple way would be to only detect the polarity electrostatically and use that to multiply the current by 1 or -1 so a positive or negative current is returned indicating direction and magnitude.
--- End quote ---
I cannot imagine any good reason to sense voltage electrostatically. I mean, you have a hundred devices connected in your home and you do not want to connect a volt meter? Why?

Another issue is that you cannot assume current and voltage will be in phase. Not at all. Homes have a lot of reactive loads.

Will a kill-o-watt indicate negative power? I have never tested that. It would be interesting to connect the load to the prongs and connect the power input to the receptacle and see what happens.
richard.cs:
I have built a device which measures net power at the mains input and if there is solar power exported servos it to zero by switching in and out loads. I have in fact been meaning to put a write-up in this forum for the last year or more but it's just not happened yet.

In my case the power measurement is current via a small current transformer, voltage via a resistive divider and a direct analogue multiplication to get a voltage corresponding to power. This is low-pass filtered (as it has a strong 100 Hz component) to get a real time power measurement for logging. To control the loads this signal is integrated to get energy and the controller switches loads in and out based on priority to keep the energy centred in a 3600 Joule window which corresponds to the (intentional) dead band in the electricity meter. The loads pulse on and off at a frequency set by their size the the solar generation with a few hundred Joules being shunted into and out of the grid and zero new usage.

This is not the cheapest measurement approach, but it is simple and works well. It has a bandwidth of a few kHz until after the multiplication so odd current and voltage waveforms don't affect it at all. The multiplier output always reflects the instantaneous power and the integrator the total energy used.
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