Electronics > Projects, Designs, and Technical Stuff

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

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CodeDog:
Here's the idea: unobtrusively measure the direction and intensity of power flowing along an a.c. line, and by unobtrusively I mean without a physical connection (e.g. clamp meter, Rogowski coils, ...), and the amount of power is not so important, just a relative value would be sufficient.

Why? ... Like a lot of others, I have solar panels on the roof. I also have a few high energy users such as a pool pump, and an airconditioner. Wouldn't it be great if I could immediately detect when I'm exporting power and flip on the pool pump? Or the aircon? And if it gets cloudy and I'm no longer exporting power, to turn off the pump?

If I could determine whether energy is flowing into my house or out of my house (on the mains active line) that would be fantastic. And if I knew how much (or even relatively how much) that would be even better. A centre-zero analog meter for a display would be sufficient.

But how to measure the direction in which energy is moving in an a.c. line? Phase angle doesn't do it. Some sort of directional coupler maybe?

One alternative would be to measure the power output of the solar array and the power consumption of the house, and calculate the difference - which would be either the power I am importing or the power being exported - depending on which is the larger of the 2 measured values. But this is not simple as I don't have easy access to the circuits feeding the house.

Another alternative would be to watch the meters in the meterbox and work out manually whether I'm importing or exporting - but this is time-consuming and doesn't provide an instantaneous indication of the situation.

Is anyone aware of a simple means to measure the direction in which energy is travelling on the a.c. feed to the house?

I sense a market opportunity here for a clever solution, if there isn't already something out there.

cheers,
Mark

viperidae:
I'm pretty sure it works if you measure the current direction compared to the voltage.
Emoncms is an open source product that can do it

CodeDog:
huh?

with ac there is no such thing as current direction is there? ... the current flows back and forth, 50 or 60 times a second
depending on your location  ... hence "alternating" current ... all day long the electrons just oscillate and barely move
much at all as far as I understand ... so there is no current direction.

But there is a current. (a movement of electrons). Just not in one direction only.
And although the current is not in one direction, energy DOES get transferred in one direction ... from the source to the load.

A.C. is weird.

the old-fashioned spinning disk meters could do this job ... if power was exported, the disk ran backwards

is there an electronic equivalent that does not require connection to the feed cable?

I totally don't understand what you mean by "measuring current direction compared to voltage". Sorry.


cheers,

Brumby:

--- Quote from: CodeDog on January 18, 2017, 06:08:39 am ---I totally don't understand what you mean by "measuring current direction compared to voltage". Sorry.

--- End quote ---

Seems there is a voltage/current relationship exploited in those old electromechanical meters....

Seekonk:
The concept of scheduling and prioritizing loads is the future of energy, grid or solar.  With smart meters they will in the future start charging more for peak demand periods. The utility grid prefers to have your panels facing west to get that extra in late afternoon when they need it.  Since a meter can only look at volts and amps, it has to be looking at phase angle. That would imply that at some phase condition the meter would just stop.  Too much to think about at 3am.

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