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Real time current measurements over PLC
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NiHaoMike:
I'm in the planning stages of a project that's more or less a grid tie inverter, except it does not export to the grid. There are two units involved in the part under discussion - the inverter itself that connects via a dedicated 240V circuit and a current sensor that installs in the breaker box. The tricky part is how to send the current measurements (split phase, two sensor coils) from the sensor to the inverter without requiring the installation of extra wiring. It must be low latency (less than 1ms) so that the inverter can instantly supply power to offset arbitrary loads. (In practice, it will still allow the grid to supply a small amount of the power - a few % or so, in order to enforce the zero export requirement and ensure that it shuts down if the grid goes down.)

I'm thinking of transmitting a pair of RF carriers (one for each phase) FSK modulated by Delta Sigma bitstreams or even just a pair of analog FM carriers, but now I'm wondering if some more sophisticated modulation scheme might be more robust and still meet the low latency requirement. If it helps, the inverter is going to use a FPGA for real time control, but the sensor should be kept simple mostly to avoid failure mechanisms and bugs that arise from sophisticated embedded code.

BTW, it's part of a personal project, not a product for sale.
NiHaoMike:
Maybe I should define the requirements? The link needs to carry a pair of signals each with a few kHz of bandwidth, but unlike audio, they're only weakly correlated to each other. (Thus the sum/difference approach used in FM broadcasting is not likely to be helpful.) It does have to be pretty robust and the receiver must have a way to verify the signal is valid.

At this point, I'm leaning on a pair of FM carriers in the few hundred kHz to few MHz range, simultaneously sent on both phases. The receiver can then use diversity to improve noise tolerance on one of the phases.
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