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Voltage reverse blocker for zero grid return

I recently had a query from a friend of mine where he needed to block the current from his micro grid tied inverter so it doesn’t get pushed back into our electricity grid. The local grid and utility does is not modern enough to be able to utilise this feature, and our meters still charge even though we are pushing back instead of using!
I then found the EmonTx firmware, which I found with some hardware mods from my side, would be ideal for this application. The result is a module that can be wired into a standard household (220V) system, though it should be scaleable for 110 and monitors the direction of current flow using the power calculation. When it detects the current flowing back, it disconnects from the mains automatically, so the house then runs solely off solar or alternative power.
I thought to mention this over here, in case there is someone else that could use the design.
PM me if you are interested.

I understand yours is not the only country where back-feeding into the grid is discouraged.

Yup, and an internet search at the time didn’t come up with any ready-made answers, so in the maker spirit, I made my own!

Of course, many grid-tied inverters are designed to work only with a mains connection, and will turn off if “islanded” (in order to prevent danger to linesmen who think they are working on an isolated and safe system), so your solution might not be available to many.

It takes that into account, it doesn’t “island” the inverter, and the total current that the grid can draw once it is disconnected is in the microamp range. Agreed that it isn’t an ideal solution, but to accomodate the linesmen the sensing circuit can also be galvanically or optically isolated in order to protect them in a situation like that where you have large numbers of these systems running. At this point in time it isn’t an issue yet, as there is only one of these in operation. (That I know of) But then this is why we have forums like this one to highlight potential issues and improve designs so they can be used widely.

Why don’t you use a diverter and run a restive heater or something?
Its going against the grain thou wasting energy.

Regards
Dave

The idea behind this is not to control waste energy, as the unit sits between the house/inverter (Grid Tied) and the grid, at the point where it switches the grid off, the house will still be drawing energy from the inverter. It is only to prevent the energy from going back into the grid. It can be expanded to include the management, but as I said, it is not intended for that.
As to the “islanding” mentioned by Robert, I went over my design notes last night and it came back to me. The purpose of the inverter switching off when “islanded” is done to protect linesmen, so if the grid goes off, the inverter will too, and the unit allows for this, so the inverter can still sense the mains voltage so as to not disable this feature, but still in such a way that only microamps of current can flow back to the grid.
Hope this answers both questions sufficiently.

Hi Dave

It still won’t allow my archive file, even though it says it is one of the allowed formats.
Uploaded the individual files.
The 103.sch and brd is the main power board, the 103A is the cpu with the arduino on board. This was designed to fit inside an OKW enclosure (1: Main enclosure
https://www.okw.com/en/Railtec-B/B6502121.htm?var=c9f11140-cb2e-11e3-89f9-00163e72470b
2: Covers front and back (2of)
https://www.okw.com/en/Railtec-B/B6608160.htm
3: top cover
https://www.okw.com/en/Railtec-B/B6602180.htm
, with a small ribbon cable connecting the two boards.I have been fruitlessly searching for the DC supply module I used for the arduino power, it is one of those small 3 to 5W LED supplies from China, just ensure the connections are the same.
I probably haven’t covered it all, but if there’s enough interest, I will sit down and do a proper build log. Just let me know if you need any more info.
Power meter 1.03.brd (67.7 KB)
Power meter 1.03A Bom.txt (594 Bytes)
Power meter 1.03A.brd (56.5 KB)
Power meter 1.03.sch (556.2 KB)
Power meter 1.03A.sch (251.6 KB)
Power meter 1.03 Bom.txt (5.0 KB)