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I have a 3 year old 1kW PV System and recently bought a 2kW system. Both are off grid. And since the batteries on 1kW system are old, and since I availed subsidies for both systems I cannot change the configurations.

Both inverters are 230V ~50Hz

How can I combine both the inverter outputs to get a 3kW output? I came across some combiner boxes but most are for grid-tie systems or way too expensive.

Is there any other way? How can I build my own combiner box if it's the best option?

I have a 3-phase electricity connection. Is there a way to make use if it for this?

John Mathew
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    Synchronizing phase is one problem but load balancing is another. I guess another way of approaching this is to switch between the two and handle the gap time with a UPS. This way you won't have 3KW peak output but you will have combined battery life. Similarly you can connect low power load to the 1KW and reserve 2KW for high load – user3528438 Jun 08 '17 at 18:20

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There's an elaborate procedure for putting a generator in sync with another. Traditional generators stay in sync because of load dynamics and backfeed forces (a generator a few degrees lagging will have its load eased, causing it to race faster and catch up). The problem is semiconductor inverters are absolutely oblivious to these forces, and will simply ignore them and bang their drums ever more out of sync until sparky smoky things happen.

I'm not a huge fan of using inverters generally in solar systems intended to run off-grid, as the overhead of the inverter is a total loss you must pay for dearly. I would be moving as many loads as possible to the DC side and see if you can put those on the smaller system and deprecate the inverter entirely. Ideally, only run the inverter when you have a non-DC-able load.

If you really must tie the two systems together into a common AC waveform, that's a hard problem. You could

  • use DC as the shared language: tie them together on the DC side, with diodes, and both feed a single inverter. This of course depends on their battery voltage being equal and their batteries being of same chemistry.
  • use inverter B's battery as the shared language: the smaller one's AC output feeds a battery charger for the larger one.
  • use shaft torque as the shared language: using a magnetic rotary machine (motor-generator set) to sync them, either each turning a motor which turns a common shaft to a generator, or the smaller one having an M-G set, which you then manually sync to the larger one, the protective circuitry here would be an adventure.
  • use water head as the shared language: install small hydro, and have both solar supplies backpump. It helps if you have a hill and a creek.

I know some of these solutions are pretty appalling, but these consumer tier inverters are just not made to do this.

Harper - Reinstate Monica
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To do this your both inverters must be capable of paralelling using a third wire for frequency synchronization and to be of same manufacturer and model/series. This feature is not common for cheap inverters. If yours dont have it you cannot tie them.

You could either buy a single 3kW inverter or split your mains to 2 different circuits - one for each inverter.

Todor Simeonov
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If both converters are grid-bound, you can simply wire them in parallel to the grid, same live wire. What you have to check is what happens when the grid fails.

Unfortunately there is not an easy answer on this. It is very unlikely any of the both converters is going to work without the grid when they are paralleled - even if they work without as singles. If they are good quality, they should detect a failure condition and switch off automatically. If not, one of them may blow.

Janka
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  • Both are off-grid. Can I really simply parallel it without knowing if it's in phase? – John Mathew Jun 08 '17 at 14:12
  • Then it is very unlikely to work. Such free-running converters cannot be paralleled because their AC phase is not coupled. In general, you may also **not** use the second and the third live wire of your installation either because the phase isn't coupled - the neutral wire would be overloaded. It may work with a 2.5mm² cross section neutral wire as the sum current is still below 16A (depends on wire cooling) then. – Janka Jun 08 '17 at 14:19
  • @John, no, that is what sparky explosions are made of. Syncing a generator onto the grid is a very big deal. The thing with the three lightbulbs, all that. https://electronics.stackexchange.com/a/308756/95888 However that does not work on semiconductor inverters because they are oblivious to backfeed forces which keep a mechanical generator in sync. – Harper - Reinstate Monica Jun 08 '17 at 14:47
  • If one is master and provides "the grid" and the weaker is grid-tie-able, you may end up with a stable result but I highly doubt it. A 1 kW inverter will have a severe influence over a 2 kW peak power grid. A real grid-tie inverter usually works against a very strong grid with MVA short circuit capability. Beyond the scope and solutions for your particular problem, some Honda kW motor gensets can be paralelled in the way to describe. I'm very curious as to how they solved the regulation. – winny Jun 08 '17 at 15:08
  • @winny: motor generators, well … they have an electrical machine. A true generator. That's why they provide a solid sinus and can be paralleled easily (though reactive power may complicate things). Solar generators are solid-state and they either need a stable sinus from the grid or produce something spiky between sinus and square wave, and they are sensitive against such a waveform from another paralleled generator, too. So … you cannot transfer know-to-work from motor-generators to these. – Janka Jun 08 '17 at 16:23
  • @Janka No, they are inverter based and they still can be paralelled. Two 1 kW gensets make one 2 kW. Very expensive though. – winny Jun 08 '17 at 16:52
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The usual way to handle those kinds of problems is to rectify both input voltages. After they were filtered, usually a thyristor or high power MOSFETs and a microcontroller are used to build a kind of PWM and another filter to build a new sinewave.

If you'd like to build your own one, I guess you should calculate like at least $ 200. And that will propably a really shitty one that won't be able to handle 3kW.

Keeping the intermediate DC-circuit clean and smoothing the outputvoltage will require high quality inductivities and capacities. Also the output MOSFETS will get pretty warm, which makes them demand some kind of cooling.

Maybe I will upload a sketch of what I was thinking of later today.

bjoekeldude
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    What does those combiner boxes do with the different inputs? – John Mathew Jun 08 '17 at 14:56
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    @JohnMathew Rectify them first thing, so they are both supplying DC. This is a tougher design than the answerer thinks, and would be better solved by getting the DC straight off the panel/battery. – Harper - Reinstate Monica Jun 08 '17 at 15:15
  • well, the thing with this setup is, that it is totally indipendent from the kind of voltageinput you get, since the energy gets combined in a DC circuit. Why did this got downvoted btw? – bjoekeldude Jun 08 '17 at 15:15
  • Oh I see that the power is given by batteries. Then leave the rectification away – bjoekeldude Jun 08 '17 at 15:16
  • This probably got downvoted because it's a really bad description of the kind of "combiner box" that the OP has already considered. Plus, it's full of typos and other errors. You need to do a better job of proofreading your posts. Feel free to use the `edit` button to make improvements. – Dave Tweed Jun 08 '17 at 20:49