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Max distance for Parallel setup with Axpert MKS 5K


Jacques1s
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 I Have 1 Axpert MKS 5K system up and running. Thanks to the help from Chris Hodson and other memebers on this siste ( Thanks !!! )

All works great, but I need more power.

I would like to add a second system, but from what I can see, if you parallel 2 systems single phase they need to be close to each other.

I have another building being build – about 200 Meters  away and would like to mount 2nd PV on the new roof for 2 reasons. A) more sunlight B) excising roof almost at max.

Now the question is – How long can parallel cable be?  If not possible, is it ok to install second as standalone connected to same AC feed as system one?

Thanks !!  

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Hi Jacques

2 Axpert in parallel must share the same battery bank and so must be close together.

It is no problem to have a separate system in the new building on same AC supply because you are not feeding back into the grid with an Axpert so there is no chance of them interacting - obviously system2 should be connected directly to the AC supply and not from output of system1.

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Thanks for the fast feedback Pilotfish. I have read so many difernt documnets on this. Si If I understand you correctly -- 2nd system should be stand alone and not have grid AC as backup. Only exsiting system will have the AC grid backup?  Now how do I get to share the output of both systems if they are far apart?

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Hi Jacques

I think you have misunderstood Pilotfish. 2nd system could have AC backup. You will not be able to combine their outputs. So both inverters could be powered from the grid but their output would have to be separate. So the second system could power the new building but that may  not solve you dilemma of enough power.

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Multiple distances might affect the answer here.

1. The inverters (master and slave) communicate with each other using a serial protocol, likely RS485. Though RS485 allows for insane distances, the propagation delay might be too large for reliable parallel operation.

2. As @pilotfish pointed out, it must be on the same battery bank, which means they should be in the same room preferably.

3. BUT... there is no reason why the PV panels connecting to the second MPPT/inverter can't be 200 meters away. Of course it is not adviseable, but if you size the cables correctly for the voltage drop and the distance, there should be no reason why you can't mount the PV panels on the new building.

4. You said you wanted "more power", which suggests that the 4kw you have now is insufficient, but could also be interpreted as "my monthly bill is still too high". If the second, then don't install an Axpert. Install a GTI. That can feed power to the rest of the house (and even to the Axpert, for charging) using the higher voltage AC wiring, which solves all your cable problems :-)

The ideal setup here would have been a Multiplus running ESS (with GTI on the output, former Hub-2 setup) and a Fronius GTI on the new building.

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Because I love arithmetic so much...

Round trip distance to the other building is 400 meter (200 times two, two cables).

6mm copper has a resistance of 3.8 ohm per kilometer, times 0.4km, that's 1.5 ohms. Let's say PV at a round 100V (cause the Axpert wants to be below 115V), then 3kwp means 30 amps. That's a 45V drop. So 6mm won't work.

16mm copper has a resistance of 1.4 ohm per km, 0.5 ohms, 15V drop. That will leave you with at least 85V which is good enough for the MPPT, but you're burning 450W over the length of that cable.

25mm has a resistance of 0.9 ohm per km, so 0.4 ohms total, 12V total drop. Still far from ideal, still burning 360W of heat over the cable.

16mm cable costs R30/meter, or 12k in wiring. 25mm cable costs R50/meter or R20k in cable.

So scrap my previous argument, this is not going to work at all!

What could work is a separate high-voltage MPPT so you can get the voltages up and the current down, but that's going to cost more than the inverter.

It would seem you're up the creek without a propulsion device?

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Sorry to butt in here. Just a suggestion. Why not install all your panels at the new building and run the axperts in parallel there? That would mean you'll have to install two cables ( one of which you would have done in any way) one to connect the axperts to the grid and one to supply the old building from the axpert's output? Way less losses that way and the advantage of having the axpert's in parallel on one battery bank. Pv can be split up between the two axperts then.

Sent from my S60 using Tapatalk

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1 hour ago, plonkster said:

1. The inverters (master and slave) communicate with each other using a serial protocol, likely RS485.

Actually, it's CAN bus. But there are also current sharing cables. So paralleling the outputs when the inverters are 200 metres apart is out of the question. Even running 230 V over that distance is questionable, though I'm sure it can and is done. You'd need at least 16 mm^2 cable, without doing the maths, possibly more. In this scenario you'd have two Axperts paralleled near each other and run 230 V cables to the other building. The cabling would be prohibitively expensive, considering  Plonkster's post. 

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1 minute ago, Coulomb said:

Actually, it's CAN bus.

Interesting. Yeah, it's usually one or the other. Victron uses RS485 for their VE-bus, but there is a rumour that they too might move to canbus. The argument would be the same though.

Are the current sharing wires analog signals? I'm not sure how it's done with the Victron stuff, but I think it's all digital.

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8 minutes ago, Coulomb said:

Even running 230 V over that distance is questionable

On my father's farm we have 230V running 1000 meters. But the load is known (one electric motor running a water pump) and the cable is sized for that load. It was cheaper than the equivalent solution with transformers on either end.

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If you install both inverters and your grid feed into the new building then and install a 40a CB feeding the existing building from the inverters you would need a 25mm cable to remain within the SANS 5% volt drop limit.

It isnt easy to get 25mm 2 core so If it was my house I would install 16mm x 2 with a 10mm earth and accept the slightly higher volt drop (and spend an extra 5min in confession on Sunday).

 

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2 hours ago, Anachem20 said:

Add seperate battery charger and charge the batteies from the new buildind

That's roughly what I envisioned with the grid-tied idea. You already need to put in a thick cable to power the new building (likely 25mm if you want full power), so you already have a channel by which that power can flow back to the first building. If your objective is more savings rather than a higher peak power, then you're done.

Some caveats of course.

Because your "main" inverter is an axpert and it is not designed for that, you cannot place the GTI on the OUTPUT of the main inverter. This means that your new building has no backup (unless you put in an extra set of cables for that, which can of course be thinner), and when the grid goes out you lose all the power made on that side too.

Also, with other inverters you can do things like prioritising solar for charging, that is, you can use the AC charger in the inverter and set the power level to be strictly less than what the GTI exports, thereby ensuring that you use only PV for charging. You can also charge from grid and PV simultaneously while prioritising the PV-inverter power and using the residual power, none of which you can do here.

Finally, if you had an inverter that allowed tying the GTI to the output, you could use the combined power of the GTI and the main inverter (even while running from battery), allowing a higher peak capacity, but with the Axpert there's no such choice.

Given the kind of costs we're talking about here (tens of thousands just on cabling), dare I suggest you install a better inverter? :-)

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  • 2 weeks later...

Thanks all for the advice. I have have looked at all the options and decided to install a stand-alone system on second building that does feed only AC output from inverter to my existing building.

I will install a manual changeover switch that allow me to use access power when needed. ( Night time ) I plan to install a larger battery bank in new building. My consumption will be very low in new building and nothing at night.

So will use it more as a backup for existing house at night. Not the best or high tec solution, but given space and distance, this will need to do the trick.     

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