2 x Multuiplus in series config

Q&A and troubleshooting
, , , , ,
1

So I want to pose a question and get some feedback.

I had 2 x 48/5000 (MP2) in parallel, something happened to the masters voltage regulator circuit (2 years in), it would no longer maintain voltage and I was getting swings of 8v under small to medium loads and out to 20 volts on larger loads. The repair is being taken care of by Victron, (Fantastic customer service by the Vendor), however this has taken both MP2 out of service. I reconfigured both to be single units and tested them, The master was cooked, the slave would get to the set voltage ok, but it was still hunting 5 to 8 volts with just a couple of lights turned on. So effectively both out of service.

I had a third which at this point had never been in service it was my backup unit, so this is running the house on its own now, and we are back to the good old days of knowing what the load is /appliances are on before turning on another (large items like kettles, air fryers etc).

So I am considering the following.

  1. Run 1 MP2 with the assistant set to start the second unit when the load gets over 3500w for 10 seconds, and stop it when it drops below 2500 for 5 minutes. So this would ultimately operate in a pass through / assist mode, (Generator / MP2 style) as the load climbs during the evening periodically to 7kw the second unit would be called. Typically my daily load is around 1kw to 1.3kw, its only when the EV gets plugged in or the AC is running that we sit around the 5kw to 6kw, maxing out around 7kw at night. During the day the fronius Adds to the system capacity…but that would need to change in this setup.

Questions.

a. My thought process is should I have another malfunction of an inverter it doesn’t take out two inverters, just one, and a simple 5 minute bypass cable connection would see one unit out of service and back to a sole source again.

b. The same load is still managed by both inverters, can setup an assistant to stop the primary from charging the battery when AC input1 has power.

Can you see any issues with this ?

Other Options

  1. Option 2 is configure exactly as before (both in parallel) and hope nothing goes wrong now that the warranty period is 1/2 way through?
  2. Option 3 buy a new 48/10000 and sell two of the 48/5000, probably end up recovering 1/3rd to 1/2 the cost of the new unit…..not ideal.

I currently have 7.8kw of Fronius inverters on the AC out1 side of the MP2(both isolated awaiting fix of MP2) 2x MPPT’s 450/100 keeping the house alive. In option 1 above I would have to do away with one of the AC coupled fronius units and make it a DC coupled solar. Not really a big issue, I have space on the shed (battery / inverter room is in shed) for those panels. Alternatively, I could up the cable size from the panels to the MPPT, and run the 40m to the shed(leaving them on the house)

Appreciate your comments and feedback / suggestions on this. A youtuber (OffgridGarage) does this cascade inverters using a 48/5000 and a pheonix 48/3000, so I know it will work, but is it a good solution.

1 Like
2

Apparently nobody has responded. I have been considering a similar approach. I have a smaller system, MPII 48/3000. I would like to accomplish two things. 1, to allow the MPII to output more power. 2, to avoid back-feeding into the grid (ie. strictly off-grid scenario). Ideally, I could use an existing self-contained 2kW power station as the “grid”. The power station would be connected to the grid, but it can’t feed in to the grid. This scenario is much simpler to implement than adding another MPII in parallel, and eliminates the concern of feeding into the grid, which buys me nothing but bureaucratic headaches. Plus, it’s free since I already have the power station.

3

Hi Larry,

So I actually did try this method, when I got one of the repaired units available, after a couple of days. What I found was the Second inverter took too long to cut in.

Option 1 testing results

By the time the first inverter(Main) enabled the second inverter (Backup), then the Main synced to Backup and closed the internal relay 15 or more seconds had passed. OK if the load was say 2kw, and turning a 2.5kw kettle on, as the extra 2.5kw would take me 1.5kw over the threshold I had set and it would manage it. But if the load was already up just under the 3kw threshold set in Main, we would end-up tripping if the kettle kicked in. The time taken to get the Backup inverter online, and synced, and close onto it was outside the high load parameters of the Main. We got away with it a couple of times, but didnt a couple as well.

Settings, close the contact to start the Backup once the load was over 3000w for 5 seconds, open the contact when below 1800w for 10 min.

Option 2 Testing results.

So I tried a second method, this was to have “Backup” (the second unit) become the main source of supply and have “Main” (in pass-through /assist mode), supply/assist when the load was up. As they are both on and synced and the relay’s closed in pass-through mode this worked. But this meant I couldn’t use my down stream AC coupled solar, as I had to set Main as not a battery charger, other wise it would just try and charge all the time.

So at the end of the day we ran with scenario 2 and no AC coupled solar for a week. Now I have ended up back were I started, 2 units in parallel. I did consider buying a whole new 10kva unit and replace both 5kva with one, but as I already had 3 of the 5kva units it didn’t seem logical.

I have moved my solar around now, so I only have 5kw AC coupled, and 16kw DC coupled and the system is working fine, if anything a little better but hard to tell as it hasn’t been long enough. Having the AC coupled unit does help with available inverter capacity, we are just going into the summer months now and we are seeing 32Deg C temps, so with the AC running and the EV charging, the additional inverter capacity during the hot parts of the day handy.

Back to your situation, if you didn’t have AC coupled solar and you had an easy way to keep the self contained power station charged Id set it up in option 2 method. You could experiment with option one trigger settings to turn on the power base if you wanted it as a backup source, I guess it depends on how well you manage your loads / type of loads.

I am running a normal Family home so we need the extra power. I am considering buying another 15kw battery and taking the spare inverter and putting it up the house down stream of the AC coupled Solar and enabling it as pass through / power assist mode. So the main system would power this smaller system, but the smaller unit could keep the battery 100% during the day, and go into power assist if required during the evening. Alternatively supply some redundancy to the house should the main unit have a failure and I am not home.