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Axpert Parallel - power down one unit


vlan_one
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Hi,

I did a search and cannot find the info I need. It might very well be that I am not searching for the correct question. I had difficulty in adding a title to this post that will make sense for the next person searching for this type of info.

I have 2x Axpert MKS 5kva running in a single phase parallel configuration. I needed to change the battery cables of both units. I thought that I could just take one down at a time and all will be fine, but seems not.

My shutdown process was to take down one unit at a time using the process below:

1. disconnect the PV
2. switch the slave inverter of at the power switch below
3. switch the AC output breaker off
4. switch the AC input breaker off
5. disconnect the main DC supply

At point 4 is where I can't seem to understand what's happening. As soon as I disconnect the AC input, the remaining inverter switches to battery mode and I can't get it to switch back. Even after disconnecting the DC supply from the powered off inverter the remaining unit does not switch back to utility. I thought if the master does not see the slave it will continue to work as normal, but only after bringing the other unit back online does it switch back to utility.

Is this normal? Would I need to power off both inverters, set both back to 'single' and power up the remaining inverter? Is there a setting that can prevent this from happening? Or am I expecting high availability from something that was not designed to perform high availability? I do understand that if you are running in a 3 phase configuration you cannot simply take one unit offline.

My question would be, what will happen under a true failure condition of one of the inverters?

Any info or past experiences would be appreciated.

 

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Do you have separate AC-in breakers for both inverters?

Does the master still show the AC icon?

I only got my second MKS running a few months ago, and just leave them running. I also only have one AC-in breaker, and I run in battery mode all the time when possible. So I don't know if this is normal. I can imagine that when the slave is switched off but still running because it has a charging source (AC in), it might send a message to the master saying that it can't stay in utility mode. That might cause the master to switch to battery mode. I can possibly check that in the firmware later.

I don't think it's so terrible to be in battery mode, I believe that's where the inverters expect to be most of the time. But then I don't have the prospect of load shedding to worry about.

In the case of a hardware fault in one unit, in most cases I believe that the master will send all units into fault mode. Paralleling Axpert units seems to be designed for extra power capacity, not reliability.

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6 hours ago, Coulomb said:

I can imagine that when the slave is switched off but still running because it has a charging source (AC in), it might send a message to the master saying that it can't stay in utility mode.

My initial reading is that no, the slave does not send any such message when it loses its AC input.

However, it's still possible that the master, when in line mode, periodically checks to see that all slaves have AC-in present.

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Hi Coulomb, I do have separate AC-in and AC-out breakers installed. I did this for the reason of being able to take one unit offline for maintenance. The master will still have AC in, but I will actually just verify that. The periodical checks will have to be every second as it almost immediately switches to battery. I still have to replace the master's battery cables, so will give this a test again.

From the parallel guide it states that the AC-out neutrals need to be bonded. I have also bonded the AC-in neutrals on recommendation of an engineering friend of mine. I wonder if this could have any impact? I have attached a diagram of my DB wiring for reference.

AC DB Drawing.jpg

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