Hi 
I have just installed 2 axpert MKS ii inverters in parallel, connected the batteries up ok, connected the PV up ok, powered both inverters up and setup params for parallel mode all ok, powered down, connected AC output with no load.
Power back on sequence, switched battery mcbs on, switched solar isolater on, turned master inverter on at switch, turned slave inverter on at switch at which point there was a pop and the battery mcb tripped, now the slave just trips the battery mcb instantaneously.

I've double checked all wiring and parallel connections, all ok.

Anybody got any ideas or do you think it was just a faulty inverter.

Thanks Andrew

Edited July 1, 20223 yr by AndrewM314

 

I was just thinking about the AC output side of the inverter, there must be some synchronization of the AC cycle, does anyone know how it works, is the synchronization performed through the parallel board, could it be the synchronization that has failed?

Hi Andrew, please send pics of the Transfer switch & AC input wiring for both inverters. This will explain a lot.

 

Hi Steve,

I've disconnected the wiring at the moment so unfortunately can't send pictures, the inverters were going to be totally off grid so no AC input available 

 

Is having no AC input a problem, the inverters should give AC output from batteries and solar only is that correct?

6 minutes ago, AndrewM314 said:

 

Is having no AC input a problem, the inverters should give AC output from batteries and solar only is that correct?

Thats Correct. No AC input required.

 

Thanks for the reply piper, you don't happen to know how the AC output synchronises when the inverters are in parallel 

Can you describe how they would get wired on the AC output to the loads? Or have a pic available. 

Thanks for the reply, at the time of failure they where temporarily wired to an open mcb essentially the 2 AC outputs where just connected together 

The current sharing green cable also connected & option 18 set to PAL?

It's better to have both inverters connected to their own MCB & then into a larger breaker for isolation reasons. Is the AC fuse maybe tripped. It's a little black button type fuse under the unit bottom left. 

5 hours ago, AndrewM314 said:

I was just thinking about the AC output side of the inverter, there must be some synchronization of the AC cycle, does anyone know how it works, is the synchronization performed through the parallel board, could it be the synchronization that has failed?

Certainly synchronisation failure would cause the slave and/or master to see massive overload currents and failure(s).

How the synchronisation works is somewhat of a mystery. I suspect a combination of a CAN packet and one of the wires on the parallel board circuit, or possibly just the latter.

When there are more than two parallel boards, all the connector terminals are effectively in parallel. The paralleling manual or manual section dictates strictly how to wire the cables, but as far as I can tell, this is merely to ensure that the inverters all get the signals with as short a delay due to cable length as possible.

My guess is that one of the pins is connected to an output controlled by the master, and the master changes its state at the zero crossings of its mains input. Of course, if you use the wrong phase, or swap active and neutral, this would cause a 180° phase shift, and certain failure. There is also the neutral to earth relay contact connection that could short circuit the other inverter(s) in the event of such a swap. But that would happen before the slave's inverter switch is turned on or pressed. The slaves should get their synchronisation timing from that pin of the paralleling cables. So certainly if there was a fault in one of the paralleling boards or cables, that could cause this sort of fault when the slave is turned on. Perhaps buzz out the cables? I have also published a partial schematic trace of the parallel boards here, in case you want to check them. They're pretty simple.

Thanks Coulomb for the reply, that is excellent information much appreciated, I'll have a good read through the post, thanks again 

1 hour ago, Steve87 said:

It's better to have both inverters connected to their own MCB & then into a larger breaker for isolation reasons. Is the AC fuse maybe tripped. It's a little black button type fuse under the unit bottom left. 

Thanks Steve, I'll bear that in mind in the final installation 

14 hours ago, AndrewM314 said:

Power back on sequence, switched battery mcbs on, switched solar isolater on, turned master inverter on at switch, turned slave inverter on at switch at which point there was a pop and the battery mcb tripped, now the slave just trips the battery mcb instantaneously.

The paralleling instructions call for turning the two units on simultaneously. It mentions that the selection of master and slave is done by themselves arbitrarily. I found out that the unit that is turned on arbitrarily first becomes the master. So I turn the one I want to be the master just a fraction of second on before the slave.

When I do this operation I have the AC and load as well as PVs switched off and turn them on afterwards.

If synchronization fails as you describes, there might be a faulty paralleling board.

One very important aspect to add to what @Beathas added. When you parallel & have each output onto its own breaker that then feeds into a larger 63A breaker. DO NOT disconnect an individual output breaker under load. What you will do is overload a single unit depending on the load. In other words, disconnect the large breaker 1st before disconnecting the smaller individual breakers. Do the same on start up in reverse. But @Beathas the startup sequence perfect. 

Many thanks for the responses, 

As suggested I'm going to add circuit breakers to each AC output.

I can't seem to find an official start up sequence in my manuals, all it refers to in my parallel manual is that the AC input breakers should be switched on simultaneously which doesn't apply in my case as I have no AC input.

Does anyone know the official power on sequence from totally switched off.

The, startup sequence for you would be battery power start first. So DC battery breakers on 1st. Then both inverters on together on the button push. Once the inverters have stabilised & HS & SL are chosen, I would turn on the PV 3rd. Only after both have sensed the Voltage on the PV. Lastly I would close the AC output breakers both together & then the master AC output breaker. 

1) Battery DC on 

2) Inverters On together 

3) PV DC breakers or isolators On 

4) Each inverter AC output together 

5) Master AC output

 

Before turning on either output breaker, I would measure output live to output live, and don't turn on unless the voltage is under 10 VAC. For extra points, test for several minutes to make sure you didn't fluke synchronisation, they might drift out of synchronisation with time. 

12 hours ago, Coulomb said:

Before turning on either output breaker, I would measure output live to output live, and don't turn on unless the voltage is under 10 VAC. For extra points, test for several minutes to make sure you didn't fluke synchronisation, they might drift out of synchronisation with time. 

Great post to ensure the tech in each inverter is actually working. Too expensive to work with trail and error around synching.

excellent advice from all, wish I had found this site before I first switched the inverters on.

I am now trying to get some replacements from the supplier, does anyone know how good the axpert tech engineers are, I have sent them a video showing that one of the inverters has a direct short on the battery input but they still are asking me to switch the battery mcb on to the that inverter which I think is very dangerous, any comments please.

Thanks Andrew 

47 minutes ago, AndrewM314 said:

does anyone know how good the axpert tech engineers are

I think it varies tremendously from supplier to supplier. You won't ever talk directly to the Voltronic engineers.

My impression is that the average competency is pretty low. It sounds like yours aren't above average.

On 2022/07/06 at 2:30 PM, Coulomb said:

My impression is that the average competency is pretty low. It sounds like yours aren't above average.

They don't even acknowledge a short circuit at the inverter battery input terminals is a fault, so I'm having a go at repairing them myself, I've got 2 inverters 1 has a short at the battery input and the other has a 09 fault, so I'm working on the 09 fault first, I've downloaded a service manual and managed to remove the main board after a bit of a tussle, I've found 2 MOSFETs are short circuit QA1 & QC1. Is it recommended to replace all 9 MOSFETs, are there likely to be any other components that fail if these MOSFETs go short.

11 hours ago, AndrewM314 said:

I've found 2 MOSFETs are short circuit QA1 & QC1.

Actually, those are IGBTs.

Quote

Is it recommended to replace all 9 MOSFETs

No, QA1 and AC1 both connect to the neutral output, so just replacing those IGBTs should be OK.

Quote

are there likely to be any other components that fail if these MOSFETs go short.

Yes, the gate driver components. Check at least all nearby diodes (including zener diodes, tested as ordinary diodes) and resistors, including the internal diode in the high side gate driver chip.