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Multi parallel generator issue

Hi all,

A client's install has two 48V, 5kVA Multi ii paralleled: up to date matching firmware, identical AC and DC cabling, CCGX, gel battery & suitable solar input. The client has an isolated, completely independent back-up system (via AC transfer switch) incl. its own solar and battery.

With the occasional need for diesel gen back-up (we're in a sunny part of Spain), the client switches the transfer switch to power the loads from the back-up system (cleaner signal to the loads & maximum input into the batt). Then when the batt is full, they switch back to run the house from the Multi system.

The system has worked fine for a few months, and then (following a generator service) last week, when charging through the Multis, the generator stalls almost every time, almost immediately before it's got up to steam.

The gen is fully tested, and works fine powering loads or when charging the separate back-up system.

I've tried swapping master and slave allocation, and this seems to have helped, but as I can't say that this has 100% fixed the issue, the client is understandably twitchy.

If the gen is also powering the loads through the Multis, the gen stalls less often, but still does. This makes me wonder if there's some backfeed issue, explaining why it's much more common when only charging the batt rather than also having a load to supply.

Any thoughts/ experiences appreciated!

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2 Answers
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simons answered ·

Thanks for the input @Paul B , but unfortunately that's not the problem. The AC input limit has been set at 20A (10A per Multi) - well below what the open diesel genset is capable of (I'd be confident running 6.5kW all day from it - as it does when powering the back-up (Studer-based) system)

In normal use when gen charging, all loads are powered by the back-up system so that (up to) 20AAC only goes for battery charging, so there's no surprise peaks: compressors etc.

With no loads on the AC1 outs, the generator usually stalls quickly after the internal transfer switches have switched on the Multis, when the gen is supplying only a couple of hundred Watts - so not an AC input limit issue I'm afraid (as much as I'd like it to be!)

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try reducing the ac input limit to 10 amps, to see if it still stalls (20 amps is 4.4 Kw) so maybe the generators engine cant output full power anymore. anyway try that. if that works go up to 15 amps etc. I cant see it being the multis. to me it sounds like the gen is being overloaded. or try turning the chargers off and then load the ac loads up and see what the gen can output before bogging down. ALSO as you are using Multipluss II if they are the early ones the AC2 out was directly connected to the AC1 in so MAYBE there is something on AC2 out thats using power , just a thought

Yeah, when the client called this week, overloading the gen was one of my theories too - so I returned with the gen mechanic who'd just done the service, and fully tested the gen (it's a large open Isuzu diesel - pretty bulletproof). Working to 6kW+ constant quite happily, and everything checked out fine.

(I must add too the Multi system has been absolutely fine, stable when inverting tested up to 8kW output.)

Testing the Multi system on gen input with a small load (300W) on the AC out, it ran up to the 4kW+ input without breaking sweat, and stayed there running fine.

Then with the load removed and AC out effectively isolated (as it has been these last few months with the loads being covered by the back-up system whilst gen charging) it would stall almost immediately and every time - only at around 2AAC.

I ran through a few settings, and by changing master and slave allocations I couldn't get it to stall under the conditions above...but can't say I've fixed it just by not replicating it while I'm there...

I'm back on site this week, so I'll see how it's been behaving. Fortunately it's an easy fault to replicate and try setting changes without knocking off the power, especially with a back-up system in place.

I actually didn't try dropping the AC input limit last week..but it'll certainly be one of my tests next week as I try to get to the bottom of it.

never get the generator serviced is the lesson here :)

Thanks again for your input, and have a great weekend.

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Paul B answered ·

Hi you have not mentioned the size of the generator, but I supect that the loads plus the charging power is greater than what the gen can supply, so change the AC1 current input to just below the genrators max output. This setting would have changed back to default if you updated the firmware on the units

Hope this is helps


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