rithym1964 avatar image

3 phase DC breakers link bar

I am currently expanding my RE system with a third multi II inverter to 3 phase from the current 2 phase in parallel. (mulltiplus II 5000/48/70-50) I have purchased some Airpax 150A DC breakers and just wondered if I need to link them all together so if one trips, they all trip. I don't have any 3 phase motors running just 3 separate circuits on the AC side to spread the loads which get quite high when we run air con,etc..

Any help much appreciated

3 phase
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Hi @rithym1964

you don't have to link the DC breakers, when one trips, all units will switch off.
on a parallel system this is different there they should ideally be linked.

Also: these breakers should never trip unless there is a short-circuit somewhere, so its also a bit of a theoretical issue.

rithym1964 avatar image rithym1964 Daniël Boekel (Victron Energy Staff) ♦♦ ·

Hello Daniel thank you for answering my actual question it seems my thread has been hijacked by somebody who thinks they know what I am trying to achieve without understanding my situation. It is a straightforward question and you answered perfectly... In a 3 phase situation the DC breaker bars it is not necessary to link them... in a parallel situation it is best to link them. Thank you

2 Answers
Paul B avatar image
Paul B answered ·

To me it would be much better to Parallel all 3 units to become 1 large 15Kw system. as this way the loads can be spread evenly over all 3 inverters

then run 3 circuits off the switch board if need be.

I am presuming by your description that you are running 2 separate inverter systems , but its not really clear what you are now doing.

are you going from 1 pair of inverters to now 3 inverters setup in 3 phase.

if this is the case I would advise against that, as you are then limited to a max of 5Kv from any one inverter, (you cant share across the 3 phases) you have to then load balance the phases.

Just some thoughts to consider

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rithym1964 avatar image
rithym1964 answered ·

Paul B Thanks for reply. At the moment I have two 5KVA multis in parallel and they are connected to the 48V bus bar via one 250A Midnite DC breaker which means both inverters go off together in the event of a trip (never happened yet) If I add a third 5KVA multi it would be asking too much to connect that to the same breaker so I have purchased three 150A DC breakers, one for each multi. My question is should I link the DC breaker on/ off switches so if one trips they all trip together?

As for the question of parallel or separate phases I hear your suggestion about spreading the loads across all three inverters in parallel but I also have a 3 phase borehole water pump 120M down which normally runs from the grid at night using cheaper rate electric and fills my water tank. This will not run off a single phase. My grid connection is 3 phase 21KVA and if that goes down and my inverters are in parallel I have no way to pump water from the borehole. Chances are that will never happen but I am trying to cover all scenarios in case it does...

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it really makes no difference really which way you do it as if one trips the other inverters will shut down anyway. as it wont run on just 2 phase's

even if they were in parallel one goes down they all shut down.

yes if you have 3 phase item then thats another kettle of fish, but you did say in the orginal heading " I don't have any 3 phase motors running"

Anyway balancing the phases will be a issue I think you will find, but give it a go its always easy to convert back.

keep in mind that if any one of the phases overloads then all 3 shut down,

Thanks for the suggestion of keeping my inverters parallel. My only issue is if I configure all three inverters in parallel I can only use one 30A phase from the grid to charge batteries when the weather is bad... this would be 10A max on each inverter. If I configure three phases then I would have up to 30A available on each inverter so could charge 1,000AH battery overnight easily on my cheaper rate electric (10pm to 8am) with a theoretical 210A (3 x 70A) which is cheaper than running petrol gennie. Predicting the weather night before can be tricky... several times this Winter the forecast was bad so I charged up battery night before only to find a lovely sunny day! doh! 14KW of solar soon charges the battery on a sunny day... bulk is usually 60-120 mins from 80% DOD.

I'm not sure how the load balancing would work out are you saying there will be problems if one inverter is doing nothing and the other two are maxed out on a 3 phase setup??

yes you would be limited to the max that can be used on one phase.

so you have to way up the pros and cons

all I can say phase ballancing can be a real pain ,

anyway in a 3 phase setup and you are OFFLINE (off grid)

Each inverter can ONLY supply MAX 5Kv and you cant pull power from the others AT ALL.

if you exceed that one phase's inverters max rating. it will shut down. and then the other 2 will also shut down as well.

so in other words you cant run more than 5Kv on any one phase when the grid is down.

This is the down side to 3 phase max 5kv on any one phase

then the down side for single phase as you say on single phase the max charge rate is limited to the grids one phase supply.

anyway as long as you understand the pros and the cons, thats all