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sesshoumaru avatar image
sesshoumaru asked

Building a backup power system

Hello

I have a 7.8 kWp solar system with a Fronius Symo 6-0-3-M inverter and a Varta Element 6 battery.

I want to add backup power capability to the system for cases of power grid failures.

The inverter is 3-phase coupled to the house and I want to provide backup power to the entire house on all three phases, within the limits of 16A per phase (230V). In addition there will be a diesel power generator (8KVA) as well, which shall be integrated. As battery I would consider a BYB LVS 12.0 kWh system for start.


Even after reading lots of information about Victron and possible configurations I still have a ton of questions. I hope somebody here is kind enough to help me through the thought process.


  1. If I want to supply all three phases with backup power, I have to have 3 Victron units?
  2. Can I use 1 Quattro for the generator to come into play on one phase (L1 e.g.) and for the other two phases use a multiplus?
  3. Will all three Victron units be conencted to the same battery stack via some master charge bus?
  4. If I keep the Fronius it would be AC coupled and deliver up to 6000W PV power, how does that affect the dimensionig of the Victrons? 3x3000 = 9000 (okay?) or each all units have to be capable of handling the full 6000W?
  5. I have an additional 5000W UPS for the most critical loads, so for this phase I would disable the Victron embedded UPS?


Thanks in advance :)

system design
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5 Answers
marekp avatar image
marekp answered ·

ad 1. For 3 phase system you will need 3 identical units.

ad 2. No, you need 3 identical units.

ad 3. Yes

ad 4. 3 phase Victron system (3 x 3000) is big enough for AC coupled 6kW 3 phase Fronius.

If you need 16A x 230V = 3680W in each phase, than Victron 3x3000VA will not be big enough.

ad 5. This additional 5kW UPS can be connected after Victron system on the critical phase.

I have similar system and I am using diesel generator only for charging battery direct with the separate charger. This way the use of the generator is the most economical.


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sesshoumaru avatar image
sesshoumaru answered ·

@MarekP

Thanks for that very useful information. A pity that you can't have different units.

I thought the VA3000 meant that it only can take in 3000VA from a PV but still can put out more to the loads from the grid. So its more like a total capacity. Thats a very good hint.


You can load the battery (BYD LVS 12) directly from a diesel? Can you elaborate a bit?
Because then it may be more economic for me to switch to DC coupling and use Multiplus II units (a bit cheaper). I guess I would have to apply some magic pouder to start the generator if the power-grid if off and the battery below some percentage and stop it onces the grid comes back or the battery is above a certain percentage.


Would be a good feature if one quattro could handle all three phases like the Fronius Gen24. Three Quattro are more than twice as pricy as one Gen24.

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marekp avatar image
marekp answered ·

@Sesshoumaru

I build first my system on 3 Multiplus-II-48/3000 units in 3 phase formation.

Soon I found out that 2400W per phase is not enough, so instead of replacing those 3000VA units with 5000VA models, I added additional 3000VA unit to each phase in parallel.

Those 6 units are connected to 20kWh LFP battery (Winston) using long brass buss-bars.

System has Fronius 10-M-3 AC coupled and two MPPT (250/70 and 150/35) DC coupled.

6mp-iifr.jpg

I have 6kW DC diesel generator (120VDC/50A)

Charging this battery directly from diesel will be done (I am working on that now) by connecting this DC generator to the charger and charger directly to the buss-bars.

Same way MPPTs are connected.

Victron does not have dedicated LFP DC charger with parameters I need (120VDC input and 48V output), so I may build my own charger.

Automaic starting of the generator, in the situation you need, is simple. It is a part of Victron software.

As for Fronius GEN-24. I looked at it and decided to go with Victron.

GEN-24 cannot be used as UPS because it needs up to 90 second to switch over from grid to batteries. Also it needs an antyislanding unit (not cheap) to be legally connected to the grid.

Victron MP-II has it build in.


6mp-iifr.jpg (384.6 KiB)
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sesshoumaru avatar image
sesshoumaru answered ·

@MarekP
Impressive setup. Thank for sharing that image


I have a 230V AC generator and I heared in generel that BYD batteries are quite picky when it comes to the charger they kindly allow the grace to load them. So playing it save for me sounds like I need:


A) Switch from AC to DC coupling

B) Use two Quattro VA5000 units for 2 phases (third phase comes later to make it cheaper now)

C) Plug diesel generator as second AC source into the Quattros

D) Put all major loads across the two phases supported by the Quattros and dispensable loads on the unspported phase 3

E) BYD LVS 12 kWh battery will be connected to both Quattros

F) My own 5KW UPS will secure phase 1 for edge cases (very importand senstive loads)


I hope that sounds like a viable plan and finally settles my endavour into backup power supply :)

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marekp avatar image marekp commented ·

@Sesshoumaru

If you are later planing to make a 3 phase system for 3 phase loads, do not wait to long with buying that third unit. For 3 phase system units must be identical.

If you do not have 3 phase loads, units do not have to be identical.

If that system is to be connected to the grid, Quatro is not certified, in most countries, to be connected directly to the grid without antyislanding unit in front of it.

I heard that there will be Quatro-II that could be certified like Multiplus-II is. It could be also less expensive than Quatro_the_original.

I do not like generator connected directly to the AC-in because it has to be a very good quality generator and therefore expensive. Victrons are very picky about the quality of the AC going into them.

This is also why I prefer to charge battery directly.

Make sure that Quatro will accept the AC from your generator.



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sesshoumaru avatar image sesshoumaru marekp commented ·

I think I have a decent generator with pure sine wave and AVR-system. And it definitly was not cheap.

In my house there are no real 3-phase loads, its just that all loads have been distributed over the 3 phases. So there is no rotary current. But I will keep that in mind, especially as the time of head pumps is upon us.

I see, there is really a Quattro II in the pipe which could come around any week now. That was a really useful hint. I saw first listings for about 2200€ for the 5000VA unit with certifaction to deliver power back to the grid. Great!

The only thing still left is now the BYD LVS 12 kWh battery back and how to connect it to all two/three Quattro units. I am a bit worried, because somehwere I did read that each unit needs its own battery tower and combined there was a minimum requirement of 48 kWh capacity -- which sounds ludacrist to me.

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marekp avatar image marekp sesshoumaru commented ·
@Sesshoumaru

If Victron units are NOT connected as 3 phase system, you CAN have separate battery for each unit. Each phase is than considered as a separate circuit.

If you connect them as a 3 phase system there can only by ONE battery.

To find out what size the battery should be, you have to calculate the max current your system will draw from the battery, and get the battery size that will provide that current without the harm done to the battery.

If you want that battery to last, make the battery 100% bigger. That is my view.

My battery is 20kWh with the max current draw of 400A (1C).

My max load is 10kW so, my max current is around 200A.

I have one battery for all units since they are connected as 3 phase system.

I have a 3 phase Heat Pump, 3 phase water pump and all kinds of 3 phase equipment at home. :)


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marekp avatar image marekp commented ·

@Sesshoumaru

I noticed that your described your generator as 230V.

Is it a single phase generator?

If it is not 3 phase generator, usually described as 400V, it cannot be used as a source of AC power for 3 phase Victron system.

You can use it to charge battery directly thou. :)

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sesshoumaru avatar image sesshoumaru marekp commented ·
My power generator is 8KVA with one or three phase support.

But I want to connect all three phases of the secondary AC input of the Quattros to the single 230V supply.. also the grid delivery 230V on each phase. Why? Because 8000KVA/3 is not safe enough per phase for me - as the living room with the video/audio system can draw easily 2000-3000W.

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marekp avatar image marekp sesshoumaru commented ·
@Sesshoumaru

230V is on one phase.

400V is between phases.

I am a bit confused with what you are saying.

1. Do you have 3 phase grid supply?

2. Do you have 3 phase loads in your house?

3. Do you want to connect your Quattros as one 3 phase system?

4. Do you want to connect your generator as a 3 phase source with each phase to each one Quattro AC-in?

If you can have 3000W load on one phase than 3000VA Quattro is not big enough.

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sesshoumaru avatar image sesshoumaru marekp commented ·
My home has a three phase power supply, but I do not have 3 phase loads.

The different loads in the house are just connected to different phases to distribute the electric loads.


Yes, I want to connect the Quattros as 3-phase system, because I need backup power on all three phases to have all things running, because they are pinned to different phases.


To have full power of the generator potentially on all phases (not at the same time of cause) I was thinking about connecting the generator one phased (same one phase on all Quattros as second AC). But I could connect it three phased, as my Generator also supports 3-phase loads. But maybe it could also be done by having a real 3phase suppy from the generator. One phase using 1/3 of the Generators power, the other 2 phases could load the battery, potentially the Guattros drawing missing power from the battery then.


Yes... I would say its better to go with Quattro 5000VA. The 3000W are only peak, but better save than sorry. Right? ;)

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marekp avatar image marekp sesshoumaru commented ·
@Sesshoumaru

If you do not have 3 phase loads, there is no point i setting 3 Quattros-II in 3 phase system. 3 phase generator connected to 3 phase system will only supply 1/3 of its power per phase. Also if something happens to one Quattro-II in 3 phase system, you will loose power in all three phases.

If you set 3 Quatros-II as a separate 1 phase units and connect 1 phase generator to all 3 AC-inputs, you will have generator's full power on all phases.

There is always an option of using 3 less expensive MP-II units and charge battery from the 1 phase generator.

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sesshoumaru avatar image sesshoumaru marekp commented ·
I understand, but I have all my loads distributed over L1, L2 and L3. So I need to get power to all three power lines. I assumed that this means I need a Quattro unit per phase, which basically is a 3-phase system, regardless if there are actual 3-phase loads (like a heat pump).


I would love to investigate possiblities to load the battery directly with the generator, but I can find nothing about Skylla where it ever has been used in such a setup. I am free on battery side, might use Pylontech or whatever fits and can be bought without studying electrotechnis :)

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sesshoumaru avatar image sesshoumaru marekp commented ·

So... could I use a Victron Skylla? Putting it between a Multiplus II and the BYD LVS 4.0 Battery system? Using the Generator a full power one phase to load the battery, while the multiplus II systems and in parallel drawing power as they see fit?

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marekp avatar image marekp sesshoumaru commented ·

You should be able to use Skylla to charge your battery directly from the generator, but ask BYD for confirmation.

Your generator is 8kW 3 phase but check if it can provide this 8kW on 1 phase output.

I suspect it is less.

Generator a full power one phase to load the battery, while the multiplus II systems and in parallel drawing power as they see fit?

That is exactly my idea how I want my system to work.


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sesshoumaru avatar image sesshoumaru marekp commented ·
I will confirm, or use Pylontech otherwise. System has to fullfill my needs, not vice versa.

Yepp, my GenSet puts out 3 phase (2000W each phase) or one phase (6000W).

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shaneyake avatar image shaneyake sesshoumaru commented ·

I would definitely keep your system as 3 phase. Having 3 single phase separate systems is a mess. You need 3 GX's, CAN bus doesn't play nice, etc.

You can then feed the 3 phase from your generator into the AC-IN1 and Grid in AC-IN2. Don't worry about other chargers, etc. The coms from the battery should command the Victron equipment to do the charge.

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shaneyake avatar image
shaneyake answered ·
  1. Yes, you will need 3 units. You don't have to start off with 3, you can do 1 and then grow. Only thing to keep in-mind is that if you ever wanted to have multiple units per phase they have to be identical but you can have same model but diffrent revision on diffrent phases.


  2. You can have single phase into 1 inverter and 3 phase out from the units but if the generator is directly providing 1 of the phases and inverters the others, there will be a synchronisation problem. You either need devices to be powered from Generator or Inverters but not half and half.

  3. Yes, all inverters will be connected to same battery bank, with BYD or Pylontech or any good battery bank there will be a CAN bus connection to the Cerbo to tell the Victron equipment about the battery.

  4. Is the Fronius 3 phase? If so, yes, 3x 3000 is 9000W, if it is single phase then no, won't work.

  5. This doesn't have an effect on the Victron.

    As you have noted in other comments, I think a Quattro-II 48/5000 is best suited for your application. There is a big investment and it is better to spend a little more now than to do an upgrade later and try sell 3 inverters.

    AC-IN 1 goes to 3 phase Genny, Battery Charging will be controlled via CAN bus.
    AC-IN 2 goes to GRID

    AC-COUPLED is good and will work and you have a genny so if you had a completely discharge your batteries you could start up from that. I personally would still install some via DC-coupled up to 50%, just as a backup in case I lost grid and the genny for some reason. Like outage prevents you from getting fuel.
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sesshoumaru avatar image sesshoumaru commented ·
Thanks for the input.

So I would plan for 3 Quattro II (once available), connect the GetSet as three-phase AC source over all three Quattros and rely on the Cerbo / Venus to handle loading and start/stop of the GenSet during power outage. Just for sake or security I would plan for 5000VA units. That sounds like the most safe / common place way to design the system.


I still like the idea of directly charging the battery with DC after something like the Syklla, but i can really find nothing about that. Makes me worry that its too exotic.

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shaneyake avatar image shaneyake sesshoumaru commented ·

Yeah, they were meant to be launched in September been delayed till December.

There is no real difference in charging from a Quattro/Multiplus vs a Skylla.
They both take AC and convert it to DC. The Inverter has way more control and can deliver the voltage and current the BMS is requesting. You are limited to the Skylla-TG which is the only one that can do 48V and it is pretty dumb, in definite need of an update. Skylla-i is nice but no 48V version. When charging the bank from a Skylla-TG you have no control, it is full power or nothing. I highly recommend that you don't use a Skylla-TG with a CAN managed battery. We do use them at sites with very bad grid but that is a limited use case.

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sesshoumaru avatar image sesshoumaru shaneyake commented ·
Sounds like good advice. I want to use a BYD or Pylontech Battery.

I guess then its safer to go with loading via the Quattros. Maybe in the future I can add a Skylla TG (II) once improved for switching to direct battery loading. If I am allowed (by government) to install more solar, than I would also add DC doupling, but stay with my AC coupled fronious for now (its also 3phase, its the Symo 6-0-3-M).

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marekp avatar image marekp sesshoumaru commented ·
Quattro-II is more expensive than Multiplus-II. It is more than 500 Euro per unit.

You can make your system using 3 MP-IIs in 3 phase setup.

Connect your house to grid through Grid/Gen switch.

That switch will alternate AC-in between grid and generator.

When grid fails, system will run automatically from battery, and when battery gets depleted you can turn on your generator and connect it to the AC-in by flipping that switch to gen position. Generator would charge the battery with its full power using all MP-IIs.


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shaneyake avatar image shaneyake sesshoumaru commented ·

Why do you want to charge the battery with a charger? I don't quite understand why you would want to add a charger if you can avoid it? Maybe I am missing something.

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sesshoumaru avatar image sesshoumaru commented ·

This is probably now an outlandish question, but my already existing system has a Varta Element 6 battery with 6kwh capacity. As far as I see from the manual, its an AC-coupled battery system.

Is there a chance to somehow integrate it with the GX Device on Victron side?
I could imagie to put it between grid and victron AC-in ports and it should not matter to the system as it would swallow only exess energy the Quattros would have feed to the grid. But maybe it could also be included on the AC-out side with the PV?

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shaneyake avatar image shaneyake sesshoumaru commented ·

From what I can tell it will be possible to integrate the Varta Element 6 as it supports modbus TCP but it will require some custom setup and basic programming. This integration looks doable but definitely not out of the box kinda setup. Would be custom stuff. With it all setup, you should be able to keep in on the AC-OUPUT of the quattros so it is still available during outage and only charge when excess is available.

This openness is the true value of Victron.

Can you request the Modbus docs from Varta? I can give you a strategy to implement control then.

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sesshoumaru avatar image sesshoumaru shaneyake commented ·
I will try to request this documentation.

I am software developer.. so coding sounds fun^^

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shaneyake avatar image shaneyake sesshoumaru commented ·
Oh sweet, then this will be easy for you. NodeRed on the GX does this type of integration quite nicely.
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