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awbower avatar image
awbower asked

Adding Batteries and Multi to Existing PV System

Starting point: grid-tied PV system on a detached garage. The system is wired into a garage subpanel that connects to the main panel on the house. It is just a standard grid-tied system: current-system.jpg


I would like to add a battery backup. I would only want to use the batteries when the grid fails. I could wire the AC-in side of the Multi into my main panel and then move all of the existing circuits into a subpanel that would be connected to the AC-out side of the Multi. Then I would end up with this system:

standard-install.jpg

However, I would prefer not to tear my main panel apart.

Here is my proposed solution: I would like to leave the AC-in side of the Multi unconnected while connecting AC-out to the main panel (no other modifications to main panel). The system would look like this:

new-system-no-breaker.jpg


However, the obvious problem with this system is that without the AC-in side connected, the system doesn't know if grid power has been lost. It would end up pushing power back onto the grid in the event of an outage. Even with a grid meter, the problem would remain.


My question is this: it is possible to get around the islanding problem by connecting the Multi and batteries to the main panel through a generator interlock? In this way, when I am connected to grid power, the Multi would be totally disconnected from the house; the system would continue to function as a simple grid-tied AC system. However, if I were to lose grid power, I could use the interlock to turn off the mcb (disconnect from grid) and then turn on the breaker connected to the Multi. I would end up with this system:


desired-system.jpg


Until grid power was restored, I would use the system as diagramed above. Is this type of install possible?

system design
current-system.jpg (32.7 KiB)
standard-install.jpg (42.2 KiB)
new-system-no-breaker.jpg (57.2 KiB)
desired-system.jpg (40.6 KiB)
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Alexandra avatar image Alexandra ♦ commented ·

@awbower

Your multiplus (and battery) needs to be equal in size to the PV inverter you are using if you want it to be downstream or supported by the Victron.

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awbower avatar image awbower Alexandra ♦ commented ·
I understand that. However, my question is whether or not it is possible to connect the multiplus and batteries to my home through a generator interlock as described above. I would like to do this to prevent islanding and to eliminate the need to modify the circuits in my existing main panel.
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sharpener answered ·

Is this scheme a replacement for the one in your other thread?

It would help if you could draw us a more comprehensive diagram of this including all the changeover/interlock switches.

Is there a particular piece of kit you are proposing? There are various automatic changeover relays and anti-islanding relays on the market but they are many £00s.

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awbower avatar image awbower commented ·

In the other thread, I had proposed putting all of the equipment (Multi, batteries, etc) in my detached garage along with the PV system. However, that proved problematic since I would have needed to connect this setup to a grid meter in the main panel. There is no easy way to run the wire for that connection. As such, I won't be configuring things that way.


I am now proposing putting the Multi and batteries in direct proximity to my main panel. The only additional piece of hardware I'm proposing is a generator interlock. Like this:

generator-interlock.jpg

In my case, the breaker labeled "Generator" would be connected to the AC-out of the Multi. In this way, the Multi and batteries can only be connected to my house's electrical system if the main breaker is shut off. This setup would prevent islanding during times when the grid is down.

Most of the time, my system would just continue to function as a grid-tied AC system with no battery storage. It would only be during periods of grid failure where I would use that interlock to run my home from the Multi, batteries and existing PV.

Would this type of system design work?

Thank You

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sharpener avatar image
sharpener answered ·

I am not sure if the Multi will work as a charger if mains is only connected to AC-Out. However...

You would get all these benefits

  • full ESS operation, including possibility of charging the batteries at night when it is cheap and discharging during the day
  • reserving a programmable portion of their charge for backup mode (could be 100% for your above use case)
  • completely automatic changeover to backup mode including keeping the existing PV generating when the grid is down
  • all with no additional hardware (since you have got the necessary current transformer and relays you are already paying for built in to the Multi)

if you do the following:

  • Divert the existing meter tails to a new small CU with suitable RCD and MCB to feed the AC-In of the Multi
  • Leave the existing CU exactly as it is but supply it from the AC-Out.

The only snag is metering the PV since it is spurred off in the garage. To do this properly you would need to wire in an ET112 somehow. The system will work fine without it, but then the remote console cannot show it separately, it will appear as another (negative) load on AC-Out.

With 3.68kW of PV and a 5k Multi it works a treat.

HTH

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awbower avatar image awbower commented ·

Yes, I had thought about connecting the house side of the meter to a new subpanel, using that to power the AC-in on the Multi, and then connecting the AC-out back into my main panel. However, I thought this would be a problem for two reasons:

1) Is there any way to make this connection code compliant? Does the NEC allow the house side of a meter in one panel to be connected to another panel?

2) I have 400A service. I imagine that putting the Multi between the grid and my MCB would lower the total power that I could draw from the grid. Do you happen to know what is the max current that the Multi can draw from the grid in this setup?

Lastly, quick question: what does CU stand for?

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sharpener avatar image sharpener awbower commented ·

Consumer Unit. What country are you based in? From your time zone and the use of the term "panel" instead, perhaps not the UK which is where I am.

Western Power Distribution (now part of National Grid) is entirely happy with the solution I described and it meets the IET Code of Practice here. US rules vary from state to state or so I have read, you need to take local advice.

400A seems massive for a domestic situation, are we talking about single phase? Current limits vary by inverter, see their respective data sheets. For the 230V 5k model there is a pass-through rating of 50A so it will supply 12kW of load with a combination of battery and grid (if available) though only 4.4kW on battery alone.

If the biggest Multi is not enough then you would have to segregate your "critical loads" and connect only those to AC-Out. In this configuration you will need an external current transformer or ET112 to measure the grid demand including the non-critical loads.

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awbower avatar image awbower sharpener commented ·
I live in California. Yes, it is 400A single phase service. It is a large house (8000+ square feet).


The pass through limitations imposed by the Multi would be a problem for me. Going back to my original question, if I were to connect only the AC-out of the Multi to my house through a generator interlock, grid power would never have to pass through the Multi... The system would continue to function as 400A service with grid-tied PV unless I manually turned off the main breaker and turned on the breaker connected to the Multi.

However, you had expressed concern that the Multi would not work as a charger if I were to do this. Why not? We would have solar connected through microinverters on AC-out along with all house loads. What would stop the Multi from charging batteries in this configuration?

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sharpener avatar image sharpener awbower commented ·

I am not totally sure but I don't think it is designed to work as a charger with the only mains connection being on the AC-Out. But it will happily send power out of the AC-In(!)

Might I suggest if you have not already done so to look at the numerous alternative configurations discussed in the Victron "Wiring Unlimited" online handbook? Almost every arrangement that has any practical value is mentioned somewhere or other.

The other fruitful source of advice is your local Victron dealer, he will also be able to tell you what will comply with US and CA codes.

Good luck

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awbower avatar image
awbower answered ·

I think I found the answer: charging batteries from AC-out using MultiPlus-II shouldn't be a problem.

https://www.victronenergy.com/live/ac_coupling:start

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Alexandra avatar image Alexandra ♦ commented ·
@awbower

The inverters are bidirectional. So you can charge backwards with a few caveats.

You will need to configure a way for the inverter to either disconnect the out or control the generator run conditon (so maybe ac 2 or from a relay on gx or inverter) as it can lead to battery overcharging and dc bus voltage rise.

Just out of curiosity though how will this inverter help you as a back up if you have not connected loads to the output? And what is stopping you from removing the ac in and out from the PV inverter and putting that onto in and out of multi, with the pv inverter also on an out? So really no db alterations.

Your DB will already have disconnects in place for the pv kit.

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awbower avatar image
awbower answered ·

I don't have a generator. I'm just going to connect the Mutli to the main panel through a generator interlock. I would only use the generator interlock to connect the loads to the output of the Multi when the grid fails. The only power that will get fed into the Multi is on AC-out. It will come from PV with Enphase microinverters. The Multi should be able to throttle the output of the microinverters through frequency shifting.

When the Multi is not connected, I want my grid-tied PV (PV + microinverters) to continue feeding back into my home and grid. If I were to wire the PV into the Multi through a charge controller or other means, because the Multi is not connected most of the time the PV would not be of much value.

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