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djdemond avatar image
djdemond asked

Grid Parallel ESS and power outage. How to setup and change over.

I have a Multiplus II 8kva with 560Ah LFP battery, UK grid parallel ESS system using AC-In as the grid connection. It charges overnight and provides grid-parallel power during the day.

We recently had a power cut. The inverter was on, and the AC1-out was energised as I have one socket attached to it next to the inverter. However, no power was flowing through into AC-In to the house, which I believe is the correct behaviour when the grid is lost (anti-islanding).

I want to be able to restore power to the house, after disconnecting the grid in the event of an outage.

Should I be using the AC1 out, as the grid connection?

What stops the inverter from becoming an island, if I do it that way?

Multiplus-IIESSoffgrid
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3 Answers
Jason - UK avatar image
Jason - UK answered ·

@DJDemonD Have a look on the below link, at my photos. I have a schamtic of my system. In the event of a power cut, my whole house keeps running as normal.

https://vrm.victronenergy.com/installation/171934/share/db6807bb

You have a 8kva unit which has a pass-thru of at least 50amps, more than enough to run most houses. I'd love to have a 8kva monster but I just couldnt make the sums add up so went for the 5kva unit. I would have my whole house on the AC-out1 and never worry about power cuts.

DO NOT CONNECT THE GRID TO AC-OUT1! - your unit will not be able to disconnect from the grid or be able to sync with the grid, this could lead to failure of the Multiplus unit.

Grid - AC-IN

House = AC-Out1.

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djdemond avatar image djdemond commented ·
Thanks, and cool system BTW. This won't really work for me due to the geography of various things, my battery and inverter are in an outbuilding and the grid comes into the front of my grade II listed house. Yeah, I had a 5kva and wanted to upgrade, I should have just bought another one and paralleled them, which I have done with an install I did for a family member. But I figured the 8kva would be enough, and it is.


Okay I won't connect the AC1-out to the grid, maybe just wire the AC1-out back to the consumer unit via a switch to only be switched in the event of a power cut after I turn the grid off.

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gazza avatar image
gazza answered ·

It is a common question when a grid parallel ESS is set up on AC IN. Of course, in an ideal world, we could all re-wire our house so that the loads are always supplied from AC OUT, in which case the "changeover" happens automatically :)

So, fundamentally what @Jason said is totally correct, AC OUT must NEVER connect to the grid..! When the grid fails the Multiplus II disconnects from the grid on AC IN, as designed. The first thing that needs to be done after that is that the "whole" house must be physically disconnected from the grid at the incoming supply, BEFORE any alternative source (AC OUT) is used to power your house consumer unit. (the Multiplus II auto-disconnect is deliberate so that you can't feed power back to the grid when there is a power cut to protect engineers working on the line).

The other issue to be aware of is that AC IN should never be connected directly to AC OUT while the grid is available!! This means that you need a 2 pole changeover switch that is break-before-make (L+N between IN and OUT need to be kept separate..). Depending on where your Multiplus is, in relation to your incoming grid/consumer unit, this can all be a bit messy.!!

Therefore, an additional grid powered contactor to disconnect at the incoming supply that in turn operates a 2 pole changeover from AC IN to AC OUT at the Multilplus is what you require in principal. Or you could just re-wire your house to be supplied from AC OUT on the Multiplus :)

This "changeover" could also be switched "manually" with sufficient labelling to get the order right, but I am not recommending that :)


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larsea-dk avatar image larsea-dk commented ·
My I ask, what about the smartmeter, which is used on grid parallel systems (only on ac-in) with ess? Is this “not needed” in case of drawing power at ac-out?

I have a 3phase grid but only running 2pcs mp2 5000 on L1 and L2, since this is sufficient for my present pv size(8kW) and battery size (16kWh)

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djdemond avatar image
djdemond answered ·

In the end, as I was not easily able to run the entire house through the Multiplus AC-out pass-through.

I implemented a rotary break-before-make changeover switch at the inverter, which is in an outbuilding. The default position connects my house and it's grid connection to AC-IN.

The handle which operates the changeover switch has been removed, preventing it from being operated at the inverter inadvertently. This stops the grid from being connected to the AC-OUT accidentally.

In the event of a prolonged outage - I have to start in the kitchen, where a large sign by the consumer unit explains that to run off-grid, the grid must be disconnected at the switch between the supply meter and the consumer unit. Once this is switched, the instructions are to turn off all non-essential loads, then take the handle to the outbuilding and switch the transfer switch.

Now, the house is running from the batteries via the inverter.

The same protocol is used in reverse to return to on-grid mode.

It works really well, when I do it, might need to be tested by someone else to see if the instructions are clear enough.

Though having read this through before posting it, another run of cable from the supply meter to the Multiplus, to go along with the run that is there already, would enable full operation with automatic failover to batteries etc.

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