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boristhespider asked

Feeding one EasySolar into another

I have an EasySolar-II GX in ESS mode, connected to the grid, but with feed in disabled and the grid setpoint set high enough to ensure no inadvertent feed in. Let's call this system the house system.

I'm considering adding more PV capacity in an outbuilding (in other words, this would be a mainly separate system, with another EasySolar or Multiplus, it's own separate MPPT controller, and separate batteries). Let's call this system the outbuilding system. I would be thinking of also running this system in ESS mode.

This downstream system (outbuilding) would get it's AC IN from the one I already have (house system), so if PV or battery in the outbuilding weren't sufficient to supply it's loads it would pull power from AC IN to supplement (up to the AC IN limit) and so would be pulling that additional power from either the house PV, house batteries, or possibly at times the grid via the house system, so far so good.

But if the outbuilding system has surplus power (it has for example good sun, fully charged batteries, and light loads in the outbuilding), can I now feed power back into the house?

I'm thinking that from the point of view of the outbuilding system this would basically be doing grid feed in. It has no idea where it's AC IN is coming from, so if it is set to export AC and DC surplus to the grid, it will just sent that power up the AC IN wires to the house, where they will meet the AC OUT of the house MultiPlus/EasySolar/Whatever.

From the point of view of the house system, it will be like using a PV inverter (https://www.victronenergy.com/live/ac_coupling:start), in that it's receiving power on both AC IN (grid) and AC OUT (downstream Multi/EasySolar).

So during a period of heavier load in the house, the downstream solar system should combine with the house EasySolar to power the loads.

During a period of lighter load in the house, suplus power coming in on the house EasySolars AC IN (from the outbuilding) would go backwards through the inverter charging the house batteries, is this correct?

What about "Frequency Shifting" as described on the page I linked to above. That page describes this as being the method used by the multiplus/easysolar to control the output from the PV inverter so as to prevent it outputting excess power that has nowhere to go (say the house load is zero, house batteries are 100% charged, house does not feed in to the actual grid). Does this same principle apply if the downstream system was not a "PV Inverter" but instead was another EasySolar or MultiPlus as I'm describing?




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Alexandra avatar image Alexandra ♦ commented ·

@BorisTheSpider

Yes you can but you would have to manage it quite seriously.

We tried this as an experiment once feeding backwards into another easy solar. The main problem is the control of the feedback. When feeding to the grid there is technically no limit as to how much that can be back fed as there is always somewhere for the power to be used in the grid. (Not always the case these day -a subject for another day - grid voltage rise being an issue in some countries from all the feedback)

Frequency shifting is really for AC PV and since the Victron is so good at synchronizing a disconnect does not always happen as expected. The one way is to limit the feedback to under the base load of the main system so it does not over feed, but you have to be careful. It was especially a problem when the batteries are full, this is not great the power has to go some where and for us it ended up overcharging batteries and causing the bms to shut down which then had knock on effects. You can connect and disconnect using AC 2 based on SOC but again it has issues and potentially big problems if not really managed.

As mentioned below it is better to get AC PV so it can be controlled properly, or run an extension from the other system so you can shifts loads onto its output at the db. Seen some clever changeover switches in DB boards for this.

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boristhespider avatar image boristhespider Alexandra ♦ commented ·

I do in fact manage the SOC in the house anyway, by using an external control loop via modbus tcp to restrict the DVCC charge current at high SOC, so as to try to avoid letting the batteries get completely full (for battery life). That said, the control loop is then a point of failure.

I think, considering what you've both said about it being an unsupported and rather experimental configuration, that I'll put a 3kVa Fronius in the outbuilding, and add extra battery capacity in the house for it to charge if daytime generation is surplus. All I really lose then (vs a pair of Multis) is nighttime inverter capacity, but since I don't have the storage to be using more than 3kVa at night anyway, that's no big deal and I will still get the added generation plus 6kVa of inverter capacity during the day.

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2 Answers
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ceriw answered ·

Hi, I'll start by saying that what you are trying to do is not a system setup that is supported by Victron, so you would not be able to get any official support from them about it. A supported system would be to add an AC inverter to your outbuilding to connect the additional panels on the outbuilding to the output of your easy solar. If you are not aloud / don't want to feed back to the grid, the AC inverter would need to be one of the models that Victron supports zero feed in mode.


If you still wanted to go ahead and add another easy solar GX to your system, it is possible, but it depends on a few things. Firstly, are you aloud to feed back to the grid?


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boristhespider avatar image
boristhespider answered ·

Hi,

I could in theory feed back to the grid, but I don't want to, so let's say that feed in is not an option.

I'm presuming the reason you're asking that is because in the risk case I'm considering (too much PV on both systems, sudden drop of load on the downstream system) there would be nowhere for the excess power to go unless it could be fed into the actual grid???

If it's a better supported / more standard to use a Fronius zero feed in PV inverter for the outbuilding I might go that route.


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