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arees asked

Grid assist, 3 phase with batteries

Hi,

Apologies for my lack of technical detail, I am more an owner rather than installer, we have had a system installed on our dairy farm that consists of:

3 quattro 48/15000, 2 fronius eco 27kw, colour controller, MPPT 450/100, 40 AGM 12-200 batteries and about 90kw of panels (cant remember exactly but roughly 55kw to the fronius and 35 to the MPPT) Lynx - power in 1000, shunt 1000 & distributor 1000.

System was sold to us as an off grid solution (i.e. no more electric bill). It has been installed with a changeover switch so we are either on gird with the fronius feeding any load first OR off grid with fronius covering load and then quattro pulling from the battery.

The system doesn't seem to cope with a large load when off grid, once its pulling about 15kw from the batteries voltage soon drops and the system cuts out so we are daily switching between on and off grid. Which is not only time consuming but creating some issues with electrical equipment.

I had a read through here and get the impression it should be possible to run batteries and grid together i.e. load first covered by fronius, then batteries and then grid. Is this correct? I asked our installer and he wanted to keep us as we are and add more batteries and panels. Our grid connection is reliable and so makes more sense to me to use it rather than a generator.

Thank you in advance for any guidance you can provide.

MultiPlus Quattro Inverter Charger
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David avatar image David commented ·
Can you elaborate regarding the voltage drop? Are you getting errors?


if I have it right, you have nearly 100kwh of AGM storage, at 48v, and if you are pulling 15kw total that’s only 1/3 of total system capacity - but still equates to 312 amps on the DC bus. If your batteries are setup with 10 in parallel, that’s over 30 amps per battery. How long are you trying to power loss that high?

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marekp avatar image marekp David commented · 
if I have it right, you have nearly 100kwh of AGM storage, at 48v, and if you are pulling 15kw total that’s only 1/3 of total system capacity - but still equates to 312 amps on the DC bus. If your batteries are setup with 10 in parallel, that’s over 30 amps per battery. How long are you trying to power loss that high?

In theory it is 100kWh but in practice its is only 50kWh. AGM battery should not be discharged below 50% its capacity if you want them to last more than 500 cycles.

Also it is not 312A but closer to 350A factoring in the efficiency of DC to AC conversion.

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David avatar image David marekp commented ·
Yep - exactly. Battery to small IMHO.
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arees avatar image arees David commented ·
Yes batteries arranged as 10 lots of 4 to give 48v. Battery voltage drop to around 47v then the system throws out an error and power goes off. Yes agree batteries are too small to get off grid, having done a few calculations myself now, shouldn't rely on the pitch of a salesman.
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marekp answered ·

@ARees

I do not see why not have the system permanently connected to the grid since you have a stable grid. Especially with a Quttro units that have separate input for generator.

If the grid feeding is a problem when you have too much power from Sun, you can set it up so no grid feeding is allowed.

It is economically beneficial to get the missing power from the grid (when present) instead of generator.

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arees avatar image arees commented ·
Thank you for your reply, we would need permission form DNO to export hence my thinking put the excess in the battery and use when needed as our consumption isn't level through the day.

Is it possible for it to be using all 3 sources at once? theoretically our max load could be 35-40 kw if everything switched on the same time, could it say be using 10kw from fronius, 10 kw from battery and 15kw from grid or is it more complicated than that?

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marekp avatar image marekp arees commented ·
@ARees

Yes you can use all 3 sources. I do.

You can take from grid and battery at the same time.

You can store "Sun" (energy from PV) in the battery and use "Sun" for your load at the same time.

If you connect your Fronius at the AC-out, you can use "Sun" for charging and loads when grid is missing up to the point when battery is full. If grid is still missing and battery is full, Quattro will limit the Fronius production if there is too much "Sun" for loads.

Same if you cannot export to grid, Fronius will be limited to balance "Sun" and loads.

You should use generator only when there is no grid and batteries are empty.

BTW. I think, today, it is a mistake using AGMs for battery.

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