ESS Design/build - See my diagram of components and configuration if best method?

DIY
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ESS Electrical Layout Neil.pdf (792.5 KB)

Hi All,

Seeking advice/confirmation of an ESS I’m building to install at my home using some Victron parts I already have surplus from jobs at my company. So, where possible, the system is being designed around what I’ve already got to make use of these parts (as listed below).

I’ve attached a drawing to detail the components layout and basic connections of my proposed system and would like to know if all think this could be a suitable option or what/how it could be ‘better’ from different products to use or configurations? I can then buy the remaining parts or reconfigure system based on the advise. Further details listed here below;

Infrastructure existing/installed; 7x 250W solar panels supplying PV inverter coupled to AC grid with feed-in metering. 6x 250W solar panels (from a redundant system) to supply SmartSolar MPPT 150/85

Parts I already have for ESS; 4x LiFePO4 Battery 12.8V/100Ah, 2x BMS Lynx Smart 500, SmartSolar MPPT 150/85 VE.Can, 6x Solar panels 250W, Generator 8KVA, Auto Transfer Switch, Android tablet.

Remaining parts in question: Multiplus 24V/5000/120, Cerbo GX

Typical maximum demand from AC loads: 5KVA

ESS functionality priority: Power backup when loss of mains/grid input. (All AC loads switched to inverter from batteries, generator enabled when batteries minimum Soc reached to supplement AC load and charge batteries with excess VA from generator.)

Secondary priority: Low level of self-consumption from 24VDC batteries and MPPT solar. (Batteries not to be cycled below approx. 80% Soc and always need to remain near to fully charged ready for use when grid fails, AC-coupled PV inverter is already connected as self-consumption and excess feed-in back to grid.)

Question 1: Is the MultiPlus 24/5000/120 the correct product? Is there benefit of configuring batteries to 48VDC to allow for a better suited product as the inverter/charger. Note that I require relay switching option for generator control from whichever product may otherwise be suggested I could use.

Question 2: Can/should the solar array as pictured supplying the MPPT (150/85) be instead configured as 3x 2-string series parallel therefore inputting the controller at 225Voc, 53.16Isc? Will the controller safely limit the input voltage to 150V when exceeded?

Question 3: Should I upgrade the existing solar PV inverter coupled to AC for a more user-controllable inverter and one which will connect to Victron MultiPlus for synchronizing? I don’t believe the existing inverter can be connected elsewhere in any AC circuit input/output without it detecting a grid failure and shutting off via anti-islanding function. The main objective of a new inverter being installed on this solar array coupled to AC would be to continue inputting it’s yield when there is a grid outage. Either way, I can do this upgrade at a later date if deemed worth while however I just wanted to mention this incase perhaps this may have come into account in considering the suitable and future proofed option of what main charger/inverter the ess should utilise (like the MultiPlus.)

Thank you in advance of any response to my post here. I will try to return and respond daily to any info provided here with queries.

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Hi answers to your questions

1: the higher the voltage the lower the current, so thinner cables and you need one less lynx bms

2: the mppt voltage is a hard limit stay well below that with VOC! 250v will blow it up, keep it 3s

3: you can use a ac pv inverter on ac out side of the multi, but it has to support frequency shift and it needs to be smaller than the multi (stay below 4000w/5000va)

If you want Pv on ac in, use an energy meter on it if its not a fronius or other supported brands

also a grid meter is adviced for ess

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Consider a Quattro rather than a MultiPlus, it has two ACin so you can save the AC transfer switch.

You could consider 48V, would let you use smaller fuses and cables, but then has other implications like the PVs 3s Voc not being much higher than the typical 54-56V in a 48V system

The 50A PV MCB will never trip, your Isc is only 17.7A. Personally id not fuse it at all, theres no point in single or two parallel strings, the fuse rating would have to fall inbetween Impp and Isc and thats a very narrow window.

If you wire it up as drawn, the bottom battery will receive a higher voltage from the MPPT, since its directly connected to it. The top battery, due to the added cabling up to the inverter and back to its own BMS will receive less charge voltage. The top battery will lag behind with SOC. Personally i would not botherr with a redundant battery setup in this case, since you also got mains and genset as alternative power sources should the DC side fail. But rather think about using a bypass switch on the AC side

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Hi Marc, thanks for the info!

Response to A1;

Something I had overlooked before questioning perhaps to use a 48VDC inverter/charger is that the MPPT 150/85 which I already have, is 12~24VDC only. So 48V simply not an option for me. Regardless of which, if the only main advantages of instead using whichever 48V unit is less DC current draw then I’d just stick with the 24V unit anyway. (I have an auto electrical business so any cable I need up to about 95mm2 is basically free, any circuit protection items I get at wholesale prices, I have 4x BMS units available to use for free)

One thing that did lead me to firstly choose the 24V MultiPlus was I can configure batteries as two banks therefor having redundancy should one bank fail.

So when considering which best inverter/charger unit to use, the main requirements based on my general application of use are;

  • AC transfer current: not less than 100A
  • AC output: not less than 5kva
  • DC input: 12~24VDC

I believe the options then available within reason is narrowed down to using either a MultiPlus 24/5000/120-100, or, a Quattro 24/5000/220-100.

If I’m correct in assuming the Quattro could function in the same manner as the auto transfer switch I already have that when there is a grid failure it will physically disconnect from the grid input into AC-1, then accept the input from generator into AC-2 (once batteries minimum Soc is reached and triggers generator to start), then this could be an option.

I’ll further study into the pro’s and con’s of which way to transfer the A/C inputs is ‘better’ and whether its justifiable to pay more for the Quattro (than MultiPlus) and perhaps sell the auto transfer switch..

Any further advise on which inverter/charger unit to use is welcomed!

Response to A2;

Noted. Keep solar array at 3s (112.5Voc) as input to MPPT at 3s Voc input at 112.5V

Response to A3;

Yes, ultimately the PV inverter would be setup and connected to AC-output circuit to form a micro grid continuing to supply AC loads and charge batteries with excess etc. I think at a later date I will purchase a new compatible PV inverter and reconfigure it to an AC-output if I want the extra juice when grid fails. I’ll just leave it there for now how its already connected at the house on grid input and I’ll accept it will be shut off when grid fails. I don’t believe whether I do this upgrade now or want to then do it later makes any difference whether I choose to use a Multiplus or Quattro because either unit will be the 5Kva model (factoring the 1.0 rule) and either unit both mention about the option to do it in their manuals.

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Hi Chrigu, thanks for your response!

I’ve since realised after my post that I’m restricted to a 24V unit because the MPPT I already have is 12~24VDC only. So the only options I see this narrows it down to based on my general requirements of the system and within reason is the MultiPlus 24/5000/120-100 or Quattro 24/5000/120-100/100.

The main consideration with the Quattro being its 2x AC inputs and its controls over them is whether it’s ‘better’ to use this instead of the ATS unit I already have. So I’ll further look into the benefits of controlling the AC transfer when grid fails with the Quattro instead if it can be justified.

Ok, I wont bother fusing PV input circuit to MPPT.

Disregard the drawing layout as being to scale or exact where connections are made. Although I do see how I’ve drawn the MPPT 24VDC circuit connected near the BMS to be misleading but it was the easiest spot those circuits could be connected on. In reality when I build the system, the MPPT output will be connected at whichever inverter/charger DC terminals. Yes, I’m aware the cables to BMS’s and then batteries will all need to be same length so the charge/discharge is balanced to both banks.