question

@sharpener

It will work but you will have to load ESS or PV inverter assistant in order for PV inverter frequency control to work.

As a standard frequency settings are for Fronius but they can be changed to match you Steca inverter.

Here are my ESS settings.

Thanks for yr quick reply @marekp. I was planning on using ESS anyway and I now see that frequency control is provided if you set it up with PV on the AC Out? = Yes.

I am a bit confused by the reference to Fronius, AIUI they have 0 - 100% modulating control designed to achieve perfect No Feed In. Whereas the Steca inverters are all-or-nothing so will generate at full power until the frequency is shifted to >51 Hz and then shut down, this will be fine provided the EasySolar can cope OK, I do not care if there is Feed-in or not (my UK FIT payment is mostly on generation not export).

Related to all this is also the size of battery. Here it explains the Factor 1.0 rule: https://www.victronenergy.com/live/ac_coupling:start, this test is met. But later it says that you must have 4.8kWh of lithium batteries for every 1.5 kW of PV. This would mean I need 3 x 4 kWh BYD LVS (not 2) which means extra cost and space. Since the charge/discharge current is <60% of the maximum the batteries can take, can you explain the reason for this rule?

TIA

Thank you, that is all very reassuring.

I was a bit worried that the 1.5kW/4.8kWh is a requirement for the warranty of either the battery or the EasySolar, but the charge/discharge current limit is fine and your experience seems to suggest the rule is unnecessary. The BYD size chart says a minimum of 1 x 4kWh is sufficient for the ES 5000 except (as you say) for off-grid use where you need 3.

Perhaps I will try and install the batteries so I can add an extra one on top if I need to!

Many thanks

Good point @MarekP. If I understand your reply correctly I had overlooked the fact that the the charger 70A limit does not include the MPPT output. This might be another 80A if I succeed in getting 4kW of extra panels on my roof (space is limited and there is some shading).

So (unless I add the third battery) your advice to limit the total charging current to less than the maximum allowed by the BMS sounds very wise.

Presumably if the grid fails the ES will shut down the MPPT first, and only trip the Steca inverters if there is still too much solar power? I haven't yet found any details of this anywhere.

@sharpener

If I understand your reply correctly I had overlooked the fact that the the charger 70A limit does not include the MPPT output. This might be another 80A if I succeed in getting 4kW of extra panels on my roof (space is limited and there is some shading).

Yes, the 70A charging current is only from ES-II charger from AC side. The included in ES-II MPPT 250/100 can provide up to 100A charging current, on the DC side, directly to the battery.

So (unless I add the third battery) your advice to limit the total charging current to less than the maximum allowed by the BMS sounds very wise.

Even wiser is to limit charge current below the "max" allowed by BYD. The battery will thank you for it with longer service.

Presumably if the grid fails the ES will shut down the MPPT first, and only trip the Steca inverters if there is still too much solar power? I haven't yet found any details of this anywhere.

When grid is down and PV production can be fully consumed by house loads and battery, frequency of AC-out is not increased. (FQ value you can see in the "critical load" box at it's bottom)

When grid is down and PV production is higher than house loads and battery "save intake", ESS will first limit the production of the MPPT charger.

If this is not enough it will increase the AC-out frequency to lower the production of the AC-coupled PV inverter.

Here you can see how this frequency shifting works. (on top there is a missing input FQ when grid failed.

Here you can see that there is no FQ shifting if PV production is low.

In my system AC-coupled Fronius has 10kWp in panels and MPPT has 6kWp.

Thank you, that looks fine.

But I am puzzled by the previous illustration, the total consumption is 4121W and the total solar is 4102, a deficit of 19W. But the inverter is shown as charging at 2547W, where is this coming from and why if there is no AC input?