Frequency not shifted when CCL = 0 - Offgrid + AC coupled inverter

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mr-manuel asked Mar 25, '24

Frequency not shifted when CCL = 0 - Offgrid + AC coupled inverter

I have some problems with the frequency shift of my MultiPlus ii-GX 5000/48/70-50. When the battery is nearly full my BMS reduces the CCL until it reaches 0. Unfortunately the MultiPlus does not shift the frequency enough to reduce the PV power. This results that the BMS blocks charging, there is a little voltage spike, a lot of electronics crash and then the MultiPlus switches to 51.54 Hz to shutdown the AC coupled PV inverter.

So my question now is, why is the whole range from 50.30 Hz - 51.50 Hz not used, to slowly reduce the power and shutdown the AC coupled inverter BEFORE the BMS cuts off the battery for loading?

MultiPlus II-GX firmware: 508

ESS Assistant version: 018C

In the GUI under DVCC only DVCC is enabled and the correct BMS is selected.

Batteries: Custom made LFP 18S with JKBMS with dbus-serialbattery (I'm one of the developers)

An additional question to the "Frequencies" and "Total solar power" page. Below 50.20 Hz (in my case) it should be 100% solar power and 51.20 Hz should be 0% solar power?

Is it using:

1) The total installed PV inverter power of 5000 W for 100 %
2) The total installed panel power of 6480 W for 100 %
3) The last seen power when frequency was below 50.20 Hz

Probably it is even more complex. I try only to understand why the system is not frequency shifting as it should.

Let me know, if I forgot any important informations.

VRM frequency ac coupled shift

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Answer 1 · 2024-03-26T02:19:26Z

snoobler answered · Mar 26 at 02:19 AM

Your battery is pretty wickedly imbalanced.

What happens when you lower the absorption and float voltage such that no cell reaches cutoff?

I'm speculating, but I think there are two driving factors here. DVCC/CCL limit what is connected to the Cerbo, i.e, a Victron MPPT or MP charging via AC will limit charge current to CCL; however, the PV inverter is governed by absorption voltage, and your imbalanced battery is allowing CCL 0A BEFORE absorption is reached, so there's no frequency shifting until the shit hits the fan when the voltage spikes because the PV continues charging and you over-volt due to the charge FETs cutting out.

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Answer 2 · 2024-03-26T02:28:28Z

snoobler answered · Mar 26 at 02:28 AM

IMHO, your battery is in severe need of balancing. You have divergent voltages as indicated by the red lines:

While charging, you have cells INCREASING and hitting limit while other cell's voltages are decreasing during charging...

You either have cells at substantially different SoC, bad connections, or they are failing.

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mr-manuel commented · Mar 26 at 07:27 AM

Yes, this is because I'm not able to stay at 3.40 - 3.55 V per cell to balance. Since I'm using the BMS there is no absorption parameter that is passed to the MultiPlus, only the voltage is changed.

The BMS is lowering the CCL and CVL when a cell hits the max voltage of 3.55 V. This means the total voltage is dynamically decreased that the max voltage of a cell should never be over 3.55 V. When grid connected this works like a charme, but only in off grid this dies not work with frequency shift. So for me it's a very valuable info that frequency shift acts when the MultiPlus is changing to absorption.

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snoobler mr-manuel commented · Mar 26 at 03:55 PM

Can't you configure your BMS for active balancing above 3.40V/15mV deviation even when not charging?

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mr-manuel snoobler commented · Mar 27 at 06:32 AM

Yes, but as soon as I stop charging the voltage gets very fast below 3.40 V, since power is taken from the battery. Therefore I would need a good setup frequency shift that is handling the right amount of power from the AC coupled PV.

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Answer 3 · 2024-03-26T04:55:48Z

JohnC answered · Mar 26 at 04:55 AM

@mr-manuel

You say "Offgrid". If really so, you should be using PV Inverter Control Assistant rather than ESS.

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mr-manuel commented · Mar 26 at 07:29 AM

Oh, ok. My setup if off grid for about 8 months and on grid for about 4 months. What would then be the best choice?

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snoobler mr-manuel commented · Mar 26 at 03:53 PM

ESS when on grid.

PV inverter assistant when off.

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mr-manuel snoobler commented · Mar 27 at 06:34 AM

Is there a way I don‘t know of that it switches automatically the assistant based on the situation? I don‘t think I can add both assistants at the same time.

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mr-manuel snoobler commented · Mar 30 at 12:06 PM

Else I could configure the MultiPlus to not allow grid feed in, configure the PV Inverter Controll Assistant and use the grid like a generator right?

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Answer 4 · 2024-03-26T05:18:47Z

snoobler answered · Mar 26 at 05:18 AM

DOH! Right there in the question... Yes... when off-grid PV inverter control assistant

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