I am running a Multiplus II 48/5000 with MPPT chargers and AC out coupled PV, using ESS Assistant. The system is running off-grid. (PV-Assistant had other bugs that ruled it out.)
The AC coupled PV inverter is a SMA Sunny Boy 3000-TL21. Its frequency response is a linear reduction in output between 51Hz and 52Hz. These are the parameters I gave when setting up the ESS Assistant.
The attached screenshot shows what happens on a typical day with low AC loads when the battery reaches the target charge voltage of 55.2V.
At about 8:50 battery charge current is gradually reduced by first reducing power from the MPPT chargers. Battery voltage remains almost constant at 55.2V during this time, as it should.
Then, from about 8:52 to 8:55 charge current remains constant, leading to an overshooting in battery voltage all the way to 55.5V. During that time the Mutliplus slowly increases frequency from 50Hz to 51Hz, with no effect on output from the AC-coupled inverter. The Multiplus knows that any frequency changes between 50 and 51Hz will have no effect, since it is has been told when setting up the ESS assistant: output reduction starts at 51Hz.
Only once the frequency gets over 51Hz the AC inverter output is reduced and the battery voltage returns to normal. If I was charging to 3.6V cell voltage (as often is done for LFP), this same overshooting behaviour would trip the BMS, causing a completely unnecessary power outage. It’s only my conservative 3.45V target cell voltage that leaves enough buffer.
In my view this is a bug in the ESS assistant. Once the MPPT chargers are effectively turned off, with still a surplus of charge current, the Multiplus frequency should instantly jump to 51Hz and rise from there. There is absolutely no benefit in slowly moving from 50Hz to 51Hz, taking 3 minutes to get there, allowing 3 minutes of over-voltage in the battery!
Interesting enough: when the battery target-voltage changes to float (53.6V) at about 9:08, the frequency does jump to 52Hz in one big step, reducing output from the AC coupled inverter to zero in an instant.
ESS has to survive a grid outage of an arbitrary length. Let’s just assume an off-grid system is one with a permanent grid outage But even when connected to a grid ESS can be set to zero exports.
In both those cases the exact same situation will arise and this software bug in the ESS assistant will cause problems.
No, the SMA inverters works exactly as specified in the SMA installation manual. There has to be some room for variation in grid frequency that does not instantly affect the inverter output. SMA chose that to be 1Hz and starts to ramp down over 51Hz.
Both the PV and ESS assistant have input fields that allow a user to specify the exact parameters of the connected inverter. I can’t remember the default values, but SMA was not too far off.
If you have read the manuals , you will find the reasons why
you can have a lot of issues like this when using ess in off grid, ess advantages wont work in off grid,
you can have generator on AC in problems and AC pv problems with freq shifting
There have been multiple topics and questions about this, search the old community
@Diederik if grids fails, ess functionality is “shutdown” and you can run on battery and PV like normal off grid users
In the beta code there is now proper control of SMA systems (via modbus iirc), similar to Fronius. Have you considered looking at that?
The main issue with ESS off-grid is actually safety considerations for mobile users.
Beyond that, it will operate without a grid, but there isn’t a lot of benefit beyond the additional control features. I run for months with grid dropped to avoid the inefficiency of the setpoint, as do others.
I would eliminate ESS as an issue by removing it and using the PV inverter assistant which is intended for off-grid use cases.
(edit, see this was done already)
Wow, I only just saw that part of your reply. That might be the explanation for the very odd behaviour that got me to install ESS assistant instead of PV assistant in the first place.
The first thing I did when the system was installed was to add PV assistant. But it seemed like it didn’t want to use the AC coupled PV at all: it kept the frequency at 53Hz (which not only throttles, but actually shuts off the inverter!).
I can’t exactly recall the SOC at that time, but it is possible it was 95% or higher.
I might give PV assistant another try.
@lxonline Why the past tense, what are you using now instead of PV assistant?
Thanks @nickdb. I didn’t know about the beta, would you have a link for me so I can read up on that?
I have integrated the SMA inverter via Modbus, but as far as I can tell for now (in the production version), that is just used as a data source. It looks like control of the inverter is still exclusively via frequency.
I will give PV assistant another go, now that I have a reason to believe it could work (thanks to the reply by @lxonline mentioning the early shut-off of the PV).
There is semi hidden category called betas, look on the main page under the “muted sections” tab. Venus OS beta category, has the change logs and issues reported during testing. To support zero feed-in, full control would be needed, aiui, not that PV inverters are my wheelhouse.
I did manage to change from ESS Assistant to PV Assistant.
And the same bug still happens. Not really surprised, I would expect the same code works in both when it comes to frequency adjustments.
To make sure things don’t get confusing I will start a new thread, and hope it catches the attention of Victron developers. It is a bug, and it is easily fixed. I do hope Victron lives up to their reputation now.
But even when connected to a grid ESS can be set to zero exports.