In an installation with a Fronius inverter on AC-Out, a schort grid failure leads to an OVP of the battery if the battery was full at the time of the grid failure.
If the grid fails, the output power of the Fronius inverter (on AC-Out) is immediately reduced. The critical loads are then supplied from the battery and discharge it in the process. However, if the grid returns quickly (approx. 30 s), the output power of the Fronius inverter is immediately increased again. However, as the microgrid is not yet synchronized with the grid, the battery must absorb the entire PV power of the Fronius inverter until the Multiplus / Quattro switches through. If the battery was already fully charged at the time of the power failure, this additional power will quickly lead to an OVP.
It would be better to start up the Fronius inverter only when the Multiplus / Quattro has switched through and not when the grid is back on.
This behavior was observed with firmware 3.50~13, but is probably also the case with the regular firmware.
Yes, the factor 1 rule is observed.
The system consists of a Quattro II 48/5000, 5 x 100AH LiFePo4 batteries and a Fronius Primo 4.6-1 (with 4.4 kW peak panels) on AC-out, plus a Fronius Symo 10.0-3 (with 5.5 kW peak panels) on AC-in.
At the time of the power outage and the test afterwards, the Primo produced an output of around 2.5kW. The two strings are aligned E/W.
The problem is that the Fronius inverter is already started up when the Micro-Grid is not yet synchronized with the grid and the Multi / Quattro has therefore not yet switched through.
If the grid is lost, the Fronius control works perfect.
Why the annoyance? What is the problem? In my view, the problem is that the Fronius inverter is switched on too early, i.e. exactly when the grid is back on. When the grid is off, the Fronius inverter is slowly ramped up to the required load. But when the grid is back, this does not happen slowly and especially before the Multi / Quattro has switched through.
Why not wait so long until the grids are synchronized?
I will spare you my BS batt story
Fronius is widely installed, even here, in a bad grid country.
A latent fault would be known by now.
A beta bug is easy to isolate by reverting to the official release.
The ESS assistant is already installed. This is why the power control of the Fronius inverter works so well when the grid is lost, as mentioned.
Only after the grid returns the Fronius inverter output is ramped up before the grids are synchronized.
Is there a reason why this is done in this way?
In order to synchronise an island to the grid, you need to not only match the angle/phase shift between both sides but also the frequency. Sure its possible to switch a 52Hz island onto the 50Hz grid at the exact moment the angle is getting close to zero, but its not advised. Its better to match frequency and angle before connecting. But this in turn means the Fronius will start ramping up as soon as the synchronising begins.
This however is an inherent issue in the whole “PV-inverter is controlled by frequency” and is present across all manufacturers. So far ive only had this issue with a generator forming an island with PV-inverters feeding to it. The solution was to deactivate the PV-inverters while synchronising with the generator.
On some models theres also the possibility to control/limit the output power trough an analog input or over modbus
Thank you chrigu for the clear explanation.
However - and perhaps I don’t know enough about this - I don’t know when the Fronius is controlled via frequency and when via modbus.
Because of course the modbus control of the Fronius (and in the cerbo) is activated.
With this, it should be possible to limit the power output of the Fronius inverter via modbus when the grid returns until the two grids are synchronized (frequency and phase).
