AC coupled PV in a Multi-2 ESS along with an automatic backup generator

Q&A and troubleshooting
, , , ,
1

Hello community,

Following situation;
An existing installation: A three-phase grid-connected system (3x40A) with an automatic backup generator. When the grid is unavailable (which is rare), the system automatically starts the generator and transfers the installation from the grid to the generator (all three phases plus the neutral) using a motorized transfer switch. An AC-coupled single phase PV system includes multiple micro-inverters (with a combined power of 2.4kW) that are connected in front of the transfer switch (towards the grid).

The idea is to add a single phase Victron ESS to the mix, so that we can use more of the generated PV energy locally (greatly reducing the PV energy injected into the grid). In the meantime, taking optimal advantage of the possibilities that the necessary components have to offer.

How? By adding an 48V-5kVA multi-2 (including a 48V battery system and a Cerbo GX) behind the transfer switch on L1 (using AC-in and AC-out 1). Adding an VM-3PT75CT to the grid connection directly behind the transfer switch will allow for measuring of the total GRID consumption and power. An ET112 will also be added to measure the PV production.

However, the remaining issue is where to connect the AC-coupled PV system, I believe I have found a satisfactory solution for this problem. Connect it to AC-out 2, but turn it (AC-out 2) off while the generator is running. This is important because the ESS will (mistakenly) consider the generator voltage as the GRID, which could cause power quality instability as a result of the simultaneous production of AC-Coupled PV and the generator (due to the missing Cos-phi regulation).

Unfortunately, I didn’t find any documentation for this type of system, nor any similar installations. I also did not find any documentation, how I should introduce the “generator running” status to the Cerbo GX to switch off the AC-out 2. Maby, this could be done using an input on the Cerbo GX (Generator active input => 0, if not, => 1)? Therefore, I would be grateful for the community’s opinion on this subject.

N.b. Please refrain from proposing the use of a quadro or injecting the PV energy via the 48VDC bus using an MPPT charger as these options are not feasible for this installation.

Thanks in advance, JB

2

Hi JB,

AC out 2 control has limitations - not severe but important enough to be a pain.

I would use Relay 2 on the Cerbo GX and Node Red.
When the Generator is running you drive a small relay (from the Generator AC) which closes an input on the GX. This then triggers Relay 2 on the GX which in turn triggers a contactor to disconnect the AC supply to the PV inverters.

You could of course just not include the GX at all and simply have the relays disconnecting the PV when the generator is running but Node Red in between would allow you to keep the PV in and assisting the generator and then disconnect them using Node Red if for example the generator gets close to reverse power (export). Using Node Red would also allow you to change the ESS mode to “Keep batteries charged” when the generator is running.

1 Like
3

Hello Ben,

Thank you very much for your input, it is greatly appreciated.

Your idea of changing the mode from ‘ESS’ to ‘Keep batteries charged’ when the generator is started is probably the best compromise here. I haven’t attempted this before, but I plan to give it a shot as soon as I have some free time.

Concerning the AC-coupled PV inverters, I think I will just connect them between the VM-3PT75CT and the generator transfer switch so that they are disconnected from the installation when the load is transferred to the generator.