Exceed factor 1 rule but with auto-shutoff

Q&A and troubleshooting
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1

Hello,

I’d like to add a ESS to a existing PV system that’s a bit complicated:

  • Loads shall have backup power from the batteries through three Multiplus 2 5000
  • PV inverter power in total 32kW (1x 8kW 1x 25kW)
  • PV inverters behind a separate meter

Since 32kW far exceeds the factor 1 rule, I’d like to switch off the 25kW inverter with a relay as soon as the grid fails.

Normally I would just connect the 25kW inverter on the AC-in side, but in this case it’s not possible because it has to go through the meter together with the 8kW inverter.

Is this possible or am I missing something? Are there requirements on how fast the relay has to switch off?

The alternative would be to not have any PV when the grid fails (when I connect both PV inverters to AC-in)

2

Connect the 25kW inverter to AC2 out, and program that to turn off if the grid fails - should be that as default. However, this would mean splitting the inverter feeds.

Why?
If the meter has current transformers, then there is a solution. Separate wires carrying the current from the 2 inverters can be run through the CT’s - but to different locations. The phase wires from the 8kW inverter can be run to AC out 1, and those from the 25kW inverter to AC out 2. This does require that the inverters have separate isolator switches.

3

The grid operator needs it for invoicing. It’s a official meter from the grid operator that’s sealed. Therefore I can’t run separate wires.

It’s set up like this:

PV inverters –> PV meter –> loads –> grid meter

The Multiplus would be connected between the loads and the grid meter

4

So your grid company would pay you for producing PV power even during island operation?

Whats the make and model of the 25kW PV inverter? Most of them have digital inputs, im sure one could be programmed for a soft shutdown

5

Yes, there was a additional monetary incentive to consume the PV yourself, therefore this setup with two meters is necessary. Due to the age of this system, it still falls under this rule. Back then the export was paid better than the import, therefore this incentive was created to increase the self-consumption.

The inverter is a SMA STP 25000TL-30. I’m sure this has inputs to do a soft shutdown, however I’m unsure how fast it will react and how fast the multiplus needs reduction.

6

The STP 25K can be controlled by Modbus.

7

great, that was not my question though. Is the reaction sufficiently fast?