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boristhespider avatar image
boristhespider asked

Fronius Zero Feed In - What happens on Network Loss?

I am considering adding some additional capacity to a setup that currently consists of an EasySolar-II.

I asked in https://community.victronenergy.com/questions/67594/feeding-one-easysolar-into-another.html about feeding one EasySolar into another, but it seems this is not such a great idea.

To recap what I said in that thread about the setup, currently an EasySolar in ESS mode supplies the house (blending grid power with inverter where necessary) and I do not want to feed into the grid. The idea is to add some generation capacity on one of the circuits (which goes to an outbuilding) coming from the existing EasySolar (so this setup would be on an AC-Out of the EasySolar).

The Fronius Zero Feed In system came up in that other thread, but I have looked further into how this works and have some concerns.

https://www.victronenergy.com/media/pg/6292-ESS_design_and_installation_manual-html5/en/configuration.html#UUID-44102771-a07a-55eb-b676-772c0d78e5fa describes using ModBus TCP to expose the Fronius control loop so that the EasySolar/Multi can control it by turning down generation on the Fronius when the power can't be absorbed by upstream loads and batteries.

The documentation also describes frequency control, but that can only be used during a grid outage (when the EasySolar can dictate the frequency of the AC waveform) and during normal operation with the grid connected to the EasySolar the only way the Fronius can be controlled is through ModBus TCP.

My concern is, what happens if the TCP link between the Fronius and the EasySolar/Multi fails. Is this a fail-safe system (meaning that if ModBus communication is lost the Fronius shuts down or regulates down to zero generation)?

I can see that this system is fail-safe from the point of view of battery state of charge, because if there's no grid to export power to, the upstream multi will vary the frequency and the Fronius will definitely receive this command on its AC lines, so can be relied upon to regulate down when there's no grid to prevent overcharging batteries.

But I'm not sure if it's fail-safe from a "no feed in to the actual grid" point of view. If the ModBus connection drops but the grid is still present, then if the Fronius kept producing excess power that the Multi didn't have battery capacity or load to dissipate, then that power would have nowhere to go except feeding in to the grid.

What happens to a Fronius configured as described in that page if the modbus TCP link is interrupted, does it shut down or regulate down, or does it keep generating power?


Fronius
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boristhespider avatar image boristhespider commented ·


Bumping this in the hope that either someone from Victron can answer, or if not perhaps someone with a Fronius on the AC-Out (configured for zero feedin) can deliberately kill it's wifi/ethernet connection for a while and see what happens.

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danielw avatar image danielw commented ·

Hi,

would be also interesting to me, too :-)

Similar configuration... and I know if only using the PV-assistant, without an "ignore AC-in" relay assistant, you will fry your batteries, until the Multi shuts down (at 36V for a 24V system)...

I don't want to repeat this with my 48V 775Ah bank :-o

Would be interesting if the ESS is more intelligent, i.e. disconnecting grid and shifting frequency. Or, as you mentioned, the Fronius has to power down if no modbus communication is active.

Daniel

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danielw avatar image
danielw answered ·

Hmmm. Seems like this question is not as easy to answer as one would expect...


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boristhespider avatar image
boristhespider answered ·

Indeed, I'll tag in @Guy Stewart (Victron Community Manager) and see if anyone from Victron knows the answer?

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Guy Stewart (Victron Community Manager) avatar image
Guy Stewart (Victron Community Manager) answered ·

Hi @BorisTheSpider & @DanielW


I reached out to Fronius to see if they could explain the situation. This was their reply.....


As far as I understood the question, it is about what happens in case of a Modbus-failure condition and grid IS present (with zero feed-in active in ESS configuration).


Fronius offers the option to reduce the inverter power to 0W in case of a Modbus failure but only if this is correctly configured and Modbus is used correctly.


The configuration in this case is usually to first set the control priority within the Datamanager to Modbus Control = priority #1, and Dynamic Power Reduction to 0W set to priority #2.


Some users like to use I/O control at priority #2 instead (all inputs open are typically configured to 0W inverter power) but with the same effect.


Please compare the attached PDF (our Easy Commissioning Guide for PV Genset systems) on pages 14 and 19-20 for the short explanation resp. screenshots for better understanding.


For additional information: on page 19-20 there is a small chapter about our Solar Net failsafe function that is reacting on a failure of the connection Datamanager <-> inverter but has nothing to do with the Modbus fallback addressed by the customer and is intended for larger multi-inverter systems.


Now the second part that is important for the function is that the controlling device writes a value to the Modbus register WMaxLimPct_RvrtTms that is NOT 0 (0 = Fronius power up default). E.g. 10 means 10 seconds -> 10 seconds after the last valid received Modbus message the Datamanger switches to the next priority (in this example: dynamic power limit to 0W - the value is freely configurable) if no new Modbus message is received within that time frame. It is the so called "fallback" time. If the value is 0, the Modbus-fallback is completely deactivated and the Modbus setpoint remains unchanged at the last valid Modbus setpoint until either a new Modbus message with a new setpoint is received or a Datamanager reboot occurs that disables the limitation completely.


I learnt in the past in a customer's system that a Colour Control device is sending 120s to this register -> 2 Minutes for the fallback time but I do not know for sure if ESS was active and/or if ESS works with a shorter fallback time as 120s is quite long (some countries are only allowing a few seconds of feeding in in case of zero feed-in).

SE_QG_Easy_commissioning_guide_PV_System_Controller_EN.pdf


se-qg-easy-commissioning-guide-pv-system-controlle.pdf (2.2 MiB)
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boristhespider avatar image boristhespider commented ·


Thanks for following this up Guy, I appreciate the information.

It seems to me I could probably manually write WMaxLimPct_RvrtTms via ModBus (from the ModBus client that is running my external control loop for the EasySolar) to set a shorter fallback period. Do you happen to know the normal polling frequency with which the EasySolar will update the Fronius with a new power limit? In other words, what I'm trying to determine is what would be the shortest practical fallback time, for example if the EasySolar polls the Fronius every second then I could set 5 or 10 seconds, but if it only polls every 20 or 30 seconds I´d need to keep the fallback time longer than that.

I don't know if the EasySolar will poll the Fronius when things are stable (ie. when it doesn't need to change the Fronius power limit) ??? Assuming the EasySolar doesn't want to change the active power limit that the Fronius is using, does it still hit the Fronius with a regular "keepalive" type ModBus poll?

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Guy Stewart (Victron Community Manager) avatar image Guy Stewart (Victron Community Manager) ♦♦ boristhespider commented ·

I don't know sorry, and it might now be some time before anyone will be able to answer further due to holidays.

So if possible, you might need to experiment (and report back your findings).

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danielw avatar image
danielw answered ·

Hi Guy,


thanks for your reply!

So indeed, if running a grid-connected zero-feed-in system, loss of communication should be taken care of... At the moment, by the installer.

It would actually be cool if, in case of communication error, the Multi would disconnect from grid and start frequency shift, but probably this would cause other problems somewhere else.

It doesn't affect my system yet, I was just curious about it. I was about to decide if better using a Fronius or a MPPT, both have their (dis-)advantages... now that the MPPT450/100 will be available soon, this sounds like the by far better solution for a PzS battery.


Daniel


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