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colj avatar image
colj asked

ESS system with grid attached Solar PV running in 'micro grid' mode during Grid power failure

Hi All,

Scenario as follows. Grid attached Solar PV (SMA 3kw) installed 12 years ago. Quattro ESS system recently installed. Testing Grid failure when batteries are full and excess PV being generated. What should happen?

My understanding is that the SMA inverter was configured as grid attached (correctly) when it was installed i.e. frequency shifting will not work. This cannot be changed since my system has a bypass switch for the Quattro so the SMA inverter must always be configured as 'grid attached'.

What appeared to actually happen was the system tried to frequency shift (per the ESS parameters) and the SMA inverter noted the frequency was outside the acceptable 'grid-attached' range and decided the grid was faulty and stopped inverting. A few minutes later (i think there is a minimum wait time in the SMA config) , the system had restored normal frequency and the SMA inverter restarted. This cycle repeated many times - it did avoid overheating the system (not least because eventually somehow it caused the PV RCD trip). So, was this just lucky? I've seen other forum entries for similar scenarios where systems have overheated and shut down.

Colin

ESSSolar Panelmicrogrid
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4 Answers
bassman avatar image
bassman answered ·

Hi Colj,

We have an Enphase S270 ESS System. As Enphase has advised (at least with the S270 range) to not rely on frequency shift in this situation. Our solution was to program an assistant in the MP2 to switch off the Enphase (on grid failure) if the batteries are at 100% SOC and to only allow them to be switched back on at 95% SOC. Has worked well so far! This may help.

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colj avatar image colj commented ·
That sounds exactly what is needed to protect the system. Thank you. How did you manage to switch off the PV - did you use a relay to switch off AC to the PV inverter or was it an Enphase specific control please?
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bassman avatar image bassman colj commented ·
Hi, We used the Primary Programmable Relay Assistant, 1 to turn off, 1 to turn on. There is a specific listing for Battery SOC. Unfortunately my designer worked out any the details and I don't know if any additional wiring is required for the relay!! We have an ET112 which I believe is triggered by the relay. I hope this helps. Cheers.
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gazza avatar image
gazza answered ·

I understand that the SMA is permanently grid tied, but could you provide a simple block diagram of the other parts showing relevant inputs and outputs connected for your "Testing Grid failure" scenario? Was the grid actually isolated at the SMA?

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colj avatar image colj commented ·

Hopefully this shows the scenario:

Edit: To clarify, the SMA is permanently configured 'grid tied' (though clearly is part of a micro grid when the Grid is isolated as shown). The situation shown below will occur automatically when the grid fails (and reconnect when the grid is restored).

gridfailtest.jpg

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gazza avatar image
gazza answered ·

In answer to your question if the batteries are full, no grid available and the PV is producing more than the load requirement the excess power has to go somewhere (to prevent damage) and there is nowhere for it to go! So ESS would try and frequency shift to reduce PV power. The SMA configuration may need looking at?
The answer may be in the setup instructions but I am not familiar enough with the SMA configuration to help I am afraid.

https://www.victronenergy.com/live/ac_coupling:start

https://www.victronenergy.com/live/ccgx:ccgx_sma

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colj avatar image colj commented ·
That's the crux. The SMA is configured at install time depending on how it is installed either as grid attached or as micro grid. It's not dynamically changeable between the two (requires a password only given to certified installers). If configured for microgrid it cannot be attached to the grid. Given I have a bypass switch (for the Quattro) as do oher ESS configs I am aware of, then it has to be configured as grid attached (in which case frequency shifting doesn't work/isn't supported). Hence my original question - was I just lucky in avoiding damage since the SMA inverter stopped inverting when the ESS system tried to frequency shift because the SMA thought the frequency shift meant the grid had failed?
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gazza avatar image
gazza answered ·

Yes, I would imagine if it still works in normal mode (ie with grid connected) then you have probably got away with it, or at least the SMA has done it's job in protecting itself.? What is the prospect of having the SMA reprogrammed, but without the changeover switch? Then you could use as microgrid (in power failure) situation and with grid available to AC IN of the Quattro ESS would feed excess to grid as usual?

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colj avatar image colj commented ·
Thanks but I don't want to lose the bypass capability so am stuck with the SMA in grid attached configuration!
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