AC PV inverter (Fronius) throttled to 0 in "inverter only" mode

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christartu asked Jun 7, '22

AC PV inverter (Fronius) throttled to 0 in "inverter only" mode

I am running an ESS system consisting of the following components:

- CerboGX at v2.84 (ESS mode 2)
- 3x MP2 48/3000/32
- 1x RS 450/100 (v1.09)
- 6x PylonTech UC3000 battery modules
- 1x Fronius Symo 10.0.3-M (connected between MP2s and GridMeter (EM24) (1:1 rule)

The general idea of the setup is to charge the batteries during the daytime using the DC coupled PV ONLY, provide the daytime energy usage with the AC coupled PV but also feed in excess energy into the grid (both AC and DC)

The ESS grid setpoint is set to -40W

AC and DC coupled feed in excess settings are both ON

Limit system feed-in is also ON

Maximum feed in is set to 15000 W (network provider requirement)

Feed-in limiting active says: YES (Fronius ModBus TCP set to control via ModBus at Prio 1)

In order to charge the batteries in the morning using the DC coupled PV energy only, I have tried to set the MP2 into "Inverter only" mode, so it wont use any of the AC coupled PV energy which I would rather feed into the grid (morning hours usually have high energy costs/profits if you feed-in)

What I have noticed though is that the AC coupled PV system (Fronius) is "running (throttled)" to basically 0-50W while the battery is not 100% charged. Once its charged 100% the throttling ends and the Fronius inverter feeds into the house/grid at full speed.

If I set the MP2s to "ON" instead of "inverter only", the Fronius inverter in the Remote Console is still showing "Running (throttled)" while in fact is it NOT throttled at all, but going full steam (not hitting the 15000W total limit yet)

While I do see the grid setpoint being set to -50W could be a reason for throttling the Fronius, but why is the "Feed in excess" not overriding this and why does the mode of the MP2s influence this behaviour?

Running the MP2 in "ON" mode, the Fronius produces much more energy than the MP2s can use to charge the battery (in addition to the RS450).

Example (rounded numbers)

Fronius produces 6000W
RS 450 provides 2500W (DC)
MP2s use an extra 2500W (AC) to charge the batteries at 100A (limit)
House uses 500W
GridMeter shows -3000 W (3000W feed in)

Switching to "inverter only" changes this to:

Fronius produces 0-50W (fully throttled down) UNEXPECTED
RS 450 provides 2500W (DC)
MP2s do NOT charge the batteries (expected)
GridMeter shows -50 to 0 W (no feed in) UNEXPECTED
House uses 500W (provided by the MP2s using the DC coupled energy and thus slowing down the charging)

Why does the Fronius not continue at 6000W feeding everything into the house / grid ?

ESS AC PV Coupling Fronius

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marekp answered · Jun 08, 2022 at 11:20 AM

@christartu

If you want all power from Fronius to be fed to the grid, install it in front of your grid meter (between grid and grid meter) and do not tell your system about Fronius. Use mostly DC coupled charger to charge the battery and send excess DC energy to grid. This way system will not be throttling Fronius down because will not know of its existence. I do not know if you can prevent the system from using AC from grid to charge the battery. You can minimize it by limiting the max charge current.

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christartu commented · Jun 09, 2022 at 06:04 PM

It is not my intention to feed in all AC PV Power.

In my ideal solution, in the morning during the "charging phase" of the battery, the AC inverter would power the AC loads in the house and feed in the rest while the DC PV charger energy is used to charge the battery.

This would avoid useless AC / DC / AC conversions and would charge the battery slower (there is no need to be at 100% at 10am).

Minimizing the charge current is also not a great solution because I want to use all available DC power to charge the battery, but just not any extra energy converted from AC. I would have to adjust the max. charge current based on the available DC PV output current. That seems like a hack and would either not use all the available DC energy (limit a bit too low), or would still trigger (limit too high) some AC power conversion (at very bad efficiency points).

Why is it not possible to simply disable AC conversion (the inverter)!?

I would imagine this to be very simple.

For now I think this is not properly supported, but isn't that the ideal solution in a mixed AC / DC PV system ?

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marekp christartu commented · Jun 09, 2022 at 06:23 PM

@christartu

In my ideal solution, in the morning during the "charging phase" of the battery, the AC inverter would power the AC loads in the house and feed in the rest while the DC PV charger energy is used to charge the battery.

That is exactly what would happen if you put Fronus outside of your system. Fronius would produce the energy and your house would take it from the grid. In reality house would take energy from Fronius and if its production is not enough the rest would come from grid. System would not control Fronius at all.

No mater what you do there always will be AC-DC or DC-AC conversion because ESS always charges the battery first. If DC PV is not enough the rest comes from AC PV AC grid.

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christartu marekp commented · Jun 09, 2022 at 06:35 PM

Yes, you are right, it would be a solution if there wasn't the 15kW feed-in limit defined by the network operator.

I need to ensure to reduce combined MP2 AC power + Fronius AC power to comply with that limit. For that reason the ESS needs to know the current Fronius power.

Given the amp limits of the system components and such it would be an edge case to pass the 15kW feed in, but today we had a few situation not far from it. The Fronius gets to 10000W easily (overprovisioned with 13kWp)

I have seen the RS450 produce above 6000W many times, 5500W quite often.

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marekp christartu commented · Jun 09, 2022 at 06:47 PM

@christartu

Leave Fronius uncontrolled outside your system and limit the feed in from your DC PV to 5000W or a bit less because that feed in control is not exact.

In your calculation you have to take the house load into consideration.

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christartu marekp commented · Jun 10, 2022 at 07:39 AM

Leaving the Fronius outside of the ESS system would have a few consequences:

- Grid Setpoint would not work anymore
- Energy Statistics would not be complete
- No insight into wether we feed in or receive energy

While this would not be super bad during the months with abundant solar energy, the rest of the year would be a problem.

We have very strongly fluctuating hourly energy prices (which are also used for feed in compensation). From -0.02 EUR to +0.60 EUR easily during the day and I expect this difference to increase over time.

For this reason I need the Grid Setpoint and I need to know the total energy balance for the current hour in order to implement my energy "trading" strategy which is very different depending on the month of the year.

In winter time we charge the battery during the night with cheaper prices than during the day. In summer time this is different, prices tend to be cheaper during the peak sun hours of the day.

I thought a bit more about your suggestion to use the "max charge current".

For now I think this is a good solution. I am now shaving off some of the DC power to charge the battery slowly during the day. The extra DC power is converted to AC and fed in / used by the house. It would be converted anyway once the battery is at 100% so there is no big difference except for the efficiency points of the AC -> DC conversion. This is good enough for me now. Thank you very much for all your suggestions!

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christartu christartu commented · Jun 10, 2022 at 08:06 AM

One more observation:

I had set the max charge current in the DCVV settings to 20A this morning and now see that it is not fully taken into account.

The DC charger now provides around 50A and the battery charges at around 50A (does not care about the 20A limit it seems).

When I set the max charge current to 100A it quickly ramps up to use 100A for battery charging by adding energy via AC -> DC.

So after all, I seem to have found my "off" switch for the AC -> DC conversion by setting the max charge current to a very low value like 1A. All the DC energy will be used anyway to charge the battery.

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Modifications Space

Node Red Space

Answer 1 · 2022-06-07T08:11:41Z

marekp answered · Jun 07, 2022 at 08:11 AM

@christartu

Can you explain closer your setup?

- CerboGX at v2.84 (ESS mode 2)
- 3x MP2 48/3000/32
- 1x RS 450/100 (v1.09)
- 6x PylonTech UC3000 battery modules
- 1x Fronius Symo 10.0.3-M (connected between MP2s and GridMeter (EM24) (1:1 rule)

If Fronius is connected between MP2s and GridMeter that means that is connected to AC-in of your MP2s therefore it is not AC-coupled.

To be AC-coupled it needs to be connected to AC-out of your MP2s.

Moreover this Fronius is 10kW so it cannot be used as AC-coupled with 3x MP2 48/3000/32 because they are only 9kVA in total.

The 1:1 rule is not met when it is AC-coupled.

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christartu commented · Jun 07, 2022 at 08:27 AM

Hm, as you pointed out yourself, the inverter is too strong to be connected to the AC out due to the 1:1 rule. That is why I moved it "outside" and IMO this is still "AC coupled" as opposed to DC coupled like the RS 450. Is this defined differently in the ESS world?

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marekp christartu commented · Jun 07, 2022 at 09:59 AM

@christartu

Inverter is "AC-coupled" when is connected "coupled" to AC-out of the inverter.

Inverter connected to the grid is "grid-coupled".

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christartu marekp commented · Jun 07, 2022 at 12:14 PM

Thank you very much for clearing that up for me !

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Answer 2 · 2022-06-07T08:35:34Z

christartu answered · Jun 07, 2022 at 08:35 AM

Some more details I skipped before

The 3 MP2s are one setup as multiphase, so one for each phase.

House <AC OUT1 (critical loads)> MP2 <AC IN1> Fronius inverter <> GridMeter <> Grid

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Answer 3 · 2022-06-08T06:57:55Z

christartu answered · Jun 08, 2022 at 06:57 AM

So, I have taken some screenshots during the charging phase in the morning. It's not done yet today (a bit cloudy), but I have learned to better understand what is happening.

1. MP2s are set to "ON", battery is not fully charged yet

Grid Setpoint is at -50W
DC Charger is used to charge battery
Fronius AC powers the house

Remaining power of the Fronius is used to charge battery via AC->DC conversion
(This is what I wanted to avoid)

2. MP2s are set to "inverter only", battery is not fully charged yet

Contrary to my earlier statement, the AC IN is actually DISCONNECTED and disconnected in a way that the flow of power is EXACTLY 0W, so it seems to be disconnected by relay (What is the reason for this?)

DC Current is -16.6A, so DC Charger power is used to power the house (DC -> AC conversion!)

Weirdly this DISCONNECTED status is not reflected in the graphical representation.
MP2 is still shown as connected to the grid!

Would be interesting to see what happens when the critical AC load rises above the level the MP2s can handle and PV charger+battery power may support!?!

My understanding is as follows:

Grid Setpoint is still at -50W

Battery is not fully charged yet
-> There is "no excess power"
-> "feed in excess AC" does not apply
-> throttle AC inverter to reach grid Setpoint

Once the battery is at 100% this changes to:

Grid Setpoint is still at -50W

Battery is fully charged
-> There is "excess power"
-> "feed in excess AC" does apply
-> Ignore the Grid Setpoint and let AC inverter ramp up (until max feed in limit is reached)

The "inverter only" mode seems to produce an edge case that the throttling logic does not fully cover and thus produces those results.

Futhermore it would be interesting to see, if -even in "ON" mode- the AC inverter power is also throttled to the MP2s maximum inverter power limit?
Would it be allowed to ramp up to 10kW production if the MP2 may only use 6-7kW to charge?
Would it be allowed to ramp up to 5kW production if the battery charge limit would allow only 2 kW usage by the MP2s?

It seems that my idea to use the "inverter only" mode is not producing the intended results.
Any idea on how to avoid the AC -> DC conversion during the charging phase?

It has many downsides:
- not efficient
- batteries get charged very quickly and are at 100% at 10am already, staying there until 8pm when battery power usage starts again.
- While I want to slowly charge during the day (using the PV DC charger only), I still want to produce as much AC power as possible for earn power credits for the winter time.

I think a new switch "Allow AC conversion to charge" would be helpful!

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christartu commented · Jun 08, 2022 at 07:14 AM

"ON" mode:

So, close to 100% but DC charger and AC inverter are producing more energy than can be used to charge (charge limit defined by PylonTech BSM to around 88A)

Grid Setpoint is ignored in this case and AC inverter is allowed to ramp up!

"Inverter only":

Charge limit would allow higher charging as before, but only DC charger power is used.

AC inverter is throttled down to apply the Grid Setpoint

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marekp christartu commented · Jun 08, 2022 at 11:07 AM

@christartu

"Inverter only" behavior is "kind of" logical.

You telling the system that you want to use only inverter for managing the loads, and system does what you ask of it.

Turns of AC -in source of energy and uses inverter only for he loads. Inverter to do that takes energy from the DC side.

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Answer 4 · 2022-06-08T07:53:27Z

christartu answered · Jun 08, 2022 at 07:53 AM

Here is the behaviour at 100% SoC and "inverter only" mode.

So, it seems it is still not considering the "AC inverter" power as "excess power" and throttles it down to apply the grid setpoint.

So, it is not connected to the SoC of the battery but only to the fact that in "inverter only" mode the AC IN is disconnected.

Original question still remains: I don't see the point in throttling down the AC inverter in this case (unless it hits the maximum feed in limit).

And finally, is there an alternative to "disable the AC > DC conversion" in the MP2s ?
Maybe a "disable charger" mode?

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Answer 5 · 2022-06-12T13:54:19Z

lucadea1985 answered · Jun 12, 2022 at 01:54 PM

I also have your configuration and similar problems. I want to charge the battery with only 20A and with the surplus power the loads so that the fronius injects everything into the network, I do not want to charge the battery with the fronius except in winter, I want the fronius to help only if the loads are higher than 20A battery + loads... For now I'm using ignore ac input to do so, with activation setting over certain loads. It seems to me that it is not possible and so I downloaded venus large node red with which I seem to have understood that you can program any logic but I am not able to do it

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marekp commented · Jun 12, 2022 at 02:07 PM

@lucadea1985@gmail.com

Do you run ESS?

In my system when I set the "limit charge current" to 0A

System is charging battery only from MPPT and not using grid or PV inverter.

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lucadea1985 marekp commented · Jun 12, 2022 at 02:40 PM

hello marekp, we have already written about another discussion a few days ago. yes I have ess but if I set 0A my mppt goes to 0W I think because I have not set feed in CC. Do you know how to use node red?

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marekp lucadea1985 commented · Jun 12, 2022 at 03:04 PM

@lucadea1985@gmail.com

It would be logical that if battery is full and DC PV is not allowed to feed the grid, MPPT will be throttled down.

But if battery is not full it should charge with MPPT.

It could be that I do not understand that setting yet fully.

For long time I did not get why when I set the limit charge current to some low number the system ignored it. Lately I noticed that it did not ignore it completely but applied it only to the MP-II's charge current and letting the MPPT charge at will.

As for node-red I am not familiar with it yet. It will be my next project when I put together small test system for playing with settings. Did not want to experiment on the main system. :)

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baxter marekp commented · Jun 24, 2022 at 07:48 AM

Limiting the charge current in DVCC effects MP and MPPT if dc coupled pv feed in excess is not enabled in ESS.

As soon as you enable it, the MPPT is not longer controlled by the charge current but by the voltage. This is a long term pain in the ass for all BMS users which can not controll the voltage (eg seplos).

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marekp baxter commented · Jun 24, 2022 at 08:05 AM

@Baxter

Thank you for this info.

I tested it and you are correct.

It is a bit illogical but it is what it is.

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lucadea1985 marekp commented · Jun 12, 2022 at 03:17 PM

yes, I understand but I really have a lot of energy wasted by pv dc so I would like to charge the battery at low power 15/20A, and with the surplus pc dc I would like to power the critical loads. In this way I can inject into the grid 10 kwh more from the fronius every day that I need for winter consumption. All this week I stayed off grid, recharged the battery and powered the loads all with the pv cc... the fronius injected everything into the grid.

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Answer 6 · 2022-06-12T14:07:40Z

mazenkhaddaj answered · Jun 12, 2022 at 02:07 PM

@christartu you are not creating a micro-grid, you are connected to the grid. So you change your FRONIUS settings to international 50 or 60 Hz or whatever country you live in. And then simply Fronius will not lower output.

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