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rrroonn avatar image
rrroonn asked

SMA grid-tied inverter and Multiplus II battery over-voltage prevention

This is more of an "this is what I did" than a question - hopefully helps others.


What is the problem?

I have a grid-tied SMA TL-5000 on the AC-Output of the MP2. When disconnected from the grid, the MP2 will modulate its AC output frequency to limit SMA output when needed. For example, the battery is full and AC loads are low, the excess power cannot be fed back to the grid, so the MP2 tries to throttle back the SMA to match power requirements.

The problem with this control method is that it is a relatively slow process and does not respond to rapid changes. This slow response can cause a total power outage as follows:

  • Grid is connected
  • Battery is fully charged
  • Solar inverter is putting out high power
  • MP2 is exporting excess energy
  • Grid fails (or disconnects)
  1. The sudden loss of the grid causes the MP2 to dump power into the battery while it tries to (slowly) gain control of the SMA output.
  2. The high power going into an already full battery can trigger a "high DC voltage" alarm.
  3. The high DC voltage can cause the battery management system to go into protection mode and disconnect the battery.
  4. Now you have no grid and no battery - the lights go out.

Solutions?

Solution 1 - Direct control. Ideally, the MP2/GX should talk directly to the SMA inverter (EG via its modbus interface) to instantly shut down power output and then turn it back on when the battery voltage and AC load conditions allow.

Solution 2 - Relays / Contactors. In a previous discussion on this forum, it was proposed to have two contactors in parallel on the SMA inverter feed. Contactor 1 would be powered by the AC-In supply. Failure of the grid would remove the SMA from the system, instantly removing the over-supply problem. The problem is that you would not have any solar power when the grid is down, so a second contactor was proposed to be driven by the MP2 / GX in response to battery SOC. IE, if SOC was less than 99%, then enable relay to allow solar power input. If SOC is >= 99% then disable the relay and remove solar power input.

This would work, through clearly Solution 1 is much easier to achieve.

Solution 3 - External control. I did go down the route of using contactors but then identified that I could achieve a Solution-1-like outcome. The SMA inverter is controllable via its modbus interface. I already have a home automation system using Home Assistant and Node Red and already use it to "talk" to the MP2 and the SMA.

By monitoring the current SMA state, the MP2 grid-relay status and the Battery SOC, I could use this to disconnect / reconnect the SMA when required.


Basically, the Node Red flow, reads the grid-relay state, the battery soc and the SMA mode; presenting this to the the control logic below:



While Solution 1 is the ideal solution, it requires Victron to do some coding to add the functionality, though it may be possible to add custom scripts to the GX device yourself.

Solution 3 is another option if you already have a home automation system.


I am happy to share further information and details if someone is interested.


Ron

Multiplus-IIgx device
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6 Answers
teemischer avatar image
teemischer answered ·

Great solution, thanks for sharing!


I acutally need the same functionality and I would very much appreciate if Victron could simply add those 2 modbus write commands to their own code. We all know that throttling works well with Fronius inverters and we also know that SMA does not 100% comply with the spec, but please Victron, could you just please write custom code for the SMA inverters so the GX can directly throttle the inverter power for feed-in limiting?

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muntanga-munkombwe avatar image
muntanga-munkombwe answered ·

Out of interest i want to find out if adjusting the Victron MPII parameters in the PV assistant to match those mentioned in the off-grid parameters stated by SMA would be of any help? The

Parameters stated by SMA under Frequency Shift Power control (extracted from "Technical Information - use of PV inverters in Off-grid and battery backup Systems)

Would it make sense to attempt to input and adapt these settings in the PV assistant? I intend on adding an SMA Sunny boy 4.0kW (AV41) and additional PV to my existing off-grid installation and would be interested in know if this would work.

Table with general suggested settings for 50Hz and 60Hz frequency Fronius SMA Fronius SMA Frequency System 50Hz 60Hz Start 50.2 51 60.2 61 Hz Minimum 52.7 52 62.7 62 Hz Disconnect 53 54.5 63 64.5 Hz


Please let me know if you think if this is feasible or helpful. In relation to the battery over-voltage condition it may be advisable to introduce lower full battery set points so as to encourage the inverter charger to taper the power allocation faster.


Muntanga







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1 comment
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rrroonn avatar image rrroonn commented ·
Sorry, old post, but I dont get notifications.


The PV assistant has little in the way of configuration, other than sizing information. The settings you refer to are available by programming the SMA inverter.

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Paul avatar image
Paul answered ·

This goes toward Vicron Rocket science :-), happy I can follow (mostly...)


Does this also count for Fronius inverters too?

Or does the Symo 5.0 have a better comms with the Victron system?

I will upgrade my MP2 5000/48 with two extra ones for a 3 phase system and want to move my SMA STP5.0 to the AC out in order to charge batteries and keep our house "alive" AND charge the batteries during a grid failure.

A Smartsolar is also included in my current setup so I should be able to charge the batteries at DC side and feed the users and/or charge the batteries from ACout as well, like done by Victron for many BIG projects

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Phil Gavin avatar image
Phil Gavin answered ·

I have used AC coupling with other brands in the past and never had this issue, but I intend to deploy SMA PV inverters in the future.

May I ask if this issue exists after Victron guidelines for AC coupling have been complied with?

Namely, the 1:1 rule, the MP2 must be a minimum of 5kVA, and the battery sizing as stated below:

3.1 Lead batteries

1 kWp installed PV power requires approximately 5kWh of lead acid battery:

  • 100 Ah at 48 Vdc

  • 200 Ah at 24 Vdc

  • 400 Ah at 12 Vdc

Each additional 1 kWp of AC PV will require an additional proportional 5 kWh increase in lead acid battery storage.

3.2 Lithium batteries

1,5 kWp installed AC PV power requires 4.8 kWh of battery storage:

  • 100 Ah at 48 Vdc

  • 200 Ah at 24 Vdc

  • 400 Ah at 12 Vdc

Each additional 1.5 kWp of AC PV will require an additional proportional 4.8 kWh increase in battery storage.


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robmeerwijk avatar image
robmeerwijk answered ·

Hi Rroonn,

I am planning on building my own 3fase MP2 installation with 64kWh batteries. I do have a few SMA inverters operational. I want to use a variable pricing contract for energy based on day ahead prices. Prices can be negative in the Netherlands. I would like to prevent the solar panels from producing in this situation. At the same time I would like to get paid for charging batteries or EV.

So controlling the solar inverters output power without losing grid connection is essential. Therefore I am interested in your solution. Do you still use it? Does it work well?

Thanks for your answer!

Rob Meerwijk

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rrroonn avatar image rrroonn commented ·
Yes, this would work. It does rely on the SMA inverters having the WebConnect module for TCP/modbus access.
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Ben Walsh avatar image
Ben Walsh answered ·

Hey Rob,

I have been successful in controlling SMA inverters with frequency shifting from Victron. You need to make sure your ESS assistance frequency settings and your SMA inverter settings match up. Have you put your SMA into island mode and made alterations to the active power gradients?

Ben

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