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)
- 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.
- The high power going into an already full battery can trigger a "high DC voltage" alarm.
- The high DC voltage can cause the battery management system to go into protection mode and disconnect the battery.
- 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
