I have a Multiplus 2 240v 5kva inverter that has been in use for years in a mobile application working fine. It is set up with an auto transformer on the output for split phase 120/240 in the vehicle.
Shore power is from the two hot legs of US split phase. 240v between them, neither bonded to ground. Grid neutral wire is not connected to the vehicle. I have a hardwired bond between Neutral and Ground in the load center immediately after the auto transformer. This is necessary because under no circumstances do I ever connect to ground bonded neutral. Ground relay disabled obviously.
That vehicle is now permanently stationary and hard wired to 240V grid power. So far the only change was the shore power plug replaced with hardwired grid input and has been working as normal which was great for a mobile set up but now I am behind an anti-islanding relay and want to export all my excess solar generation. 3kw of panels on the top.
Today I updated the firmware, set the grid code to Other: Neutral Externally Joined, and added an ESS assistant. The inverter will not accept the AC input, reporting Under Voltage in the IP (NS) Protection Log. It retries periodically and makes a loud rapid clicking noise from inside the inverter, a relay I assume. In VE Config UMains is yellow despite showing the same voltage as UOut. Frequency is also well within range.
For testing I ungrounded the Multiplus and used the Other grid code which has the ground bond relay permanently enabled. Everything worked perfectly including grid export for a few minutes until the ground relay test failed error was tripped.
So, any ideas why the perfectly good grid power is being rejected? No momentary voltage drop and there is properly sized wiring all the way back to the service entrance, only about 150ft away. Could this have to do with there being no neutral bond on either of the incoming hot wires for some reason?