I've recently added a second Multiplus-II 48/5000 unit in parallel to one that's successfully been running for nearly 2 years, in ESS mode.
I've read through the guide (https://www.victronenergy.com/blog/2021/02/02/victronconnect-parallel-three-split-phase-setup-and-more/), ensuring the wiring for each unit was the same length. They both have the same firmware (updated to latest version, 494), and have successfully been configured in parallel mode by VE Quick Configure. They also both have the same settings (through VEConfig), including the same ESS assistant.
ESS mode is set to "external control", like it was for the 2 years running on the single unit.
The system works well to charge (can get the full charge current), as well as power loads. The slave unit is showing as slave (correct LED codes).
However, the problem is when I try and set the grid setpoint to negative: after a while (timing of how long it takes is random, but a few minutes is the best I've had) I get a strange oscillating behaviour. Within about 0.8 seconds, the power draw from the battery can spike to a few kW, both positive and negative (and lights flicker accordingly on AC out). The transformers within both Multiplus units also emit a very noticeable crescendoing buzz. This keeps cycling, until the minute timeout of sending a grid setpoint.
Even applying a grid set point set to a value within what the single Multiplus unit was able to achieve before exhibits the behaviour.
Any idea what could be causing this issue?