Dual RT Box 3 Offline Simulation Divergence Issue with Two Parallel Single-Phase Cascaded H-Bridge Converters

General
1

I have developed a single-phase cascaded H-bridge converter model that runs successfully on a single RT Box 3. I am now extending this to a dual-system configuration where two identical single-phase cascaded H-bridge converters operate in parallel. Each converter runs on a separate RT Box 3 unit, and the two units communicate via SFP fiber optic links.
While the single-unit model runs without issues in both offline simulation and hardware-in-the-loop (HIL) operation, the dual-unit parallel model exhibits divergence during offline simulation. Specifically, the AC grid voltage and current waveforms become unstable and diverge shortly after the simulation starts.
Could you please explain why this occurs? We would appreciate your assistance in resolving this issue. The model I am using is attached for your reference.

SigPETcomb_1A.plecs (640.1 KB)

2

The polarity of the current source from the SFP connection is reversed. This leads to the runaway condition.

If I set the current to zero (test just the upper RT Box subsystem) I noted that the DC voltages on the paralleled side look incorrect, which is likely a control issue.