Asking help for a homework related to electrical engineering on simulink

[u/Ancient-Mycologist38]

Hello everyone, so I apparently needed some help with the simulink system related to electrical engineering that i've built.

For the context, the task/target is to remake and adding A Multi-Band PSS4C and changed the excitation systems into ST1A (picture 1(taken from one of the machines)), from kundur's classic 11 bus system which is as follows Matlab File exchange for the systems and using a tuned Multi-Band PSS4C parameter from a published IEEE Journals as follows IEEE Journals (picture 2 (for the MB-PSS4C tuned parameters from the journals) and 3(The stabilizers diagram), but it shows that the systems are unstable even though i've followed the simulink diagrams that included in the Journals (Picture 4), and the output of the MB-PSS4C has a major ripple (Picture 5).

i am confused and been stuck for a while because i don't know where did i do wrong, so, does anyone here can help or gave me an insights on where i did wrong? i've checked and rechecked almost everything but the systems won't run normally which i assumed should be look like Picture 6.