This work aims to develop a robust controller design for the optimal operation and stability of the diversified interconnected power system (IPS). With regards to this connection, a cascade one plus proportional, integral (1 + PI), and proportional-integral-derivative (PID) ((1 + PI)-PID) is presented to regulate the deviations in the behavior of the diversified IPS. However, a recent soft computing technique called the butterfly optimization algorithm (BFOA) is utilized for the parameter tuning of the cascade (1 + PI)-PID controller to obtain the most optimal performance. The diversified IPS examined in this work has two areas with four types of generation units, such as thermal, hydro, gas, and wind energy conversion systems, in each area. The performance of the presented cascade controller is tested on the two-area, four-source system, and its efficacy is showcased. Further, to obtain an improvement in the diversified IPS performance, a high-voltage DC (HVDC) line is operated as the tie-line. The simulation analysis confirmed the considerable enhancement in the IPS performance with the HVDC line incorporation. Finally, the cascade (1 + PI)-PID and the incorporation of the HVDC line robustness are validated through the sensitivity analysis.