## 4 Conclusions  
In this paper, a new GA-FLC controller was proposed for frequency stability problems in a two area multi source interconnected power system. The power system to be controlled was consisted of a thermal, a reheated thermal, and a PV unit. In addition to the proposed controller, conventional PID, FL, and GA-based PID controllers were designed. All systems and controllers were designed and simulated with the Matlab-Simulink program. In the simulations, a change in the frequencies and the tie-line power were observed by implementing each controller. Since there was a two-area power system and they were connected by a tie-line, the outputs of \(\Delta F_{1}\), \(\Delta F_{2}\), and \(\Delta P_{tie}\) of the system were individually plotted. As can be seen from the data in Table 6, the proposed controller produces better results than the other controllers, both in terms of overshoot and settling time values. These values were taken during the load increase and the solar power decrease periods. Therefore, this demonstrates the reaction of the system at the moment of operation and a more realistic simulation was realized. The proposed GA-FLC has three membership functions with an optimized GA, and twenty-five rules. Owing to the proposed controller, the system frequency is affected at a minimum rate when the power changes on the PV-SPP because of climatic conditions. Moreover, the ability of the system to adapt to sudden power fluctuations at the operation point was also improved. In the meantime, the optimum values of the \(K_{p}\), \(K_{i}\), and \(K_{d}\) parameters for the PID controller were determined by the Zigler-Nichols method.  
In conclusion, the frequency stability problem caused by solar energy sources in the interconnected power systems was abolished. According to these results, for the frequency stability, the proposed GA-FLC yielded an efficiency of about 90% compared to the conventional PID controllers. Moreover, it was observed that the overshoot value of the proposed control technique was 75% better than the FLCs, and approximately 40% better than the GA-PID controllers. Ultimately, since the the overshoot values and the settling time values directly affect the operating lifetimes, operating costs, and efficiencies of the grid, the proposed GA-FLC would be beneficial and advisable for an interconnected power system with solar power sources.  
**Author Contributions:** [PERSON] proposed the main idea of the method. [PERSON] and [PERSON] established and validated the method, including the set up of the analysis model, and conduction of the simulation study. [PERSON], [PERSON], and [PERSON] actively contributed in the writing of the manuscript.  
**Conflicts of Interest:** The authors declare no conflict of interest.  
**Abbreviations and Nomenclatures**  
\(A\): Curve fitting factor \(P_{PV1},\ P_{PV2}:1\) and \(2\) areas power values derived from PV  
\(ACE_{i}\): Area Control Error of i \(P_{\Delta F}\): The balance of power i  
\(B_{1},\ B_{2}\): Frequency bias constant of area \(1\) and \(2\)  
\(c\): Electron charge \(R_{g}\): Series resistance of cell (\(\Omega\))  
Figure 23: The power change over the connection line between area \(1\) and area \(2\).